The role of arthropod vectors in the transmission of lumpy skin disease in cattle

Abstract

Lumpy skin disease (LSD) is an economically crippling disease of cattle with epizootic occurrence particularly after the onset of the rainy season. Biting arthropods have been implicated as vectors of lumpy skin disease virus (LSDV) due to observations that there is no transmission of LSD when cattle are confined to insect-proof houses. Nevertheless, the method by which LSDV is transmitted under field conditions is not clear and no specific vector has been conclusively pinpointed. The main objectives of this study were to identify and incriminate biting insects as possible vectors of LSDV in outbreak areas and also determine the capacity of vectors thus implicated, in the transmission of LSDV, under laboratory conditions. Biting arthropods that are closely associated with livestock were trapped in three outbreak areas, namely Machakos, Kiambu and Kajiado Districts of Kenya. The insects were identified, dissected and inoculated onto prepubertal Lamb testis cell cultures to isolate virus from them. Blood meal analysis was carried out on engorged insects. LSD experimental transmission was done using Zebu cattle (80S indicus) and insects of the species Glossina morsitans morsitans, Glossina morsitans centralis, Stomoxys calcitrans, Phlebotomus dubosqui and Aedes aegypti. The Neethling strain of virus was used in challenging the animals, feeding the insects and in the virus neutralisation tests. Time series dissections were performed on the insects to isolate virus from various insect parts. Meteorological data from the study areas and other Kenyan districts were recorded. The distribution of biting arthropods in the study districts revealed a total of more than twentynine species of insects. Stomoxys niger species had the highest frequency of occurrence (18 %) while the tabanids species were the least frequent (0.2 %). The average number of females trapped at any given time was significantly greater than that of the male insects (27 compared to 18, P < 0.0001). The blood meal analysis showed that various insect species had , fed on human (14.9 %), bovine (20.7 %), goat (14.9 %), sheep (4.1 %), and lizard (33.6 %,) blood. A female field caught Prostomoxys species insect yielded a positive result for the presence of LSDV. There was a strong association between the insect species and insect parts from which virus was isolated (P = 0.000 Cramer's V = 0.5596). The largest proportion of the virus was recovered from the heads. The crops and hindgut pools had the smallest proportion of virus recovered from them. The source of virus had an effect on seroconversion of the animals (Pearson chi2 (3) = 8.6152, P < 0.035). There was a significant difference between the different species of insects as far as seroconversion of the animals they fed upon was concerned-P < .043. The association between days post virus feeding by the insects, and seroconversion was statistically not significant (P- value = 0.321). Climatic variables had different effects on LSD outbreaks in the areas studied (P < 0.05); however, relative humidity, maximum temperature and wind-speed had significant effects on occurrence of LSD in all the areas. Vaccination and quarantine significantly reduced LSD occurrence. The results showed that that several species of insects are able to transmit the LSD virus. Integrated vector management and application of meteorological information in planning LSD control programmes may have an effect on reduction of LSD outbreaks.