A novel apicomplexan parasite was serendipitously discovered in horses at the United States - Mexico border. Phylogenetic analysis based on 18S rDNA showed the erythrocyte-infective parasite to be related to, but distinct from, Theileria spp. in Africa, the most similar taxa being Theileria spp. from waterbuck and mountain zebra. The degree of sequence variability observed at the 18S rDNA locus also suggests the likely existence of additional cryptic species. Among described species, the genome of this novel equid Theileria parasite is most similar to that of Theileria equi, also a pathogen of horses. The estimated divergence time between the new Theileria sp. and T. equi, based on genomic sequence data, is greater than 33 million years. Average protein sequence divergence between them, at 23%, is greater than that of Theileria parva and Theileria annulata proteins, which is 18%. The latter two represent highly virulent Theileria spp. of domestic cattle, as well as of African and Asian wild buffalo, respectively, which differ markedly in pathology, host cell tropism, tick vector and geographical distribution. The extent of genome-wide sequence divergence, as well as significant morphological differences, relative to T. equi justify the classification of Theileria sp. as a new taxon. Despite the overall genomic divergence, the nine member equi merozoite antigen (EMA) superfamily, previously found as a multigene family only in T. equi, is also present in the novel parasite. Practically, significant sequence divergence in antigenic loci resulted in this undescribed Theileria sp. not being detectable using currently available diagnostic tests. Discovery of this novel species infective to equids highlights exceptional diversity within the genus Theileria, a finding with serious implications for apicomplexan parasite surveillance.

Right dorsal colitis in horses has been associated with administration of phenylbutazone. Although reports of right dorsal colitis in this species have described surgical treatment associated with a poor prognosis, we have had success treating this condition medically. This report describes 5 horses with right dorsal colitis confirmed during celiotomy that were initially managed medically. All horses had a history of intermittent abdominal pain; weight loss was noted in only 1 horse. The doses (2.0 to 4.6 mg/kg PO bid) and duration (5 to 30 days) of administration of phenylbutazone were not unusually high relative to those recommended (4.4 mg/kg PO bid). Hypoproteinemia and hypoalbuminemia were observed in all horses at the time of admission; packed cell volume was low in 4 horses, and hypocalcemia was also observed in 4 horses. Three ight dorsal colitis has been experimentally and clini-R cally associated with administration of phenylbutazone to horses.'" Clinical signs associated with right dorsal colitis include anorexia? weight loss, intermittent or sporadic episodes of acute abdominal pain, and diarrhea.'-3 Surgical treatment is advocated in prior reports; however, the prognosis for affected horses is poor.'" At the Texas Veterinary Medical Center (TVMC), some horses with right dorsal colitis identified during celiotomy have been managed medically. The purpose of this retrospective study is to review the signalment, history, physical examination, clinicopathologic findings, and response of treatment in horses with right dorsal colitis managed medically. Materials and MethodsAll horses with right dorsal colitis managed medically at the TVMC between January I , 1985, and December 31, 1993, were included in the study. Five horses were identified by reviewing the medical records. All horses were used for athletic performance. The diagnosis of right dorsal colitis was made on the basis of surgical findings in 4 horses and at necropsy in 1 horse. Diagnosis of right dorsal colitis was based on gross findings of marked edema, thickening, or reduction in diameter of the intestinal tract restricted to the right dorsal colon. In each case, other gross intestinal lesions were absent, and the cause of abdominal pain was attributed to abnormalities of the right dorsal colon. Horses that had right dorsal colitis, but that were not managed medically, and horses that had other visible intestinal abnormalities accompanying right dorsal colitis were excluded from the study.The medical record of each horse was reviewed to abstract the following information: ( I ) date of admission; (2) gender; ( 5 ) character and duration of clinical signs prior to admission; (6) history and duration of treatment with nonsteroidal antiinflammatory drugs (NSAIDs); (7) history of training and performance; (8) hematologic and serum biochemical data obtained at the time of admission; (9) results of cytological and biochemical analysis of peritoneal fluid obtained at the time of admission; (10) results of microbiologic culture of feces during hosp...

Abstract. Ocular contents from a horse with a 4-week history of severe unilateral uveitis were submitted for histopathologic examination. A severe unilateral granulomatous chorioretinitis with intralesional Halicephalobus deletrix was diagnosed. The horse developed progressive neurologic signs several days following the surgery to remove ocular contents and implant a prosthesis and was subsequently euthanatized. A severe multifocal granulomatous encephalitis with intralesional H. deletrix, localized primarily to the optic chiasm, thalamus, and brain stem, was diagnosed from tissues acquired at necropsy. The other eye was not affected. This is the first report of ocular parasitism by H. deletrix and suggests possible systemic dissemination from a primary site in the eye.

Control of a naturally occurring lentivirus, equine infectious anemia virus (EIAV), occurs in most infected horses and involves MHC class I-restricted, virus-specific CTL. Two minimal 12-aa epitopes, Env-RW12 and Gag-GW12, were evaluated for presentation by target cells from horses with an equine lymphocyte Ag-A1 (ELA-A1) haplotype. Fifteen of 15 presented Env-RW12 to CTL, whereas 11 of 15 presented Gag-GW12. To determine whether these epitopes were presented by different molecules, MHC class I genes were identified in cDNA clones from Arabian horse A2152, which presented both epitopes. This horse was selected because it is heterozygous for the SCID trait and is used to breed heterozygous females. Offspring with SCID are used as recipients for CTL adoptive transfer, and normal offspring are used for CTL induction. Four classical and three putative nonclassical full-length MHC class I genes were found. Human 721.221 cells transduced with retroviral vectors expressing each gene had equine MHC class I on their surface. Following peptide pulsing, only cells expressing classical MHC class I molecule 7-6 presented Env-RW12 and Gag-GW12 to CTL. Unlabeled peptide inhibition of 125I-labeled Env-RW12 binding to 7-6-transduced cells demonstrated that Env-RW12 affinity was 15-fold higher than Gag-GW12 affinity. Inhibition with truncated Env-RW12 demonstrated that amino acid positions 1 and 12 were necessary for binding, and single substitutions identified positions 2 and 3 as possible primary anchor residues. Since MHC class I 7-6 presented both epitopes, outbred horses with this allele can be immunized with these epitopes to optimize CTL responses and evaluate their effectiveness against lentiviral challenge.

Efficacious lentiviral vaccines designed to induce cytotoxic T lymphocytes (CTL) in outbred populations with a diverse repertoire of MHC class I molecules should contain or express multiple viral proteins. To determine the equine infectious anaemia virus (EIAV) proteins with epitopes most frequently recognized by CTL from seven horses infected for 0n5 to 7 years, retroviral vectortransduced target cells expressing viral proteins were used in CTL assays. Gag p15 was recognized by CTL from 100 % of these infected horses. p26 was recognized by CTL from 86 %, SU and the middle third of Pol protein were each recognized by 43 %, TM by 29 %, and S2 by 14 %. Based on these results, it is likely that a construct expressing the 359 amino acids constituting p15 and p26 would contain epitopes capable of stimulating CTL in most horses.