Since May 2013, the U.S. swine industry has been hit hard by diarrhea outbreaks, which have caused significant economic losses including the recorded deaths of at least 8 million piglets. Although two different viruses -- porcine epidemic diarrhea coronavirus and delta coronavirus -- already have been isolated from affected swine, disease has been reported in 32 American states as well as Mexico, Peru, Dominican Republic, Canada, Columbia, Ecuador and Ukraine, with repeated outbreaks in previously affected herds. Despite strict, intensive biosecurity measures adopted by many farms to control the epidemics, disease has continued to spread.
The situation became a talking point between two researchers knowledgeable about animal viruses at Virginia Tech’s Virginia-Maryland College of Veterinary Medicine in Blacksburg, Va. Elankumaran Subbiah, MVSc, PhD, and Xiang-Jin Meng, MD, PhD, said they started to wonder if additional viruses were playing a role in the spread of disease. Their adjoining labs agreed to work together to investigate, and asked affected farms to send fecal samples.
“They gave us samples and lo and behold, we found a novel virus,” said Subbiah, an associate professor of virology.
They found this virus, mammalian orthoreovirus 3 (MRV3), not only in fecal matter but also in samples of ring-dried swine blood meal, a protein source used in pig feed. They published their findings this week in mBio.
“It remains to be determined whether MRV3 is responsible on its own or in conjunction with other viruses for the current epidemic of diarrhea in piglets in the United States, but the disease-causing nature of the virus warrants further investigations about its origin and prevalence,” Subbiah said.
Investigators tested 48 fecal samples from piglets living on farms in North Carolina, Minnesota and Iowa affected by swine epidemic diarrhea outbreaks, and 11 samples of ring-dried swine blood meal from multiple sources. They found presence of MRV3 in 18 of the 48 (37%) fecal samples and nine of the 11 (82%) blood meal samples.
The virus was not found in 36 samples of pig feces and plasma from farms in Indiana, Ohio, Iowa and Illinois that had no ongoing swine diarrhea infections or whose swine had recovered from diarrhea epidemics.
In additional lab tests, neonatal piglets experimentally infected with the virus or given a chloroform extract of blood meal (to remove enveloped viruses) developed severe diarrhea and gastroenteritis and died within three days of infection. Genetic and other analyses of the MRV3 isolates from fecal and blood meal samples revealed that they were identical to each other but significantly different from other, non-disease-causing mammalian orthoreoviruses circulating in the United States.
Although MRVs don’t commonly cause severe disease outbreaks in livestock, related disease-causing strains have been isolated from pigs in China and Korea, said Meng, university distinguished professor of molecular virology. This is the first time it has been identified in the United States, he said. MRV3 strains also have been reported in bats in Europe.
Continuing studies are necessary to determine how the virus originated; how prevalent it is; if it is a pathogen or an opportunistic infection; and if additional blood meal lots have been contaminated. For now, Subbiah said, there is no evidence that the virus can be transmitted to humans.
Subbiah’s and Meng’s laboratories are now collaborating to make a vaccine to control the spread of MRV3.
-- Karen Blum