Researchers have determined that a German Warmblood mare aborted her foal in the third trimester due to a rare infection with cowpox. The foal’s entire body surface, including membranes, showed multiple reddish-brown papules about a half-inch in diameter and larger papules covered in crust. The mare was apparently asymptomatic (showing no signs).

“Indeed, this is a rare case,” said Donata Hoffman, PhD, of the Federal Research Institute for Animal Health Institute for Virus Diagnostiques, in Greifswald, Germany.

Despite its name, the reservoir hosts—the animals that carry and transmit the virus—for cowpox are not cows, but voles (small mouselike animals) and other wild rodents, Hoffman said. Occasionally, domestic cats can become infected with cowpox by consuming infected voles. Equine infections are extremely rare, with only two other cases ever reported, she said. Horses might acquire the virus by eating grass or hay that has been contaminated with infected rodent droppings or urine.

In 1999, a 7-year-old Arabian horse died from complications of a cowpox virus infection that caused multiple papules all over its body. And in 2001, a premature foal was born with weakness, low body temperature, and gasping respiration, but no lesions. However, laboratory analyses confirmed an infection of the cowpox virus. Both cases were reported in Germany, and the cowpox disease is currently limited to continental Europe, Hoffman added.

In this third, most recent case, the cause of death was “most probably multifactorial,” Hoffman said. “There was likely placental undersupply, possibly in combination with fever in the mare that went unnoticed, in addition to any harm to the fetal organs themselves by the virus.”

Because of her infection with the cowpox virus, the mare is now immune to the disease and can go on to carry healthy foals, Hoffman added. In their investigation, Hoffman and her fellow researchers collected tissues from the mare’s placenta and from various organs from the fetus including the lung, liver, spleen, thymus, and skin, to carry out further pathological, virological, and molecular investigations. They found the highest viral load in the skin, she said.

The researchers also sequenced the genome of the virus found in the skin, resulting in a complete genome of 222,069 base pairs, Hoffman added.

More knowledge about this poorly understood virus could lead to medical and veterinary improvements in managing viruses and viral infections, Hoffman said.

“Why is this disease so rare?” she said. “You might expect to detect clinical cases more frequently, as many livestock animals probably come into contact with voles. So what’s happening that’s making it get transmitted to horses so exceptionally?”

Knowing why the vole is such a “suitable reservoir for this particular virus” could also help scientists get a better handle on virology, she added.

“In addition, this case study is a perfect possibility to emphasize the relevance of this virus, which has been known for decades, to people and their animals,” she said. This is of particular importance as people are no longer routinely immunized against the closely related smallpox virus.

However, there’s no reason to panic. José Esparza, MD, PhD, former senior advisor on global health and vaccines at the Bill & Melinda Gates Foundation, in Seattle, Washington, said the cowpox virus—and possibly even the horsepox virus—were the first “vaccines” used to effectively inoculate people against smallpox.

The last recorded existence of the horsepox virus was in a herd of Mongolian horses in 1976, Hoffman said.

As far as cowpox is concerned, there’s no detailed data on the prevalence of the virus infecting horses at this time, said Hoffman. In the national reference laboratory where she works, they receive far more samples from diseased cats than horses.

“To the best of our knowledge there are only isolated cases that do not spread within a herd,” she said.

The study, "Fatal Cowpox Virus Infection in an Aborted Foal,” was published in Vector Borne and Zoonotic Diseases