Diagnosis

■ Adult parasites. Onchocercomas can be identified by palpation and ultrasonic imaging. Presence of the parasites can be confirmed by surgical removal and examination of the cutaneous nodules.

■ Microfilariae can be found in skin snips after the prepatent period. A PCR is now available for species-specific detection of Onchocerca DNA (0ncho-150 repeat sequence) in skin specimens. Living or dead microfilariae can be seen in the anterior chamber of the eye with the help of a slit lamp or an ophthalmoscope. Various techniques and antigens (e.g., recombinant antigens) can also be used to detect serum antibodies (Table 11.5, p. 625).

Therapy. Onchocerca nodules can be removed surgically. Suramin—a quite toxic substance effective against macrofilariae—is now no longer used. It was recently discovered that O. volvulus, Wuchereria bancrofti, Brugia species, and several other filarial species contain endosymbionts of the genus Wol-bachia (order Rickettsiales) that are transovarially transmitted from females to the following generation. Studies in animals and humans have shown that prolonged therapy with doxycyclin damages both the endosymbionts and the filariae. These results could lead to a new therapeutic approach. Ivermectin in low doses is highly effective against microfilariae (see prevention), and has some effect on macrofilariae in repeated higher doses.

Prevention and control. Protective clothing and application of repellents to the skin can provide some degree of protection from blackfly bites (see Malaria). WHO programs involving repeated applications of insecticides to streams and rivers with the aim of selective eradication of the developmental stages of Simuliidae in western Africa have produced impressive regional results. Mass treatment of the population in endemic areas with low-dose iver-

mectin (0.15 mg/kg of body weight) administered once or twice a year has been practiced since 1987. This can drastically reduce the microfilarial density in human skin for up to 12 months. Microfilariae in the eyes are also influenced. These measures prevent disease and reduce, or even interrupt parasite transmission. Simultaneous application of vector control measures and mass therapy with ivermectin has eliminated the parasite reservoir in the populations of seven of the 11 African countries participating in the above-mentioned control program (WHO, 2000). The manufacturer of ivermectin is providing the drug at no cost for the WHO-coordinated program. In further disease control campaigns, the vector control measures are to be stopped, but mass treatments with ivermectin or other potent drugs are to be continued once a year over the longer term (WHO, 2000).

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Essentials of Human Physiology

Essentials of Human Physiology

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