Researchers are working to uncover just how prevalent the disease may be in seropositive horses.

Although piroplasmosis, an equine disease transmitted primarily by ticks, is not currently endemic in the United States, efforts are underway to determine how many horses may have been imported when testing methods weren’t entirely reliable.

The disease is caused by a protozoan that lives in red blood cells and is endemic in many countries such as the Caribbean area, South and Central America, Africa, Europe and Asia. In addition to the United States, equine piroplasmosis is currently not endemic in Australia, Canada, England, Ireland and Japan.

For many years, horses imported into the United States have gone through quarantine and testing, to try to determine their health status (and whether or not they might be unapparent carriers of piroplasmosis) before allowing them to enter our country. The traditional testing methods, however, were not always accurate. Thus there may be a number of horses allowed into this country that have this disease and the potential to infect other horses. Efforts are now underway to try to determine how much of a problem this might be.

Kent Fowler, DVM (Animal Health Branch Chief for the California Department of Food and Agriculture), is on the Infectious Diseases of Horses Committee for the U.S. Animal Health Association. Under that IDOHC committee, Fowler is chairman of the Equine Piroplasmosis Subcommittee, and he was in private practice for 26 years on the Monterey Peninsula of California before joining the CDFA four years ago.

He said that prior to Feb. 1, 2004, horses coming into the United States from other countries entered with the “official” Compliment Fixation test.

“Unfortunately, that test occasionally yielded some false negatives, especially in horses that had recently been treated with corticosteroids and/or some of the treatments for piroplasmosis,” he explained.

Some of these imported horses were actually seropositive. “For years we were accepting these seropositive horses.  This is one of the reasons that the C-ELISA (Competitive Enzyme-Linked Immunosorbent Assay) test was made the “official” import test in August 2005. There were some bumps in the road regarding getting it finally designated as the official test, and there were some adjustments needed. Since then, the C-ELISA test has been the official test for horses entering the United States.”

This test has a high sensitivity and specificity for determining which horses are seropositive.

Tracing Imported Horses

The U.S. Animal Health Association’s piroplasmosis sub-committee would like to ensure that piroplasmosis remains a truly foreign-animal disease. Yet there is currently no effective means of traceability to follow up on all the horses that were imported.

“They move around, and some have left the country,” said Fowler. “Also, it is important to remember that these horses entered the country legally under the ‘official’ CF test.  So what we are proposing, and in the process of organizing, is a national survey, in which banked residual Equine Infectious Anemia serum from National Animal Health Laboratory Network  laboratories throughout the nation be sent to the National Veterinary Services Laboratory for C-ELISA testing for equine piroplasmosis.”

The USDA Center for Epidemiology and Animal Health is helping establish the ground rules for the proposed survey. “We are hoping to test this residual serum from all parts of the country. The serum will not be identified in any manner (not by horse name, gender or region of the country). It’s a random, blind study just to see what the prevalence of equine piroplasmosis is in the United States.”

More than 30 states and 39 labs are involved in the study. “We are requesting, and at this point, getting a very positive response from the labs, that they send unmarked serum to the NVSL at Ames, Iowa. Funding will hopefully be provided by USDA and by their research branch, to help run the C-ELISA test for both organisms causing piroplasmosis (Babesia equi and B. caballi) on each serum sample. Any positive test will undergo confirmatory testing at the research center, as well,” said Fowler, who expects the survey to take six months to a year.

Most of the horses imported into the United States come from Canada, Mexico and about eight other countries, for a total of about 30,000 horses per year.

“This is a fairly large group we’re dealing with, and I think most experts feel more comfortable with the C-ELISA test as the official import test. These are issues that our subcommittee is working on. The survey results could have a dramatic impact and resolve the uncertainty of potential prevalence of EP in the domestic U.S. horse population. If survey results indicate significant prevalence of EP infection in our horse population, then responsible efforts to address the disease can be made.  A survey result finding no indication of infection would be very beneficial and welcomed information,” said Fowler.

Handling Carriers

Although few clinical cases of piroplasmosis are being seen in the United States, the experts are saying that this may be because the disease has not yet reached the threshold of infectivity, regarding seropositive horses.

“This is what we obviously want to avoid, so it won’t become an endemic disease in this country,” Fowler said.

Successful treatment for this disease has not been validated. “There are some treatment options out there that might be recommended to an owner with a seropositive horse or a clinically ill horse, but at this point research has not found a treatment. Most of the options available appear to regress the disease for a period of time, but then the disease returns,” he said.

The protozoa are not totally eliminated from the blood.

The best defense is to prevent it by not bringing in any more horses that harbor the protozoa. This leaves one question of what do we do about the carrier horses that are already here?

“That’s a huge topic of discussion that I think will be spearheaded by the regulatory agencies involved, once we know what the prevalence is. Currently USDA’s philosophy is that a positive piroplasmosis horse in an import center (found positive upon arrival and testing) should be exported back to the country it came from, or euthanized. If it turns out that we have a lot of seropositive horses that have been here for a number of years, I suspect that approach will have to be re-examined, to see what other options can be made available. Many of the imported horses are valuable animals and loved as part of a family,” he said.

With piroplasmosis, there can be a certain number of horses that are unapparent carriers; they’ve never been ill but can still provide a source of protozoa that could then be transmitted to other horses via ticks. Today, with the C-ELISA test, these horses are discovered at the import centers and turned back.

But those horses that legally entered the country with a false positive test may still be here. Most owners who discover they have a seropositive horse will obviously be resistant to exporting or to euthanizing the horse.

This could not only lead to a new look at regulations regarding what to do with seropositive horses but should also spearhead more research on treatment and vaccination. The dilemma of dealing with this disease may help fund more research efforts.

“In many instances the owner of a seropositive horse (imported earlier and never showing clinical signs) would have no idea that the horse was positive.

We just need to get a general idea of what the prevalence is, in this country, and hopefully the survey will help us do that. We are hoping to test about 15,000 samples from across the United States.

The epidemiologists tell us this will give us a reasonable number to base a prevalence estimate upon, with 95 percent confidence,” he said. “There are several different roads you can take when you start talking about a disease like piroplasmosis,” said Fowler. “Before you get too far down any one of those roads, it’s best to have some idea what the prevalence is, so this is the first step.”