A Sidewinder is a term describing a neurologic older horse that has an unusual clinical presentation. The horse has a lateral hemi-paresis that results in a gait that makes them list to one side or the other. Most Sidewinders are depressed. The clinical presentation (because they are depressed) classifies them having a multi-focal, diffuse neurologic disease (encephalomyelitis) of undetermined etiology. In our consulting practice we saw a pattern to the clinical presentation of these idiopathic encephalopathies and wrote about them in a blog posted in February of 2012. Sidewinding horses have recently reached the level of recognition of clinicians at one university, we expect more discussion will be forthcoming.
To facilitate the veterinary interests and research community we are providing access to our database of the cases we have gathered data over several years. This syndrome is recognized by clinical presentation and is a non-reportable disease. The limits of our data are similar to all statistical databases, which includes under reporting and misclassification of disease. Our data is captured by state of residence and not where the animal was exposed. These cases are reported by veterinarians from field observations and are therefore termed anecdotal.
Three years ago our impression was that horses diagnosed with EPM were part of a larger group of ataxic horses and the weakness/hemi-paresis group were due to a subset of horses with an inflammatory syndrome. Most of the horses are presumably diagnosed as suspect EPM, some of them have antibodies to S. neurona in the serum and some don’t. There is usually a history of extensive clinical work up, referral to a university, CSF fluid analysis, and treatment for protozoa. Generally these horses don’t respond to NSAID’s, steroids, or anti-protozoal drugs and show progressive disease. The treatment protocol varies from the standard EPM treatment.
Clinically recognizable features are signalment (age is the only statistically related factor, 20-35 years old), depression, and a twisted gait. Often the horse will use a stall wall for balance. A complete blood count and clinical chemistry values are generally normal. Antibody against S. neurona is not a significant factor. There can be a varied and incomplete response to anti-protozoal drugs, anti-protozoal treatment does not maintain the horses and they are considered treatment failures. An elevated C reactive protein is present in most, but not all of the animals and may be a significant factor.
We have not yet found the causal molecule in sidewinding horses. Histopathology was unremarkable in 4 cases (we concentrated on the choroid plexus), one case showed mild Wallerian degeneration. A small percent of animals show signs related to vaccination and vaccination accompanied by specific treatment prevented recurrence of signs in these horses. The resolution of signs with treatment can be directly associated with vaccination, there is no correlation with a specific adjuvant or manufacturer. Note that vaccination is not the precipitating factor in most horses.
Some treated horses remained symptom free post-treatment and were had a good quality of life. Due to the age of these animals most are trail riding, breeding stallions, and pasture pets. Some animals show recurrence of signs after 12-15 months.
Our interpretation is that Sidewinders are a subset of neuromuscular diseases in horses, 20-35 years old with chronic inflammation due to unidentified causes. Our differentials include infectious, metabolic, and immune mediated causes. Infectious causes include chronic protozoa infection or chronic herpes viral infection.