Equid Blog : Horse Health & Diseases : University of Guelph : Ontario Veterinary College : Equine Infectious Diseases

Following a repeating trend that has been ongoing over the past couple of years, piroplasmosis has been identified in more US horses with no clear source. The latest cases were in North Carolina, which has reported 11 infections in four different locations.

This is quite concerning because it now seems to be a regular event to get a report of piroplasmosis in US horses from different areas and with no clear link to other outbreaks. The disease, caused by the blood parasites Theileri equi and Babesia caballi, can produce a wide range of clinical signs, and kills approximately 20% of infected horses. Some horses can become healthy "silent" carriers, complicating recognition and control. The bloodborne parasites are naturally transmitted by ticks, but can also be spread by other forms of bloodborne transmission, most notably the reuse of needles. Often, poor needle handling practices are blamed by officials for spread of piroplasmosis among groups of horses on a farm, but that doesn't address the most important question - where did it come from in the first place?

PIroplasmosis is considered a foreign disease in the US, but the repeated cases of unknown origin in at least 19 US states suggests that the disease is actually endemic (established) in the US. Any explanation of where the causative parasites keep coming from is pretty much always missing from US piroplasmosis reports. It certainly seems possible, if not likely, that there is an unidentified pool of infected horses and tick species in the affected areas that are able to transmit the infection.

If the US is going to get piroplasmosis under control, a serious effort needs to be undertaken to understand how widespread the infection is and how it's being spread. Tracking known cases and their contacts is important, but a more concerted effort is required. That takes time and money, and often governments are reluctant to spend much of either when it comes to horses (as opposed to food animals), but considering the potential implications of endemic piroplasmosis and the often overlooked economic importance of the horse industry, it's a worthy investment.

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Yet another case of piroplasmosis has been identified in the US. The horse was identified as infected with the bloodborne parasite Babesia caballi through routine testing for interstate movement of horses. Babesia caballi and Theileria equi are both causes of piroplasmosis, and both have been found multiple times in the US over the past year or so. The infected horse is currently under quarantine.

The good news:

• The infected (and infectious) horse was identified so proper measures can be taken to reduce the risk of spread to other horses.
• The surveillance program that is in place is effective at identifying (at least some) positive horses.

The bad news:

• Still, no one has a good explanation as to why these unrelated cases keep occurring. I can't see any other explanation other than this disease must be present (endemically) in the US and is probably being spread by unrecognized insect vectors. 
• The surveillance that is underway only tests a very small percentage of horses. So, for every infected horse that is detected, there may be many more that go undetected.

Clearly, a concerted and coordinated effort needs to be undertaken to determine the scope of the problem and, more importantly, the source and route of transmission (i.e. the insect vector) of these infections. If at least the vector is not identified, there's no way to properly control this disease.

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Another case of piroplasmosis has been identified in the US. The latest case was detected during routine screening of racehorses in New Mexico, which has been implemented because of the ongoing and poorly contained outbreak. The latest case, a Quarter Horse, tested positive and was euthanized. However, what makes this case different from the large number of other recent piroplasmosis cases in the US is that a different cause of infection was identified: Babesia caballi.

Both Babesia caballi and Theileri equi are parasites that cause piroplasmosis. The US has been considered free of these two parasites, but it's clear that's not the case. Piroplasmosis caused by T. equi seems to be well established in some regions, with large numbers of cases having been reported over the past year or two. There is no indication that the recent B. caballi case was acquired in a foreign country, so it's possible this parasite is also established in the US, at least in the region (in this case in New Mexico) from which the infected horse came. Clearly, much more work needs to be done to determine how these two organisms are infecting American horses. Poor needle handling and other potential routes of bloodborne contamination have been frequently blamed for piroplasmosis transmission in these US outbreaks, but these don't explain where the disease actually came from in the first place. Close investigation of potential insect (tick) vectors is required to determine if recognized vector species of ticks are transmitting these pathogens to horses, and whether in some regions there is an unrecognized reservoir in ticks of other species that are capable of transmitting piroplasmosis.

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The rather large, long-lasting and remarkable outbreak of piroplasmosis continues in the US. A report from the OIE on June 10 describes 23 new cases in 3 states (Colorado, Texas, Oklahoma). None of these are linked to the large outbreak in Texas that has been previously reported, which is not good news.

The origin of infection and route of transmission are unclear. The report states "Preliminary results of these investigations indicate that the transmission of the organism may have resulted from management practices (use of shared needles or substances between horses) rather than by a tick vector." That's what's been said for many of the cases in the US. Piroplasmosis is naturally transmitted by certain tick species. Finding cases in areas where those ticks are not found indicates one of a few possibilities:

• Horses were infected in other regions where infected ticks are present: This is relatively easy to rule out based on travel history, and there's no indication that these are travel-associated infections.
• The cause of piroplasmosis, Theileria equi, is being transmitted by human activity: Specifically, this blood-borne parasite can be transmitted by things like reusing needles and syringes between horses.
• The ticks are actually there, they just haven't been identified.
• Other types of ticks are able to transmit the infection, we just don't know it yet.

Typically, these reports have blamed management practices (e.g. re-using needles) for transmission of T. equi. One problem I have with that is it doesn't tell me where the infection came from. Needle sharing can certainly result in T. equi transmission, but that doesn't account for how the pathogen came to be in the horse population in the first place. Without a clear link to an endemic area or earlier outbreak, there's got to be concern that this organism is more widely disseminated than we realize, and/or that there are other natural sources of transmission that aren't know.

Regardless of how T. equi got to these regions, poor management practices need to be addressed. It's just common sense. Anything that could result in transmission of blood between horses must be avoided. The small amount of money and/or time saved by re-using needles or other instruments isn't worth the risk associated with disease transmission.

At some point, they may have to stop calling this an outbreak, and recognize that piroplasmosis is an endemic disease in the US. There's evidence that this outbreak has been underway for years and the presence of cases unrelated to the large Texas outbreak that was reported earlier suggests that there are multiple foci of infection in the US.

(Picture source: http://icanhascheezburger.com)

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At some point, the US is going to have to admit that piroplasmosis, the bloodborne parasitic infection caused by Theileria equi, is endemic in some regions of the country. It's a declaration that will have major impacts on horse movement to some areas but, it's better for everyone to know what's going on. Piroplasmosis is technically still considered an exotic disease in the US, but there have been many cases identified over the past year and a clear source for the individual outbreaks in lacking, indicating there must be a reservoir in some part(s) of the country.

The latest incident involves the diagnosis of piroplasmosis in three race horses in New Mexico, which were picked up as part of routine screening. There were only three positives out of about 1200 horses tested, so the disease is still rare, but the fact that it was there and none of the positive horses had any link with previous outbreaks is definitely a concern. The OIE report states that transmission is suspected to have been from "artificial" means like sharing needles between horses, not natural tick transmission. This could account for the multiple horses affected but doesn't explain where the disease came from it the first place, and it's unclear how solid that hypothesis really is.

It's quite interesting (surprising, frustrating...) that few comments are put forth in any of these outbreaks indicating where the infections may have originated and why we are seeing recurrent, unrelated infections. Is increased testing in different areas helping to pick up cases that would otherwise have been missed (i.e. were already there), or is piroplasmosis in the US an emerging problem? How confident are they that there are no ticks capable of transmitting T. equi in some of these areas? Is wider screening of horses required to determine the extent of the problem and to determine whether it can be controlled? Is broader screening of ticks in the affected areas needed to see if there are ticks known to be able to transmit T. equi? Are studies needed of other tick species in areas where unexplained cases have occurred to determine if some tick species that are not currently known to be able to spread the parasite can actually do so? Lots of questions... hopefully someone's trying to find some answers.

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A large number of infected horses has now been identified in association with the ongoing outbreak of piroplasmosis in the US. Three hundred seventeen (317) positive horses have been identified in 11 states: Texas, Alabama, California, Florida, Georgia, Louisiana, Minnesota, North Carolina, Tennessee and Wisconsin. Two hundred eighty eight (288) of the positive horses are from the index farm in Texas. All positive horses are under quarantine (and will likely be euthanized), and testing of other in-contact horses is ongoing.

This report is very concerning for several reasons:

• A previous report indicates that positive horses were likely on the index farm for at least a year or two, making it likely that more infected horses are out there. The longer a disease like this goes unnoticed, the farther it can be spread before it's detected and controls are put in place. It's possible that an even larger number of infected horses are in the US now.
• The widespread infection makes it more likely that the disease will establish a true foothold in the US. This bloodborne parasitic disease (caused by Theileria equi) is naturally transmitted by certain types of ticks (not all tick species are able to transmit the parasite). The more areas in which infected horses are found, the greater the chances that infected horses will be bitten by ticks that can transmit the parasite. If it gets established in the tick population, it becomes much harder to control.
• It's not known how so many horses became infected. With this number of infected horses, it seems to me that tickborne transmission is more likely, rather than human-associated transmission through re-use of needles or other means of transmitting bloodborne pathogens. If the disease is being spread through it's natural route, it's harder to control.
• It is still not clear where this outbreak (or the other recent US outbreaks) originated. If you don't know how something started, it's hard to prevent it from happening again.

Piroplasmosis may be on its way to becoming an endemic (established) disease in the US. Broad investigation is required to see if it is present beyond the affected premeses identified so far.

Image source: http://animalphotos.info

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Following on the heels of a few outbreaks of piroplasmosis in the US over the past year is a report of two piroplamosis-positive horses from New Jersey. This bloodborne parasitic disease is officially a foreign disease in the US, but the number of recent outbreaks and their unknown origin certainly suggest that this disease has a solid foothold in the US. The latest situation in New Jersey involves two horses purchased from the Texas farm which is currently under quarantine due to its involvement with the most recent outbreak of piroplasmosis in that state. However, these two horses were purchased in 2008, which strongly suggests that this disease has been in the Texas herd (and presumably elsewhere) since at least that time. Odds are this disease is actually wide-spread in some areas of the US.

Losing national piroplasmosis-free status could have significant repercussions on horse movement to and from the US, and a major impact on the equine industry overall. For this reason, some people might prefer to try to ignore the problem and hope it goes away. But, as I've said before, hope is not an infection control strategy.  It's much better to investigate this carefully and transparently to figure out what is going on. Without knowing the scope of the problem, it's impossible to control it.

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The following report was provided by the Texas Animal Health Commission:

--------------------------------------------------------------

November 4, 2009

Equine Piroplasmosis Disease Investigation Continues

Canada and a number of U.S. states have imposed movement restrictions or additional entry requirements for horses from Texas after equine piroplasmosis, a tick-transmitted blood disease of equine animals, such as horses, donkeys, mules and zebras, was detected in South Texas in mid-October.  Equine piroplasmosis may be carried and transmitted by as many as 15 species of ticks.  Although ticks have been collected from the South Texas ranch for testing, final results are not complete, and it is not known whether any of the ticks can serve as a host for the disease.

“Before moving horses from Texas, we urge you and your veterinarian to check with animal health officials for any state of destination, to ensure the animals have met all entry requirements,” said Dr. Bob Hillman, Texas’ state veterinarian and head of the Texas Animal Health Commission (TAHC), the state’s livestock and poultry health regulatory agency.  “Regulatory requirements can be fluid as disease situations evolve, so it is essential to call each state each time you haul.”  As states provide entry restrictions and requirements, the documents are posted on the TAHC web site at http://www.tahc.state.tx.us.  Dr. Hillman urged equine owners and veterinarians to call state animal health officials directly before hauling, as many states have not yet distributed entry requirement information. Contact information for state veterinarians may be obtained from the TAHC at  800-550-8242, ext. 710, or by emailing ceverett@tahc.state.tx.us.

“We are continuing the equine piroplasmosis disease investigation initiated in October in South Texas. No horse movement is being allowed from or to the ranch where the infection was detected,” said Dr. Hillman.  “While this tick-borne disease has not been considered endemic in the U.S., cases of the disease, scientifically known as Theileria equi, and previously called Babesia equi, have been detected in the U.S.   Our epidemiologists are tracing the movement of specific equine animals. Blood tests will be conducted, and the animals will be examined for ticks. Individual equine owners will be contacted, if their horse needs to be tested by animal health officials.”

Dr. Hillman refrained from speculating on how many equine animals will be tested or how many may be exposed or infected.  “Until the epidemiological work and testing of potentially exposed horses is completed, there is no way to predict how many horses may be affected with this tick-borne illness,” he said.

Dr. Hillman said horses infected with equine piroplasmosis may appear well, while others may exhibit a host of non-specific clinical signs, such as fever or anemia. These clinical signs also could be attributed to a variety of other diseases or causes. Blood tests are needed to diagnosis the disease. 

“Equine owners should talk with their private veterinary practitioners about complying with interstate movement requirements, testing recommendations and protecting their horses from ticks.  If a horse appears to be ill, it should be evaluated by an accredited private veterinary practitioner,” said Dr. Hillman.

Texas Animal Health Commission
Bob Hillman, DVM * Executive Director
For info, contact Carla Everett, information officer, at
1-800-550-8242, ext. 710, or ceverett@tahc.state.tx.us

We thank Carla Everett, Texas Animal Health Commission for sending this report.

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The ongoing large piroplasmosis outbreak on a Texas farm, following on the heels of a few other US outbreaks, raises concern that this infection is not really a "foreign" disease in the US (at least not anymore). Although the source of the earlier outbreaks was not identified, they did not raise too much concern because ticks capable of transmitting this bloodborne infection were not found (which reduces the risk of uncontrolled spread of the disease). However, the situation may be different in Texas. Following identification of the first case, 31 more positive horses were identified on the affected farm. That's a lot of infected horses, and given the number it's probably less likely that the infection was spread by human activity (e.g. reusing contaminated needles) than by ticks. Concern has been expressed over the past couple years that Boophilus microplus and Rhipicephalus microplus, ticks capable of transmitting this parasite, might be establishing themselves in the state of Texas. If this is the case, this may not be an easily controllable situation, and the US may lose its piroplasmosis-free status. Intensive investigation of the source of this outbreak, how it was spread on this farm, whether there are other affected horses in the region and whether these tick species are present, is urgently needed.

Image: A "hard tick" (male brown dog tick (Rhipicephalus sanguineus)) from the same genus as R. microplus, one of the tick species capable of transmitting piroplasmosis (Theileria equi) to horses. (CDC Public Health Image Library #7646)

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For a country that is "piroplasmosis-free," the US sure has a lot of piroplasmosis. The Texas Animal Health Commission has reported that this bloodborne disease, caused by Theileria equi, has been confirmed on a ranch in south Texas. The farm is quarantined and testing is under way to determine the scope of the problem. They are presumably also looking at ticks in the area to see if the types of ticks that are able to transmit the infection are present, and trying to figure out where the infection came from.

This is one of a couple of recent outbreaks of piroplasmosis in the US. The source of infection in these outbreaks is typically not found, but careful testing and quarantine of animals is usually effective at containing the disease. Unfortunately, positive horses can be infected for life, and are therefore usually euthanized. Long-term isolation or shipping infected horses to a country where the disease is endemic are other options.

Interestingly, I've not seen any comments about whether this outbreak could be linked to the recent outbreak is Missouri, where some infected horses were taken out of quarantine and have disappeared. It's certainly logical to consider that one of the fugative horses may have been  smuggled into Texas and been the source of this latest infection. Hopefully this is a small, contained outbreak that doesn't result in widespread death of horses and economic disruption. Hopefully the source is identified promptly and measures are taken to reduce future problems. Only time will tell.

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The recent piroplasmosis outbreak in Missouri and Kansas has been declared resolved. The concluding report has a few interesting findings:

Evidence indicates that in this outbreak, ticks, the typical vector of the disease, did not play a role in the transmission of the bacterium that causes piroplasmosis, Theileria equi. No infected ticks were identified and all ticks that were found on the farms were species that have not been shown to transmit the infection.

Disease transmission was attributed to poor management practices such as needle sharing between horses, which can result in transmission of bloodborne disease, including piroplasmosis. (Needle sharing is always a bad idea. There aren't too many bloodborne diseases in horses, but it's still stupid to reuse needles. They're incredibly cheap and there is no valid reason to do this).

The failure to find ticks that can transmit disease suggests that the risk peiod is over in the affected area. No one has explained, however, where this infection came from in the first place. That's an important issue that hasn't received a lot of attention.

There's also the sticky issue of the vanishing quarantined horses. Three infected horses were illegally taken out of quarantine and still have not been recovered. Therefore, "resolved" is a questionable term. The local outbreak is probably over, but these three escapee horses could still be a threat to the horse population wherever they are. The report states that unsubstantiated information indicates the horses may have been taken out of the country. That's obviously a concern for neighbouring countries like Canada.

Hopefully, this episode is really over. Hopefully the infected horses have been taken to an area where there are no ticks capable of transmitting disease. Hopefully people that have them are smart enough not to reuse needles. Hopefully the illegal and reckless action of sneaking out infected horses doesn't cause an outbreak elsewhere and result in the deaths of more horses. However, hope should not be an infection control strategy.

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TheHorse.com reports that a third horse that tested positive for the reportable, foreign disease equine piroplasmosis (Theileri equi infection) was illegally removed from quarantine in Kansas... in June. Two other horses were broken out of quarantine shortly after they were diagnosed in June, and have yet to be found. These horses could be anywhere spreading this disease, and may result in the US losing its piroplasmosis-free status. The latest OIE report indicates that the third horse was reported missing the day after the other two horses. That's one more potentially infectious horse on the loose, and one more threat.

This appears to be a good example of what can happen when you combine recklessness, carelessness and stupidity (perhaps with a little bit of laziness on the side). Any effective quarantine needs to be properly implemented and monitored. Any breaches of quarantine need to be immediately identified, reported, investigated and communicated. I haven't seen any previous information about the missing Kansas horse; I can't find anything in earlier press releases or other statements. If this information was withheld, it shows pretty blatant disregard for effective communications. People need to know when there are disease threats. Knowing that a horse with piroplasmosis is on the loose is important because this horse has to have gone somewhere - possibly a public stable or other facility with other horses. If people knew a horse was broken out of quarantine one night and someone showed up at the barn with a horse the next day, they could ask some questions or call the authorities to help protect their farm and their animals. Without this information, they're helpless.

(Image source: www.funnyphotos.net.au)

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There is apparently still no evidence regarding the location of two horses with piroplasmosis that were stolen out of quarantine in Missouri. I've been trying to find out more information, to no avail. TheHorse.com quotes a senior USDA veterinarian who states that, as of June 30, the location of the horses was still not known.

This is a pretty concerning situation. While piroplasmosis is not highly transmissible, these two missing horses could be putting many other horses at risk, as well as the piroplasmosis-free status of other regions of the US (if they have been moved to other states or infected other horses that have subsequently traveled to other states).

Piroplasmosis, which is caused by the bloodborne parasite Theileria equi, is transmitted by ticks and through contaminated needles.  If these horses are in an area where there are tick species that are capable transmitting this parasite, piroplasmosis could be silently spreading. Whoever took these horses, and anyone associated with this situation, are incredibly irresponsible and are putting many other horses at risk. This is another good reminder of why you need to be careful and ensure you know as much as possible about all horses (and their owners) that you allow on your property.

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The Canadian Food Inspection Agency (which, despite the name, is not only in charge of food inspection) has announced import restrictions on horses from Missouri and Texas. Horses that have been in those states in the past 21 days or who have passed through those states during transportation are being flagged because of concerns regarding exposure to piroplasmosis and vesicular stomatitis, respectively. Canadian horses traveling to those states will have to wait until the restrictions are lifted or will have to be moved to a non-affected state for 21 days before being eligible to come home.

If these two outbreaks spread beyond these two particular states, it's fairly certain that these restrictions will be expanded.

There's still no word on the location of the two horses that were "busted out" of their piroplasmosis quarantine in Missouri. It's surprising how little information as been forthcoming on this matter.

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TheHorse.com reports that two horses from Missouri that tested positive for equine piroplasmosis were removed from their quarantined stable without authorization on Wednesday night. Quarantine and testing of horses on the farm were implemented following identification of piroplasmosis in a Quarter Horse at a Missouri equine clinic. Five other positive horses were also identified and were euthanized with their owners' consent. The only other alternatives for piroplasmosis-positive horses in the US are long-term quarantine until negative or shipping them to a country where the disease is endemic. The Missouri Department of Agriculture, local officials and FBI are investigating.

Taking horses from a reportable disease quarantine is incredibly stupid.

Firstly, someone's going to notice. The authorities know what horses are quarantined and who owns them. No one has said who is suspected to have cut the padlocks to get at the horses.  I'm not saying that it was necessarily the owners who did it, but who would go to such effort to break an infected horse out of quarantine? I don't imagine they thought that the FBI would be involved.

Secondly, the horses are not being quarantined just because someone's trying to give the owners a hard time. This is being done because of the presence of a severe disease that is not normally present in the US. While it's not highly transmissible, the fact that multiple horses on this farm were postiive certainly shows that transmission can occur, be it through the appropriate tick vectors or other routes (e.g. contaminated needles).

Let's hope the people who took the horses come to their senses or get caught soon, so that the disease is not spread further. I assume that if there are other horses stabled where the escapees are eventually found, that those horses will need to be quarantined as well. I wonder if the people who might be harbouring the fugutive horses have thought of that.

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Equine piroplasmosis, a foreign reportable disease, has been identified in a horse in Missouri.  On June 2, the affected horse (a seven-year-old Quarter Horse) was presented to an equine hospital with signs consistent with a bloodborne disease (although the exact signs have not been reported). The horse was isolated because of the potential for piroplasmosis, and an investigation was started. The sick horse and other horses from the same farm were examined for ticks (the vector of piroplasmosis) and none were found.  On June 10, the diagnosis of equine piroplasmosis was confirmed. The other 63 horses on the farm are currently being tested, the results are pending.

No source of infection has yet been suggested. The US was considered free of piroplasmosis after the last horse in a Florida outbreak cleared the infection earlier this year. This newly affected horse was apparently purchased six months ago, but no information was provided about where it came from. The lack of a clear source of infection is concerning because it could indicate that there are unknown cases somewhere else in the US.

Piroplasmosis is a tickborne disease cause by the protozoal parasite Theileria equi. It can cause signs of illness such as fever, anemia (decreased red blood cell count), jaundice, respiratory signs, reddish urine and weight loss. Up to 20% of affected horses may die. The parasite is naturally transmitted by ticks. It can also be transmitted through the re-use of needles or other blood-contaminated medical equipment, and perhaps through breeding if the semen contains blood. One problem with identification and control of piroplasmosis is that many infected horses show no signs of disease. Further, horses that recover can carry the parasite for prolonged periods of time and become long-term sources of infection for other horses, if the appropriate ticks are around to transmit the pathogen. Horses that are carrying this parasite therefore need to be strictly quarantined. Horses that become persistent carriers of piroplasmosis need to  be quarantined for life, euthanized or sent to a country where the disease is endemic. More information about the source of infection in this case and how far it has spread is anxiously awaited.

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