Onchocerciasis of Horses in Southeastern Louisiana

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Bloodless skin biopsies obtained from the neck of 232 horses from 17 locations in urban and suburban areas of New Orleans, Louisiana, were examined for microfilariae of Onchocera. The skin samples were teased apart and soaked in Tyrode's solution for 24 to 30 hr, fixed in 2.0% formalin, and concentrated by centrifugation. The sediment was spread on a slide, dried, stained for 30 min in Delafield's hematoxylin, and examined microscopically. Microfilariae were found in 52 horses (22.4%) in 16 of the locations. Several of the infected horses had lived from birth at their respective stables, suggesting that the infection could be acquired locally. The observed microfilariae, described in detail, appeared to represent only one species, probably 0. cervicalis. The biology of the Onchocerca infecting horses is poorly understood, probably because they are not considered to be important as a cause of disease (Soulsby, 1968). However, several pathological conditions such as periodic ophthalmia (Lagraulet, 1962; Roberts, 1963), various skin lesions (Riek, 1954), and fistulous withers (Chambers, 1932) have been associated with the presence of these worms. The potential for cross-infections with human and animal Onchocerca is also largely unknown. Caballero and Berrera (1958) reported Onchocerca, probably 0. volvulus, from a spider monkey (Ateles geoffroyi vellerosus) collected in Chiapas, Mexico. Duke (1962) obtained a patent infection of 0. volvulus in a champanzee. Siegenthaler and Gubler (1965) discovered Onchocerca in a nodular swelling of the knee joint of a 25-year-old Swiss farmer who had not been in an area of human onchocerciasis. They suggested that it probably was 0. gutturosa. An Onchocerca removed from the external part of the eye of a woman who had not been outside the Crimean region was reported by Azarova et al. (1965). The worm was presumed to be a species found in domestic animals, the first such case from the USSR. More recently, in Illinois an adult Onchocerca was found in a nodule removed from the wrist of a woman who denied traveling outside the immediate area (P. C. Beaver, Department of Parasitology, Tulane Medical Center, New Orleans, Received for publication 6 March 1973. * This study was supported by Grants AI-00002 and AI-04919 from the NIH, U. S. Public Health Service. Louisiana, 1972, pers. comm.). The specific diagnosis in these cases is uncertain but the worms are likely to represent one or more species commonly found in horses or cattle. In the United States the horse population is increasing rapidly (Irving, 1966). Moreover, the growth of suburban communities has made the horse an amenity living in close proximity with large numbers of people. This is especially true in suburban areas of Orleans and Jefferson parishes in the vicinity of New Orleans, Louisiana, where zoning codes and health laws do not forbid the stabling of horses in populated areas. A survey of the horse population in this area for microfilariae of Onchocerca was therefore conducted to determine the species, the potential public health importance, and some of the biological characteristics of these worms. MATERIALS AND METHODS Microfilariae were first detected in the skin of 2 horses utilized in a preliminary study to develop suitable methods for obtaining, processing, and examining skin samples. The hair was trimmed and a small piece of skin was raised up with needlepoint forceps and cut off with a razor blade without drawing blood. Skin samples were taken from 24 sites over the body of each horse to determine the best area for detecting the microfilariae in a single sample. Blood samples were examined for microfilariae by the Knott (1939) technique. Three concentrations were done on each horse with blood obtained at about 0800, 1200, and 1900 hr. After processing, the skin samples were blotted dry, weighed, and fixed for histologic examination. The samples of skin were examined for microfilariae by the method of Eichler and Nelson (1971), with slight modifications. Each sample was teased apart in a wide-mouth vial containing