VECTOR COMPETENCE OF NIGERIAN STRAIN OF Aedes aegypti Linn. (DIPTERA: CULICIDAE) FOR RIFT VALLEY FEVER VIRUS.

Abstract

Studies were carried out to determine the vector competence of Nigerian strain of Aedes aegypti for Rift Valley fever virus. Oral infection was carried out by feeding mosquitoes on (1) viraemic mice and (ii) suspension of RVF virus and blood through animal membrane. Mosquitoes fed with virus dose of 3.5log₁₀ LD₅₀/0.20ml became infected. The virus was recovered from them after 7 days of extrinsic incubation. The rate of infection ranged from 13% to 73% when 3.5log₁₀ LD₅₀/0.20ml and 7.0log₁₀ LD₅₀/0.20ml respectively. Dissemination of infection studies showed that RVF virus was inseminated to the legs of Ae.Aegypti mosigitoes 10 days after infection. Out of the 245 mosquitoes that got infected, 147 (60%) disseminated the virus. In this study, viral dissemination was dose and time dependent.. In addition to injection and dissemination Ae.aegypii mosquito also transmitted RVF virus to baby mice by bite after 12 days of extrinsic incubation period. Transmission rate increased significantly with increased viraemic titres ingested (P<0.05). However, transmission rates obtained in the artificial feeding was significantly lower than rows obtained from mosquitoes that fed on viraemic mice. Studies on the effect of environmental temperature on vector competence of Ae.aegypti for RVF virus showed that infection, dissemination and transmission rates increased significantly (X² , P<0.05) with increasing extrinsic incubation temperature. In this study also Ae.aegypti mosquito transmitted the virus both transstadially and transovarially. Transstadial studies showed that (no adult mosquitoes transmitted the virus after ingestion of liver of mice infected with RVF virus. Transovarial transmission (TOT) was detected only in the progeny from the first 3 ovarian cycles (OVCs) with the transmission rate decreasing as the number of OVC increased. A total of 51 (23%) isolates were obtained from 220 pools comprising 11,093 progeny of parents infected with zinga strain of RVF virus. There was no significant difference in the minimum filial infection rates (MIRs) obtained from the progeny from sterilized and non-sterilized eggs. On the other hand TOT increased as the temperature of progeny rearing water and titre of virus ingested increased. Results obtained from this study showed that Ae.aegypti is a competent vector of Zinga strain of RVF virus. However it is not known why the virus is not transmitted by this prevalent mosquito naturally in Nigeria. Two reasons may be adduced for this; either the mosquito does not feed on the reservoir host or the virus does not attain enough titre in this host to effect infection of the mosquito. It is therefore suggested that further studies be done to establish the reservoir host of the virus, and to determine the course of the virus in this host.