Imagine being bitten by a mosquito, not in your hometown but in one of the countries you have always longed to visit. After a few days, you may not feel well. This is because you have been invaded by the tiny micro-larvae offspring of a worm that lives in another person. You may or may not develop malaise and fever, but your symptoms should subside pretty soon, anyway.

Unfortunately, the long-term effects of filariasis and elephantiasis can be debilitating. The microfilariae, injected into the body by a mosquito bite, settle in the inguinal lymph nodes. There, they mature into adult worms, male and female, that reproduce and live for about 6–8 years. This process damages the lymphatic vessels, causing inflammation and blockage, which leads to the characteristic massive enlargement of the legs and scrotum. The diseases also increase the susceptibility to infections, particularly streptococcal and fungal, which cause skin changes and abscesses. The adult worms constantly produce microfilariae, which can be transmitted to other humans through mosquito bites.

In epidemiological parlance, the infected humans are the primary hosts, and mosquitoes are the intermediate hosts. The worm is about 23 to 46 mm long, very narrow, and cannot be seen without a microscope. It is classified as a nematode or roundworm of the Filarioidea type. Wuchereria bancrofti is the most common, causing 90% of clinical cases. Less common subtypes are Brugia malayi and Brugia timori. They are transmitted by one of the five genera of mosquitos (Aedes, Anopheles, Culex, Mansonia, and Ochlerotatus). Adult worms can live up to nine years, and their females can give off eggs for about five years.

Filariasis has been a scourge for thousands of years. The earliest evidence comes from statues, inscriptions, and medical texts from Egypt, China, India, Japan, and Persia, dating from as early as 2000 BC and depicting the characteristic swollen limbs. In 1588, the Venetian physician and botanist Prospero Alpini described the disease he observed in Egypt; about the same time, Huygen van Linschoten wrote about it after visiting Goa. In 1862, Jean-Nicolas Demarquay found the microfilaria in the liquid of the hydrocele of a Cuban in Paris, and in 1866 Otto Henry Wucherer discovered them in Brazil in the urine of a woman with chyluria. In 1873, Timothy Lewis reported finding microfilaria in urine and in the blood of patients in India.

In 1877, Sir Patrick Manson found microfilariae in the stomachs of bloodsucking mosquitoes, leading to an understanding of the life cycle of these parasites. Other investigators reported finding microfilariae or adult worms in urine, blood, or lymph nodes, including Joseph Bancroft, who in 1877 sent worms from Australia to London, where they were named Filaria Bancrofti.

As the parasite’s life cycle came to be understood, efforts began to be made to prevent the disease and the ravages it was causing. At the last count, filariasis affected 250 million people in seventy-two countries worldwide, mostly in sub-Saharan Africa, Southeast Asia, the Pacific Islands, South America, and the Caribbean. The disease remains endemic in Haiti, the Dominican Republic, Guyana, and Brazil, where some one-third of children are infected. Symptomatic disease occurs predominantly in the young, in their thirties or forties, and there is a 10:1 predilection for men to women. Effective treatment became available in the 1940s, first with diethylcarbamazine (DEC), then with combinations of ivermectin, albendazole, and DEC, including single combined doses, as well as tetracycline. There have been many worldwide public health efforts such as Mass Drug Administration (MDA) programs and the WHO Global Programme to Eliminate Lymphatic Filariasis designed to eradicate this distressing disease.

References

• Thomas Newman and Andrew Juergens. Filariasis. This is an excellent update provided by the National Center for Biotechnology Information in August 2023.
• Kanaan al-Tameemi and Raiaan Kabakli. Lymphatic filariasis: an overview. Asian Journal of Pharmaceutical and Clinical Research 12, no. 12, 2019.