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Case Description: A 7-year-old Quarter Horse gelding used for unsanctioned racing was examined because of fever and anorexia., Clinical Findings: Physical examination revealed fever, tachycardia, and tachypnea. Results of a CBC indicated anemia and mild thrombocytopenia. Results of microscopic examination of a blood smear indicated piroplasms in erythrocytes, consistent with Babesia spp. Regulatory authorities were contacted, and results of serologic testing at the National Veterinary Services Laboratories confirmed acute Babesia equi infection., Treatment and Outcome: Equids on the home premises of the index horse were placed under quarantine. Those equids were tested for piroplasmosis, and 6 of 63 horses had positive results for B equi. Another horse that had previously been housed on the index premises also had positive results for B equi. Competent tick vectors for piroplasmosis organisms were not identified. All 8 horses with piroplasmosis were Quarter Horses that participated in unsanctioned racing and were trained by the same person. Two of the horses were illegally removed from the index premises; these 2 horses and the other horse with piroplasmosis that was previously housed on the index premises could not be found. The other 5 horses with piroplasmosis were euthanized. Investigators concluded that transmission of B equi among horses was most likely iatrogenic., Clinical Relevance: The United States has been considered piroplasmosis free. However, veterinarians should consider piroplasmosis in horses with signalments and clinical signs similar to those of the index horse of this report. Regulatory authorities should be contacted regarding horses in which piroplasmosis is suspected.

Case Description: A 7-year-old Quarter Horse gelding was hospitalized in Ocala, Fla, because of lethargy, fever, anorexia, and swelling of distal aspects of the limbs. A tentative diagnosis of equine piroplasmosis (EP) was made on the basis of examination of a blood smear. The case was reported to the Florida State Veterinarian, and infection with Babesia equi was confirmed. The subsequent investigation included quarantine and testing of potentially exposed horses for B equi and Babesia caballi infections, tick surveillance, and owner-agent interviews., Clinical Findings: 210 horses on 25 premises were tested for infection with EP pathogens. Twenty B equi-infected horses on 7 premises were identified; no horses tested positive for B caballi. Seven horses, including the index case, had clinical findings consistent with EP Dermacentor variabilis was considered the only potential tick vector for B equi collected, and all D variabilis specimens tested negative for Babesia organisms via PCR assay. Results of the epidemiological investigation suggested that B equi was spread by use of shared needles and possibly blood transfusions. All horses that tested positive were involved in nonsanctioned Quarter Horse racing, and management practices were thought to pose substantial risk of transmission of blood-borne pathogens., Treatment and Outcome: Final outcome of B equi-infected horses was euthanasia, death from undetermined causes, or shipment to a US federal research facility., Clinical Relevance: This investigation highlights the importance of collaboration between private veterinary practitioners, state veterinary diagnostic laboratories, and regulatory officials in the recognition, containment, and eradication of foreign animal disease.

We report an outbreak of equine piroplasmosis in southern Texas, USA, in 2009. Infection prevalence reached 100% in some areas (292 infected horses). Amblyomma cajennense was the predominant tick and experimentally transmitted Theileria equi to an uninfected horse. We suggest that transmission by this tick species played a role in this outbreak.

A competitive-inhibition enzyme-linked immunosorbent assay (cELISA) was developed for detection of equine antibodies specific for Babesia caballi. The assay used recombinant B. caballi rhoptry-associated protein 1 (RAP-1) and monoclonal antibody (MAb) 79/17.18.5, which is reactive with a peptide epitope of a native 60-kDa B. caballi antigen. The gene encoding the recombinant antigen was sequenced, and database analysis revealed that the gene product is a rhoptry-associated protein. Cloning and expression of a truncated copy of the gene demonstrated that MAb 79/17.18.5 reacts with the C-terminal repeat region of the protein. The cELISA was used to evaluate 302 equine serum samples previously tested for antibodies to B. caballi by a standardized complement fixation test (CFT). The results of cELISA and CFT were 73% concordant. Seventy-two of the 77 serum samples with discordant results were CFT negative and cELISA positive. Further evaluation of the serum samples with discordant results by indirect immunofluorescence assay (IFA) demonstrated that at a serum dilution of 1:200, 48 of the CFT-negative and cELISA-positive serum samples contained antibodies reactive with B. caballi RAP-1. Four of five CFT-positive and cELISA-negative serum samples contained antibodies reactive with B. caballi when they were tested by IFA. These data indicate that following infection with B. caballi, horses consistently produce antibody to the RAP-1 epitope defined by MAb 79/17.18.5, and when used in the cELISA format, recombinant RAP-1 is a useful antigen for the serologic detection of anti-B. caballi antibodies.

The competitive inhibition ELISA (CI-ELISA) format overcomes problems associated with antigen purity since the specificity of the CI-ELISA depends solely on the monoclonal antibody (mAb) used. Therefore, the CI-ELISA format is well suited for use with recombinant antigens. Molecular clones expressing a conserved 19 kDa protein of Anaplasma marginale and a 34 kDa protein of Babesia equi were derived and characterized. The 19 kDa A. marginale protein, conserved in all recognized Anaplasma species, and present in the infected tick salivary gland, was reactive with all bovine immune sera tested. The 34 kDa B. equi protein contains a protein epitope bound by antibody in equine immune sera from 19 countries. Monoclonal antibodies reactive with these proteins were derived and applied with recombinant copies of the 19 kDa A. marginale and 34 kDa B. equi proteins in a CI-ELISA format.

Horses infected with Babesia equi were previously identified by the presence of antibodies reactive with a merozoite surface protein epitope (D. P. Knowles, Jr., L. E. Perryman, L. S. Kappmeyer, and S. G. Hennager. J. Clin. Microbiol. 29:2056-2058, 1991). The antibodies were detected in a competitive inhibition enzyme-linked immunosorbent assay (CI ELISA) by using monoclonal antibody 36/133.97, which defines a protein epitope on the merozoite surface. The gene encoding this B. equi merozoite epitope was cloned and expressed in Escherichia coli. The recombinant merozoite protein, designated equi merozoite antigen 1 (EMA-1), was evaluated in the CI ELISA. Recombinant EMA-1 bound antibody from the sera of B. equi-infected horses from 18 countries. The antibody response to EMA-1 was then measured in horses experimentally infected with B. equi via transmission by the tick vector Boophilus microplus or by intravenous inoculation. Anti-EMA-1 antibody was detected 7 weeks post-tick exposure and remained, without reexposure to B. equi, for the 33 weeks of the evaluation period. The data indicate that recombinant EMA-1 can be used in the CI ELISA to detect horses infected with B. equi.