Equine Neurological PCR Panel

Test code: P0014

Test name: Equine neurological PCR panel

This panel includes qualitative detection and differentiation of:

EEE and WEE are transmitted by mosquitoes. Infected horses may exhibit somnolence, lethargy, ataxia, incoordination, recumbency or death. Diagnosis is based on blood or cerebrospinal fluid testing, or by postmortem examination of the brain. There is no treatment for these diseases and affected horses are managed symptomatically. Although some horses recover from these diseases, many are eventually euthanized. Vaccines are available and are generally effective. Young foals need special attention, as their immature immune systems cannot yet develop immunity in response to vaccines. Mosquito control is also an important aspect of prevention.

EHV-1 usually manifests as a respiratory disease, but can occasionally mutate to a form that affects the nervous system. Infected horses may develop symptoms such as weakness or paralysis of the hind legs giving rise to the "dog-sitting" position, loss of tail and anal tone, inability to urinate or defecate, urine dribbling, cranial nerve deficits, recumbency, and death. No specific treatment is available for EHV-1, but general supportive therapy and care can aid recovery of affected horses. Anti-inflammatory agents may be helpful in minimizing damage to the spinal cord. The human drug acyclovir was used in a recent EHV-1 outbreak in Ohio, and efficacy of this treatment seems promising.

WNV is a mosquito borne virus first identified in the USA in 1999. The virus affects the horse’s central nervous system, causing a range of symptoms including somnolence, violent behavior, toe dragging, recumbency, hypo- or hyper-sensitivity to sound, touch, or light, falling down on the front knees in a “prayer position”, single limb lameness, and fever. A fever is not always detected before neurological signs appear. Treatment is symptomatic. Approximately 50% of horses survive the infection and survival is not highly correlated with severity of symptoms. An effective WNV vaccine is now available.

S. neurona, N. caninum, N. hughesi and T. gondii are 4 related coccidian parasites associated with equine protozoal myeloencephalitis (EPM). The source of infection for N. hughesi is unknown, whereas opossums, dogs, and cats are the definitive hosts for S. neurona, N. caninum and T. gondii, respectively.

A recent serological study of 276 horses in central Wyoming (Dubey et al., 2003) indicated that the serological prevalence of T. gondii was less than 1%, suggesting that this parasite is not a major cause of EPM in horses. Serological prevalence of N. hughesi is also low at 6.5%. In contrast, 31.1% of horses show serological reactivity to N. caninum and >50% of horses show serological reactivity to S. neurona.

S. neurona is carried by a number of hosts at different stages of its life cycle. The opossum is the major end host and feces from infected opossums can transmit the disease to horses. The horse is a dead end host for S. neurona -- i.e. the protozoa are unable to complete their life cycle in the horse. However, they can cause severe neurological damage during their development within a horse’s central nervous system.

N. hughesi and N. caninum are very similar in their genomic organization and biochemical characteristics, making clinical differentiation of the two species very difficult. Clinical differentiation of S. neurona from N. hughesi/caninum is also difficult, as the range of symptoms overlap broadly. Although N. caninum seems to have wide serological prevalence, EPM cases are most often attributed to S. neurona.

Typical symptoms of EPM include asymmetrical ataxia, toe dragging, circumduction of the hind limbs, hypermetria, and less frequently recumbency and cranial nerve deficits. In the past, definitive diagnosis of EPM in horses was quite difficult. Since many horses have been previously exposed to one or more of these parasites, serological testing does not yield a definitive diagnosis of current infection. Nearly 90% of horses in some populations have positive antibody titers. Cerebrospinal fluid (CSF) sample may be tested, but false positive result occurs frequently because even a few blood cells contaminating the CSF sample can result in false positive signal. A very clean sample is therefore necessary.

Treatment of EPM is a long process that should begin as soon after clinical presentation as possible. Accurate identification of the causative agent is critical to successful treatment. Various anti-parasitic drugs are used and generally must be administered for several weeks to months. With proper, quick and aggressive treatment, 60% to 70% of horses make a significant or complete recovery. Recently an EPM vaccine has become commercially available under a conditional license from the USDA. Studies are underway to determine the efficacy of the vaccine.

PCR detection and identification of equine neurological viruses and parasites is highly sensitive and specific. In particular, PCR-based molecular techniques are more useful than immunological methods in testing cerebrospinal fluid or brain tissue biopsies. Furthermore, serological testing is of little use in diagnosing active infection in an animal that has been previously exposed to these pathogens; PCR is a more appropriate testing technique for this application.

Utilities:

• Confirm the pathogen causing generalized neurological symptoms
• Selection of appropriate treatment regimen
• Shorten the time required to confirm a clinical diagnosis
• Ensure that horse populations are free of listed neurological pathogens
• Early prevention of spread of listed neurological pathogens
• Minimize personnel exposure to listed neurological pathogens
• Safety monitoring of biological products that derive from horses

Specimen requirements: Preferred samples: 1 ml cerebrospinal fluid or brain tissue, shipped overnight at room temperature; or frozen CSF or tissue, shipped frozen.

Less preferred sample: 1 ml whole blood in EDTA (purple top) or ACD (yellow top) tube, shipped overnight at room temperature.

For specimen types other than those listed here, please call to confirm specimen acceptability and shipping instructions.

For all specimen types, if there will be a delay in shipping, or during very warm weather, refrigerate specimens until shipped and ship with a cold pack unless more stringent shipping requirements are specified. Frozen specimens should be shipped so as to remain frozen in transit. See shipping instructions for more information.

Turnaround time: 3 business days

Methodology: Qualitative PCR, qualitative real time PCR and qualitative reverse transcription real time PCR

Normal range: Nondetected