African swine fever (ASF) was first described in 1921 as a highly fatal and contagious disease that caused severe outbreaks among settlers' pigs in British East Africa, according to a "Knowledge Bank" post from the Swedish University of Agricultural Sciences (SLU).
Since then, the disease has expanded its geographical distribution and is currently present in large parts of Africa, Europe and Asia and considered a global threat, SLU said.
Although ASF is typically associated with very high case fatality rates, a certain proportion of infected animals will recover from the infection and survive, and it was speculated that those survivors could act as virus carriers, SLU noted.
The importance of such carriers for disease persistence and spread has since then almost become an established truth, said researchers Karl Stahl and Erika Chenais with the National Veterinary Institute in Sweden, Susanna Sternberg-Lewerin with SLU, Sandra Blome with the Friedrich-Loeffler-Institut in Germany, Arvo Viltrop with the Estonian University of Life Science and Mary-Louise Penrith with the University of Pretoria in South Africa.
However, they questioned the scientific basis for such a role of carriers and conducted a study to review the available literature in a systematic way to evaluate the available scientific evidence about the ASF viral carrier theory.
Stahl et al. selected publications based first on a database search, followed by a stepwise screening process in order to exclude duplicates and non-relevant publications based on pre-defined exclusion criteria. By this process, the number of publications finally included was reduced from the 3,664 hits identified in the initial database search to 39 publications, from which data was then extracted and analyzed, SLU noted.
Based on this analysis, Stahl et al. said it was clear that a definition of an ASF virus carrier was lacking, and that in general any survivor or seropositive animal has been referred to as carrier. It was also clear that evidence of any significant role of such a carrier is absent, the researchers added.
Stahl et al. defined two types of "survivors": (1) pigs that do not die but develop a persistent infection, characterized by periodic viremia and often, but not always, accompanied by some signs of subacute to chronic disease, and (2) pigs that clear the infection independently of virulence of the virus and that are not persistently infected and will not present with prolonged virus excretion.
The researchers concluded that there was no evidence suggesting that any of these survivor categories could be considered as "healthy" carriers, i.e., pigs that show no sign of disease but can transmit the virus to in-contact pigs. However, localized virus persistence in lymphoid tissues may occur to some extent in any of the categories of survivors, which, in theory, may cause infection after oral uptake, Stahl et al. said.
To what extent this is relevant in reality, however, can be questioned given the virus dose generally needed for oral infection, they added.
Stahl et al. published their findings in Virus Research.