African river blindness nematode

Onchocerca volvulus

ORDER Spirurida

FAMILY

Onchocercidae

TAXONOMY

Filiria volvulus (Lenckart in Mason, 1893), originally Filaria Homo sapiens, West Africa.

OTHER COMMON NAMES English: River blindness nematode.

PHYSICAL CHARACTERISTICS

Filarial parasites of primates, primarily humans. They have an adult length of 0.8-27.6 in (2-70 cm) with females measuring 13.0-27.6 in (33-70 cm) in length by 0.011-0.016 in (270-400 pm) in width; males measure 0.8-1.6 in (2-4 cm) in length by 0.00512-0.00827 in (130-210 pm) in width. Microfilariae measure 0.00866-0.0142 in (220-360 pm) in length by 0.000197-0.000354 in (5-9 pm) in width, are unsheathed, and have a lifespan of almost two years. The epicuticle is folded separately from the underlying cuticle, with the adult male epicuticle showing a honeycomb-like pattern. The intestinal cells of adult females are very thick. The body is slender and blunt at both ends. Lips and a buccal capsule are absent, and two circles of four papillae each surround the mouth. The esophagus is not conspicuously divided. The female vulva is behind the posterior end of the esophagus. The male tail is curled and lacks alae; it bears four pairs of adanal and six or eight pairs of postanal papillae. The microfilariae are unsheathed.

DISTRIBUTION

Distributed throughout the world, including central Africa, Central America, northern South America, and Mexico. (Specific distribution map not available.)

HABITAT

Live primarily in the tropics and subtropics near fast flowing rivers where the Simuliam black fly breeds. (They are normally transmitted by the flies' bites.) They accumulate in raised nodules found under the skin and in the lymphatic system of connective tissues of the human host. Also found occasionally in peripheral blood, urine, and sputum. They can also enter the eye, leading to the formation of lesions and cataracts.

BEHAVIOR

Complex lifecycle begins when an infected female takes a blood-meal from a human host. The larvae enter the host's subcutaneous tissues and slowly mature into adults in one year. Adults can live for 15 years, and many males and females can live together. The microfilariae reach about 11.8 in (300 mm) in length by 0.03 in (0.8 mm) in diameter, and are sheath-less with sharply pointed, curved tails. Many thousands of microfi-lariae migrate in the subcutaneous tissue. When the infected host is bitten by another female fly, microfilariae are transferred from the host to the black fly where they develop into infective larvae, and the lifecycle continues.

FEEDING ECOLOGY AND DIET

Live off the nutrients located in such places as subcutaneous connective tissues, peripheral blood, urine, sputum, and skin found within the host.

REPRODUCTIVE BIOLOGY

Females may produce 1,000 microfilariae per day, which are already hatched when they are born (unsheathed). After mating eggs inside the female develop into microfilariae ovivipariously, which leave the worm one by one.

CONSERVATION STATUS Not listed by the IUCN.

SIGNIFICANCE TO HUMANS

Causes onchocersiasis, which has infected an estimated 18 million people worldwide (mostly in Central and South America and sub-Saharan Africa). It has also caused more than 270,000 cases of bilateral blindness and more than one million cases of visual impairment. It rarely causes death, and is the second most common cause of infectious blindness. The severity of this disease has far reaching economic consequences but, fortunately, advancements have been recently made in reducing the disease. Controlling black flies is the prime way to control the disease. As a result, this species has almost been eliminated from some locations. ♦

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