The USA is on a short list of countries where equine piroplasmosis has been deemed "eradicated", with no local source for new infections in horses. However, as of 2011 at least 409 horses, in multiple states have tested positive for equine piroplasmosis. One source of infection was traced to horses that had illegally entered the USA from Mexico without screening at a federal animal disease inspection and testing import station.
Other horses' infection was traced to sharing of needles or other mechanical transmission directly from these illegally imported horses. Another source in question involved a ranch in Texas where local ticks may have been the cause of that disease outbreak.
As a direct result of these recent equine piroplasmosis positive cases, several states, Canada, and several racing jurisdictions now require proof of a negative piroplasmosis test to enter the state, or race track.
Equine piroplasmosis is a devastating disease deserving of vigilance by our regulatory agencies. If the vector responsible for transmission is in our environment, authorities will need to be aware of this so that steps can be taken to control the disease by eliminating this vector, as Florida and California did in the 1960s. This disease can be devastating with a high mortality rate.
Only certain ticks are known to be able to transmit this disease and these ticks have been considered eradicated from the USA. Regular testing of ticks must be made to ensure that they cannot transmit this disease.
Positive horses whose source of infection was outside of the USA must be identified so that regulatory agencies will be able to establish that the disease is not re-emerging in our environment, and so that horses entering the USA will not bring the disease organism with them as a potential source of infection.
In anticipation of the growing list of states and equestrian associations that will require proof of a negative test, horsemen may choose to have any horse tested prior to purchase, especially if they originated in a piroplasmosis endemic country, or from an area with a local outbreak in the USA, regardless of prior testing history.
It is reasonable to expect that a negative piroplasmosis test will be required in the coming years by many states and equine sports associations, in the same manner as we currently require proof of negative Coggins. For example the Alltech/FEI World Equestrian Games, Kentucky 2010 required proof of negative piroplasmosis test for all horses entering the competition grounds including horses from the USA.