Isolation of African swine fever virus from ticks of the Ornithodoros moubata complex (Ixodoidea: Argasidae) collected within the African swine fever enzootic area of Malawi

Abstract

SUMMARYTicks of the Ornithodoros moubata complex were collected from domestic pig sties and dwelling houses, and from a warthog habitat, and tested for the presence of African swine fever (ASF) virus. Collections were made in 9 of the 24 districts of Malawi, these being primarily the districts in which 0. mowbata is most numerous. ASF virus was isolated from ticks collected in both domestic pig sties and houses in certain villages in Mchinji district where ASF outbreaks had recently occurred. Mchinji district is in the centre of a large ASF enzootic area which stretches into other districts of Malawi and -.also into Zambia and Mozambique. The high titre of virus in some of the ticks demonstrates that 0. moubata can act as a virus reservoir and potential vector of disease in the field situation in Malawi.

Molecular epidemiology of African swine fever in East Africa

“…Wild pigs were implicated in the first report of the disease in East Africa, and these wild suids, especially warthogs, and Ornithodoros ticks, are present, in many of the localities where ASF outbreaks occur in the region. ASFV specific antibodies and viraemias were detected, and virus was isolated from some of these reservoirs, confirming the active roles they played in outbreaks (Montgomery, 1921;Plowright, 1977;Haresnape, 1984;Thomson, 1985;Dixon and Wilkinson, 1988;Haresnape et al, 1988;Wilkinson et al, 1988). Despite the presence of sylvatic reservoirs in East Africa, the location of ASF outbreaks, distribution of warthogs and to a lesser extent bushpigs, dispersal of argasid ticks in warthog burrows and pig sties, and the isolation of ASFV from argasid ticks inhabiting these burrows seldom have a positive correlation (Plowright, 1977;Allaway et al, 1995).…”

“…Midgut replication and escape of ASFV is extremely important for long-term maintenance of infection in ticks (Kleiboeker et al, 1999). The virus is transsexually, transovarially and transtadially transmitted in these ticks, thereby ensuring that the virus can be maintained within the tick population even in the absence of an infected blood meal (Plowright et al, 1970;Plowright et al, 1974 Malawian situation best describes this particular cycle (Haresnape et al, 1988;Haresnape and Wilkinson, 1989). …”

A study of African swine fever virus infected ticks (Ornithodoros moubata) collected from three villages in the ASF enzootic area of Malawi following an outbreak of the disease in domestic pigs

“…However, titres of 4 6 HAD50 or more were noted in three pools of adult females in the November 1983 collection made 3 weeks after the ASF outbreak. It is interesting to note that a tick collected from Tikoliwe in March 1982, approximately 1 month after an ASF outbreak there, which had an extremely high titre of 6 4 HAD50' was also an adult female and it is suggested that infected ticks may have transmitted ASF virus to the pigs in both Tikoliwe and Chalaswa (Haresnape, Wilkinson & Mellor, 1988). In general, titres decreased with the passage of time but it appears that virus multiplied to high titres in a small proportion of infected ticks including adult females in the Chalaswa area in November 1983 and in Tikoliwe in March 1982, and also in the three nymphae identified in this study in July 1984, 8 months after the ASF outbreak.…”

“…Adults from later collections were assayed individually, and maximum pool sizes of 6 for large nymphae and 13 for small nymphae were used. Virus was assayed by haemadsorption (HAD) in pig bone marrow cultures at Pirbright (Haresnape, Wilkinson & Mellor, 1988). Titres were expressed as log1o 50 % haemadsorbing doses (HAD50) per tick or per pool.…”

“…None of the 977 ticks collected from houses in the same villages were infected. The three infected ticks were from kholas in three different villages in the west part of Mchinji district and there had been suspected cases of ASF in all three just prior to the time of collection (Haresnape, Wilkinson & Mellor, 1988). The overall prevalence of infection in the 364 ticks from these three kholas was 0 8 %.…”