Domesticated riding horses dwelling in the same regions of the Hustai National Park appear to resist the disease. These free-roaming horses belonging to nomadic populations have similar infection rates—but no deaths, said Katsuro Hagiwara, PhD, of the Rakuno Gakuen University School of Veterinary Medicine, in Hokkaido.
“Susceptibility to pathogens (like piroplasmosis) depends on the horse species,” Hagiwara said. “The wild horses (which we call locally ‘takhi’) in particular are highly susceptible to the responsible protozoa (Theileria equi). Nomadic horses do not progress to deadly diseases after infection as wild horses do.”
The reason for this difference is unclear, he said. But it does highlight the need for better surveillance of the takhi, which appear to be infected by local ticks after arriving in Mongolia.
“Control of the vector (pathogen-mediated ticks) is important,” he said. “It’s already done in other countries in the management of livestock (through drug use), but it’s very problematic concerning the wild takhi.”
Because the local nomadic horses don’t die from the disease, there’s been little budget for and education about tick management, Hagiwara said. More efforts could help improve survival in this endangered species, which now numbers only 336 in the wild.
In a recent study, Hagiwara and his fellow researchers collected samples from 92 domestic horses from five nomadic families residing in the same park as the Pzrewalski horses in Mongolia. They also examined and sampled 19 Pzrewalski horses. They found that 80% of the nomadic horses had a T. equi infection, compared to 84% of the takhi.
Babesia caballi, which can also cause piroplasmosis, was practically nonexistent. Only one of the 92 nomadic horses had a B. caballi infection, and none of the Pzrewalski horses did.
They also performed genomic analyses of the T. equi strains and found that the domestic and wild horses were infected with the same strains of the pathogen, Hagiwara said.
“On average, 50% of newborns die each year,” Hagiwara and his fellow researchers reported. “Among takhi of other ages (yearlings, juveniles, and adult horses), approximately 10 to 15 die each year. The mortality incidence from Piroplasma infection was 19% (24/127) over three years. All individuals that died of piroplasmosis were young takhi born in the park. The rates of mortality from piroplasmosis were 19% (10/54) in 2012, 14% (7/49) in 2014, and 29% (7/24) in 2015 of total deaths.”
Yearlings that died showed common clinical signs: loss of appetite, depression, fever, nasal discharge, anemia, and hair loss due to louse (Damalinia equi) infection. Each horse was infected with approximately 12 to 17 ticks, they said.
No vaccine currently exists against piroplasmosis. Tick control is the preferred management technique.
The study, “Problems in the Protection of Reintroduced Przewalski’s Horses (Equus ferus przewalskii) Caused by Piroplasmosis,” was published in the Journal of Wildlife Disease.