Cause and prevention: vector-mediated encephalitis

Protecting our equine patients from infectious encephalomyelitis requires some knowledge of the pathogen(s) of concern, risk to the host and products available to induce protection. The subsequent summary is intended to provide the reader with an overview of some of the potential causes of togaviral encephalitic diseases in horses. In addition, protective strategies will be discussed in an effort to prevent or control disease outbreak.

This horse is suffering from West Nile Virus encephalitis. The animal is demonstrating depressed mentation and poor urinary function.

Among the pathogens inducing encephalitis in horses, those categorized among Togaviridae include alphaviruses and flaviviruses. Although phenotypically similar and capable of inducing disease with similar clinical signs, the epizoology and antigenic structures are diverse. Those categorized as alphaviruses tend to be more infectious and more commonly associated with epidemics compared to those viruses classified among flaviviruses, which are more commonly associated with sporadic disease.

Examples of viral encephalitis in horses:

• Eastern equine encephalomyelitis (EEE)

• Western equine encephalomyelitis (WEE)

• Venezuelan equine encephalomyelitis (VEE)

• West Nile Virus (WNV)

• Japanese B encephalitis

Reservoir species for such pathogens include various small mammals, birds and reptiles. Mosquitoes play a role in transmitting disease and will be involved with viral replication in some instances. Disease transmission to horses and people results from mosquito feeding activity of a blood meal from an infected host (sylvatic host) that is followed by feeding on a horse or human (dead-end host). Togaviridae have been noted for their ability to overwinter in various sylvatic hosts, such as birds, small mammals and reptiles. It is incompletely understood how they survive the non-vector season, but upon warmer temperatures, the pathogen is transmitted by and will potentially amplify within their specific vector (see Suggested Reading).

When exhibiting signs of encephalitis, an indwelling urinary catheter ensures that the bladder empties adequately when an animal experiences poor urinary function.

It is important to recognize that not all vectors should be created equal! Specificity of virus for vector definitely occurs. For instance, Culisetamelanura is the common vector for EEE, yet this vector might not generate a virulent state of WEE even though it can successfully transmit the virus. An additional vector for EEE includes the Aedes species. C. melanura is most typically located in regions of fresh water, such as lakes and swamps, since this mosquito feeds mainly on water fowl, uncommonly this insect is found in regions of dense horse populations. From an epidemiologic point of view, C. melanura is an important vector for enzootic cycle, whereas Aedes spp. play a role in epizootic and epidemic outbreaks of disease. Important vectors for WEE include Culex tarsalis, Dermacentor andersoni and Triatoma sanguisuga. Vectors for VEE include Culex melanoconium, as well as Mansonia, Aedes and Phosphora species. WNV is transmitted primarily by species of the Culex mosquitoes with birds acting as the wildlife reservoir.

Disease progression

Following host inoculation with the particular virus, localization will occur in the regional lymph node. Pathogen clearance will be initiated at the level of the mononuclear phagocytic system by macrophages, neutrophils and natural killer cells. Low levels of viral exposure will be controlled at this level; however, neutralizing antibodies also are produced at this time. Incomplete viral clearance results in endothelial concentration, particularly in various organs, such as the liver and spleen. Viral replication is more likely to occur at this level, which may lead to dissemination within general circulation potentially involving the central nervous system (CNS); disease development (incubation period) may occur within three to five days of initial challenge.

Naïve horses are less likely to clear the pathogen with more severe disease occurring in such patients. Nonspecific early clinical signs may include pyrexia, depression, poor appetite and perhaps generalized soreness and lethargy. During the early phases of disease, viremia is greatest; this is why therapies aimed at controlling viremia, such as hyperimmune plasma or serum products, are most efficacious if administered early in the course of disease.

When clinical signs involving the CNS develop, the viremic phase is likely to be complete. Disease progression signs suggesting CNS involvement include compulsive stall walking, anxiety, hyperesthesia and altered mentation. Gait analysis will reveal conscious proprioceptive deficits early in the course of disease. Later stages of disease may be more disease specific but will reflect the level of damage that has occurred, such as head pressing, blindness, circling, head tilt and generalized or focal muscle fasciculations. Dysphagia may occur resulting from cranial nerve dysfunction associated with muscles of mastication and swallowing. Persistent recumbency is typically a poor prognostic sign irregardless of the inciting cause of virally induced encephalomyelitis. Complete recovery requires weeks to months of gradual motor improvement; peak athletic performance is unlikely in less than six months following disease onset.

Making the diagnosis

The differential diagnoses for horses demonstrating acute onset neurologic disorders, such as cortical dysfunction, spinal ataxia and hyperesthesia among others, should include: rabies, EEE, WEE and VEE, WNV, equine protozoal myeloencephalitis (EPM), equine herpesvirus-1, botulism, heat stress, trauma, bacterial meningitis, cervical vertebral stenopathy and equine degenerative myelopathy.

Serologic evidence of disease is considered the standard for diagnosis. A four-fold rise in serum antibody titer is consistent with the diagnosis of EEE, WEE or VEE. Antibody production against viral antigens is initiated within 24 hours of pathogen challenge. Antibody levels increase substantially during the initial several weeks; then, antibodies typically decline during the subsequent six months. Samples should be collected at the time of initial evaluation; this occasionally can coincide with peak titer levels. If increased titers exist when using the hemagglutination inhibition, complement fixation and neutralizing antibody tests, then a presumptive diagnosis for the inciting agent of encephalomyelitis can be established.

An enzyme-linked immunosorbent assay (ELISA) is commercially used for establishing the diagnosis of VEE, which detects immunoglobulin M (IgM) antibodies as soon as three days after the onset of clinical disease. Additional testing, such as fluorescent antibody, enzyme-linked immunosorbent assay, viral isolation and PCR, can be performed on tissue samples if the diagnosis remains open (see Suggested Reading).

Serologic tests used to diagnose WNV include the plaque reduction neutralization assay (PRNT), virus neutralization, hemagglutination inhibition, complement fixation; ELISA and antigen (IgM and IgG) capture ELISA. Virus or viral DNA can be detected from CNS tissue with virus isolation, PCR or immunohistochemistry. The IgM-capture ELISA is considered the most reliable test for confirmation of recent exposure to WNV in a horse demonstrating clinical signs consistent with this disease. Following exposure, horses typically develop a sharp rise in WNV-specific IgM that can persist for several weeks to months following infection. Incidentally, following vaccination, little IgM is induced, so vaccination should not interfere with establishment of diagnoses in a horse that has been vaccinated. In contrast, antibody measurement using the PRNT method can be influenced by exposure or vaccination; therefore, interpretation of test results can be difficult if the horse has been vaccinated.

Cerebrospinal fluid analysis will reveal similar changes among the various causes of viral encephalomyelitis that include a pleocytosis (50-400 mononuclear cells / microliter; normal < 5-8 cells / microliter) and elevated protein concentration (100-200 mg/dL; normal < 50 mg/dL).

Prevention of disease

The goals of disease prevention associated with virally induced encephalomyelitis include control of insect-vector populations and implementing effective vaccination programs (Table 1). Of the currently available vaccines, killed products are most commonly selected and are available for protection against a variety of viral pathogens. Vaccination selection should be based on the pathogens of concern. For instance, in endemic regions, a monovalent, divalent or trivalent vaccine is available for protection of alphaviruses EEE, WEE and VEE, respectively. If a horse is located in region of the country where VEE is not a threat, then the divalent vaccine should be selected for EEE and WEE. Interestingly, horses previously vaccinated against EEE or WEE are less responsive to vaccination against VEE; however vaccination against VEE has not been demonstrated to reduce protection from EEE or WEE vaccines. The goal of vaccination against any of the viral encephalitides should be to complete vaccinations several weeks before the onset of the insect season. Therefore, in a previously unvaccinated horse, a series of two vaccines should be completed two to three weeks before the onset of mosquitoes and other blood-meal insects. This recommendation is based on the fact that a secondary immune response in a healthy horse will require approximately seven to 10 days to be complete. In order to have optimal protection against viral infection, ample time should be allowed to provide maximal benefit from vaccine administration.

Table 1: Vaccination frequency guidelines

Prior to foaling, mares should be boosted approximately one month prior to foaling to allow for maximum colostral antibody concentrations. Foals that have received adequate colostrum are considered to be protected for the first four months of life. Vaccination of foals should be based on potential risk of disease; a series of three vaccines should be administered at four-week intervals to provide protection in a previously unvaccinated foal. After 1 year of age, depending on the climate, the goal of vaccination should be to maintain high antibody titers against viral pathogens during warm seasons of the year. On average, vaccination is required every four to five months during these seasons (Table 1).

WNV: protection from disease

Protection against WNV-induced encephalomyelitis, similar to other vector-mediated encephalitides, requires strict attention to control of insects as well as an effective vaccination program (see Suggested Reading).

Two vaccines currently are licensed for use in horses to aid in the protection against WNV. A killed vaccine has been approved for use in horses for several years. Original data demonstrated that the vaccine was effective at protecting against viremia. An additional investigation demonstrated a reduced death rate in vaccinated horses. Because a model is still under investigation, the rate of protection from natural disease has yet to be determined.

Suggested Reading

The second vaccine is a live canarypox vector vaccine. Similar to studies investigating the killed vaccine, vaccination protects against the development of viremia.

Label recommendations suggest a series of two vaccines be administered at four-to-six-week intervals followed by annual vaccination. In endemic regions where the insect (mosquito) season is prolonged (>6 months), a booster vaccination is recommended to maintain high titers throughout this season. In many areas of the United States, this requires vaccination at the beginning of spring and summer. In more southern climates, vaccination has been performed safely as frequently as every four months.

Neither of the licensed vaccines is approved for use in pregnant broodmares, so mares should be ideally vaccinated prior to breeding whenever possible.

A recent publication described the findings of a retrospective study that evaluated 595 mares for the potential for reproductive failure associated with administration of the killed vaccine. The findings from this study suggested that vaccination of broodmares during any period of gestation was not associated with increased incidence of pregnancy loss. Therefore, when the risk of disease is weighed against the potential risk of adverse events, the use of vaccination in pregnant mares has been performed.

Booster vaccines given to mares that have been vaccinated properly prior to pregnancy one month prior to their expected foaling date is expected to provide foals with humoral protection for the initial three to four months of life.

Insect protection

Owners routinely should use insecticides and repellents whenever possible, and standing water should be eliminated from the property. In endemic regions during an outbreak, heightened precautions should include more attention to insect control, which can include constant use of insecticides, fly sheets applied to at-risk horses with screening placed over windows and doors to reduce dusk and dawn exposure to horses.

Conclusion

A variety of vector-borne viral pathogens have been associated with the development of viral encephalomyelitis in horses. Goals of protection should include insect-control strategies, as well as effective vaccine programs. Specific vaccine recommendations will be influenced by patient age, geographic location and potential risk of disease. A plan for effective control programs will require good communication between veterinarian and horse owner.

Dr. Elizabeth Davis earned her DVM from the University of Florida, College of Veterinary Medicine in 1996. She then completed an internship at Kansas State University (KSU) and then pursued a combined equine medicine residency and PhD program in the Departments of Anatomy and Physiology and Clinical Sciences. Currently, Dr. Davis is an assistant professor in KSU's departments of clinical sciences, and anatomy and physiology. Clinical activities, such as evaluating and managing internal medicine patients admitted to the equine hospital and teaching fourth-year veterinary students during their equine rotation, compose a significant proportion of her time, and additional teaching responsibilities include didactic lectures given to the underclass veterinary students during their first three years of veterinary training.

When not working with clinical cases, Dr. Davis focuses on her research activities, characterizing immune function in horses. She has explored the areas of innate immunity that combine antimicrobial peptide expression and function with the effects of systemic immunomodulation. These investigations also have explored effective alternative methods of immune stimulation. Additional research investigations have related to equine respiratory disease. In combination with Dr. Bonnie Rush, these studies have evaluated effective therapies for horses suffering from heaves. Most recently, these investigations explored how the genome is expressed in horses suffering from heaves, as well as how current therapies alter the genes that are expressed.