Study reveals pigs can transmit FMD prior to signs of sickness.
Foot-and-mouth disease (FMD) virus spreads much more aggressively in pigs than previous research suggests, according to a new study by USDA scientists.
The study, recently published in Scientific Reports, shows that pigs infected with the FMD virus were highly contagious to other pigs just 24 hours after infection — long before showing any clinical signs of infection, such as fever and blisters.
Foot-and-mouth disease continues to be the most important foreign disease of livestock worldwide, said Jonathan Arzt, lead investigator and veterinary medical officer with USDA’s Agricultural Research Service (ARS). Although the United States has not had an FMD outbreak since 1929, this highly contagious viral disease, which is sometimes fatal, is still considered a serious threat to U.S. agriculture. If introduced into FMD-free countries like the United States and Europe, it could cost billions of dollars in losses to the economy from trade bans and eradication, which often includes unavoidable euthanasia of a huge number of affected animals.
Vaccine protection of pigs is notoriously challenging. Vaccinated pigs still shed infectious virus and potentially transmit infection, according to Arzt, who works in ARS’s Foreign Animal Disease Research Unit at the Plum Island Animal Disease Center in Orient Point, N.Y. Prior to this research, it was believed that transmission of FMD did not occur during the preclinical phase — before visible signs of sickness.
This research is critical for infectious disease experts, who use such information to provide the right data and guide the resources to protect livestock against foreign animal diseases if there is an outbreak. A variety of disease-dynamics models have been developed in recent years to identify critical targets for control efforts, predict impacts and estimate resource requirements for specific outbreak scenarios for FMD, Arzt said. However, none of these models included the impact of preclinical transmission.
Working with scientists at the Center for Epidemiology and Animal Health in USDA’s Animal and Plant Health Inspection Service (APHIS), Arzt and his team used a mathematical modeling approach to estimate the occurrence of FMD preclinical transmission amongst pigs. They found that transmission occurred approximately one day prior to development of visible signs of disease.
These updated disease data were then incorporated into a second model that simulates disease spread. The results showed that simulation of FMD outbreaks in the U.S. pig production sector, including a preclinical infectious period of one day, would result in a 40% increase in the number of farms affected. That’s 166 additional farms and more than 664,000 pigs euthanized compared to the existing scenario of no preclinical transmission, Arzt added.
Failure to account for information like this could make the difference between a limited, well-controlled FMD outbreak in the United States with a cost of $3 million over two months as opposed to a catastrophic nationwide epidemic with a cost of $20 billion over one year, Arzt added.
The goal is to prevent FMD from invading the United States and to be prepared if it enters the country, Arzt said. Infectious disease modeling is a critical part of preparedness and protection of U.S. livestock. This research provides another tool — vital information — to help build better models to protect pigs, cattle, sheep and industries from FMD.
The study describing this work can be found at https://go.nature.com/2TKKTym.
Editor’s note: Sandra Avant is a public affairs specialist with USDA ARS.