OnCulicoidesas a Vector ofOnchocerca gibsoni(Cleland & Johnston, 1910)

Abstract

The problem of the vector ofOnchoccrca gibsoniwas investigated in Kuala Lumpur, F.M.S., where the parasite is endemic and a high percentage of cattle is infected.For reasons which are outlined, the experimental work was confined to species ofCulicoidesandLasioheleaof which 18, possibly more, species were found on cattle, some of them being found to bite cattle in very large numbers daily.The distribution of the microfilariae of 0.gibsoniin the skin of live cattle was found to be very irregular, even within small areas of skin. In transverse sections of skin the microfilariae appear to have a maximum concentration just under the epidermis at a depth of ·05 to ·2 mm.0·52% of 1,523Culicoides pungens, which were dissected soon after an infective blood meal, were found to have picked up the microfilariae. 0·96% of 1,670 of this species, dissected some days after an infective blood meal, were found to have developing or mature filarial larvae in the thorax or head. 0·35% of 3,734 of this species, were found to be naturally infected with filarial larvae in the thorax or head. The difference between the experimental infection rate and the natural infection rate is shown statistically to be significant and it is concluded thatC. pungensis an intermediate host ofO. gibsoni.Culicoides oxystoma,C. shorttiandC. orientaliswere also dissected in large numbers and were found to pick up the microfilariae ofO. gibsoni; in each of these species the experimental infection rate exceeded the natural infection rate, but the difference in none of these is significant. It is concluded however, from the results obtained and from the morphological similarity of the mature larvae found in these species with that inC. pungens, that these species are also intermediate hosts ofO. gibsoni.C. peregrinusandC. buckleyiare suspected but not proven intermediate hosts.C. anophelisand/orC. anophelisv. flavcscensandLasiohelea stimulanswere found to pick up the microfilariae but only partial development was found in them.C. sumatraeandC. raripalpisalso picked up the microfilariae but no development was found in them. The remaining species worked with were entirely negative. In view, however, of the very minute percentage of infection which occurs inCulicoidesspp. and the large numbers which must be dissected to obtain positive results, the negative results given by these less common species are not necessarily conclusive.