Question

Q. Compare and contrast the epidemiology and the impact of the following parasitic diseases: Malaria, schistosomiasis, ascariasis and tapeworm infection.

Answer 1

Malaria

It occurs primarily in tropical and some subtropical regions of Africa, Central and South America, Asia, and Oceania . In areas where malaria occurs, there is tremendous variation in the intensity of transmission and risk of infection. For example, over 90 percent of clinical malaria infections and deaths occur in sub-Saharan Africa (World Health Organization, 1996a). However, even there the risk varies widely. Highland (>1,500 m) and arid areas (<1,000 mm rainfall/year) typically have less malaria, although these areas are prone to epidemic malaria if climactic conditions become favorable to mosquito development (World Health Organization, 1996a). Although urban areas have typically been at lower risk, explosive unplanned population growth has been a major factor in making urban or peri-urban transmission an increasing problem (Knudsen and Sloof, 1992).

Human malaria is caused by one or more of four parasites: Plasmodium falciparum, P. vivax, P. ovale, and P. malariae. Distribution of these parasites varies geographically, and not all species of malaria are transmitted in all malarious areas. P. falciparum, the species most commonly associated with fatal malaria, is transmitted at some level in nearly all areas where malaria occurs. It accounts for over 90 percent of all malaria infections in sub-Saharan Africa, for nearly 100 percent of infections in Haiti, and causes two-thirds or more of the malaria cases in Southeast Asia. P. vivax is a relatively uncommon infection in sub-Saharan Africa. Duffy antigens, which are required by the parasite to invade red blood cells, are lacking in many ethnic groups, especially in West Africa. Vivax malaria, however, is the predominant species in Central America, most of malarious South America, and the Indian subcontinent .

Tapeworm Infection:

The tapeworms that cause taeniasis (Taenia saginata, T. solium, and T. asiatica) are found worldwide. Eating raw or undercooked beef or pork is the primary risk factor for acquiring taeniasis. Persons who don’t eat raw or undercooked beef or pork are not likely to get taeniasis.

Infections with T. saginata occur wherever contaminated raw beef is eaten, particularly in Eastern Europe, Russia, eastern Africa and Latin America. Taeniasis due to T. saginata is rare in the United States, except in places where cattle and people are concentrated and sanitation is poor, such as around feed lots when cattle can be exposed to human feces. Tapeworm infections due to T. soliumare more prevalent in under-developed communities with poor sanitation and where people eat raw or undercooked pork. Higher rates of illness have been seen in people in Latin America, Eastern Europe, sub-Saharan Africa, India, and Asia. Taenia solium taeniasis is seen in the United States, typically among Latin American immigrants. Taenia asiatica is limited to Asia and is seen mostly in the Republic of Korea, China, Taiwan, Indonesia, and Thailand.

A disease called cysticercosis can occur when T. solium tapeworm eggs are ingested. For example, people with poor hygiene who have taeniasis — with or without symptoms — will shed tapeworm eggs in their feces and might accidentally contaminate their environment. This can lead to transmission of cysticercosis to themselves or others.

Ascariasis

It is caused by infection with the giant roundworm Ascaris lumbricoides, with around 760 million cases worldwide . Although infections are particularly common in developing countries where sanitation and hygiene is poor, ascariasis exhibits a cosmopolitan distribution, with cases also described in developed countries . It was recently estimated that ascariasis contributes 1.31 million disability-adjusted life years to the global burden of disease . The closely related parasite Ascaris suuminfects innumerable pigs across the globe and is especially common in organic and extensive farming systems []. Infections in pigs are associated with production losses owing to reduced growth and low feed conversion efficiency, with livers unfit for human consumption

Because adult A. lumbricoides and A. suum worms are morphologically indistinguishable, there has been much debate as to whether they represent the same or different species . In addition, the extent of natural cross-transmission of worms between pig and human hosts is unclear. Experimental cross-infections have demonstrated that A. suum can infect humans and that A. lumbricoides can infect pigs, with host preference in the efficiency of infection establishment. Although no definitive molecular marker has been found that can clearly distinguish between so-called human worms and pig worms, a combination of markers has proven useful ]. Molecular studies have demonstrated that human Ascaris infections in developed countries are predominantly of pig origin, whereas in developing countries human-to-human transmission predominates]. However, in China, 14% of human worms were derived from pigs (ie, were of zoonotic origin), indicating that cross-transmission in areas of endemicity may be more common than originally thought . Hybrids between human and pig worms have been detected, indicating that mating can take place between Ascaris from the 2 host species .

Schistosomiasis:

It is an important cause of disease in many parts of the world, most commonly in places with poor sanitation. School-age children who live in these areas are often most at risk because they tend to spend time swimming or bathing in water containing infectious cercariae.
If you live in, or travel to, areas where schistosomiasis is found and are exposed to contaminated freshwater, you are at risk.

Areas where human schistosomiasis is found include:

• Schistosoma mansoni
  • Distributed throughout Africa: There is risk of infection in freshwater in southern and sub-Saharan Africa–including the great lakes and rivers as well as smaller bodies of water. Transmission also occurs in the Nile River valley in Sudan and Egypt.
  • South America: Including Brazil, Suriname, and Venezuela.
  • Caribbean (risk is very low): Dominican Republic, Guadeloupe, Martinique, and Saint Lucia.
• S. haematobium
  • Distributed throughout Africa: There is risk of infection in freshwater in southern and sub-Saharan Africa–including the great lakes and rivers as well as smaller bodies of water. Transmission also occurs in the Nile River valley in Egypt and the Mahgreb region of North Africa.
  • Found in areas of the Middle East.
  • A recent focus of ongoing transmission has been identified in Corsica.
• S. japonicum
  • Found in Indonesia and parts of China and Southeast Asia.