Identifying Risk Factors for Stephanofilaria-Caused Ulcerative Dermal Lesions, in Black and White Rhinoceros’ Meta-Population in Kenya

Abstract

Ulcerative skin lesions caused by Stephanofilaria dinniki are common in populations of the critically endangered eastern black rhinoceros (Diceros bicornis michaeli). Although considered benign, they have been associated with loss of body condition and mortality in rhinoceros. Stephanofilarial lesions in rhinoceros may also lead to delayed puberty, reduced milk yields, and prolonged intercalving periods as observed with a similar disease in cattle. In this study the prevalence and predilection sites of stephanofilarial wounds were examined and the influence of age, sex, normalized difference vegetation index (NDVI), rainfall, temperature, rhinoceros’ species, and population density on the prevalence of stephanofilarial skin lesions was evaluated in 10 rhinoceros’ populations in Kenya. The results revealed that the prevalence of stephanofilarial skin lesions was 0% in the lowland sanctuaries such as Ngulia Rhino Sanctuary, Tsavo West Intensive Protection Zone, and Tsavo East National Park to ≥90% in the highland rhinoceros’ sanctuaries including Solio Wildlife Sanctuary, Ol Pejeta Wildlife Conservancy, Lewa-Borana Wildlife Conservancies, and Nairobi National Park. Generalized linear models revealed that monthly minimum temperature and NDVI negatively influenced the probability of stephanofilarial skin lesions. However, spatial heterogeneity in NDVI, and rhino density were positively related to the probability of rhinoceros’ infection with Stephanofilaria. Black rhinoceros had a higher average prevalence of stephanofilarial wounds (50.9%, n = 228) than white rhinoceros’ (3.6%, n = 110). Temperature and heterogeneity in NDVI can directly influence the presence of the filaria vector Rhinomusca dutoiti as corroborated by the previous studies on their distribution. Moreover, the inverse relationship between NDVI and the prevalence of stephanofilarial skin lesions suggests that nutrition negatively impacts rhinoceros’ immunity. Comprehensive studies on the vectors of Stephanofilaria and their bionomics may illuminate the epidemiological patterns of stephanofilariosis in rhinoceros.