Filarial diseases cause chronic disease and disabilities in millions of people, leading to a lifetime of suffering and social stigmatization. Filariasis is a disease of the poor, and fosters poverty by decreased productivity, increased healthcare expenses and the abandonment of fertile land. In areas where two or more filarial diseases are endemic, people are frequently co-infected with more than one type of parasite.
There are three parasites responsible for causing lymphatic filariasis (elephantiasis) in humans;
- Wuchereria bancrofti causes about 90% of all cases
- Brugia malayi accounts for most of the rest
- B. timori also causes the disease
Each is transmitted by their own specific insect vector.
An infected insect deposits the larval stage parasites (microfilariae) on the skin next to the bite, they enter the body and migrate to the lymphatic vessels to mature into adults where they live and breed for many years. The disease is frequently caught during childhood, and can go undetected for years, although there may be localized swelling, fever and effects on the immune system. As the number of worms present in the human body increases over time, the worst effects of chronic disease are felt during adulthood, when the dilated lymphatic vessels cause swelling of the limbs and genitals, particularly in men. Over 25 million men are affected with genital disease and over 15 million people with lymphoedema. It causes significant pain and leads to decreased productivity, discrimination, social isolation, and emotional and psychological problems.
Onchocerciasis affects the skin and eyes and is a major cause of blindness in African countries. The Onchocerca volvulus parasites are spread by infected female Simulium blackflies, which are abundant in riverside areas, giving the disease its common name of “river blindness”. The injected microfilariae take about a year to mature in fibrous nodules under the skin, and adults may live as long as 14 years. After mating, the mature female can produce up to 1000 microfilariae each day, which migrate through the skin and eyes. As they die, the large numbers of dead microfilariae produce severe itching and eye lesions. After many years, this may lead to disfiguring skin diseases and permanent blindness, and can shorten life expectancy by up to 15 years. In some West African communities about half of the men over 40 years of age have been blinded. As a consequence, people tend to avoid the fertile river lands for less productive areas resulting in a huge economic burden.
Human loaisis occurs in the rain forest and swamps of West and Central Africa, where Loa loa parasites are spread by infected Deer flies or Mango flies. Mature adults migrate through the subcutaneous tissues of the skin, and may cause red itchy swellings. Occasionally worms can be seen when they move about in the eye, which can be painful, but does not cause damage. In itself, loaisis is not particularly debilitating and is frequently untreated.
In each of these diseases, adult worms in the human host produce millions of juveniles which spread throughout the body via the blood and lymph systems. These microfilariae infect the insect host and pass through several stages of maturation before they can infect humans where they complete the maturation cycle and survive as adults for many years. The adult worms continue to produce new microfilariae in the body.
Filarial disease patients and doctors share their stories