Piroplasmosis in Horses

The parasites that cause piroplasmosis and their tick vectors are spreading to previously nonendemic areas, such as the UK, due to climate change and increased global transportation of horses.

Most horses infected with piroplasmosis don’t develop clinical signs of illness. These horses carry the parasite for long periods, and are a source of infection for other horses. Horses that recover from severe symptoms of piroplasmosis also continue to carry the parasite in their bloodstream, even after symptoms resolve.

Piroplasmosis is extremely rare in North America, but more common in areas like southern Europe, Mexico, South America, Africa, and the Middle East. The geographic distribution of piroplasmosis is growing as transmission increases.

Horses who test positive during quarantine after being imported to North America from endemic areas are subject to strict, lengthy quarantine protocols and a risky treatment process. Ultimately, these horses have an extremely poor prognosis due to the high risk associated with the treatment protocols rather than piroplasmosis itself.

In areas where piroplasmosis is endemic, it is uncommon for horses to develop serious disease. Most horses show no symptoms despite testing positive for the disease. These horses have a good prognosis, but cannot be imported into piroplasmosis-free countries. These horses act as a source of infection for other horses when ticks feed on them.

Piroplasmosis is a tick-borne disease, transmitted by numerous tick species. Ticks acquire the pathogenic blood parasites that cause piroplasmosis when they feed on an infected animal. The next time the tick feeds, which may be up to a year later, the parasite enters the new host’s bloodstream, causing infection. Horses that receive blood transfusions are also at risk if the donor horse is infected.

Horses that do develop symptoms of piroplasmosis typically suffer from anemia. Anemia results because the parasites that cause piroplasmosis infect red blood cells, targeting them for removal by the immune system.

The symptoms of piroplasmosis are not specific to this disease, so testing for the parasite is required to make a diagnosis. Testing methods for piroplasmosis include:

• Examining the blood under a microscope
• DNA identification of the parasite

It is important to note that piroplasmosis is a reportable disease in North America. Cases of piroplasmosis must be reported to relevant animal health authorities. If a horse tests positive, the case is managed according to local legislation.

Piroplasmosis infection continues until treated. Horses that test positive for piroplasmosis must be quarantined away from all other horses immediately. In some areas, treatment programs are available. These treatment protocols often require one or more years of quarantine, making them unfeasible for many owners.

Additionally, the chemotherapeutic medication used to treat the parasitic infection has serious potential side effects, including colic and sudden death. For these reasons, the prognosis for piroplasmosis is extremely poor, even in horses who do not show symptoms.

Piroplasmosis is not directly contagious between horses, as a tick is required to transmit the disease. Appropriate tick-control strategies, such as topical tick products and environmental management, are the best methods of piroplasmosis prevention in areas where the disease is common.

In North America, prevention focuses on imported horses. All horses imported to North America undergo a quarantine period, and must test negative for piroplasmosis before release from quarantine. Local legislation dictates the outcome for horses who test positive, but often the horse must be returned to its country of origin.

North America is largely considered piroplasmosis-free, making the disease extremely rare in this region. The disease is endemic in Africa, the Middle East, South America, Mexico, and southern Europe, and is relatively common in these areas. Most cases of piroplasmosis in North America are a direct result of horse imports from endemic countries.

Treatment varies according to specific protocols depending on jurisdiction.