Immunoconversion against Sarcocystis neurona in normal and dexamethasone-treated horses challenged with S. neurona sporocysts.

This study looked into how horses responded to the Sarcocystis neurona parasite, both under normal conditions and under the influence of the immunosuppressive drug dexamethasone. It was found that all horses exposed to the parasite did build an immune response, with those given dexamethasone possibly exhibiting a faster response. The research is important as it helps develop understanding about equine protozoal myeloencephalitis – a neurological disease in horses common in North and South America.

Objective and Methods

• The research aimed to monitor immune response (immunoconversion) in horses against the parasite Sarcocystis neurona, the main cause of the equine protozoal myeloencephalitis (EPM) disease.
• The study used sporocysts (an infective life stage of the parasite) obtained from naturally infected opossums, the definitive host of S. neurona.
• Eight horses were fed sporocysts for seven days and then regularly examined for signs of illness.
• Both blood and cerebrospinal fluid (CSF) samples were collected from the horses over a 90-day period to monitor immune response through both qualitative (western blot) and quantitative (anti-17kDa) methods.

Using Dexamethasone

• Half of the challenged horses were given dexamethasone, an immunosuppressive drug, throughout the experiment. This was done to assess whether a suppressed immune system would affect the horse’s ability to defend against the parasite or impact the progression of potential disease symptoms.

Results

• All horses that were fed the sporocysts showed an immune response to the parasite within 32 days.
• The horses that were treated with dexamethasone appeared to immunoconvert earlier than those that were not, though this was not statistically significant (P = 0.057).
• The signs of neurological disease were mild to equivocal across all challenged horses, with those given dexamethasone displaying more severe signs of limb weakness.
• During the post-mortem examination, damage was seen in the spinal cords of the challenged horses; however, organisms were not visible in either routinely processed sections or via immunohistochemistry.
• Control horses remained seronegative throughout the period, although one eventually immunoconverted on day 75 in blood and day 89 in CSF.

Conclusion and Implications

• The immune responses observed occurred both systemically and within the Central Nervous System and indicate that a S. neurona infection had taken place.
• The study did not succeed in producing EPM-like neurological disease in the horses. It did, however, provide valuable insights into the progression from S. neurona infection to potential EPM disease in horses.
• The evidence gained could guide future research to refine the understanding of EPM and devise prevention or cure strategies.