Susceptibility of Sus scrofa to Rift Valley fever virus : implications for animal and human health in Africa

Abstract

Rift Valley fever (RVF) is a vector-borne viral zoonotic disease which affects domestic ruminants and camels. It is characterised by abortion storms and neonatal deaths in animals, and has severe economic impacts. While humans mostly present with self-limiting flu-like symptoms, the disease can progress to more severe signs and even death. Following the discovery of RVF in the early 1930s, initial investigations considered various animal species, including pigs, for susceptibility to the disease by infecting them with the causative agent, RVF virus (RVFV). Using various routes of infection, transmission, pathogenesis, and discernible immune response data established that sheep, goats, cattle and mice were susceptible to infection, but that pigs were not. Also generated were field epidemiological data involving different invertebrate vectors, and various mosquito and animal species. The information gathered was used to devise the current control measures, which mainly focus on ruminants to the general exclusion of domestic pigs. However, reports on the susceptibility of domestic pigs (Sus scrofa domesticus) to RVFV were conflicting and those which concluded that pigs were amenable to infection did not describe the clinicopathological course of infection in this species. Given the epidemiological links of pigs to known RVF hosts in mixed farming set-ups in endemic countries, this study aimed to use current diagnostic laboratory techniques such as blocking enzyme linked immuno-sorbent assays (ELISA), virus neutralisation test (VNT), histopathology and immunohistochemistry, conventional and real time RT-PCR, and sequencing, as well as infection experiments and electron microscopy to determine: (i) the effect of RVFV infection on the pregnant sow; (ii) the effect of RVFV infection on neonatal piglets and weaners and (iii) to establish and apply methods that are suitable for assesing RVF seroprevalence in domestic and wild suids in order to establish their natural exposure to the virus. Results showed that pregnant sows can abort following infection with high doses of RVFV under experimental conditions, and that neonatal pigs and weaners can also be successfully infected, although the infection is sub-clinical. Evaluation of the widely used blocking ELISA for determining anti-RVFV antibodies was shown to be less sensitive than the VNT in experimentally infected pigs. Finally, application of VNT revealed that domestic pigs and warthogs in South Africa were naturally exposed to RVFV and seroconverted to the virus, albeit at lower rates than ruminants. These findings demonstrate the merit of using pigs in RVF biomedical research and contribute to our understanding of the role of suids in the epidemiology of RVF in Africa. The results further revealed the VNT to be the best serological test to use for sero-surveillance of RVF in suids, in the absence of faster and more efficient assays. Demonstration of RVFV in aborted and newborn piglets highlights the need for exercising safety precautions when handling not only ruminant, but also swine aborted materials during outbreaks, to avoid possible virus transmisson to humans. Further studies to isolate and quantify RVFV from blood and oronasal and faecal secretions of infected pigs will assist with determining their potential to infect vector mosquitoes and shed virus in the environment, since viral RNA was shown to persist for at least 28 days post infection in such secretions in this study.