The persistence of African swine fever virus in the argasid tick Ornithodoros moubata.
African swine fever virus (ASFV) usually causes a fatal haemorrhagic disease in domestic
pigs. However ASFV has no apparent adverse effects on its vertebrate reservoir hosts,
warthogs (Phacochoerus aethiopicus) and bushpigs (Potamochoerus porcus). The virus is
maintained in the wild by a campestral cycle between wild members of the family Suidae,
especially warthogs, and the argasid ticks which inhabit their burrows.
Infection of Ornithodoros moubata with the VIC T90/1 or LIV 13/33 isolate of ASFV did
not have an adverse effect on the oviposition rates of infected female ticks. Hatching rates
of eggs laid by infected females were also unaffected. In addition infection with ASFV did
not cause a significant reduction in the feeding and moulting rates of first nymphal stage
ticks from infected females. The feeding rates of adult ticks infected with either of the two
isolates of virus were also unaffected. However, infected adult ticks showed an increase in
mortality rates over uninfected ticks. After the second blood meal 40.0% of ticks infected
with the VIC T901l isolate of ASFV died and 33.3% of ticks infected with LIV 13/33 died,
whereas only 4.0% ofuninfected ticks died. Similarly after the third blood meal 93.3% of
ticks infected with the VIC T901l isolate of ASFV died and 84.2% of ticks infected with
LIV 13/33 died whereas 9.1% ofuninfected ticks died.
ASFV is capable of being passed transovarially and transstadially within the Pirbright
colony of O. moubata. Filial infection rates were highly variable between individual ticks.
After the second blood meal filial infection rates ranged from 1.8% to 31.8% for ticks
infected with the VIC T90/1 isolate of ASFV and from 1.2% to 35.5%. After the third
blood meal filial infection rates ranged from 15.0% to 32.4% and 1.7% to 44.0%
respectively. An increase in the number of blood meals increased the number of females
capable of laying infected eggs and increased the filial infection rates of these females.
Transstadially infected individual second nymphal stage ticks were capable of excreting up
sHADsoitick, which is sufficient to infect a pig or warthog.
Dissemination and localisation of ASFV within the tick vector was monitored by using anindirect immunoperoxidase staining technique. In ticks which had been infected either
orally or by direct inoculation of virus into the haemocoel, viral antigen was detected first
in the sexual organs of both male and female ticks. When comparing orally infected ticks
and ticks infected by direct inoculation of virus into the haemocoel, the rate at which the
various organs become infected did not differ substantially. Furthermore, since infection
of ticks either by inoculation of virus directly into the haemocoel (bypassing the gut) or by
oral ingestion of virus, resulted in similar infection rates in the organs in the organs (31.7%
and 31.0% respectively). It seems unlikely that 0. moubata possesses a gut barrier to