Published Date: 2019-03-06 19:01:19
Subject: PRO/AH> Foot & mouth disease: pig, epidemiology, research
Archive Number: 20190306.6353216
FOOT AND MOUTH DISEASE: PIG, EPIDEMIOLOGY, RESEARCH
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Date: Mon 4 Mar 2019
Source: USDA/Agricultural Research Service - Research News [edited]
https://www.ars.usda.gov/news-events/news/research-news/2019/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 U.S. Department of Agriculture (USDA) scientists.
The study, recently published in [Nature's] 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.
FMD 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, New York. Prior to this research, it was believed that transmission of FMD did not occur during the pre-clinical 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, 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.
This updated disease data was 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-percent 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 USD 3 million over 2 months as opposed to a catastrophic nationwide epidemic with a cost of USD 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://tinyurl.com/y3tgo5hj [see commentary].
The Agricultural Research Service is the U.S. Department of Agriculture's chief scientific in-house research agency. Daily, ARS focuses on solutions to agricultural problems affecting America. Each dollar invested in agricultural research results in USD 20 of economic impact.
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[The experimental basis for the analyses presented in the above paper consisted of a transmission study, published in 2016 (Ref. 1), in which groups of naive pigs were exposed to a group of FMDV-infected "donor pigs" through successive 8-hour periods. FMD serotype A was applied. The progression of infection was monitored and quantitated in both donors and contact-exposed pigs through measurements of viral genomic RNA in serum and oropharyngeal fluids (OPF). All pigs in the donor group were confirmed to have been infected by needle-free oropharyngeal inoculation with FMDV and were shedding FMDV RNA in OPF throughout the contact transmission trial. Viremia in donor pigs was 1st detected at 24 hours post inoculation (hpi), whereas fever (rectal temperatures above 40 C [104 F]) and vesicular lesions were observed simultaneously at 48 hpi. The study experimentally demonstrated that transmission from incubating donor pigs had occurred at least 24 hours prior to detection of them presenting clinical disease.
When FMD is suspected, knowledge on the exact time of the initial introduction of the virus into the suspected holding, either by an incubating/infected animal or otherwise, is of crucial importance, allowing tracing back contacts with other animal holdings during the period which preceded the diagnosis, when animal movements were unlimited, and allowing timely control measures. In many cases, the index case remains unnoticed, secondary cases being the 1st to be recorded. The success of the applied measures depends, to a high degree, upon the accuracy of the established timeline.
Pigs are considered very prolific FMD virus amplifiers and disseminators, allowing virus-laden aerosol to spread along considerable distances. When wild boar are infected, their uncontrolled movements combined with abundant virus generation may present a highly hazardous situation.
It deserves to be mentioned that different FMD virus serotypes/strains may behave differently from other serotypes. This is related also to studies such as the animal infection trial described in the above report.
References
1. Stenfeldt C, Pacheco JM, Brito BP, Moreno-Torres KI, Branan MA, Delgado AH, Rodriguez LL and Arzt J. (2016) Transmission of Foot-and-Mouth Disease Virus during the Incubation Period in Pigs. Front. Vet. Sci. 3:105. doi: 10.3389/fvets.2016.00105. - Mod.AS]
