Equine herpes virus infections (equine rhinopneumonitis, abortions)

Equine herpes virus infections in horses and foal are caused by Alphaherpes virus of herpesviridae family producing different disease syndromes, which are characterized by abortion, neonatal disease, myoencepathalopathy and respiratory disease. Equine rhinopneumonitis is caused by equine herpes virus-4 (EHV-4) and the disease is characterized by upper respiratory tract infection of horses during the first 2 years of life. The EHV-1 is similar in morphology to EHV-2, EHV-3 and EHV-4 with which it shares common structural features. However, there are differences in immunological properties. The respiratory disease caused EHV-1 and EHV-4, is more common in foals and is manifested as an acute febrile disease with rhinopharygitis and tracheobronchitis. Abortion mainly caused by EHV-1, occurs in mares as a sequel to respiratory infection. It also causes paralysis in young horses.Various outbreak of EHV-1 infection have been reported from India. In a national survey, 13.5% of horses were found seropositive for EHV-1 infection.

Clinical signs: Abortion in EHV-1 infected mares occurs at 6-11 months of gestation without showing any clinical sign. It is not generally preceded by respiratory form of the disease. It may occur as early as fifth month of gestation. Foal born to infected mares may be weak and die soon after birth.Prenatal disease syndrome due to EHV-1 infection involves still birth, the birth of weak, depressed foal which dies within 24 hr after birth. Such foals generally die due to respiratory infection. Complete recumbency with fore- and hind-limb paralysis may be seen in horses infected with neurological syndrome. Affected horses have variable degree of ataxia and paresis. Stumbling, toe  dragging and pivoting are more severe in hind limbs.Respiratory form of EHV-1/4 infection is characterized by fever, conjunctivitis, coughing and mild inflammation of the upper respiratory tract. The temperature varies from (39-40.50C; 102.5-105.50 F).The nasal discharge and cough persists for 1-3 weeks. Secondary bacterial infection may lead to development of pneumonia.

Diagnosis: The disease is diagnosed by isolating the virus and demonstrating the presence of intranuclear inclusion bodies in the exfoliated epithelial cells of the respiratory tract and specific antibodies in paired sera samples. Demonstration of four fold rise in antibody titre by complement fixation test (CFT) and virus neutralization test (VNT) in paired serum samples may be a criteria for diagnosis of EHV-1 infection in horses. Virus isolation or demonstration of viral nucleic acid by PCR from nasal swab materials, aborted foetal tissues, buffy coat of the infected horses can be done.

Prevention and control: Live attenuated and killed vaccines are available for immunization. All pregnant mares should be vaccinated with EHV-1 killed vaccine on 5, 7 and 9 months of gestation or as per the vaccination program advised by the farm veterinarian. Isolation and segregation of infected animals and strict hygiene may be followed in the farms.