Published June, 2013

The necropsy records of the University of Kentucky Veterinary Diagnostic Laboratory (UKVDL) for the years 2011 and 2012 were searched for equine cases having a clinical diagnosis of neurologic disease. The search revealed 38 cases in 2011 and 50 cases in 2012. These represent 2.2% and 3.1%, respectively, of the total equine necropsy cases examined at the UKVDL during the last two years. Although the percentages are small, the accurate diagnosis of equine neurologic disease is important because these cases often result from infection with specific pathogens, some of which are communicable to other horses and might even pose a zoonotic risk to people.

The most common diagnosis by far was a non-infectious condition, cervical vertebral stenotic myelopathy, or wobbler syndrome. There were 42 total cases of stenotic myelopathy (21 each year). Thirty-six of the cases (86%) were in male horses and five (12%) were in female horses (in one case the gender was not recorded). The ages of the horses at time of necropsy were skewed to younger animals. Six were juvenile (less than one year old), 20 were yearlings, and 10 were two year olds. Four horses were three years or older (ranging from three to seven years of age). In keeping with the predominate breed in Central Kentucky, 18 of the cases were in Thoroughbreds; however, other breeds were also involved. Five of the cases were Quarter Horses, and there were individual cases in American Saddlebreds, Tennessee Walking Horses, and Standardbreds.

Wobbler syndrome is associated with abnormalities of the cervical vertebrae, which result in compression of the spinal cord in one or more areas leading to neurologic signs. No predominant site of compression in the cases was recorded in this two-year period. Based on the findings at necropsy and microscopic examination of the spinal cord, 13 of the cases had compression at C 3-4 (the junction of the 3rd and 4th cervical vertebrae), 13 cases had compression at C 4-5, 11 cases at C 5-6, and 12 cases at C 6-7.

The next most commonly diagnosed group of neurologic diseases was encephalomyelitis/myelitis cases. Equine protozoal myeloencephalitis (EPM) caused by Sarcocystis neurona was the principal disease that was diagnosed. Some 12 cases occurred in 2011 and 18 in 2012. Most involved adult horses ranging in age from a juvenile to a 20-year-old mare. Six yearling horses and several different breeds were represented. The causative agent was visualized in tissue in a small percentage of the cases, while diagnosis in the majority was based on the characteristic microscopic pattern of inflammation and damage in the spinal cord and brain stem, which was typical of EPM. Many of the horses had received treatment for EPM and this likely contributed to the difficulty in demonstrating the agent.

Other causes of encephalitis/myelitis were West Nile virus (WNV, four cases in 2012), one case of equine herpesvirus (EHV-1) myeloencephalopathy in 2011, and one encephalitis case of undetermined etiology in 2012. The WNV-associated disease was seen in three adult horses and one yearling. The diagnoses of WNV and EHV-1 were made on the basis of microscopic examination of the brain and spinal cord tissue and a positive polymerase chain reaction (PCR) test result.

Inflammation of the membranes covering the brain (meningitis) or the membranes plus the brain tissue (meningoencephalitis) were occasionally diagnosed in horses submitted to the UKVDL. There were five cases in 2011 and 10 cases in 2012. Eight were in adult horses and seven were in foals. The majority were caused by bacteria with eight cases attributed to bacterial infection. Isolates included the genera ofStreptococcus, Staphylococcus, Salmonella, Klebsiella, and Actinobacillus. Two cases involved fungal infection and five were idiopathic cases (no known cause).

Encephalopathy was diagnosed in three cases in 2011 and six in 2012. Encephalopathy refers to neuronal dysfunction resulting from a variety of causes. Pathologically, there typically is neuronal degeneration with inflammation not being a component. In all cases the encephalopathy was believed to be secondary to another process. The cases included four adult horses, four foals, and one juvenile horse. The adult cases were all secondary to liver disease and were classified as hepatic encephalopathy. The cause in the foals was believed to be encephalopathy secondary to neuronal degeneration as the result of hypoxia at birth.

Over the two-year period, 46 horses were tested for rabies because of antemortem clinical signs but no cases of rabies were diagnosed. There were two cases of cauda equina neuritis (polyneuritis equi), in a yearling and an adult horse. This idiopathic condition results from inflammation of the peripheral nerve roots at the termination of the spinal cord.

In a prior issue of the Equine Disease Quarterly (July 2003, Volume 12, Number 3) that reviewed cases over a three-year period, neurologic diseases (excluding trauma to the central nervous system) comprised 5% of the equine cases, which is slightly more but similar to the current report. As in 2003, when wobbler syndrome accounted for a third of the neurologic cases, it was also the most common diagnosis in the present group. EPM remains the most common infectious/inflammatory condition as was the case in 2003.

This is an excerpt from Equine Disease Quarterly, funded by underwriters at Lloyd's, London, brokers, and their Kentucky agents.