Equine Piroplasmosis

Take Home Message  

Equine piroplasmosis (EP) is the disease caused by protozoan hemoparasites, Babesia caballi and/or Babesia (Theileria) equi. The natural transmission of these parasites is through competent  tick vectors. These blood parasites can also be transmitted by transfer of blood from infected to  naïve horses through iatrogenic means. AAEP members have played a key role in the  identification of clinical cases of EP in the U.S. in 2008 and 2009. Through regulatory response  and epidemiologic investigation, all known infected horses in the U.S. are under quarantine as of  the fall of 2010. Interim guidance on management of positive and exposed horses has been  developed. The interim guidelines are available through State Animal Health Officials and  Federal Area Veterinarians in Charge in each state. Long term guidance has been developed and  implementation of the guidance by VS is currently in the planning stages. Equine Practitioners  can play a role in the identification and work-up of suspect cases and the education of their  clients about EP.  

Backgrounder on Equine Piroplasmosis (EP)  

Josie L. Traub-Dargatz DVM, MS, Diplomate ACVIM  

Causative Agents  

Equine piroplasmosis is a tick-borne disease that affects horses, donkeys, mules, and zebras.  There are two distinctive EP causative agents, Babesia caballi and Babesia (Theileria) equi. It  has been proposed that Babesia equi be given a taxonomic classification of Theileria equi¹ while  other researchers have suggested that a more accurate classification for this parasite falls  between Babesia and Theileria based on genomic analysis.^2,a Until additional studies are available that determine a final taxonomic classification, for this proceeding paper we will refer  to the parasite as Babesia (Theileria) equi or B. equi.  

Methods of Transmission  

B. caballi and B. equi are transmitted by many of the same tick vectors, with multiple types of  ticks implicated in the transmission of EP agents on a global perspective. Ticks serve as a  reservoir of B. caballi because the organism persists in the ticks through several generations,  with transtadial and transovarial transmission occurring in some types of ticks.³ In contrast,  horses are the primary reservoir of B. equi with no transovarial transmission demonstrated to  date.³ Few countries are considered free from native transmission of EP agents via ticks and the  prevalence of EP infections is consistent with the distribution of known competent tick vectors.³  

Dermacentor (Anocentor) nitens, the tropical horse tick, was reported to be the primary vector of  B. caballi transmission during the 1960’s outbreak of EP in Florida. A joint USDA-Animal and  Plant Health Inspection Services (APHIS) and State of Florida eradication program for B. caballi brought the outbreak under control through a multifaceted approach that emphasized tick control  on horses and equine premises.⁴ Prior to 2009, no tick transmission of B. equi was recognized in  the U.S.  

The EP agents can also be transmitted by iatrogenic means. Procedures that move blood from an  infected horse to a naïve horse through the reuse of equipment such as needles and syringes have  been implicated in transmission of EP agents.⁵ In addition the use of horses that are carriers of  the EP agent as a source of blood for transfusion could result in transmission.  
Clinical Signs  

Horses infected with either agent have similar clinical signs. Clinical signs of EP can include  fever, anemia, icterus and anorexia.³ Digestive tract signs can occur including colic, constipation  or diarrhea.³ It has been reported that Babesia equi can be transmitted by intrauterine infection  leading to abortion or neonatal infection but how often this occurs is not well documented.⁶  

In countries where EP is endemic, foals born to infected mares maybe protected from clinical  disease through ingestion of protective colostral antibodies; this is called premunition.³ Thus, in  some regions of the world where infection is common, little or no clinical disease maybe  observed in native horses. However, disease is frequently observed in adult horses suddenly  introduced into areas with large numbers of infected ticks.  

It is important to recognize that inapparent carriers represent the majority of infected horses.  Because these horses appear clinically normal, diagnosis of infection relies on laboratory testing  that will be described in a separate section of these proceedings.  

Diagnosis  

Clinical signs can alert the veterinarian to the possibility of EP. However, the clinical signs of  EP are consistent with other diseases including equine infectious anemia (EIA), purpura  hemorrhagica, idiopathic immune mediated anemia, and intoxications. In cases having clinical signs consistent with EP, examination of blood smears can assist in diagnosis. Giemsa staining  of blood smears followed by careful microscopic examination can reveal the intraerythrocytic  parasites in acute cases. B. caballi can appear pyriform shaped and occurs in pairs (Figure 1)  while B. equi appears as four pyriform parasites in a Maltese cross formation (Figure 2).³ 

Because the parasitemia can be very low in horses that have recovered from clinical disease the  examination of direct blood smears may not allow for detection of the infection in chronic  carriers.

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