3.5.3 Filarial worms, elephantiasis and river blindness
Adult filarial worms are thread-like parasites measuring 40–100 millimetres in length, whose description ‘filarial’ [fill-air-ee-uhl] comes from the Latin for filament. The diseases they cause are all vector-borne, transmitted when a biting invertebrate transfers microscopic worm larvae (around 300 micrometres long) to a human host as it takes a blood meal.
The disease once known as elephantiasis because of the appearance of infected limbs (Figure 8) is properly called lymphatic filariasis [lim-fatt-ik fill-arr-eye-ass-iss]. Most cases are due to one species, Wuchereria bancrofti, transmitted to humans by mosquitoes. The filarial worm larvae block the fine lymphatic tubules that collect tissue fluid from all over the body and return it to the blood stream. The blockages cause painful inflammation and swelling, as fluid collects in the lower limbs and genitals. More than 120 million people are infected with filarial worms in 73 countries, including Bangladesh, India, Indonesia, Ethiopia, Nigeria and the Philippines, and 40 million people are severely disabled by them.
The microscopic larvae of another species of filarial worm (Onchocerca volvulus) are transmitted by blackflies that bite humans. The larvae cause skin lesions that itch relentlessly, but the larvae also invade the eyes and cause ‘river blindness’, so-called because blackflies breed in fast-flowing rivers, so this disabling condition only occurs in riverside communities.