Pubic Healt

100honestywork

chapter_7.docx

Home >Science homework help >Pubic Healt

CHAPTER 7

Communicable Diseases

LEARNING OBJECTIVES

By the end of this chapter, the student will be able to:

• describe the burden of disease caused by communicable diseases.

• describe the criteria that are used to establish that an organism is a contributory cause of a disease.

• identify factors that affect the transmissibility of a disease.

• identify the roles that barrier protections play in preventing communicable diseases.

• identify the roles that vaccinations can play in preventing communicable diseases.

• identify the roles that screening, case finding, and contact treatment can play in preventing communicable diseases.

• identify the conditions that make eradication of a disease feasible.

• describe a range of options for controlling the HIV/AIDS epidemic.

Your college roommate went to bed not feeling well one night, and early the next morning, you had trouble arousing her. She was rushed to the hospital just in time to be effectively diagnosed and treated for meningococcal meningitis. The health department recommends immediate antibiotic treatment for everyone who was in close contact with your roommate. They set up a process to watch for additional cases to be sure an outbreak is not in progress. Fortunately, no more cases occur. You ask yourself: Should your college require that all freshmen have the meningococcal vaccine before they can register for classes?

As a health advisor to a worldwide HIV/AIDS foundation, you are asked to advise on ways to address the HIV and developing tuberculosis (TB) epidemics. You are asked to do some long-range thinking and to come up with a list of potential approaches to control the epidemics, or at least ways to reduce the development of TB. The first recommendation you make is to forget about eradicating HIV/AIDS. How did you come to that conclusion?

Your hometown of 100,000 is faced with a crisis as an airplane lands containing a passenger thought to have a new form of severe influenza that has recently gained the ability to spread from person to person through airborne transmission. As the mayor of the city, what do you decide to do?

You are a principal at a local high school. One of your top athletes is in the hospital with a spreading bacterial infection due to staphylococcus bacteria resistant to all known antibiotics. The infection occurred after what appeared to be a minor injury during practice. As the principal, what do you decide to do?

Diseases due to infection form a large part of the history of public health and are again a central part of its present and its future. Infections of public health importance are primarily those that are communicable; that is, they can be transmitted from person to person or from animals or the physical environment to humans. Communicable disease may be caused by a wide variety of organisms, ranging from bacteria, to viruses, to a spectrum of parasites, including malaria and hookworm. Let us examine the burden of disease due to communicable diseases. a

WHAT IS THE BURDEN OF DISEASE CAUSED BY COMMUNICABLE DISEASES?

For many centuries, communicable diseases were the leading cause of death and disability among all ages, but especially among the young and the old. Communicable diseases are not only the causes of great epidemics, but they can also become endemic and become routine causes of death. b These diseases included a key role in maternal deaths associated with childbirth, infant and early childhood deaths, as well as deaths of malnourished infants and children.

The last half of the 1900s saw a brief respite from deaths and disabilities caused by communicable diseases and other infections. This was due in large part to medical efforts to treat infections with drugs and public health efforts to prevent infections (often with vaccines) and to eradicate or control other infections. Even as these great accomplishments were underway, warning signs of bacteria resistant to antibiotics began to appear. Staphylococcus organisms resistant to current antibiotics began to plague hospitals in the 1950s until new antibiotics were developed. Resistance of gonorrhea and pneumococcus to a range of antibiotics became widespread. World Health Organization (WHO) and U.S. government–sponsored programs, such as those to promote the eradication of malaria and TB, were not able to have sustained impacts, and the goals were trimmed back to control rather than eradicate.

The early 2000s have seen the return of infections that were previously under control, as well as an emergence of new diseases. Tuberculosis, the great epidemic of the 1700s and 1800s, has returned in force, partially as a result of HIV/AIDS. Box 7-1 looks at the history of TB and the historical and current burden of disease caused by it. 1 , 2

Over a dozen previously unknown infections have emerged in recent decades—the majority of which are believed to have originated in animal species. In the United States, the presence of Lyme disease and the West Nile virus were unknown until the late 1900s, but have now spread to extensive areas of the country. Long-established diseases, such as malaria, are extending their geographic range, and farther extension is expected with global warming.

Influenza is anticipated to return again in pandemic form, as has occurred repeatedly in prior centuries. A pandemic is an epidemic occurring worldwide, or over a very wide area, crossing international boundaries and affecting a large number of people. A pandemic of influenza is most likely to occur when ongoing mutations produce new strains capable of person-to-person transmission. Whether you live in a dorm, are a public health professional, a clinician, a politician, a high school principal, or are involved in almost any profession, communicable diseases are part of your present and your future.

BOX 7-1 The Burden of TB

“If the importance of a disease for mankind is measured by the number of fatalities it causes, then tuberculosis must be considered much more important than those most feared infectious diseases, plague, cholera and the like. One in seven of all human beings die from tuberculosis. If one only considers the productive middle-age groups, tuberculosis carries away one-third, and often more.” 1

—Robert Koch, March 24, 1882

The history of tuberculosis (TB) goes back to ancient times, but beginning in the 1700s, it took center stage in much of Europe and the United States. It has been estimated that in the two centuries from 1700 to 1900, tuberculosis was responsible for the deaths of approximately one billion human beings. The annual death rate from TB when Koch made his discovery was approximately seven million people. 1 Today, that would be the equivalent of over 30 million people, considering today’s population.

Robert Koch’s discovery of the association between the tuberculosis bacilli, its culture and isolation, and its transmission to a variety of animal species provided a clear demonstration that the bacilli are a contributory cause of the disease. While the tuberculosis bacilli are clearly a contributory cause of the disease tuberculosis, they are not sufficient alone to produce disease. A large percentage of the world’s population harbors tuberculosis. Other factors are needed to produce active disease. These factors include reduced immunity and nutrition, as well as genetic factors.

The discovery of the tuberculosis bacilli actually followed the development of what was called the sanitarium movement, which began in Europe and the United States. Sanitariums isolated tuberculosis victims, while providing good nutrition and clean air. The sanitarium movement was coupled in the early 1900s with the use of the Bacillus Calmette-Guérin (BCG) vaccine, purified protein derivative (PPD) skin tests, and the recently invented chest X ray. These three early and rather crude technologies are still in use today and are designed to prevent and diagnose TB. In addition, the understanding of the epidemiology of the tuberculosis bacteria led to a clear victory for public health, with the near elimination of TB from the milk supply early in the 1900s.

Thus, even before the ability to actively treat tuberculosis, public health interventions were able to dramatically reduce the frequency of the disease, at least in Europe and the United States. A second round of efforts to control TB began in the 1940s with the discovery of streptomycin, the first anti-TB drug, followed over the next decade by para-aminosalicylic acid (PAS) and isoniazid (INH). Combination drug treatments proved highly effective. In addition, INH was found to be effective on its own to prevent skin-test-positive TB from progressing to active disease. Public health and medical efforts to conduct screening for positive skin tests and selectively treating with INH became widespread.

Thus, by the late 1950s and 1960s, TB was brought under control by the combination of medical and public health advances. The success of this effort resulted in the closing of TB sanitariums, gradual cutbacks in screening and treatment programs, and a general loss of interest in TB. Beginning in the mid-1960s, there was little interest in or research on TB. Tuberculosis became a treatable disease usually handled as part of routine medical care. Most medical and public health practitioners regarded it as a disease ready for eradication.

Unfortunately, TB was prematurely pronounced dead. It had never come under control in many parts of the world, and approximately 33% of those living in developing countries today are estimated to harbor the TB bacillus. That is, they carry TB organisms that may multiply and spread in the future.

Soon after the beginning of the AIDS epidemic in the 1980s, active TB came back with a vengeance. AIDS patients with latent TB often developed active TB quite early in their battle with AIDS. They then became contagious to others. Tuberculosis may be more difficult to diagnose and may progress faster in AIDS patients.

Coupled with the return of TB as a public health problem, resistance to TB drugs began to emerge. The problem was successfully combated in the 1990s by the simple public health intervention known as directly observed therapy, or DOT. DOT helped ensure that patients received all the prescribed treatment, thus greatly increasing adherence to effective treatment.

Nonetheless, in the early years of the 2000s, resistance to multiple TB drugs increased all over the world. We were faced with a triple threat of limited recent research, leaving us without modern diagnostic aids or new drugs; a rapidly emerging threat from multiple-drug-resistant tuberculosis; and a spreading epidemic of HIV/AIDS, predisposing patients to active tuberculosis. Each of these can be addressed by coordinated public health and medical actions, which have been occurring in recent years. In the past, TB has been exceptionally responsive to a wide range of public health and medical efforts to control its spread. There is hope that with increased awareness and further research, this will happen again. 2

History suggests that public health and medical interventions have and will continue to have major impacts on the burden of communicable diseases. Let us look at how we establish that an organism is in fact capable of transmitting a communicable disease.

HOW DO WE ESTABLISH THAT AN ORGANISM IS A CONTRIBUTORY CAUSE OF A COMMUNICABLE DISEASE?

Establishing that an organism is a contributory cause of a disease traditionally relied on Koch’s postulates. Koch’s postulates hold that in order to definitively establish a cause-and-effect relationship, all of the following four conditions must be met:

1. The organism must be shown to be present in every case of the disease by isolation of the organism.

2. The organism must not be found in cases of other disease.

3. Once isolated, the organism must be capable of replicating the disease in an experimental animal.

4. The organism must be recoverable from the animal.

Ironically, TB could not be shown to fulfill Koch’s postulates. However, a very useful set of Modern Koch’s postulates has been developed by the National Institute of Allergy and Infectious Disease. Modern Koch’s postulates require:

1. Evidence of an epidemiological association

2. Isolation

3. Transmissions to establish that an organism is a contributory cause of the disease 3

At times, researchers have been able to directly isolate and transmit diseases from person to person. For instance, fluid from chicken pox skin was collected and transferred to other individuals, resulting in active cases of chicken pox.

This type of direct evidence of transmission is unusual in humans. Nonetheless, outbreaks of disease can often provide the evidence needed to fulfill Modern Koch’s postulates, as illustrated in the case of the bacteria Neisseria meningitidis, which has been established as the organism that causes meningococcal meningitis and sepsis.

Meningococcal meningitis, an important cause of meningitis and blood-borne infection, or sepsis, was first recognized during an epidemic in Geneva, Switzerland, in 1805 and occurred in epidemic form especially during military conflicts throughout the 1800s. The bacteria Neisseria meningitidis was first cultured from patients with acute meningitis in 1887. Efforts to fulfill Koch’s postulates were not successful because transmission to animals was not possible and a large number of humans were carriers without developing the disease.

The ability to culture or isolate the bacteria in sick individuals in the meninges and the blood, areas of the body that should be free of bacteria, was key evidence in establishing the causal role of Neisseria meningitidis in the disease. Transmission from person to person has been documented in outbreaks of the disease and has been used to confirm the final criteria of Modern Koch’s postulates.

Understanding the mechanism of transmission is important for establishing etiology, but it is also important for appreciating the degree of communicability of a disease and in designing interventions to reduce its spread. Let us take a look at the factors that affect the ease with which a communicable disease is transmitted.

WHAT FACTORS AFFECT THE EASE WITH WHICH A COMMUNICABLE DISEASE IS TRANSMITTED?

Three factors have major impacts on the ease of transmission of communicable disease. These are:

• Route of transmission—the anatomical and physiological methods for transmission from person to person and from animal species to humans

• Asymptomatic transmission—the ability to transmit the disease while humans or animals are free of symptoms of the disease

• Reproduction ratio (R0)—the number of new cases one individual with the disease generates on average over the course of its communicable period

Let us take a closer look at each of these factors.

Route of Transmission

A wide range of possible routes of transmission has been demonstrated for communicable disease. Communicable diseases may be transmitted from person to person or from animal species or the physical environment to humans. Table 7-1 outlines the major methods of transmission of human disease and provides examples of each of the methods. 4

Asymptomatic Transmission

Diseases with the potential to create human epidemics are often diseases that can be transmitted from person to person while the individual is free of symptoms. Classic examples include influenza, chicken pox, measles, and mumps. Transmission often occurs in the days or weeks prior to the development of symptoms, but it can also occur from individuals who never develop signs or symptoms of the disease.

TABLE 7-1 Methods of Transmission of Human Disease

Method of transmission	Examples
Insects	Malaria, Lyme disease, West Nile virus, yellow fever
Other animals	Rabies, avian flu, anthrax, plague, other zoonotic diseases
Airborne—person-to-person	Influenza, SARS, measles, chicken pox, tuberculosis, common cold
Sexual transmission/open sores	Gonorrhea, syphilis, herpes genitalis, chlamydia, hepatitis B, HIV
Water/food	Hepatitis A, cholera
Fecal/oral	Polio, salmonella
Transfusions/blood/contaminated needles	Hepatitis B and C +
Transplacental	Rubella, HIV
Breastfeeding	HIV
Contaminated articles (“fomites”)	Chicken pox, common cold, influenza

Data from Timmreck T.C., An Introduction to Epidemiology 3rd Edition Jones and Bartlett, Sudbury Mass 2002 p. 34.

In addition to those who transmit the disease as part of their initial exposure to it, other individuals can transmit the disease after they have recovered from the clinical disease or after they are infected with it but do not develop symptoms. Those individuals without symptoms but with the ability to chronically transmit the disease are called chronic carriers. Examples of disease that often lead to chronic carriers are HIV, hepatitis B, and hepatitis C.

Reproduction Ratio

The reproduction ratio, often called R0, provides a measure of the inherent transmissibility of a disease. It indicates the number of new cases one individual with the disease generates on average over the course of its communicable period. The reproduction ratio depends heavily on the disease’s route(s) of transmission as well as the presence or absence of asymptomatic transmission, but the reproduction ratio provides a measure of the potential for an epidemic’s development given the patterns of transmission in a particular society.

When R0 is less than 1, each infected individual results, on average, in less than one new infected individual. Therefore, a communicable disease with an R0 less than 1 is thought to have a low potential for becoming an epidemic disease. If the R0 is greater than 1, the infection is likely to become endemic or epidemic in the population. Diseases with high R0s have great potential for developing into epidemics. While the R0 is not a perfect predictor of the chances that a disease will become epidemic, a number of historical epidemic diseases have a high R0, including measles (R0 12 to 18), pertussis (R0 12 to 17), polio (R0 5 to 7), and mumps (R0 4 to 7). 5 , 6

Now let us turn our attention to the tools that are available to public health to deal with communicable diseases.

WHAT PUBLIC HEALTH TOOLS ARE AVAILABLE TO ADDRESS THE BURDEN OF COMMUNICABLE DISEASES?

A range of public health tools are available to address the burden of communicable diseases. Some of these are useful in addressing noncommunicable diseases as well, but they have special applications when directed toward infections. These include:

• Barrier protections, including isolation and quarantine

• Immunizations designed to protect individuals as well as populations

• Screening and case finding

• Treatment and contact treatment

• Efforts to maximize effectiveness of treatments and prevent resistance to treatment

Let us look at each of these tools.

HOW CAN BARRIERS AGAINST DISEASE BE USED TO ADDRESS THE BURDEN OF COMMUNICABLE DISEASES?

Examples of barriers to the spread of infections are as old as hand washing and as new as insecticide-impregnated bed nets, which have had a major impact on the rate of malaria transmission. Barrier protection, such as condoms, is believed by many to be the most successful intervention to prevent sexually transmitted diseases. The use of masks may be effective in reducing the spread of disease in healthcare institutions, such as hospitals. The same measures may be preventative in the community at large and are a routine part of winter weather habits in much of Asia.

A special form of barrier protection consists of separating individuals with disease from the healthy population to prevent exposure. As we discussed previously, sanitariums had a major impact in reducing outbreaks of TB in the 1800s and the first half of the 1900s. Today, once again, we are faced with issues of isolation and occasionally have to legally enforce quarantine.

A second traditional public health approach to communicable and noncommunicable infections is the use of immunizations. Let us take a look at a range of ways that immunizations can be used to address the burden of communicable disease.

HOW CAN IMMUNIZATIONS BE USED TO ADDRESS THE BURDEN OF COMMUNICABLE DISEASE?

Immunization refers to the strengthening of the immune system to prevent or control disease. Injections of antibodies may be administered to achieve passive immunity, which may provide effective short-term protection. Inactivated (dead) and live vaccines (attenuated live) can often stimulate the body’s own antibody production. Live vaccines utilize living organisms that also stimulate cell-mediated immunity and produce long-term protection that more closely resemble the body’s own response to infection.

Vaccines are now available for a wide range of bacterial and viral diseases and are being developed and increasingly used to prevent infections as varied as malaria and hookworm. 7 Unfortunately, it has been difficult to produce effective vaccines for some diseases, such as HIV/AIDS. Vaccines, like medications, are rarely 100% effective and may produce side effects, including allergic reactions that can be life threatening. Live vaccines have the potential to cause injury to a fetus and can themselves produce disease, particularly in those with reduced immunity. Some vaccines are not effective for the very young and the elderly. Therefore, the use of vaccines requires extensive investigations to define their effectiveness and safety as well as to identify high-risk groups for whom they should be recommended.

For instance, college students and military recruits who tend to live in close quarters represent two high-risk groups for meningococcal disease. This bacterial infection can be rapidly life threatening, and when present, it requires testing and antibiotic treatment of close contacts. Effective vaccination is now a key tool for controlling this disease.

Ideally, vaccination occurs before exposure. However, when an outbreak occurs, vaccination of large numbers of potentially exposed individuals living in the surrounding area may be key to effective control. Thus, public health uses of vaccines need to include consideration of who should receive the vaccine, when it should be administered, and how it should be administered.

Vaccine administration has traditionally been limited to injections as shots or ingestion as pills. New methods of administration, including nasal sprays, are now being developed. In addition, it is often possible to combine vaccines, increasing the ease of administration. Inactivated vaccines may not produce long-term immunity and may require follow-up vaccines or boosters. Thus, the use of vaccinations requires the development of a population health strategy that gives careful attention to “who,” “when,” and “how.”

Some infections, especially those viruses that are highly contagious, can be controlled by vaccinating a substantial proportion of the population, often in the range of 70 to 90%. In this situation, those who are susceptible rarely, if ever, encounter an individual with the disease. This is known as herd immunity or population immunity. When a population has been vaccinated at these types of levels for diseases—such as chicken pox, measles, and polio—those who have not been vaccinated are often protected. For some vaccines, such as live polio vaccine, herd immunity is facilitated by the fact that the virus in the vaccine can itself be spread from person to person, providing protection for the unvaccinated. Thus, public health authorities are interested in the levels of protection in the community—that is, the level of protection of the unvaccinated as well as the vaccinated.

In addition to tools for preventing disease in individuals and populations, public health efforts are often directed at screening for disease and conducting what is called case finding.

HOW CAN SCREENING AND CASE FINDING BE USED TO ADDRESS THE BURDEN OF COMMUNICABLE DISEASE?

Ideally, screening for communicable diseases fulfills the same criteria for noncommunicable diseases. Screening for communicable diseases has played a role in controlling the spread of a number of infections. For example, screening for tuberculosis and syphilis has been an effective part of the control of these infections even before they could be cured with antibiotics. Today, screening for sexually transmitted diseases, including gonorrhea and chlamydia, are a routine part of clinical care. HIV screening has long been recommended for high-risk individuals and for populations with an estimated prevalence above 1%. Today, universal screening is increasingly being discussed as a basic strategy for controlling HIV/AIDS.

Screening for communicable diseases has often been linked with the public health practice known as case finding. Case finding implies confidential interviewing of those diagnosed with a disease and asking for their recent close physical or sexual contacts. Case finding techniques have been key to the control of syphilis and to a large extent TB both before and after the availability of effective treatment. The advent of effective treatment meant that case finding was of benefit both to those diagnosed with the disease and those located through case finding.

Successful case finding aims to maintain confidentiality. However, when following up with sexual contacts, confidentiality is difficult to maintain. The potential for public recognition and the attendant social sigma has inhibited the use of case finding in HIV/AIDS in many parts of the world. The reluctance to utilize case finding may change in coming years, as early diagnosis and perhaps early treatment become more effective in controlling the epidemic.

In addition to the use of barrier protections, vaccination of individuals and populations, as well as the use of screening and case finding, public health tools also encompass treatment of those with disease and their contacts.

HOW CAN TREATMENT OF THOSE DIAGNOSED AND THEIR CONTACTS HELP TO ADDRESS THE BURDEN OF COMMUNICABLE DISEASE?

Treatment of symptomatic disease may in and of itself reduce the risk of transmission. Successful treatment of HIV has been shown to reduce the viral load and thereby reduce the ease of transmission. Similarly, treatment of active TB reduces its infectivity. In addition to direct treatment, a public health tool known as epidemiological treatment, or treatment of contacts, has been effective in controlling a number of communicable diseases. Sexual partners of those with gonorrhea and chlamydia are routinely treated, even when their infections cannot be detected. This approach presumably works because early and low-level infections caused by these organisms may be difficult to detect. As we saw in the meningococcal scenario, epidemiological treatment may be the most effective way to halt the rapid spread of a disease.

Contact treatment of HIV/AIDS may become a routine part of controlling the disease. It is already recommended and widely used for treatment of needlestick injuries in healthcare settings. Let us look at one additional public health tool being increasingly used to maximize effectiveness of treatment and prevent resistance.

HOW CAN PUBLIC HEALTH EFFORTS MAXIMIZE EFFECTIVENESS OF TREATMENT AND PREVENT RESISTANCE?

In recent years, the impact of antibiotic resistance has become painfully obvious—resistant pneumococcus, gonococcus, and tuberculosis have become widespread. In fact, extensively drug-resistant strains of TB exist in many areas of the world today. Efforts to control resistance have been modest compared to the forces encouraging resistance. Forces encouraging drug resistance include overuse of prescribed antibiotics, over-the-counter sales of antibiotics in many countries, and widespread use of antibiotics to stimulate modest growth in agricultural animals.

As we have seen, one successful effort known as directly observed therapy (DOT) has led to a more effective treatment of TB. As the name implies, DOT aims to ensure complete adherence to TB treatment by observing individuals taking treatment at daily or at less frequent intervals. This effort has been credited with success even in the presence of drug resistance, perhaps on the basis that the body can handle resistant TB if most of the organisms are effectively treated. Efforts are now underway to reduce or eliminate the use of antibiotics for animal growth and to place increased restrictions on the prescribing of highly useful new antibiotics.

The recent emergence of methicillin-resistant staphylococcus aureus (MRSA) outside hospitals has drawn long-overdue attention. It is spreading to communities and beginning to affect otherwise healthy individuals, including athletes and others simply in close physical contact. An effective program to control MRSA will require the use of a range of interventions. This is the situation for many of today’s increasingly complex communicable diseases.

Let us now turn our attention to strategies that combine many of the specific public health tools designed to address the problems of communicable diseases. We will look at two basic strategies for combating complex infections: elimination and control.

HOW CAN PUBLIC HEALTH STRATEGIES BE USED TO ELIMINATE SPECIFIC COMMUNICABLE DISEASES?

Smallpox was the first human disease to be eradicated. An international effort is hopefully nearing completion to eradicate polio. These two viral diseases are the only ones that have been successfully targeted for eradication. As we have discussed, programs to eradicate TB and malaria have never come close to meeting their goals. Talk of the end of HIV/AIDS is even more unrealistic. Let us see what it takes to successfully eradicate a disease and why so few diseases are on the short list for potential eradication.

The history of smallpox has a unique place in public health. The disease goes back thousands of years, and it played a prominent role in the colonial United States, where epidemics often killed a quarter or more of their victims, especially children, and left most others, including George Washington, with severe facial scars for life. The concept of vaccination and the first successful vaccination were developed for smallpox. During the 1800s and early 1900s, smallpox was largely eliminated from most developed and developing countries through modest improvement on Jenner’s basic approach to vaccination in which fluid from cowpox sores was placed under the skin to protect individuals against smallpox, despite the many side effects of this quite crude treatment. c

Despite the control of smallpox in most developed countries, there were still over 10 million cases annually of the disease in over 30 countries during the early 1960s. In 1967, the WHO began a campaign to eliminate smallpox. The success of the campaign over the next decade depended on extraordinary organizational management and cooperation, but the prerequisites for success were the unique epidemiological characteristics of smallpox that made it possible. Let us outline the characteristics of smallpox that made eradication possible:8

• No animal reservoir—Smallpox is an exclusively human disease. That is, there is no reservoir of the disease in animals. It does not affect other species that can then infect additional humans. This also means that if the disease is eliminated from humans, it has nowhere to hide and later reappear in human populations.

• Short persistence in environment—The smallpox virus requires human contact and cannot persist for more than a brief time in the environment without a human host. Thus, droplets from sneezing or coughing need to find an immediate victim and are not easily transmitted except by human-to-human contact.

• Absence of a long-term carrier state—Once an individual recovers from smallpox, he or she no longer carries the virus and cannot transmit it to others. Smallpox contrasts with diseases such as HIV/AIDS and hepatitis B, which can maintain long-term carrier states and be infectious to others for years or decades.

• The disease produces long-term immunity—Once an individual recovers from smallpox, very effective immunity is established, preventing a second infection.

• Vaccination also establishes long-term immunity—As with the disease itself, the live smallpox vaccine produces very successful long-term immunity. Smallpox has not mutated to become more infectious despite the extensive use of vaccination.

• Herd immunity protects those who are susceptible—Long-term immunity from the disease or the vaccine makes it possible to protect large populations. At least 80% of the population needs to be vaccinated to interrupt the spread of the infection to the remaining susceptible people.

• Easily identified disease—The classic presentation of smallpox is relatively easy to identify by clinicians with experience observing the disease, as well as by the average person. This makes it possible to quickly diagnose the disease and protect others from being exposed.

• Effective postexposure vaccination—The smallpox vaccine is effective even after exposure to smallpox. This enables effective use of what is called ring vaccination. Ring vaccination involves identification of a case of smallpox, vaccination of the individual’s household and close contacts, followed by vaccination of all those within a mile radius of the smallpox case. In the past, households within ten miles were typically searched for additional cases of smallpox. These public health surveillance and containment efforts were successful even in areas without high levels of vaccination.

The presence of all of these characteristics makes a disease ideal for eradication. While fulfilling all of them may not be necessary for eradication, the absence of a large number of them makes efforts at eradication less likely to succeed. Table 7-2 outlines these characteristics of smallpox and compares them to polio—the current viral candidate for eradication—as well as to measles. Based upon the content of the table, you should not be surprised to learn that the polio campaign has been much more difficult and has taken much longer than that of smallpox. The potential for a successful measles eradication campaign is still being debated. d

TABLE 7-2 Eradication of Human Diseases—What Makes It Possible?

Finally, take a look at Table 7-3 , which applies these characteristics to HIV infection. It demonstrates why the eradication of HIV/AIDS is not on the horizon.

Unfortunately, eradication of most diseases is not a viable strategy. Thus, public health measures are usually focused on control of infections. In order to understand the range of strategies that are available and useful for controlling communicable diseases, we will take a look at three important and quite different diseases—HIV, influenza A, and rabies.

WHAT OPTIONS ARE AVAILABLE FOR THE CONTROL OF HIV/AIDS?

HIV/AIDS has been a uniquely difficult epidemic to control. An understanding of the biology of the HIV virus helps us understand many of the reasons for this. The HIV virus attacks the very cells designed to control it. The virus can avoid exposure to treatments by residing inside cells and temporarily not replicating. Many treatments work by interrupting the process of replication and thus are not effective when replication stops. The virus establishes a chronic carrier state, enabling long-term infectivity. High mutation rates reduce the effectiveness of drugs, as well as the effectiveness of the body’s own immune system to fight the disease.

Despite these monumental challenges, considerable progress has been made by reducing the load of virus through drug treatment and preventing the transmission of the disease through a variety of public health interventions. To appreciate the efforts to control transmission of HIV/AIDS, it is important to understand the large number of ways that it can be transmitted.

TABLE 7-3 Potential for Eradication of HIV/AIDS

	HIV/AIDS
Disease is limited to humans, i.e., no animal reservoir?	No—Animal reservoirs exist
Limited persistence in the environment?	No—May persist on contaminated needles long enough for transmission
Absence of long-term carrier state?	No—Carrier state is routine
Long-term immunity results from infection?	No—Effective long-term immunity does not usually occur
Vaccination confers long-term immunity?	No—None currently available and will be difficult to achieve
Herd immunity prevents perpetuation of an epidemic?	No—Large number of previously infected individuals increases the risk to the uninfected
Easily diagnosed disease?	No—Requires testing
Vaccination effective postexposure?	No—None currently available

HIV is most infectious when transmitted directly by blood. Blood transfusions were an early source of the spread of the virus. The introduction of HIV virus testing in the mid-1980s led to a dramatic improvement in the safety of the blood supply. Nonetheless, the safest blood transfusions are those that come from an individual’s own blood. Thus, donation of one’s own blood for later transfusion when needed has become a routine part of elective surgery preparation in many parts of the world. The most dangerous forms of transfusions are those that come from blood or blood products pooled from large numbers of individuals. Hemophiliacs in many developed countries used pooled blood products to control their bleeding in the 1980s. They suffered perhaps the world’s highest rate of HIV infection before this hazard was recognized and addressed. A more recent pooling of blood products occurred in China and contributed to a surge of the disease.

Unprotected anal intercourse is a highly infectious way to transmit HIV. This may help to explain the early spread of the disease among male homosexuals. Today, however, heterosexual transmission is the most common route of infection; additionally, there is a higher risk of transmission from male to female than from female to male. A series of public health interventions has now been shown to be effective: properly used latex condoms, male circumcision, and abstinence are being promoted in efforts to control the disease throughout the world. Additional interventions to provide protection before, during, and after intercourse are being investigated. Aggressive treatment of AIDS at an early stage reduces the viral load and the ease of transmission to others. e

Maternal-to-child transmission of HIV was a common, but not universal, event before the advent of effective drug treatment. The use of treatment during pregnancy and at the time of delivery has dramatically reduced the maternal-to-child transmission of the infection. Today, this route of transmission is close to being eliminated, which is an important public health achievement. Breastfeeding represents an ongoing and more controversial route of transmission. Up to 25% of HIV-positive breastfeeding women may transmit HIV to their children. In countries where breastfeeding provides an essential defense against a wide range of infections, the issue of whether or not to breastfeed has been very controversial. Fortunately, drug treatment of HIV infections during breastfeeding has been shown to greatly reduce, but not eliminate, transmission.

Finally, HIV can be transmitted through contaminated needles. Thus, the risk of HIV transmission needs to be addressed in two very different populations—healthcare workers and those who abuse intravenous drugs. New needle technologies and better disposal methods have reduced the likelihood of needlestick injuries in healthcare settings. Postexposure treatment with drugs has been quite successful in reducing healthcare-related HIV infections. Reductions in HIV transmission through intravenous drug use have also occurred in areas where public health efforts have focused attention on this method of transmission. Needle exchange programs have met resistance and remain controversial, but most likely contribute to transmission reductions when the programs are carefully designed and administered.

Thus, a range of existing interventions linked to the method of transmission of HIV have been moderately successful in controlling the disease. New methods of control are needed and are being investigated and increasingly applied. Unfortunately, vaccination is not yet a successful intervention. In fact, early randomized controlled trials demonstrated no substantial degree of protection from vaccinations and raised the concern that vaccination may actually increase the probability of acquiring HIV. More recent studies combining two or more types of vaccines have again provided hope for at least a partially effective vaccine in the years to come.

The recognition that highly effective vaccinations are not likely in the foreseeable future has brought forth a wide array of ideas on how to control the spread of infection. Antiviral creams, postcoital treatments, and early testing and case finding may become effective interventions. Antiviral creams may become both an adjunct to condom use, as well as a substitute in those situations where condom use is not acceptable. The success of postneedlestick interventions in the healthcare setting has raised the possibility that postexposure treatment may also be effective after high-risk sexual contact.

Finally, new diagnostic tests for HIV that allow for detection of the disease in the most contagious early weeks of the infection are being investigated for widespread use. To be effective, testing for early disease would need to be coupled with rapid case finding to identify and ideally treat contacts.

It is encouraging to know that existing and emerging interventions for HIV hold out the possibility of effective control. Public health and medical interventions complement each other and are both needed if we are to effectively address the most widespread epidemic of the 2000s. Table 7-4 summarizes the routes of transmission and the estimated transmission rates per exposure. 9 Finally, it outlines the potential interventions that we have discussed.

TABLE 7-4 Mode and Chances of Transmission of HIV and Existing Interventions

Route of transmission	Estimated transmission rate per exposure	Potential interventions
Blood transfusion Blood and blood products, such as pooled blood products previously used in United States by hemophiliacs	Contaminated blood over 90% chance of transmission with infected blood; pooling of blood dramatically increases infection risk	Screening of blood to detect HIV early Use of individual’s own blood for surgery
Sexual contact—anal higher than vaginal, which is much higher than oral	Range from 0.1% to 10% per contact, with unprotected receptive anal intercourse posing highest risk Vaginal male to female greater than female to male Circumcision reduces risk by half Other sexually transmitted diseases may increase risk	Latex condom Circumcision Abstinence Serial monogamy reduces spread compared to two or more concurrent partners
Mother-to-child transmission	15% to 40% higher in developing countries Highest rate of transmission at time of vaginal delivery	Cesarean delivery Drug treatment during pregnancy and at time of delivery for mother and child
Breastfeeding	Very low per exposure, but up to 25% over year or more of breastfeeding	Continuation of drug treatment reduces, but does not eliminate, transmission
Needlestick exposures Healthcare occupational risk	Less than 0.5% of HIV-positive needlesticks result in transmission	Postexposure treatment with drugs established as effective prevention
Injection drug use	Less than 1% per episode of needle sharing	Needle exchange programs

Data from Population Reference Bureau. Facing the HIV/AIDS Pandemic. Population Bulletin 2002: 57(3).

BOX 7-2 The Influenza Pandemic of 1918

The history of the influenza pandemic of 1918 is summarized by the United States National Archives and Records Administration as follows:10

World War I claimed an estimated 16 million lives. The influenza epidemic that swept the world in 1918 killed an estimated 50 million people. One fifth of the world’s population was attacked by this deadly virus. Within months, it had killed more people than any other illness in recorded history.

The plague emerged in two phases. In late spring of 1918, the first phase, known as the “three-day fever,” appeared without warning. Few deaths were reported. Victims recovered after a few days. When the disease surfaced again that fall, it was far more severe. Scientists, doctors, and health officials could not identify this disease which was striking so fast and so viciously, eluding treatment and defying control. Some victims died within hours of their first symptoms. Others succumbed after a few days; their lungs filled with fluid and they suffocated to death.

The plague did not discriminate. It was rampant in urban and rural areas, from the densely populated East coast to the remotest parts of Alaska. Young adults, usually unaffected by these types of infectious diseases, were among the hardest hit groups along with the elderly and young children. The flu afflicted over 25 percent of the U.S. population. In one year, the average life expectancy in the United States dropped by 12 years.

WHAT OPTIONS ARE AVAILABLE FOR THE CONTROL OF INFLUENZA?

Pandemic influenza is not a new problem. The influenza epidemic of 1918 is estimated to have killed 50 million people in a world populated with 2 billion people. Today, that would translate to over 150 million deaths. The history of the 1918 influenza pandemic is briefly summarized in Box 7-2. 10 The 1958 pandemic of Asian flu caused a similar, if less deadly, pandemic. Thus, we should not be surprised if pandemic flu returns in the coming years.

Influenza A is a viral infection that has long been capable of pandemic or worldwide spread. f Its ability to be rapidly transmitted through the air from person to person and its short incubation period have made it an ongoing public health problem. It often kills the very young, the very old, and those with chronic illnesses, particularly those with respiratory diseases and suppressed immune systems. In addition, the disease continues to mutate, creating new types against which previous infections and previous vaccinations have little or no impact. Thus, new vaccines are required every flu season. Seasonal influenza kills over 30,000 people in the United States alone in the average year despite the increasingly widespread use of vaccinations.

A variety of public health and medical interventions have been and continue to be used to address the current and potential threat posed by influenza. They may well all be needed to address future threats. Let us take a look at a number of these interventions.

Inactivated or dead vaccines have been the mainstay of immunization against influenza. Unfortunately, current technology requires approximately six months lead time to produce large quantities of the vaccine. Thus, influenza experts need to make educated guesses about next year’s dominant strains of influenza. In some years, they have been wrong and the deaths and disability from seasonal influenza have increased. New technologies for vaccine production are now available and should be able to help with this issue in the future.

In recent years, live vaccines administered through nasal spray have been developed and increasingly used. These vaccines are more acceptable than shots to most patients and are now considered safe for a wide range of age groups. They raise the hope of greater acceptance and wider use of influenza vaccinations in coming years.

Medications to treat influenza and modestly shorten the course of the disease have also been developed. Influenza experts view these drugs as most useful to temporarily slow the spread of new strains, providing additional time for the development of vaccines to specifically target the new strain. Widespread use of influenza drugs has already resulted in resistance, raising concerns that these drugs will not be effective when we need them the most. Efforts are underway to develop new drugs and reserve their use solely for potential pandemic conditions.

Despite our best efforts, influenza is expected to continue its annual seasonal epidemic and to pose a risk of pandemic spread. The use of barrier protection such as masks, isolation methods, and even quarantine has been considered part of a comprehensive effort to control influenza. It is clear that we have a variety of public health methods to help control the impact of the disease. It is likely that we will need all of these efforts and new ones if we are going to control the potential deaths and disabilities due to influenza in coming years. 11 Now, let us look at our last example of the development of public health strategies to control communicable diseases—that of rabies.

WHAT OPTIONS ARE AVAILABLE FOR THE CONTROL OF RABIES?

Rabies is an ancient disease that has plagued human beings for over 4,000 years. It is caused by a ribonucleic acid (RNA) virus that is transmitted through saliva of infected animals and slowly replicates. It spreads to nerve cells and gradually invades the central nervous system over a 20- to 60-day incubation period. Once the central nervous system is involved, the disease progresses almost inevitably to death within one to two weeks. Any warm-blooded animal can be infected with rabies, but some species are particularly susceptible—most commonly raccoons, skunks, and bats. Cats and dogs can also be infected and transmit the virus.

A multicomponent vaccination strategy has been very successful in preventing the development of rabies in humans. In most recent years, there have been between one and five fatal cases of rabies per year in the United States despite the persistence and periodic increase in rabies among wildlife populations. Let us take a look at how this quite remarkable control effort has occurred.

The ability to successfully vaccinate humans against rabies after the occurrence of a rabies-prone bite has long been a component of the success of rabies reduction among humans. The use of postexposure vaccination was first demonstrated by Louis Pasteur in 1887 and was used to dramatically save the life of a young victim. Early live vaccines had frequent and severe side effects. They were sequentially replaced by inactivated vaccines grown in animal nerve tissue. These replacement vaccines still led to occasional acute neurological complications and gave the treatment a reputation of being dangerous. The development of a vaccine grown in human cell cultures in the 1970s led to safety records comparable to those of other commonly used vaccines. Today, over 30,000 rabies vaccination series are administered annually in the United States.

The success of rabies control is a result of a series of coordinated efforts to utilize vaccinations in different settings. Vaccines are administered to individuals who are bitten by suspicious species of wild animals, including raccoons, bats, skunks, foxes, and coyotes. Victims of suspected rabies bites by dogs and cats may await the results of quarantine of the animal and observation over a 10-day period. When substantial doubt still exists after this time frame, vaccination is recommended. Laws requiring rabies vaccination of dogs and cats have been enforced in the United States for decades and have greatly reduced the number of reported infections in these animals. Today, only 10% or less of suspect rabies-prone bites come from dogs and cats.

Wildlife remains the greatest source of rabies—wildlife epidemics occur with regularity. Rabies-prone bites still occur especially from raccoons, which regularly feed from garbage cans in rural, suburban, and occasionally urban parts of the United States. The recent development of effective oral vaccinations that can be administered to wildlife through baits has been credited with reducing the number of infected animals, especially those residing in close proximity to humans.

Rabies illustrates the variety of ways that a key intervention—vaccination—can be used to address a disease. As with many complex diseases of public health importance, a carefully designed and coordinated strategy is required to maximize the benefit of available technology. In addition, ongoing research is needed to continue to develop new and improved approaches to the control of communicable diseases. 12

HIV/AIDS, influenza A, and rabies represent three very different communicable diseases. However, they all require the use of multiple interventions, close collaboration between the public health and healthcare systems, and continuing efforts to find new and more effective methods for their control.

Efforts to control communicable diseases have increased in recent years along with the increase in emerging and reemerging infectious diseases. Technological advances have provided encouragement for the future but at times have raised concerns about the safety of our interventions. g New technology, new strategies for applying technology, and new ways to effectively organize our efforts are needed to ensure the effectiveness and safety of our efforts to prevent, eradicate, and control communicable diseases.

Now let us turn our attention to our third category of disease: that of environmental diseases and injuries.

KEY WORDS

• Communicable disease

• Infectious disease

• Infections

• Epidemic

• Endemic

• Pandemic

• Koch’s postulates

• Modern Koch’s postulates

• Route of transmission

• Asymptomatic transmission

• Reproduction ratio (R0)

• Chronic carriers

• Immunization

• Passive immunity

• Inactivated vaccine or dead vaccine

• Live vaccines (attenuated live)

• Antibody

• Cell-mediated immunity

• Herd immunity or population immunity

• Case finding

• Infectivity

• Epidemiological treatment

• Ring vaccination