The argasid tick,Ornithodoros moubata porcinus, is a natural reservoir and vector of African swine fever virus. The stability of ASFV in dead ticks was first determined and then laboratory-reared, nymphal ticks were infected by feeding on viraemic domestic pigs; “Uganda” strain virus multiplied and persisted over periods of 13–15 months in 70–75% of ticks. Infected tampans regularly transmitted ASF to domestic pigs, even when fed singly; in only about two-thirds of cases was virus excreted in the coxal fluid. “Tengani” strain virus, however, produced persistent infection in but 5% of ticks, following the ingestion of approximately 104 to 105 HAD50.
The minimum infective dose of 3 strains of ASFV for ticks collected in Uganda, was investigated by the capillary-feeding technique. Some ticks were infected by 100.9 to 101.9 HAD50 of “Uganda” virus and virtually all by doses ⪖ 104.0 HAD50; a recent warthog isolate in a dose of 105.0 HAD50 caused persistent infection in 60% of ticks, whilst 104.0 HAD50 of a tick isolate produced only 40% persistent infections. Some Uganda ticks exhibited virus proliferation of at least 3–4 log10 units and transmitted ASFV to pigs during natural feeding; virus was sometimes excreted in the coxal fluid.
These results were discussed in relation to the maintenance of ASFV and the probable sites of localisation in the tick; ASFV has the essential characteristics of a tick-borne arbovirus.