Towards an improved understanding of African swine fever virus transmission

Abstract

African swine fever is a haemorrhagic disease of swine caused by African swine fever virus (ASFV). Estimates of virus transmission (direct or indirect) parameters for ASFV are necessary in order to model the spread of the virus, and to design more efficient control measures. Results presented on this thesis show that ASFV is consistently present in the oropharyngeal swabs (for at least 70 days after infection), showing two peaks. The first peak coincides with the onset of clinical signs and fever, lasts approximately 15 days; and the second peak occurs after a period of lower titres, is unrelated to the re-appearance of clinical signs or fever, and lasts approximately 25 days. ASFV is also constantly present in blood, whereas in faeces it is intermittently present (occasionally with high titres) and in nasal, ocular and vaginal excretions it is present in low titres. Neither the dose nor infection route (inoculated or naturally infected) significantly influence ASFV excretion. Airborne ASFV can be detected and quantified under experimental conditions and viral DNA can be detected from 4 dpi, with titres as high as 103.2 median tissue culture infective dose equivalents (TCID50 eq.)/ m3 during acute disease, remaining detectable in the air while persistently infected pigs were present. Airborne ASFV shows maximum titres and half-lives (average 19 min by qPCR or on average 14 min by virus titration) that are comparable to classical swine fever virus. The transmission rate of ASFV by direct contact under experimental conditions is high (ranging from 0.45 to 3.63 new infections per infectious pigs per day) stressing the importance of animal contacts in ASFV spread. Depending on whether persistently infected pigs can infect others, infectious period can range from approximately 20-40 days (maximum – if persistently infected remain infectious), or from 6-7 days (minimum – if persistently infected are not infectious). The reproduction ratio (R), the average number of pigs infected by one infectious pig during its infectious period, varies between ASFV isolates, and depends on the infectious period used for its estimation, ranging either from 4.9 to 24.2 (minimum) or from 9.8 to 66.3 (maximum). Another relevant aspect of ASFV transmission is determining the virus inactivation rate in different contaminated matrices, under different conditions. Results presented show that, when stored at 20°C, viable ASFV can be detected up to 7, 5, 3 and 2 days in respectively the spleen, lymph nodes (retropharyngeal and parotid), the tonsil, and liver. Virus DNA, however, could be detected much longer. Half-lives of ASFV DNA in tissue samples mostly range from 1.7 to 7.4 days (at 20°C), in urine from 19 to 5 days (between 5oC to 30oC) and in faeces stored 5°C or 12°C there is no decrease in ASFV DNA. ASFV is a tick-borne disease; therefore, possible ASFV vectors should be identified. Using a valid in vitro setup, there is no evidence of ASFV replication in I. ricinus and D. reticulatus, in contrast to the known vector Ornithodoros moubata. Nevertheless, D. reticulatus and I. ricinus cannot be excluded as mechanical vectors of ASFV.