Previous challenge studies performed at Ohio State University involved a transport-stress model where the study animals were dosed with Sarcocystis neurona sporocysts on the day of arrival. This study was to test a second transportation of horses after oral inoculation with S. neurona sporocysts. Horses were assigned randomly to groups: group 1, transported 4 days after inoculation (DAI); group 2, at 11 DAI; group 3, at 18 DAI; and group 4, horses were not transported a second time (controls). An overall neurologic score was determined on the basis of a standard numbering system used by veterinarians. All scores are out of 5, which is the most severely affected animal. The mean score for the group 1 horses was 2.42; group 2 horses was 2.5; group 3 horses was 2.75; and group 4 horses was 3.25. Because the group 4 horses did not have a second transport, they were compared with all other groups. Statistically different scores were present between group 4 and groups 1 and 2. There was no difference in the time of seroconversion between groups. There was a difference between the time of onset of first clinical signs between groups 1 and 4. This difference was likely because of the different examination days. Differences in housing and handling were likely the reason for the differences in severity of clinical signs. This model results in consistent, significant clinical signs in all horses at approximately the same time period after inoculation but was most severe in horses that did not experience a second transport.

Sarcocystis neurona and Sarcocystis fayeri infections are common in horses in the Americas. Their antemortem diagnosis is important because the former causes a neurological disorder in horses, whereas the latter is considered nonpathogenic. There is a concern that equine antibodies to S. fayeri might react with S. neurona antigens in diagnostic tests. In this study, 4 ponies without demonstrable serum antibodies to S. neurona by Western immunoblot were used. Three ponies were fed 1 X 105 to 1 X 107 sporocysts of S. fayeri obtained from dogs that were fed naturally infected horse muscles. All ponies remained asymptomatic until the termination of the experiment, day 79 postinoculation (PI). All serum samples collected were negative for antibodies to S. neurona using the Western blot at the initial screening, just before inoculation with S. fayeri (day 2) and weekly until day 79 PI. Cerebrospinal fluid samples from each pony were negative for S. neurona antibodies. Using the S. neurona agglutination test, antibodies to S. neurona were not detected in 1:25 dilution of sera from any samples, except that from pony no. 4 on day 28; this pony had received 1 X 107 sporocysts. Using indirect immunofluorescence antibody tests (IFATs), 7 serum samples were found to be positive for S. neurona antibodies from 1:25 to 1:400 dilutions. Sarcocystis fayeri sarcocysts were found in striated muscles of all inoculated ponies, with heaviest infections in the tongue. All sarcocysts examined histolog- ically appeared to contain only microcytes. Ultrastructurally, S. fayeri sarcocysts could be differentiated from S. neurona sarco- cysts by the microtubules (mt) in villar protrusions on sarcocyst walls; in S. fayeri the mt extended from the villar tips to the pellicle of zoites, whereas in S. neurona the mt were restricted to the middle of the cyst wall. Results indicate that horses with S. fayeri infections may be misdiagnosed as being S. neurona infected using IFAT, and further research is needed on the serologic diagnosis of S. neurona infections.

Sarcocystis neurona and Sarcocystis fayeri infections are common in horses in the Americas. Their antemortem diagnosis is important because the former causes a neurological disorder in horses, whereas the latter is considered nonpathogenic. There is a concern that equine antibodies to S. fayeri might react with S. neurona antigens in diagnostic tests. In this study, 4 ponies without demonstrable serum antibodies to S. neurona by Western immunoblot were used. Three ponies were fed 1 x 10(5) to 1 x 10(7) sporocysts of S. fayeri obtained from dogs that were fed naturally infected horse muscles. All ponies remained asymptomatic until the termination of the experiment, day 79 postinoculation (PI). All serum samples collected were negative for antibodies to S. neurona using the Western blot at the initial screening, just before inoculation with S. fayeri (day 2) and weekly until day 79 PI. Cerebrospinal fluid samples from each pony were negative for S. neurona antibodies. Using the S. neurona agglutination test, antibodies to S. neurona were not detected in 1:25 dilution of sera from any samples, except that from pony no. 4 on day 28; this pony had received 1 X 10(7) sporocysts. Using indirect immunofluorescence antibody tests (IFATs), 7 serum samples were found to be positive for S. neurona antibodies from 1:25 to 1:400 dilutions. Sarcocystis fayeri sarcocysts were found in striated muscles of all inoculated ponies, with heaviest infections in the tongue. All sarcocysts examined histologically appeared to contain only microcytes. Ultrastructurally, S. fayeri sarcocysts could be differentiated from S. neurona sarcocysts by the microtubules (mt) in villar protrusions on sarcocyst walls; in S. fayeri the mt extended from the villar tips to the pellicle of zoites, whereas in S. neurona the mt were restricted to the middle of the cyst wall. Results indicate that horses with S. fayeri infections may be misdiagnosed as being S. neurona infected using IFAT, and further research is needed on the serologic diagnosis of S. neurona infections.

West Nile virus emerged in the United States in the fall of 1999 and has since spread from four to 44 states. Horses, humans, crows, and blue jays are particularly susceptible to this virus, which causes severe encephalitis and death in some cases. Many species of birds have been affected in the United States, and many mosquitoes have detectable virus. In the United States, the reservoir host for this virus is unknown, and the number of mosquito species that transmit the virus effectively is unknown as well. Vaccination may prevent the development of clinical signs in horses. Both horses and humans may be effectively treated for this disease.

The effect of inoculation dose of Sarcocystis neurona sporocysts on the development of clinical neurologic disease in horses was investigated. Twenty-four seronegative weanling horses were subjected to the natural stress of transport and then randomly assigned to 6 treatment groups of 4 horses each. Horses were then immediately inoculated with either 10(2), 10(3), 10(4), 10(5), or 10(6) S. neurona sporocysts or placebo using nasogastric tube and housed indoors. Weekly neurologic examinations were performed by a blinded observer. Blood was collected weekly for antibody determination by Western blot analysis. Cerebrospinal fluid was collected before inoculation and before euthanasia for S. neurona antibody determination. Horses were killed and necropsied between 4 and 5 wk after inoculation. Differences were detected among dose groups based on seroconversion times, severity of clinical neurologic signs, and presence of microscopic lesions. Seroconversion of challenged horses was observed as early as 14 days postinfection in the 10(6) sporocyst dose group. Mild to moderate clinical signs of neurologic disease were produced in challenged horses from all groups, with the most consistent signs seen in the 10(6) sporocyst dose group. Histologic lesions suggestive of S. neurona infection were detected in 4 of the 20 horses fed sporocysts. Parasites were not detected in equine tissues by light microscopy, immunohistochemistry, or bioassay in gamma-interferon gene knockout mice. Control horses remained seronegative for the duration of the study and had no histologic evidence of protozoal infection.