Scientists found that more than 20% of the tested equids were positive for antibodies against any “orthopox” virus. However, many of these animals apparently never showed clinical signs of infection, their owners reported. | Photo: iStock

Scientists eradicated smallpox, caused by the variola virus, in the 1980s using a similar virus that caused pox in horses, cows, and rodents. Unfortunately, that similar virus—vaccinia—could very well have started its own series of pox outbreaks just a few years later in India, Pakistan, and Brazil. Since the first cases appeared in dairy cattle (and the humans that milked them) in 1999 in Brazil, the numbers have been increasing steadily. To date, the country has now had to manage three vaccinia pox outbreaks in equids.

A breeding center in southern Brazil reported the country’s first official pox outbreak in horses in 2008, and several farms in southeastern Brazil reported the second such outbreak three years later. Then a handful of donkeys and mules in the northeastern part of the country fell ill in 2014.

But mystery continues to surround these episodes—how the equids got the vaccinia virus (VACV), why humans don’t seem to get it from equids, and why some equids get it in a population and others don’t. Iara Borges, PhD, of the Federal University of Minas Gerais Institute of Biological Sciences, in Belo Horizonte, Brazil, and colleagues have committed themselves to finding answers