Spring 2018
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- EHV-1 and EHV-4: Their Roles in Equine Viral Respiratory Disease, Abortions, and Equine Herpes Myeloencephalopathy (EHM)
- Preventing Six Common Mistakes in Small Poultry Flocks
Equine herpesvirus-1 (EHV-1) and equine herpesvirus-4 (EHV-4) are large double-stranded DNA viruses that are ubiquitous pathogens of horses. Estimates of prevalence show most adult horses are infected with EHV-1, EHV-4, or both throughout their lifespan and establishment of lifelong latency is detected in up to 70% of infected horses. Infection with EHV-1/4 is one of the most common causes of viral respiratory disease in horses worldwide.
Clinical Signs
Clinically, the respiratory disease caused by EHV-1 and EHV-4 is indistinguishable and can be mild or asymptomatic in older or previously exposed horses. In contrast, the respiratory disease observed in young immunologically naïve horses is often severe, lasts for two to three weeks, and is characterized by a biphasic fever, depression, anorexia, coughing, and oculonasal discharge that is initially serous and then becomes mucopurulent. Further, EHV-1 causes equine herpesvirus myeloencephalopathy (EHM), late-term abortions in the last trimester of pregnancy, death of neonatal foals, and chorioretinopathy. In contrast to respiratory disease, the risk for abortions in the third trimester of pregnancy and outbreaks of neurological disease is of greater significance in middle aged or older horses. EHV-4 has been implicated occasionally in causing late term abortions and EHM, but its etiological role is considered to be very minor and this occurs far less commonly than is observed with EHV-1.
Differences in Pathogenesis
![EHV-1 and EHV-4: Their Roles in Equine Viral Respiratory Disease, Abortions, and Equine Herpes Myeloencephalopathy (EHM) (1) EHV-1 and EHV-4: Their Roles in Equine Viral Respiratory Disease, Abortions, and Equine Herpes Myeloencephalopathy (EHM) (1)](https://i0.wp.com/cvm.msu.edu/assets/images/vdl/_imageFit400/vdl_newsletter_pathogenisis.jpg)
Pathogenesis of EHV-1 (virus in blue) and EHV-4 (virus in red) not EHV-4, infects local lymphoid cells via cell-to-cell contact. A cell-associated viremia is established for EHV-1 and transports the virus to secondary replication sites. The vascular endothelium of the CNS , pregnant uterus or eye is infected via contact with infected lymohocytes, leading to EHM, abortions, and chorioretinopathies.
(Courtesy of Dr. Gabriele Landolt, Colorado State University)
Outbreaks of EHM
In recent years, outbreaks of EHM have increased in North America. Because of this, the Center for Emerging Issues released an emerging disease notice report regarding the neurologic form of EHV-1 in January 2007. Despite this knowledge and strong efforts to control this disease, EHM outbreaks continue to be a problem. In May of 2011 the largest outbreak ever was reported and in the first quarter of 2018 (January - March), the Equine Disease Communication Center (www.equinediseasecc.org/) lists more than 20 outbreak alerts for confirmed or suspected cases of EHM nationwide. The reason for this increase in the prevalence of EHM is not clear, and there are a number of identified viral, host, and environmental components that factor into the incidence of EHM. These include age, breed, gender, season, past exposure, a secondary fever several days after primary exposure, stress, magnitude and duration of viremia, and infection with the D752 genotype. To date, the most important identified factor is the essential role of viremia in transmitting the virus to the vasculature of the CNS. Allen et al. found that EHV-1 strains with high neuropathogenic potential are characterized by a longer duration and greater magnitude of viremia when compared to EHV-1 strains with a low neuropathogenic potential. This evidence supports the argument that prolonged exposure of the CNS vascular endothelium to high viral loads increases the risk of EHM. Further, the identification of a single nucleotide polymorphism in the viral polymerase gene that results in a coding change (N752 to D752) has been associated with increased neuropathogenicity and high levels of viremia.
Diagnostic Options
Diagnosis of equine herpesvirus infections in the laboratory include methods to detect viral DNA, infectious virus, viral proteins, and antibodies. Our laboratory has the capability to test for the presence of equine herpesviruses by all the approaches described above. In addition, type-specific neutralization tests, which differentiate between antibodies induced by EHV-1 or EHV-4, will be introduced later this year. For more information on specific collection protocol, sample and shipping requirements, and other relevant details, please see our list of available tests at animalhealth.msu.edu or contact the laboratory at 517.353.1683.