CHARACTERISATION AND SERO-EPIDEMIOLOGY OF POTISKUM VIRUS

Abstract

Some aspects of biochemical and biophysical characteristics of Potiskum (POT) virus (a flavivirus isolated from Cricetomys gambianus) were studied in experimental animals, mosquitoes and tissue culture systems. The laboratory and domestic animals infected included Swiss albino mice, rabbits, chicks and goats. The tissue culture systems used were BHK-21, Vero, HEP-2, LLC-MK₂, E6, MA 104 MDCK, MDBK, and MEC. Aedes aegypti was the only mosquito species tested. The Nero-epidemiological pattern of the virus were also studied in both human and animal populations. POT virus passed freely through 200 and 450 nm diameter sized membrane filters. The virus titre was reduced by 2.0 logs when supsended in a diluent supplemented with serum collected from local cattle. The virus titre in tissue culture was not as high as observed in Swipe albino mice. Potiskum virus lost its infectivity to ether and chloroform. The virus lose was between 3.3 and 4.5 lost of virus particles to Ether and Chloroform respectively. POT virus also lost appreciable virus titre when suspended in glycerin buffer. The virus was also shown to be highly heat labile as more than 5.0 logs of POT virus was lost to heat (56°C) under 30 minutes. However, the virus loss at 4°C was minimal ( 2.0 logs) when stored for 120 hours. Potiskum virus haemagglutinated goose, sheep, pig and chick erythrocytes, optimally at either room, +4°C or 37°C using pH values ranging from 6.4 to 6.8; the POT haemagglutinating antigens were only detectable in sucrose-acetone extracted mouse brain materials. The enzyme-linked immunosorbent assay (ELISA) was shown to be sensitive but it could not detect POT virus antigen beyond 10⁻⁴ dilutions. The results of complement fixation and haemaglutination inhibiting antibodies assayed in either mouse or rabbit sera showed that the titres increased with POT virus dose/number of virus inocura. It was was observed that POT virus cross-reacted extensively with Uganda S, West Nile, Wesselebron and Yellow fever; antibodies to POT virus cross-reacted broadly with theme flaviviruses while their antibodies selectively reacted with POT virus. It was observed that POT virus was related to Uganda S, West Nile, Wesselsbron and Yellow fever viruses in descending order. The results of experimental infections of both laboratory and domestic animals indicated that mice, goats and chicks tested were susceptible to POT virus infection. The infection resulted in clinical disease; the animals developed viraemia of variable duration (4 - 8 days). Their susceptibility was however age and dose- dependent. The Swiss albino mice aced 2 - 15 days succumbed to IOLD₅₀ of the virus while the 7 - 15 days old chicks were refractory to infection with 1OLD₅₀ and 100LD₅₀ of POT virus. Variations in susceptibility were also observed in different routes of infection. The virus was however shown to be highly neurotropic in most of the POT virus-infected animals. High virus titres ( 6.5 logs) and marked histopathological lesions were confined to the brains of infected animals. There was 50-100% mortalities in chicks infected with 1000LD₅₀ or more of POT virus using subcutaneous (s.c.) or oral routes. The s. c. inoculated birds succumbed faster than the orally inoculated chicks. Striking pathological lesions and infective virus particles were detected in most of the organs assayed. Similarly the West African Dwarf goats showed 50% mortality when infected s.c. Their response was also dependent on age, the younger the more susceptible. The infected animals developed fever, starry hair coats, leucopaenia, nervousness, hindlimb paralysis and lateral recumbency among other signs. Infective virus particles and striking lesions were detected in the brain, lungs, liver and small intestine. Generally, complement fixing antigens were detected in the organs of infected mice and goats, however, CF antigen was not detected in the organs of POT virus infected chicks. The birds also failed to circulate CF antibodies. Potlskum virus multiplied in four of the 9 cell culture systems tested namely, BHK-21, E6, Vero and HEP-2. The virus produced cell rounding cytopathology and also formed plaques under Carboxy-methyl Cellulose (CMC) overlay medium. CF antigens were also detected in these four cell cultures. The cytological examinations of two of the susceptible cultures (BHK-21 and E6) suggested the replication of the virus both in the cytoplasm and the nucleus of these cells. The experimentally infected mosquitoes (Aedes aegypti) successfully transmitted POT virus from day 5 extrinsic incubation period (EIP). Transmission of the virus was delayed till day 10 in mosquito that received low virus dose. POT virus was circulated in the mosquitoes for 35 days when the last batch of the mosquitoes was harvested and in the suckling mice exposed to these infected mosquitoes. The sero-epidemiological studies showed that HI antibodies to flaviviruses were prevalent in both the human and animal populations tested. However the prevalence varied with different ecological zones. In humans, high POT virus activity (70%) was observed alongside yellow fever, Wesselebron, Uganda S, Dengues 1 & 2, West Nile and Banzi viruses. Few monotypic HI reaction, were found with POT virus despite its high cross-reactivity with other flaviviruses tested. The result of the plaque reduction test performed on some sera showed that 22%, of these sera reduced 50% or more of the POT virus plaques. The prevalence of flavivirus infection in animals were also moderate and POT virus activities were highest in camels, pigs and dogs as against the low response observed in goats, sheep, cattle and fowls. Generally, the prevalence observed in this study in both human and animal populations in 5 different ecological zones further accentuates the hyperendemicity of flavivirus Infections in this environments.