Researchers have identified a novel circovirus in a Kentucky horse with liver disease, although it is uncertain whether the virus was responsible for the animal’s health problems.
Circoviruses are small viruses whose circular single-stranded DNA genomes are among the smallest known. They are associated with diseases ranging from symptom-free to lethal in birds, pigs, elks, cows, dogs, panda bears, bears, bobcats, pumas, foxes, minks, palm civets, fur seals, numerous rodents, as well as in reptiles and fish. The virus has also been detected in many bat species.
Alvin Hui and his fellow researchers, writing in the journal Viruses, described analyzing pooled sera from four healthy and one sick horse, all from Kentucky.
DNA sequences from parvovirus-H, equus anellovirus, and some sequences distantly related to mammalian circoviruses were recognized.
Further testing revealed that the circovirus came from a pregnant mare with a fever and hepatitis. The horse’s serum was also positive for equine parvovirus-H, which could have caused the liver disease, but negative for the equine hepacivirus.
The complete genome of equine circovirus 1 strain Charaf (EqCV1-Charaf) was completed by the researchers.
EqCV1 showed 73–74% identity to pig circoviruses 1 and 2, and the elk circovirus. The closest capsid proteins were from the same ungulate circoviruses, with 62–63% identity.
The overall nucleotide identity of 72% to its closest relative indicates that EqCV1 is a new species in the circovirus genus, they said, and the first reported in a horse.
Hui and his colleagues said the equine circovirus genome is most closely related to a Canadian elk circovirus, which has been reported once, and to pig circoviruses PCV1 and PCV2, which are distributed world-wide.
Four pig-related circovirus species are currently known that are either non-pathogenic (PCV1), pathogenic (PCV2), possibly pathogenic (PCV3) or still of unknown pathogenicity (PCV4).
PCV2 has been extensively studied due to its significant impact on pig health, often in the context of co-infections with other viruses.
“The common detection of PCV1 and PCV2 in pigs world-wide, therefore, provides a readily available source of virus from which the divergent EqCV1 may have evolved,” they said.
“Based on the estimated mutation rate of PCV2 the about 30% nucleotide genetic distance between EqCV1 and PCV1, PCV2, or ElkCV indicates that cross-species transmission between these ungulates may have occurred as long as centuries ago.”
The author said the disease-causing ability of the new equine strain remains unclear.
The horse in question had a mild case of hepatitis, but it also had low levels of the equine parvovirus, which has also been linked to liver disease.
Although many symptom-free infections with the equine parvovirus have been reported, its detection in this horse does provide a conceivable explanation for its hepatitis.
Whether the equine circovirus played a leading, supporting, or no role in this case of hepatitis, or has the potential to cause other equine diseases, will require further work, they said.
The study team comprised Hui, Eda Altan, Xutao Deng and Eric Delwart, all with the Vitalant Research Institute in San Francisco; and Nathan Slovis and Caitlin Fletcher, with the Hagyard Equine Medical Institute in Lexington, Kentucky.
Hui, A.; Altan, E.; Slovis, N.; Fletcher, C.; Deng, X.; Delwart, E. Circovirus in Blood of a Febrile Horse with Hepatitis. Viruses 2021, 13, 944. https://doi.org/10.3390/v13050944