Tropical Diseases

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Tropical Diseases

Battles against malaria

The deadly sandfly and leishmaniasis

Dangerous worms
Ascariasis and hookworm

Schistosomiasis

Microscopic hazard

Diarrhea and cholera

Infectious disease killers

Stemming tropical diseases

Resources

Nowhere is the prevalence of certain illnesses more striking than in areas where tropical diseases flourish. In many parts of Africa, South America, and Asia, diseases exist that are rarely seen in the United States. Young people are highly susceptible to death from disease in tropical and semi-tropical areas. Children under five account for 40-60% of all deaths in Africa, Asia, and South America, although they make up only about 15% of the population. Such high death rates reflect the fact that tropical diseases are most prevalent in poor areas where health care is limited. Efforts to improve community health in areas where tropical diseases thrive include the establishment of health clinics and the development of new vaccines.

Battles against malaria

Malaria infects up to 500 million people annually and kills from one to three million people every year. A parasitic disease spread by mosquitoes, malaria is the best-known tropical disease and infects the largest number of people internationally. It has been called the ‘most devastating disease in history’ based on the number of people it has attacked or killed. Currently, malaria has been identified in about 100 countries, although 80% of the clinical cases are reported in Africa.

The history of malaria is different than the history of other tropical diseases, due to its immense reach and the concerted effort over time to defeat the disease. But in as much as it has been eliminated or controlled in developed countries and remains a major killer in those that are poor and less developed, its history is also similar to that of other tropical diseases.

The ancient Chinese wrote about malaria as early as 2700 BC, describing the symptomatic fever and characteristic enlargement of the spleen. Ancient Greeks and Romans also suffered from malaria. Hippocrates (460-375 BC), offered an accurate account of malarial symptoms and theorized that the disease was caused by a miasma, or poisonous cloud, rising up from marshy land.

The first effective treatment for malaria was developed in the seventeenth century and utilized a traditional Peruvian treatment for fever, the cinchona treebark. In 1630, Jesuit missionaries in Peru introduced the bark of the cinchona tree to Europeans. The active substance in the bark, quinine, was isolated in 1820. Though there were common, serious side effects to the drug, such as ringing of the ears and hearing loss, the substance was effective in treating malaria, which was common in Europe at that time.

Development of a treatment for malaria cleared the way for large scale exploration of tropical areas by Europeans. Development of a synthetic quinine, called chloroquine, in the 1940s, offered effective treatment with fewer common side effects. In the 1960s, malarial parasites became resistant to chloroquine, and the substance no longer worked in many areas.

Efforts to control malaria has been effective in many areas, including the southern United States. The pesticide DDT was used successfully, in the Tennessee River Valley, Greece, Puerto Rico and other locations to kill the mosquitoes carrying malaria and eliminate the disease in the late 1940s. A worldwide campaign to eliminate malaria used DDT from 1956-1969. However, mosquitoes became resistant to DDT and the effort failed in some areas.

There are four different types of malaria protozoa. One of the four, P. falciparum, is responsible for almost all deaths from the disease. Symptoms of the disease do not occur for one to two weeks. They often include chills, muscle aches, fatigue, and abdominal discomfort. They may include tremors and high fever, which comes and goes at regularly spaced intervals. Complications from severe malaria include renal failure, pulmonary edema, coma, and hypoglycemia.

The delayed appearance of malaria’s symptoms reflects the measured course the disease takes in the body. The activity of the malaria parasite in the body is aggressive and thorough. The disease is spread by female Anopheles mosquitoes that inject malaria parasites into the bloodstream with their bite. The parasites, called sporozoites, travel to the liver, where they enter cells of the liver tissue. Once in the liver, the sporozoite changes to a spore, which replicates itself until there are thousands of spores in a cyst like structure which has replaced the cell. Malaria manifests no symptoms while this process occurs.

Symptoms develop when the cyst bursts and the spores, called merozoites, are released into the blood stream. At this point, sweating and high fever can occur as the spores enter the host’s red blood cells. The parasite consumes hemoglobin from within the red blood cells.

As the parasite grows from consuming the hemoglobin, its nucleus divides into from six to 32 parts, each of which becomes a spore of its own. The red cell bursts, and the spore moves to another cell.

After the merozoites have launched their process of reproducing in the red blood cells of the host, some of the merozoites are transformed to male or female gametocytes. These sexual cells form the base for more parasites if the host is bitten again by the female Anopheles mosquito. When this occurs, the gametocytes develop into a number of differentiated cells, called gametes, zygotes, ookinetes and oocysts, within the gut of the mosquito. Eventually the parasite reproduces and sporozoites that travel in the mosquito are formed. These sporozoites move up to the mosquito’s salivary gland, where they are ready to infect another host.

Treatment of malaria is usually effective. Because many malaria parasites are resistant to chloroquine, physicians have returned to using quinine in many instances. Travelers and others at risk of malaria receive chloroquine or other drugs, including mefloquine or doxycycline, if journeying to areas where the drugs still work. However, there is no totally effective preventive measure against malaria. Philanthropic organizations such as the Bill and Melinda Gates Foundation have made conquering malaria through vaccine development and distribution among their top priorities

The deadly sandfly and leishmaniasis

Another disease spread by insects is leishmaniasis, a sickness caused by several types of protozoa carried by sandflies. Leishmaniasis infects about one million people worldwide per year. It is spread when sand flies draw blood from individuals who already are infected. Hosts can be humans, dogs, or other mammals.

Each of the four major clinical syndromes of the disease have a long incubation period that ranges from three months to 24 months. The disease is marked by one or more skin lesions and varies in severity depending on the type. Kala Azar, one of the types, is commonly found in East Africa and the Sahara. Kala Azar causes fever, diarrhea, enlarged liver, and anemia. Another type, Old World leishmaniasis, usually heals on its own and is marked by multiple lesions.

Treatment depends on the type of leishmaniasis and may include the use of drug therapy and transfusions for several weeks. The need for extended medical care makes treatment of leishmaniasis impossible for many poor individuals in developing countries, raising the mortality rate of the disease.

Dangerous worms
Ascariasis and hookworm

A common disease in tropical countries is ascariasis. The annual death rate due to ascariasis is about 20,000, most due to complications within the intestine, where the worms settle as adults. An estimated 700 million individuals are affected at any given time by

the disease, making ascariasis the most common worm infection in humans.

The disease can cause difficulties, such as bronchial asthma, in the lungs, when the parasite settles in that part of the body. The greatest threat to the human host is the loss of food nutrients which go to the worm instead of the host. Individuals whose diet is already sparse may suffer from malnutrition after infection with ascariasis.

The life cycle of the creature is similar to the life cycle of the blood fluke, though there are some differences. The disease is spread when Ascaris eggs leave the human host in feces. The new host ingests the eggs through soil or by soil-contaminated hands or food. The eggs then travel through the liver, lungs, and throat, before ending up in the intestine. The worm, which grows in the intestine, the roundworm Ascaris lumbricoides, is the largest such parasite, growing to a length of 19.3 in (49 cm). It can live about a year in the human body. Once they are in the intestine, more eggs are laid, clearing the way for further spread of the disease.

Another parasite that is spread through dirt and grows in the human body is hookworm, which affects approximately 800 million individuals in the developing world and kills about 50,000 annually. Hookworm was at one point common in the American South, but is now seldom reported in the area.

The hookworm, which grows up to 0.5 in (13 mm) in size, sucks blood and normally lives in the small intestine. Establishment of the worm in the host generally results in iron-deficiency anemia because of blood loss, and may cause bronchitis, peptic ulcers, or even heart problems.

Hookworms enter the body as larvae through the skin, taking advantage of hair follicles or other openings to help the creature pierce through the skin with its boring movement. The parasite enters the circulatory system, moving first to the lungs then to the pharynx, where it is swallowed. The final larvae stage typically occurs in the small intestine, where adult worms emerge and ultimately lay eggs. Adult worms normally live from one to 5 years if the host is not treated. They cannot multiply in the body.

Successful control of the hookworm depends on improvements in plumbing and the use of footwear, a challenge in countries where shoes are a luxury. Medication is available to destroy hookworm, but providing it in areas where hookworm infestation is severe does little to alter the problem, as those who are medicated can easily be reinfected.

Schistosomiasis

one of the most common tropical diseases is schistosomiasis, also known as bilharzias, a disease caused by a worm called a blood fluke. The worm, which can be about 0.47 in (12 mm) long, can live in the human body from five to 20 years. About 200 million people are infected by the worm in developing countries.

Symptoms include abdominal pain, diarrhea, and weight loss. If untreated, the disease can cause enlargement of the liver, bleeding from blood vessels in the esophagus, and problems to the central nervous system.

The worm, which is usually hosted by Biomphalaria snails, depends on fresh water to survive. The disease is common in areas where bathing occurs in freshwater contaminated by human feces and where many individuals have the disease.

The life cycle of the blood fluke entails travel throughout the human body and through the body of its other host, the snail. The worm enters the human body in a larval stage in fresh water through contact with human skin. The larvae enter a blood or lymph vessel and move to the heart and lungs, where they grow for several days. Then they move to the liver and the portal circulation, where they grow for several weeks.

Ultimately, the worms move to the intestinal wall, where they settle. The creatures lay eggs, which are released in feces and become new larva after contact with fresh water. These larvae enter the host snail, where they eventually produce cercariae, the final larval stage. These are the creatures that enter the human body after contact with skin.

Drug therapy is an effective treatment for the disease. Prevention efforts depend on controlling infection through drug therapy and on convincing individuals to steer clear of fresh water which may be infected.

Microscopic hazard

Amebiasis, a disease caused by a microscopic protozoa spread through dirty water, infects as many as 500 million individuals in the developing world. The disease kills about 70,000 people annually.

The protozoa can live in the large intestine and not cause damage to the host. But it is also capable of causing ulceration of the colonic wall and damage to the liver and other organs, including the brain.

The disease is spread through water contaminated by fecal matter, by contaminated flies, or by other contaminated substances. Hosts swallow cysts, which divide in the small intestine and again in the large intestine to form amoebae. These amoebae divide and form trophozoites, which feed on fecal bacteria. These trophozoites form cysts and are passed out in the feces of the host. This entire cycle takes from 48 hours to four months.

Diarrhea and cholera

Best known in the United States as a benign but unpleasant condition, diarrhea is one of the leading causes of death in developing countries. Indeed, diarrhea causes about 4.3 million deaths each year, more than malaria or many of the other serious tropical illnesses. Though most of the approximately 28 billion individuals who develop diarrhea survive, the health consequences of diarrhea are great in tropical areas.

Infants and small children are most likely to die of diarrhea. This is because loss of even a small amount of water can be life threatening to small children. Death due to diarrhea stems from loss of fluid, loss of plasma, and the collapse of the cardiovascular system. Infants being weaned from the mother’s breast are particularly susceptible to malnutrition and dehydration. Diarrhea can be caused by bacteria or viruses.

Residents of tropical disease areas are less likely than residents of developed areas to have access to emergency treatment for extreme diarrhea. They are also more likely to be exposed to pathogens that cause intense diarrhea in contaminated food or water. Cholera, a life-threatening condition whose major symptom is watery diarrhea, is spread through dirty water.

Oral rehydration therapy, an approach which uses a solution of glucose or sucrose and salt to hydrate the body, is an effective treatment for life-threatening diarrhea. One problem with the approach is that it must be made available immediately after a child gets sick, as diarrhea can cause enough fluid loss within the first 12 to 24 hours to cause death. Many individuals in tropical disease areas live many hours by foot from medical care. Therefore, health experts are working to make oral rehydration therapy, along with education about the therapy, available at home to residents of tropical-disease areas.

The most infamous type of diarrheal disease is cholera, which is also marked by vomiting, intense thirst, and abdominal cramping. Cholera is so common it is endemic in some parts of the world, such as India. In other parts of the world, it occurs as an epidemic or a pandemic.

The disease is caused by the Vibrio cholerae bacteria, which typically spreads through dirty water, food, or seafood from areas with contamination. The bacteria that causes cholera multiplies in the human intestinal tract, but has also been known to multiply in water.

Symptoms of cholera are a direct result of the cholera enterotoxin, the toxic substance elaborated by cholera bacteria that affects the cells of the small intestine. The substance forces the mucosal cells to secrete large quantities of fluid, resulting in diarrhea.

Control of cholera calls for good sanitation measures, specifically clean water and good food hygiene. Boiling water eliminates the bacteria. Several cholera vaccines are being developed, but only one, Dukoral^®, is likely to be effective for long periods. Dukoral is an oral vaccine, which is more easily delivered to populations in the developing world than injectable vaccines. Neither vaccine is effective against a relatively new type of cholera bacteria that emerged in Asia in the early 1990s known as Vibrio cholerae O139.

While treatment is effective, involving antibiotics, nutritional support, and treatment for dehydration, the death rate in epidemics may reach 30%.

Infectious disease killers

The most common killers in tropical areas are not exotic diseases. They are infectious diseases, many of which are not considered life threatening in developed countries such as the United States. In the least industrialized countries, 40% or more of all deaths are caused by infectious disease. In the United States, about 1% of all deaths are caused by such illnesses. The huge difference in death rates stems from the fact that most infectious diseases are among the easiest diseases to treat using modern medical care.

Approximately 10 million people die every year from respiratory diseases in the developing world.

Such diseases are caused by the influenzavirus or bacteria such as pneumococcus and Hemophilus influenza B. Measles, which can be prevented by immunization of infants, kills two million people in the developing world annually. The course of the disease in developing countries often includes complications such as diarrhea, ear infection, pneumonia, and weight loss. For some, weight loss leads to malnutrition. Aggressive efforts to curb measles deaths through immunization have reduced the number of potential deaths in recent years.

Whooping cough or pertussis is another early childhood disease which is seldom seen in the United States but is common in developing countries. About 600,000 people die of whooping cough annually in the developing world, with most of these deaths occurring among small children. Whooping cough is also prevented by immunization.

Another infectious disease which is a common killer in developing countries is tuberculosis, which kills about 900,000 people every year and infects about seven million people annually. Tuberculosis can be treated through immunization and drug therapy, although drug resistant strains are becoming increasingly prevalent. Though deaths have been reduced internationally over the last 30 years, the disease is still very common in tropical areas.

KEY TERMS

Cyst —Refers to either a closed cavity or sac or the stage of life of some parasites during which they live inside a walled in area.

Dehydration —The condition of having lost too much fluid from the body.

Larva —Immature developmental stage of various species.

Lesion —An injury or wound.

Parasite —Species which depends on another species to survive.

Peptide —A class of chemical compounds that form proteins.

Protozoa —One-celled creatures and the simplest forms of animal life.

Spore —A dormant form assumed by some bacteria, such as anthrax, that enable the bacterium to survive high temperatures, dryness, and lack of nourishment for long periods of time. Under proper conditions, the spore may revert to the actively multiplying form of the bacteria.

Tropical-disease areas have also been hit hard by AIDS and HIV infection. An estimated 40 million people internationally are infected by the HIV virus, the World Health Organization estimated in 2006. Over 20 million more people have died of the disease.

A key emphasis of prevention programs in Africa has been promoting the use of condoms during sexual intercourse, a practice which reduces the risk of AIDS. In Africa, condom use increased from two million per year in 1986 to about 70 million in 1993. But problems remain, particularly in places where poverty limits the safety of the blood supply, where condoms are in short supply, where access to accurate information about the disease is limited, and where health providers are far away. Antiretroviral treatment reduces both disability and death from AIDS, but access to these drugs is often not available in developing countries.

Stemming tropical diseases

While World Health Organization efforts have boosted the rate of immunization for many diseases, but there are still millions of children who do not receive adequate immunization for common early childhood diseases such as measles and whooping cough. Efforts to boost access to health providers and educational information also stand to benefit residents of tropical disease areas. Knowing how to care for children with diarrhea, adults with hookworm, or many other diseases could make a vast difference. Finally, the prevalence of diseases linked to dirty water and dirty soil drive home the importance of better sewage systems and basic public health measures which western countries generally adopted years ago. Diseases carried by insects are tackled through insecticides, clearing brush and standing water where the carriers may proliferate; sexually transmitted diseases are addressed by educating communities about safer sex practices, and providing condoms; immunization programs may cut down on some diseases; efforts to create vaccines for other diseases are underway; donor blood is undergoing increasingly more stringent screening to avoid transmitting disease through this route.