Managing infectious equine neurologic disease (Proceedings)


Managing infectious equine neurologic disease (Proceedings)

Table 1. Diagnostic testing for equine neurological diseases
Neurological disease represents 0.3% (affecting between 0.2 and 0.5% of horses depending on age) of all health problems identified by owners in the latest 2005 Equine National Animal Health and Monitoring Study (NAHMS).14 Likely this is much higher given losses in young horses due to non-infectious neurological causes, in all ages of horses from underreporting of encephalitis, and misdiagnoses of these diseases as lameness and trauma. The actual contribution of neurological impairment to the health of US equids overall is actually unknown due to poor diagnostic tools, loss of animals to post-mortem testing if destroyed, and reporting mechanisms that emphasize diseases only of human significance. In fact, while owners in the 2005 NAHMS study only identified 5.0% of the non-ambulatory horses as neurological, another 30.6% were due to a "lameness" problem which may have been incorrectly categorized. Neurological impairment in the horse, even more so than lameness or trauma, is more apt to foreshorten the animal's athletic career, result in permanent deficit, monetary investment by the owner for treatment, and ultimately destruction of the animal. Sixty-one percent of those horses were rendered unusable in the 1998 NAHMS data on EPM, either these horses relapsed, did not respond to treatment, were destroyed or sold due to the original diagnosis. If one estimates for 2010 that approximately 6M equids reside in the US and if one uses an overall estimate of 0.3% population incidence of neurological disease, at least 180,000 cases of some sort of neurological disease occur annually in horses. If a conservative 50% of those animals do not survive, then this represents a loss of 90,000 horses per year to neurological impairment. In direct value, utilizing the AHC data, we performed a survey during the WNV outbreak (unpublished data) and based on client reporting, the direct value of the horses averaged $1500/horse thus a loss of 90K horses represents 270M in livestock loss alone. We further estimated based on American Horse Council statistics that each horse represents $14K/year in combined direct and indirect benefits to the economy. Thus the loss due to equine neurological impairment just from mortality is conservatively 1.2B per year.

Current diagnostics for equine neurological diseases

Table 2. Summary of CSF Findings in Infectious Neurological Diseases
Antemortem testing for the cause of equine neurological disease is still more art than science. Causes of neurological diseases in the horse are infectious (RNA viruses, DNA virus, parasitic, bacterial and fungal) and noninfectious (cervical vertebral malformation, various congenital malformation, degenerative, toxic, and trauma).13 Given the substantial loss due to equine neurological disease, evidence-based identification of the causes of equine neurological diseases poses a constant challenge and this is estimated at 60% for a confirmed diagnosis (based on our database of over 2900 post mortem cases of neurological disease) while less than 30% are found to have had the etiology identified ante-mortem.14-16 Furthermore, the CNS is an immunologically privileged site which is difficult to access in the standing normal or neurologically impaired horse, thus "organ-specific" diagnosis is a challenge without invasive procedures. Although the singlemost valuable test one can obtain is a CSF analysis, many times this procedure is difficult to perform for a variety of circumstances in the neurological horse.

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