SHIC Grant-Funded Study in Vietnam Indicates Rodents May Be Low Risk ASF Vector

A research project on African swine fever (ASF) in Vietnam, made possible by a USDA Foreign Ag Service grant obtained by the Swine Health Information Center (SHIC) with support from the National Pork Producers Council, examined the potential for rodents to serve as vectors of the devastating virus. Rodent vectors are a possible transmission route long-established for other swine diseases, but uncharacterized for ASF. Work conducted by researchers from South Dakota State University and the Vietnam National University of Agriculture (VNUA) on Vietnamese farms with differing biosecurity levels provided information that suggests rodents are not a high risk of being ASF vectors.    

One objective of the study was to determine whether rodents trapped in and around ASF-infected farms harbored ASFV (African swine fever virus), and if so, the optimal samples to obtain from rodents to detect viral infection. Five commercial farms located across four provinces in northern Vietnam with recent ASF outbreaks were identified. The farms ranged in size from 26 to 851 pigs. Biosecurity procedures varied among the farms; two farms featured closed housing, one open housing, and two combined closed and open housing. 

Live traps were placed nightly at each study farm, outside facilities at entry points, and inside facilities near feed sources. Trapping continued at each farm for 10 to 36 days. Rats were euthanized on the farm, stored in freezers, and transported to VNUA for necropsy and PCR testing. 

Samples from rats in and around farms undergoing active ASF outbreaks were negative for ASF across several different sample types (examining biological as well as mechanical vector potential). This work suggests that rats (and presumably similar rodents) do not serve as important vectors for ASFV. 

Building on the negative results from Experiment 1, the second experiment examined whether rats were susceptible to challenge with ASFV, and if so whether they were able to transmit the virus to susceptible rodents. 

Rats were randomly assigned to one of six ASF challenge cages. In each challenge cage, rats were challenged with a dose of 10⁵ 50% hemadsorption doses (HAD₅₀) ASF either intraperitoneally or orally, while additional rats in each cage served as susceptible contacts. One cage of rats served as non-inoculated controls. Body temperatures and clinical signs were recorded three times daily for each animal. 

Clinical signs were not present in any rats during the observation periods. Initial analysis indicates there were no differences between the body temperature of control and inoculated rats, although temperatures of all rats (control, inoculated, and contacts) tended to climb during the second week of the experiment, then fall back to baseline. Despite robust challenges both intraperitoneally and orally, rats were not observed to become ill from nor infected with ASFV out to an incubation period of 21 days. 

This characterization provides insight for biosecurity protocols surrounding prevention of ASF outbreaks. While rodents do not appear to be a significant risk as ASF vectors, other work has proven them to be disease carriers and appropriate biosecurity protocols for elimination continue to be recommended.

As the world deals with the COVID-19 pandemic, SHIC continues to focus efforts on prevention, preparedness, and response to novel and emerging swine disease for the benefit of US swine health. As a conduit of information and research, SHIC encourages sharing of its publications and research. Forward, reprint, and quote SHIC material freely. SHIC is funded by America’s pork producers to fulfill its mission to protect and enhance the health of the US swine herd. For more information, visit http://www.swinehealth.org or contact Dr. Sundberg at psundberg@swinehealth.org.