INITIATIVE FOR VECTOR AND INSECT SCIENCE AT BOSTON COLLEGE
By John P. Roche
WHAT IS LEISHMANIASIS?
Leishmaniasis is a group of diseases caused by intracellular protozoan parasites of the genus Leishmania. Twenty-one species of the genus infect humans, and all are spread by bites of infected sand flies of the subfamily Phlebotominae. There are two major forms of leishmaniasis: cutaneous leishmaniasis and visceral leishmaniasis. Cutaneous leishmaniasis, which newly afflicts 1.5 million people globally, causes sores on the skin, which can be disfiguring. Cutaneous leishmaniasis can spread to mucus linings of the nose and throat to cause a less common disease known as mucosal leishmaniasis. In contrast, visceral leishmaniasis causes a disruption of the immune system, resulting in fever, enlargement of the liver, enlargement of the spleen, anemia, and loss of weight. There are half a million new cases of visceral leishmaniasis in the world each year. If untreated, visceral leishmaniasis is fatal.
HOW DO PEOPLE CONTRACT LEISHMANIASIS?
The leishmaniasis parasite is transmitted to humans (or other hosts) when a phlebotomine sand fly, usually of the genus Phlebotomus or Lutzomyia, bites that host to obtain a blood meal. The sand flies have cutting mouth parts, and when they feed, they cut the skin and then feed from bleeding capillaries. While feeding, the flies regurgitate compounds into their hosts that serve to reduce inflammation, increase capillary dilation, inhibit blood clotting, and inhibit immune responses. They also regurgitate into their host’s capillaries a form of the parasite called the promastigote. Once inside the host, these promastigotes enter macrophages in the host’s circulatory system. Within the macrophages, the promastigotes develop into another form of the parasite called the amastigote, and these amastigotes reproduce asexually within the host. When the parasite-containing macrophages die, amastigotes are released, and they then infect more macrophages.
The life cycle is continued when another sand fly bites the host and picks up macrophages infected with amastigotes in a blood meal. Once in the fly, the amastigotes enter the fly's gut. Within 24 hours of entering the fly, the amastigotes turn into promastigotes. Promastigotes are more elongated than amastigotes, and they have a flagellum that they use to move or attach to the fly's gut wall. About three days after a blood meal, the fly defecates and expels remains of the blood meal; parasites that are not attached to the gut of the fly are evacuated at this time. Thus, for parasites to remain in the fly and have a chance of being transmitted to a host, they have to be attached to the gut wall. Seven days following the blood meal, parasites move into the esophagus and pharynx of the fly. The life cycle is completed when promastigote parasites are regurgitated into a host when the infected vector fly takes a blood meal. (It takes at least seven days after entry into the vector for parasites to be capable of infecting a new host.)
WHAT IS THE GEOGRAPHIC DISTRIBUTION OF LEISHMANIASIS?
Leishmaniasis is found in Central America, South America, Western Asia, and the Middle East. The preponderance of cases (over 90%) occur in Bangladesh, Brazil, India, Nepal, and Sudan. The disease is also found in southern Texas within the United States. Leishmaniasis is most prevalent in rural areas.
HOW CAN LEISHMANIASIS BE TREATED AND CONTROLLED?
Medicines: Drugs used to treat leishmaniasis include pentavalent antimony compounds such as sodium stibogluconate and meglumine antimoniate, as well as pentamidine isethionate and amphotericin B. Each of these medicines is given intravenously or intramuscularly, or is injected into the regions of individual sores. Additional anti-protozoal drugs are being tested for the treatment of leishmaniasis, including drugs that can be taken orally. The evolution of drug resistance is becoming an increasing problem in some areas, including Sudan and India. The effectiveness of individual drugs varies with different species of Leishmania, so consideration of the parasite species involved (which varies with geography) is an important factor in deciding on effective treatment regimens.
[This description of medicines is given for general information purposes only; contact your health care provider for details on specific treatment options.]
Vector Control: Little is known about the breeding sites of most species of sand flies, so source reduction of the type so effective with mosquito vectors is extremely difficult. Most control programs work at eliminating adult flies. Insecticide applications to houses have proven effective, whereas spraying of larger areas has generally been less effective. Insecticide resistance has been observed in some species of Plebotomus.
Personal Protection: As with most species of insect vectors, one of the most effective ways of avoiding being infected with leishmaniasis is to avoid being bitten by sand flies. Because they are active at night, limiting the amount of time spent outdoors from dusk to dawn considerably reduces one’s chance of being bitten. Also, wearing long pants, wearing long sleeves, and applying insect repellent when one is out at night can reduce transmission. In addition, the use of house screens and bed nets with very fine meshes (sand flies are one third the size of most mosquitoes) can be effective in reducing infection rates.
WHERE CAN YOU FIND OUT MORE ABOUT LEISHMANIASIS?
Thompson, R. C. A., and Reid, S. A. 2000. Is Leishmaniasis extending its range? Parisitology Today 16: 370.
Ashford, R. W. 2000. The leishmaniases as emerging and reemerging zoonoses. International Journal for Paristology 30: 1269-1281.
Guerrant, R. L., Walker, D. H., and Weller, P. F. (Eds.) 2001. Essentials of Tropical Infectious Diseases. W. B. Saunders, Philadelphia.
Beaty, B. J., and Marquardt, W. C. (Eds.) 1996. The Biology of Disease Vectors. University of Colorado Press, Niwot, Colorado.
The Centers for Disease Control: http://www.cdc.gov/ncidod/dpd/parasites/leishmania/factsht_leishmania.htm.
The World Health Organization: http://www.who.int/health-topics/leishmaniasis.htm.
UNDP-World Bank-WHO-Special Programme for Research and Training in Tropical Diseases:
(Reprinted from the Initiative for Vector and Insect Science website.)