STUDIES ON CONGO AND DUGBE VIRUSES IN THE IXODID TICKS, HYALOMMA RUFIPES KOCH AND AMBLYOMMA VARIEGATUM FABRICIUS VIRUS-HOST RELATIONSHIP

Abstract

The capsule technique was used to rear all stages of Hyalomma rufipes Koch, and amblyomma variegatum fabricius. A comparison of the suitability of three vertebrate hosts, sheep, rabbit and guinea pigs for rearing these ticks showed that the rabbit was the most suitable host. Dugbe and congo viruses did not circulate in the body of the sheep and rabbits. Guinea pigs circulated Dugbe virus but not Congo virus. Antibodies to both viruses were produced in rabbits, after intraperitoneal and intravenous inoculation of either of these viruses into the animals. At room temperature the titres of both Dugbe and Congo viruses remained constant for a period of 8 hours, which was sufficient for the effective feeding of ticks, by capillary tubes. Nymphs and adult of H. rufipes as well as larvae, nymphs and adults of A. variegatum were infected with congo and dugbe viruses by intraccolonic inoculation. Adults of both species were infected with Congo and Dugbe viruses by capillary feeding. Groups of larvae, nymphs or adults of H. rufipes and A.variegatum inoculated with various virus concentrations showed infection threshold phenomenon. The 1 to 5% and 50% infection thresholds were calculated for each stage of ticks. Infection rates were also calculated. The multiplication of both Congo and Dugbe viruses in H. rufipes and A. variegatum followed a similar pattern, regardless of method of infection. There was first a rise in titre of virus followed by fall. At metamorphosis there was a drop in titre of virus followed by a rise. There was no significant difference in the titres of virus on unfed male and female adults. After a blood meal however, the titre in the females was significantly higher. A comparison of the titre of virus in fed and unfed adults showed that after a blood meal, there was usually an increase in the titre of both congo and dugbe viruses in infected H. rufipes and of dugbe virus in infected A. variegatum. Transovarial transmission occurred for both congo and Dugbe viruses in H. rufipes after infection of parents by inoculation as well as by capillary feeding. There was a threshold phenomenon in transovarial transmission for both viruses. The transovarial infection rate 77.3% for Congo virus and 87.3% for Dugbe virus. The filial infection rate for both viruses was between 30% to 60% for first generation (f₁) adults and 71% to 100% for f₁ eggs. After inoculation of a high dose of Congo and Dugbe viruses in H. rufipes and A. variogatum, both species of ticks transmitted, the two viruses equally well to rabbit, during feeding.