Research brings hope for horses with piroplasmosis. A drug used to treat protozoal infections in cattle has been shown to be effective against one form of the disease in horses when used at relatively high doses.
Piroplasmosis in horses is known to be caused by two blood-borne parasites, Babesia (Theileria) equi and Babesia caballi. The protozoa are spread by the bites of infected ticks of various genera, in particular Dermacentor, Rhipecephalus and Hyalomma. They can also be spread on contaminated needles and surgical equipment.
The organisms infect the red blood cells but usually can be identified only in the acute stages of the disease.
Affected animals show signs of fever, anaemia and jaundice. Their urine is discoloured as a result of the destruction of the red blood cells and the release of haemoglobin.
Infected horses may remain carriers of the infection after recovering from the initial signs of disease. Such horses can transmit the disease even though the organisms can no longer be found in blood smears.
So to be successful, any treatment must not only alleviate signs of disease; it must remove all of the parasites, and make the horse incapable of transmitting the infection.
Unfortunately few antimicrobial agents are effective against protozoal parasites.
Currently available treatments cannot guarantee a cure. So countries in which the disease is not endemic take extreme measures, including slaughter of infected animals, to prevent the disease becoming established.
Now researchers at the Agricultural Research Service Animal Disease Research Unit in Pullman, Washington, led by Dr Don Knowles, have found that relatively high doses of imidocarb dipropionate are effective for treating horses infected with Babesia caballi.
Writing in the journal Antimicrobial Agents and Chemotherapy, they report convincing evidence that the treatment cleared the infection and eliminated the risk of treated animals transmitting the disease.
They could not detect parasites in any of the treated horses using quantitative PCR and nested PCR amplification (These tests are more sensitive than microscopic examination of blood smears.)
After treatment, horses reverted to having no B caballi antibodies. (Antibody levels would be expected to remain high if the infection were still present.)
The scientists were unable to transmit infection by direct inoculation of blood from treated horses into susceptible recipients. Neither could they transmit the disease by feeding ticks first on treated horses and then on susceptible ones.
On the other hand, untreated horses remained infected and could transmit B caballi using the same tests.
“These findings establish that imidocarb dipropionate treatment clears B. caballi infection with confirmation of lack of transmission risk either by direct blood transfer or a high tick burden” they concluded.
“Importantly, the treated horses revert to seronegative status according to the international standard for serologic testing and would be permitted to move between countries where the pathogen is endemic and countries that are free of the pathogen.”
Imidocarb diproprionate is not licensed for use in horses. If it is approved it could prove to be a useful tool in the control of Babesia caballi infections.
For more details see:
Antimicrobial Agents and Chemotherapy (2009) 53(10): 4327- 4332. Epub 2009 Jul 20.
Imidocarb dipropionate clears persistent Babesia caballi infection with elimination of transmission potential.
Schwint ON, Ueti MW, Palmer GH, Kappmeyer LS, Hines MT, Cordes RT, Knowles DP, Scoles GA.