US Piroplasmosis Outbreak Widens
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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
Three or 4 years ago there was a plan in place to survey slaughter horses for piroplasmosis, however before that plan could be put in place the restriction on slaughter houses prevented that plan coming to fruition. So I think there has been a suspicion for some years that piroplasmosis could be present in the USA, and certainly the numbers and distribution of infected horses now would suggest that the disease has been present in the USA for some time. If this is the case then traditional tracebacks may not be the most appropriate way of tackling the problem and a more general survey may be indicated especially in the southern states.
If the disease is widely disseminated is euthanasia of infected horses the most appropiate method of dealing with the problem? There are treatment options available, even though admittedly of variable efficacy, but I suggest that owner compliance with a survey, or willingness to submit to quarantine, if euthanasia of their horse is the likely outcome, would be very limited.
Dr A Kettle
Excellent comments. I agree fully. With a disease that appears to have been present for a couple years and which is likely widespread, contact tracing is not an efficient tool. Some form of broader population-based sampling is required. As you say, getting compliance for a disease where the main current approach is euthanasia of positive animals is a challenge.