TROPICAL INFECTIOUS DISEASES

The tropics are usually defined as that part of the equatorial world bounded by the tropics of Cancer and Capricorn. Defining a tropical infectious disease is not as straightforward. Almost all infectious diseases can be found in the tropics; there are a great number that occur predominantly in the tropics; and there are a few, such as sleeping sickness, that are only found in the tropics. Before discussing some of the more prevalent tropical infections of today, it is worth taking a brief look at the history of a few of these infections.

BACKGROUND

Many of the infections that we now consider tropical used to be found throughout the more temperate climates of North America and Europe. For example, yellow fever epidemics swept through American cities from the 1600s through 1905, killing thousands of people. Cholera devastated the cities of America and Europe before the days of sewers. The Plague, known as the "Black Death," decimated the populations of medieval Europe. In the early part of the twentieth century, it was estimated that there were between 5 million and 7 million cases of malaria a year in the United States between Florida and Connecticut. Malaria was a disease familiar to the British of the sixteenth century and is even described by Shakespeare in Henry V (II. i. 123). Hookworm was the most common cause of anemia in the American south at beginning of the twentieth century and was only controlled after a massive public health campaign. Good sanitation, hygiene, and vector-control methods, as well as a rise in the standard of living, were responsible for the virtual eradication of these diseases from North America and Europe. The fact that so many of these infections are now considered "tropical" and are found mainly in poorer, developing countries is more a result of economics than it is of climate. There are several countries with high rates of "tropical" infections that do not have tropical climates (for example, Iran and Afghanistan).

The original designation of certain diseases as being tropical can be dated back to the 1898 publication of Sir Patrick Manson's Tropical Diseases: A Manual of the Diseases of Warm Climates. This volume identified twelve tropical infectious diseases, as well as a few other noninfectious diseases such as pellagra. The book was aimed at British physicians working in the warmer climates of many of the British colonies. Since then, the list of tropical infections has expanded to include well over one hundred infections. There is no list in existence that definitely identifies which infections are classified as tropical; most lists include a combination of those infections that are found exclusively in the tropics as well the large number that, though also found in more temperate climates, are predominantly a problem of developing countries with warmer climates. A discussion of tropical infectious diseases is essentially a discussion of the infectious diseases of the developing world.

As is true of all infectious diseases, the causative pathogens include viruses, bacteria, parasites, and fungi. Table 1 is a list of some tropical infections, divided by pathogen. Some infections, such as measles, human immunodeficiency virus (HIV), and tuberculosis, though found throughout the world, are included in the list as they cause such severe morbidity and mortality in tropical countries.

It is estimated that infections cause over 13 million deaths a year in developing countries, accounting for approximately 50 percent of all deaths. Only six diseases cause over 90 percent of the deaths attributed to infections: pneumonia, tuberculosis, diarrheal diseases, malaria, measles, and HIV/AIDS (human immunodeficiency virus/acquired immunodeficiency syndrome). Of these, only pneumonia is also a leading cause of death in developed countries. The others are found in developed countries but are now controlled either through vaccination programs (as with measles) or with effective medications and public health programs. Unfortunately, lack of effective health infrastructures, poor economies, and lack of access to affordable medications mean that these infections will continue to be the cause of significant morbidity and mortality in developing countries.

Approximately 2 billion people, one-third of the world's population, have latent tuberculosis (TB) infection. Of these, 8 million people a year will develop active infection and 2 million people a year will die from the infection. The rates of infection and subsequent death are increasing worldwide, with new outbreaks occurring in regions such as Eastern Europe for the first time in over forty years. In regions with high rates of HIV, the spread of tuberculosis is greatly accelerated, as are the death rates attributable to tuberculosis. The emergence of strains that are resistant to first-line drugs used in the treatment of TB means that it is becoming more difficult, and more expensive, to successfully treat the infection.

A variety of bacteria and viruses, such as cholera, rotavirus, and typhoid fever, may result in diarrheal illness. The burden of disease caused by

Table 1

these illnesses is huge—with an estimated 1.5 billion bouts of diarrhea a year and 2 million deaths a year, mainly in children under the age of five. The morbidity associated with diarrheal illnesses is the result of repeated episodes of dehydrating and malnourishing infections in children during their formative years. One study done in Brazil showed that such children lost over five centimeters of growth when compared to a healthy group.

Malaria, a parasitic disease caused by Plasmodium spp., is spread by the bite of the Anopheles mosquito and infects 300 to 500 million people a year, killing 1 to 3 million. The most vulnerable groups are children under five and pregnant women. Over 90 percent of lethal cases of malaria occur in sub-Saharan Africa. The deterioration in public health systems, the movement of peoples as a result of wars and civil unrest, and worsening economies in malaria-endemic countries have resulted in an increase in the rates of malaria infection. Global warming has led to an expansion of the normal range of the mosquito habitat, and thus to an expansion of the regions where infection is a risk. Increasing resistance to drugs used to treat malaria has also meant further spread of the disease and higher mortality rates.

Measles is a viral infection for which there is a very effective vaccine, which unfortunately is often not available in developing countries, partly because of difficulty maintaining the cold chain. Measles accounts for almost 1 million childhood deaths a year, especially among malnourished individuals. Survivors of the infection may have resulting disabilities such as brain damage, blindness, or deafness.

The human immunodeficiency virus was first recognized in the early 1980s. By the year 2000, over 33 million people were infected, with sub-Saharan Africa being one of the worst affected regions. There is no cure. Medications that control the infection are expensive and therefore inaccessible to the majority of infected individuals.

Other tropical infections that cause significant morbidity and mortality include leprosy, leishmaniasis, schistosomiasis, filariasis, and onchocerciasis. Although the mortality associated with these infections is not as great as those described above, they can lead to severe disfigurement and disability as well as a large economic cost and negative social impact. For example, leprosy, a bacterial infection caused by Mycobacterium leprae that affects more than 500,000 people a year, can result in significant disfigurement to the tissues of the face and extremities, often leading to severe disability. It is found in Africa, Latin America, and Southeast Asia. Early diagnosis and appropriate medication cure the infection and prevent disability.

Leishmaniasis, a parasitic infection spread by the bite of sand flies, is another infection that can result in severe disfigurement and disability. It causes a spectrum of clinical manifestations ranging from discrete cutaneous ulcers to disseminated visceral involvement, which causes death. It is estimated that 12 million people worldwide are infected. The most severe and lethal form of leishmaniasis, visceral leishmaniasis (also known as kala azar), is increasing in frequency in countries that also have high rates of HIV.

Lymphatic filariasis, caused by blood-borne parasites spread by biting arthropods, is one of the leading causes of long-term disability in the world. Infection with these parasitic worms can result in impairment of lymphatic drainage with resultant marked enlargement of limbs and genitals, a condition sometimes called elephantiasis. Onchocerciasis, another filarial infection afflicting almost 18 million people, can cause visual impairment and blindness.

The increasing movements of people and goods worldwide are facilitating the spread of infectious diseases. It was estimated that by the late 1990s there were over 1.4 billion airline passengers annually. It is inevitable that along with the movement of people, there will be movement of diseases. There have been well-documented cases of diseases such as tuberculosis, poliomyelitis, meningococcal meningitis, malaria, and influenza being brought to Western countries by travelers. An Asian ship that emptied contaminated ballast water off the shores of Peru in 1991 was responsible for the subsequent cholera outbreak of more than 1 million cases in South America, an area that had been cholera free for over one hundred years.

PREVENTION AND TREATMENT

Many tropical infections are preventable by means of simple, inexpensive, and currently available methods. For example, 25 percent of malaria deaths can be prevented by the use of insecticide-impregnated bednets. Comprehensive childhood vaccination programs would virtually eliminate infection with the measles virus. Clean water and good sanitation and hygiene would significantly reduce the burden of diarrheal illness as well as other water-associated infections such as schistosomiasis. Unfortunately, many of these prevention strategies are not being implemented for a variety of reasons. Some governments do not make health care a priority, and the cost of these programs is often beyond the means of some of the worst affected countries—due to poor and/or deteriorating health infrastructures, programs often cannot be undertaken. Armed conflict also often leads to an interruption or deterioration of health services.

Inexpensive and effective medications are available to treat most tropical infectious diseases. However, cost remains a significant barrier, and these medications remain unavailable to the vast majority of the world's population. Contributing to this problem are global patent protection laws that prohibit the manufacture and distribution of inexpensive copies of expensive medications patented by multinational pharmaceutical companies. Also, treatment of many of these diseases, such as tuberculosis and malaria, is now complicated by the emergence of resistance to first-line, traditional medications. Unfortunately, and perhaps understandably for the pharmaceutical industry, financial incentives to develop new products are lacking for a marketplace in which the world's poorest reside. Without the development of new drugs and vaccines, it is possible that we may not be able to effectively treat resistant tropical infectious diseases, relegating a significant proportion of the world's population to suffer or die needlessly.

The list of infections that we need to be concerned about is not static. New and reemerging infectious diseases have become a worldwide problem, the most important of which is HIV. From its initial identification in the early 1980s, HIV has become a leading cause of death in a significant number of developing countries. Other examples of new infections include hantavirus, cryptosporidium, and Ebola virus. Dengue fever, an arbovirus almost eradicated from the Americas by 1980, has made a major comeback in Central and South America as well as in Southeast Asia. Well-developed surveillance systems, accurate diagnostic tools, and an effective public health response are needed in order to identify and contain new infections as they occur.

In summary, many of the infections that we currently consider tropical were once endemic in the more temperate climates of developed countries but were successfully eradicated with a combination of public health, good sanitation and hygiene, and accessible medications. However, prevention and treatment of these infections is being hampered by war, poverty, and, perhaps most importantly, by the lack of political will on both a local and global level.

Martha Fulford

Jay Keystone

(see also: Communicable Disease Control; Contagion; Vector-Borne Diseases; Waterborne Diseases )

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