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Pathogenic Protozoa in Horses

It is a common thought that infection by more than one disease-causing protozoa is linked to increased severity of disease.  This was shown in marine mammals with protozoan encephalitis, is it similar in horses?

One study sought to understand if horses with presumptive EPM had antibody evidence of infection with other protozoal infections. Documenting more than one protozoal infection is possible because horses get infections with Neospora and/or Toxoplasma.  The criteria for EPM in dead animals rested on the presence of lesions (not parasites) in central nervous tissues.  They included cases from live horses that had a presumptive diagnosis of EPM.  EPM was presumed when clinical signs were present, excluding other disease based on ancillary testing, and SAG ELISA positive serum and CSF for Sarcocystis neurona (in this study the ratio was less than or equal to 50).  The scientists compared the presence of Neospora or Toxoplasma antibodies in their selected cases to a group that were not expected to have protozoal infections.  The expected-negative group was comprised of horses with a diagnosis of cervical vertebral malformation (CVM) and negative for SAG ELISA of serum and CSF (in this study the ratio was less than or equal to 50).

Overall, they found 12.9% of horses were antibody positive for Neospora.  The proportion of EPM cases that tested positive did not differ from the proportion of CVM cases that tested positive.  A similar finding with  Toxoplasma, antibodies were found in 14.9% of suspect EPM cases and CVM horses.  A qualifier in the study was that these horses were from the eastern US.  It is possible that Neospora is more common in the west, however one study does not support that.

They found no horses with co-infections! Another interesting finding was that they didn’t find sub-clinical infection’s either. A sub-clinical infection was a case in which antibody was detected in the serum but not enough antibody in the CSF to be considered EPM.  They concluded that their data did not support protozoal co-infection as a common finding in horses with neurologic disease from the eastern US, and further co-infection with a protozoal species is unlikely to play an important role in development of clinical disease caused by S. neurona infection.

You had us nodding our heads in agreement until they said “protozoal species”.  We assumed that an implied qualifier might be “the data did not support protozoal co-infection with Toxoplasma or Neospora”…yes.

We disagree that co-infections aren’t present in horses with S. neurona! We presented evidence to the attendees at the 2nd EPM Society Workshop in Tahoe City, CA on October 26, 2017 to show that Sarcocystis fayeri and Sarcocystis neurona antibodies were present in horses with clinical neuromuscular disease.  And in some horses with sub-clinical disease, antibodies present and not ataxic. And in normal horses!  We showed that 87% of normal horses have co-infections with Sarcocystis and is in direct opposition to the findings in the above experiment. Two protozoal infections in one animal.  We might speculate that S.fayeri is protective against developing EPM in S. neurona exposed horses.  We can find some data to support this speculation because the protein we use to measure S. fayeri is a protein that protects animals against Toxoplasma infections. Our data would not argue against their findings, we just disagree that horses can’t harbor two species of protozoa.

Some of the diseased horses we examined had two Sarcocystis infections and met the bar of serum/CSF ratio (<100), confirming EPM.  In diseased horses, we found 74% were Sarcocystis positive!  In diseased horses dual infections were more common than single species infections. When we only looked at single species infections we found that neurona was more common in diseased horses. These data still support the finding that a single infection is more pathogenic or sometimes S. fayeri is present but not protective. We found that some S. fayeri strains may not produce the protective protein under some as yet undefined conditions. The EPM-Neospora-Toxo study discussed above may have been stronger if they considered S. fayeri as a factor or stated that there are other protozoa that co-exist in normal and diseased horses. The relationship between Sarcocystis that infect horses remains undefined.

We, and UC Davis, have reported finding S. fayeri in horses with neuromuscular disease, although we look for the disease in different ways. We ran a study to compare our way (serum ELISA) with the UC Davis method (post-mortem tissue exam) finding a pretty good correlation between the methods.  Good enough to suggest pre-mortem serum testing is more palatable to the owner. The point is that multiple protozoa infect horses, multiple protozoa are in diseased horses and it takes a good (diagnostic eye) to discern the cause of clinical signs.