INITIATIVE FOR VECTOR AND INSECT SCIENCE AT BOSTON COLLEGE
By John P. Roche
WHAT IS LYMPHATIC FILARIASIS?
Lymphatic filariasis is a debilitating disease caused by nematode worms of the genera Wucheria and Brugia. Larval worms circulate in the bloodstream of infected persons, and adult worms live in the lymphatic vessels. Lymphatic filariasis is not life threatening, but it does cause discomfort, swelling of the limbs and genitals, damage to the kidneys and lymphatic system, impairment of the body’s ability to fight infection, and general malaise. In addition, it causes immeasurable emotional and economic costs in terms of the disruption of family and community life. Approximately 120 million people in the world have the disease, and infection rates are increasing with the continued expansion of urbanization that is underway in the tropics.
HOW DO PEOPLE CONTRACT LYMPHATIC FILARIASIS?
Humans contract filariasis when they are bitten repeatedly by mosquitoes infected with filarial worms. Over 70 species of mosquitoes in the genera Culex, Anopheles, Aedes, and Mansonia can infect humans with the disease. Mosquitoes pick up the tiny, microfilarial form of the parasite when taking blood meals from infected humans. In the mosquito, the microfilariae develop within 7–21 days into members of the next stage of the parasite’s life cycle, which are known as filariform larva. The filariform larva are infective to humans. When the larval worms move to a mosquito’s mouth, and then the mosquito bites humans, the parasites can spread through a human community. Fortunately, however, many bites from infected mosquitoes are required before a person is infected with the disease.
Once a human does pick up filariform larvae from mosquito bites, the larvae move to the lymphatic system, where they develop into adult worms. It usually takes 8–16 months after infection for symptoms of the disease to appear. The life span of adult worms is approximately seven years (microfilariae have a life span of from 3–36 months). The adults range in size from 2–50 cm in length. In the human, the adult worms mate and then the females produce millions of new microfilariae, which then circulate in the blood stream. Microfilariae circulating in the bloodstream can then be picked up by mosquitoes taking blood meals. In most endemic regions, microfilariae show peak abundance in the human bloodstream between 10 p.m. and 2 a.m., which corresponds with the time when Culex mosquitoes are most active. In some regions of the South Pacific, however, where the vectors of filariasis are active primarily in the daytime, microfilariae are most abundant during the day. These observations are consistent with the hypothesis that the microfilariae-abundance cycle in the bloodstream has evolved to maximize transmission to mosquitoes.
WHAT IS THE GEOGRAPHIC DISTRIBUTION OF FILARIASIS?
Lymphatic filariasis occurs in the tropics of India, Africa, Southern Asia, the Pacific, and Central and South America. The largest fraction of cases occurs in Southeast Asia, with the second largest fraction occurring in Africa. The disease has increased in frequency with a global expansion of urbanization; urbanization brings an increase in breeding sites for vector mosquitoes such as Culex pipiens.
HOW CAN LYMPHATIC FILARIASIS BE TREATED AND CONTROLLED?
Screening. Screening for the disease has traditionally been difficult, requiring a microscopic examination of a blood sample. Often, this blood sample had to be collected in the middle of the night in order to correspond with the time of peak microfilariae abundance. However, a simple effective ELISA test for antigens of the parasite in blood samples collected any time of the day is now available, making screening far easier.
Treatment. Treatment of filariasis involves two components: (1) getting rid of the microfilariae in people’s blood, so that the transmission cycle can be broken and (2) maintaining careful hygiene in infected persons to reduce the incidence and severity of secondary (e.g., bacterial) infections. Anti-filariasis medicines commonly used include diethylcarbamazine, which reduces microfilariae concentrations and also kills adult worms, albendazole, which kills adult worms, and ivermectin, which kills the microfilariae produced by adult worms. The disease is usually treated with single-dose regimens of a combination of two drugs, one targeting microfilariae and one targeting adult worms (i.e., either diethylcarbamazine and albenadazole, or ivermectin and albendazole). If a high enough coverage of anti-filariasis drug treatment can be achieved (treating greater than 80% of the people in a community), the disease can be eradicated from an area.
Attempts to eliminate the disease are being helped considerably by Merck and Co., which is donating ivermectin to treatment efforts, and Smith Kline Beecham, which is donating albendazole. The widespread treatment of populations in endemic areas with albendazole has the added benefit of reducing the incidence of intestinal parasite infections, which will serve to dramatically improve the health of individuals suffering those infections, particularly women and children. Attempts to reduce, and eventually eliminate lymphatic filariasis will be facilitated by the fact that humans are essentially the only reservoirs, and that the parasite does not increase in numbers in mosquitoes, but only in humans. In addition, the inefficiency with which filariasis is transmitted (many bites from infected mosquitoes are required to infect a human) further improves the chances of eradicating the disease.
[This description of medicines is given for general information purposes only; contact your health care provider for details on specific treatment options.]
Vector Control. Control of lymphatic filariasis rests in part on control of mosquito vectors. Covering water-storage containers and improving waste-water and solid-waste treatment systems can help by reducing the amount of standing water in which mosquitoes can lay eggs. In addition, killing eggs (oviciding) and killing or disrupting larva (larviciding) in bodies of stagnant water can further reduce mosquito populations. People in endemic areas can reduce the probability of being infected with filariasis by decreasing the number of times they are bitten by mosquitoes. Such personal protection measures can include wearing long sleeves, applying insect repellent, using insecticide-impregnated bed nets, and remaining inside when mosquitoes are most active.
WHERE CAN YOU FIND OUT MORE ABOUT LYMPHATIC FILARIASIS?
Dreyer, G., Noroes, J., Figueredo-Silva, J., Piessens, W. F. 2000. Pathogenesis of lymphatic disease in bancroftian filariasis: a clinical perspective. Parasitology Today 16: 544-548.
Ottesen, E.A. 2000. The global programme to eliminate lymphatic filariasis. Tropical Medicine & International Health 5: 591-594.
Cox, F.E. 2000. Elimination of lymphatic filariasis as a public health problem. Parasitology Today 16: 135.
Lalitha, P., Ravichandran, M., Suba, S., Kaliraj, P., Narayanan, R. B., Jayaraman, K. 1998. Quantitative assessment of circulating antigens in human lymphatic filariasis: a field evaluation of monoclonal antibody-based ELISA using blood collected on filter strips. Tropical Medicine & International Health 3: 41-45.
Nutman, T. B. (Ed.). 2000. Lymphatic filariasis. Imperial College Press, London.
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. Univ. of Colorado Press, Niwot, Colorado.
The World Health Organization: http://www.who.int/health-topics/lymphfil.htm.
UNDP-World Bank-WHO-Special Programme for Research and Training in Tropical Diseases:
The Centers for Disease Control: http://www.cdc.gov/ncidod/dpd/parasites/lymphaticfilariasis/default.htm.
(Reprinted from the Initiative for Vector and Insect Science website.)