Horses are susceptible to infection with the influenza D virus, but it does not appear to cause ill health, researchers have found.
Four two-year-old horses experimentally infected with the virus produced antibodies against the pathogen and showed evidence of virus shedding.
Equine influenza occurs in most of the world, causing a significant economic burden to the equine industry.
Among the four influenza types, equines are affected by type A, which is a highly contagious respiratory infection. The main subtypes involved are H3 and H7.
Of these, H7 is reportedly extinct, and the H3 subtype has diverged into several lineages and is associated with equine infections globally.
Both H3 and H7 subtypes evolved from influenza viruses originating from birds.
Equine influenza A virus causes nasal discharge, fever, cough, lethargy, and loss of appetite, with occasional lower respiratory complications due to secondary bacterial pneumonia.
The influenza D virus, with cattle as its reservoir host, has been found to have circulated in the United States in midwestern equine populations, Chithra Sreenivasan and her fellow researchers reported in the journal Viruses.
Apart from horses, the influenza D virus exhibits a broad host range which includes pigs, sheep, goats, camels, and humans.
A study in 2015 involving 464 samples collected from the equine farms and ranches in Iowa, Minnesota, North Dakota, Nebraska, South Dakota, and Wyoming revealed 11 to 12% seroprevalence against the two predominant influenza D lineages in horses.
Despite this finding, little is known about the susceptibility of horses to the influenza D virus.
The scientists, with the Maxwell H. Gluck Equine Research Center at the University of Kentucky and the Department of Veterinary and Biomedical Sciences at South Dakota State University, used four healthy two-year-old horses, all of whom tested seronegative for both influenza A and D viruses before the experiment.
The animals were infected with the virus, of bovine origin, via the nose and then monitored in subsequent days for clinical signs and virus shedding. They were also tested to see whether they seroconverted (developed specific antibodies as a result of exposure to the virus).
None of the horses showed any clinical signs of disease. There was no nasal discharge, coughing, lung sounds or unusual changes in heart rate in any of the horses. Rectal temperatures were normal.
Nasopharyngeal swabs collected in the first eight days showed virus shedding. The horses showed evidence of seroconversion as early as 13 days following infection.
“In summary, the findings of our study indicate that bovine origin influenza D virus did not cause any respiratory disease in equids; however, the virus can replicate in the respiratory tract and horses undergo successful seroconversion after experimental exposure.”
This suggests that the interspecies transmission of influenza D virus to equids can occur in nature, they said.
In the United States, influenza D strains are largely isolated from cattle compared to swine, but swine were the most affected species in Eurasia.
“Recent evidence suggests that influenza D virus is also circulating in feral swine populations in the United States,” they noted.
The species specificity is an important factor governing cross-species transmission, and it was already reported that interspecies transmission between calves and pigs depends on the species-specific strain.
“In the light of the findings of our study, the susceptibility of horses to influenza D virus needs further investigation in different lineages, as slight phenotypic characteristics can change the species preferences and cross-species transmission, which in turn can affect the viral ecology.
“Hence, more studies are needed to study the species specificity, receptor distribution, and natural susceptibility of influenza D virus in horses.”
The study team comprised Sreenivasan, Tirth Uprety, Stephanie Reedy, Dan Wang, Feng Li and Thomas Chambers, all with the Gluck Center; and Gun Temeeyasen and Ben Hause, with South Dakota State University.
Sreenivasan, C.C.; Uprety, T.; Reedy, S.E.; Temeeyasen, G.; Hause, B.M.; Wang, D.; Li, F.; Chambers, T.M. Experimental Infection of Horses with Influenza D Virus. Viruses 2022, 14, 661. https://doi.org/10.3390/v14040661