Horse Care: In the Blood

Equine piroplasmosis may be rare in the United States, but the longhorned tick is a concern for the future

by Allison Armstrong Rehnborg

If you’ve never heard of equine piroplasmosis (EP), then you’re not alone. Classified as a foreign animal disease by the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service (USDA APHIS), EP is a blood-borne protozoal infection that can affect horses, donkeys, mules and zebras. It can be transmitted via tick bite—which is common in countries where the disease is prevalent—or via mechanical transmission, such as infected blood transfusions or contaminated medical equipment.

The disease is found in South and Central America, the Caribbean, Puerto Rico, Africa, the Middle East and Eastern and Southern Europe. By contrast, the United States is one of a handful of countries where the disease is not considered common. Any occurrence of EP in the United States is carefully documented, monitored and managed by state animal health officials and the USDA.

“A foreign animal disease is a disease that is not considered to be endemic in the United States, but it is considered to be a high-risk or high-consequence disease, meaning it could have far-reaching impact on our ability for international and domestic trade as well as the overall health and well-being of our livestock,” explained Andy Hawkins, DVM, assistant animal health commissioner for the Kansas Department of Agriculture’s Division of Animal Health. “That means any veterinarian who has knowledge of or suspects a reportable disease, like EP, must notify the animal health commissioner or the USDA.”

The incidence of EP cases in the United States is low. For example, in 2020, USDA APHIS tested approximately 30,000 horses for EP and found only 23 horses infected with Theileria equi (T. equi), which is one of two parasites responsible for the disease. The other parasite, Babesia caballi, is more common in Puerto Rico and the U.S. Virgin Islands.

According to Angela Pelzel-McCluskey, DVM, MS, equine epidemiologist for USDA APHIS Veterinary Services, both T. equi and Babesia caballi have been detected in horses in the United States.

“T. equi is more commonly seen in America right now in the cases we’ve had,” Dr. Pelzel-McCluskey said. “We don’t see much of Babesia caballi, but we have had a handful of cases from horses that have been imported from countries where the disease is endemic.”

According to Dr. Pelzel-McCluskey, who has firsthand experience handling outbreaks of EP, EP cases most commonly crop up in previously imported horses as well as in clusters of racing Quarter Horses used for bush track, or unsanctioned, racing. These unsanctioned racetrack cases have occurred in Florida, Kansas, Tennessee and other states, and are typically associated with the improper use and subsequent contamination of medical equipment such as needles, syringes and IV tubing.

However, EP can also be spread by ticks, including the Cayenne tick and the American dog tick. There is currently no tickborne transmission of EP in the United States, but the threat of EP is another good reason for horse owners and handlers to be vigilant about the presence of ticks on horses.

While it’s unlikely that EP could become an issue at Standardbred racetracks, it’s crucial for horse owners, trainers and veterinarians alike to become familiar with the transmission risks, clinical signs and management of the disease so that the occurrence of EP in American horses can continue to be low. Here’s what you need to know about this disease.


Equine Piroplasmosis 101 

The symptoms of EP are often nonspecific. Although a few horses can become extremely ill, most horses are asymptomatic. After the initial point of infection, horses may take anywhere from 5 to 30 days to start showing symptoms.

“In most horses, nothing is going to draw your eye to EP right away. Instead, you’re likely to start looking into other differentials first,” said Dr. Hawkins. “Of the cases I’ve dealt with here in Kansas, all the horses were asymptomatic. There was nothing that drew our attention to testing those horses other than routine surveillance testing. Having said that, after testing, the owners mentioned the horses having experienced exercise intolerance or overall poor performance recently. One horse had a dull hair coat and looked unthrifty.”

Because the parasites attack the red blood cells, horses with EP primarily become anemic. Common symptoms include fever, weight loss, weakness and loss of appetite. According to the American Association of Equine Practitioners, other symptoms include central nervous system problems, constipation, colic and hemoglobinuria, which produces a red or dark brown color in the urine.

Typically, cases are detected not because of sick horses, but because of testing that may be required at certain racetracks or during import procedures. When a horse tests positive, the state veterinarian and animal health official must be notified. Those individuals will then notify the USDA, and testing and quarantine procedures will begin for the affected horse and any other horses in contact with that horse.

If a horse tests positive, there are four options. One is euthanasia. The other is official lifelong quarantine, where the horse is confined to a single property for the rest of its life. The third option is to treat the horse. Treatment is expensive, time-consuming and can produce serious side effects in the horse if not adequately prevented with other supporting drugs. The fourth option is to export the horse to countries that accept EP-positive horses, but this is seldom attempted.

“Lifelong quarantine can be difficult, especially considering that the disease can be transmitted via ticks,” Dr. Hawkins said. “You’d have to ensure the horse isn’t in close contact with any other horses and you’d also want to look into doing some brush control or other types of methods to decrease ticks on the property. Treatment also takes a lot of commitment from the owner and from state regulatory personnel. Horses have to be enrolled in the EP treatment program, and the treatment is administered by an accredited veterinarian with regulatory oversight.”

During treatment, horses are given a series of injections of imidocarb, a canine chemotherapy drug, which is an off-label use of the drug. That’s because there is currently no drug in the United States labeled for use in EP.


Pioneering a Treatment

Before veterinarians and epidemiologists discovered that imidocarb could be used to clear the parasites from a horse’s system, the most common solution for a positive horse was euthanasia. The use of imidocarb to treat EP in horses was pioneered by a team of veterinarians, epidemiologists and other USDA officials, led by Dr. Pelzel-McCluskey, in order to manage a large outbreak of tickborne EP on a south Texas ranch in 2009.

“In order to avoid euthanizing an entire ranch’s worth of valuable Quarter Horses, we decided to build a high-dose imidocarb treatment protocol for EP,” Dr. Pelzel-McCluskey said. “Other endemic countries use imidocarb in a low dose on horses that are very sick with EP, but their goal is not to completely clear the organism from the horse’s bloodstream. We wanted to administer the imidocarb at a high enough dose to create permanent clearance and remove the risk of transmission.”

If administered to an EP-positive horse in a high enough dose, imidocarb does clear the organism from the horse’s system. However, it may take years for the horse to test negative for the disease. Until the horse tests negative, that horse must stay in quarantine.

“It’s a long-term process,” Dr. Pelzel-McCluskey noted. “Setting expectations is the most difficult part for owners of an EP-positive horse. People need to realize their horse may be under quarantine for anywhere from one to two years, because that’s how long it takes for most horses to test negative. Once you remove the organisms from the horse, the immune system has to stop making antibodies in order for the horse to test negative, and that takes time. The treatment also doesn’t work for all horses. Some horses are treatment failures. Owners have to buy in and understand that.”

Since the high-dose imidocarb treatment can produce serious side effects, standard EP treatment protocol calls for pre-treating horses with anti-spasmodic drugs.

“The pre-treatment helps eliminate most of those side effects and helps make the horse more comfortable,” Dr. Pelzel-McCluskey explained. “Horses can get nasty spasmodic colic and develop watery diarrhea. You would never give imidocarb to a horse unless you really had to, but it does work.”

Longhorned Tick Threat

While the chances of EP becoming endemic in the United States continue to be low, there is a new threat on the horizon that could contribute to the spread of EP here—and that’s the arrival of the Asian longhorned tick.

“That’s the future scare for us related to tickborne diseases, and that’s also why we don’t want to leave any horses out there positive with T. equi,” said Dr. Pelzel-McCluskey. “Haemaphysalis longicornis, or the Asian longhorned tick, was formerly a tick exotic to the United States, but we found her here in 2017. We suspect she’s been here longer than that. She is established in at least 15 states so far.”

According to the Centers for Disease Control, as of October 2020, Asian longhorned ticks have been found in Arkansas, Connecticut, Delaware, Kentucky, Maryland, New Jersey, New York, North Carolina, Ohio, Pennsylvania, Rhode Island, South Carolina, Tennessee, Virginia and West Virginia. They were first documented in New Jersey in 2017.

The Asian longhorned tick is parthenogenic, which means it can reproduce asexually. According to Dr. Pelzel-McCluskey, this tick is also capable of transmitting many different types of diseases and pathogens that are dangerous for humans and animals.

“She is the scariest-looking tick I’ve ever seen, and she has exsanguinated (caused death by loss of blood) a number of animals before transmitting anything to them,” Dr. Pelzel-McCluskey said. “She could easily transmit T. equi or Babesia caballi and a number of other pathogens to horses, so that’s why we really need to up our game here in the U.S. with piroplasmosis. Eventually, we may have a perfect storm where we combine the wrong kind of tick with the wrong disease.”

Longhorned ticks are more commonly found in tall grass and pasture environments and will feed on wildlife, livestock, pets and humans.

Although no efforts are currently being made to eradicate the Asian longhorned tick, there is a way that horse owners can help epidemiologists like Dr.  Pelzel-McCluskey track the tick. Horse owners can request tick collection kits from the National Veterinary Services Laboratories (NVSL) in Ames, Iowa, or they can make their own kits by preserving ticks in 70 percent ethanol.

“If I was a horse owner in a state with a tick problem, I would start sending ticks into the NVSL for tick identification on a regular basis to find out what kinds of ticks I have in my area and what those ticks are capable of transmitting,” Dr. Pelzel-McCluskey recommended. “There’s a lot of concern about ticks in the industry, and that’s one way horse owners can help with this issue.”

To learn more about the National Veterinary Services Laboratories or how to participate in tick collection and submission procedures, visit HB

Allison Armstrong Rehnborg is a freelance writer living in Tennessee. To comment on this story, email us at


364 More posts in Hoof Beats Magazine category
Recommended for you
Wicked Awesome

Owner-breeder David McDuffee reflects on the ‘magical’ life that took him to the Hall of...