Equine influenza is a highly contagious, epizootic disease, which affects equines of all age groups and is caused by influenza virus A/equine virus of family Orthomyxoviridae. The disease is characterized by sudden onset of cough and rapid spread of the disease. Influenza A/equine-1 (H7N7) or Influenza A/equine-2 (H3N8), RNA virus of family Orthomyxoviridae (RNA) are causative agents of this disease. Equine influenza virus A (EIV-A)/ equine 2 (H3N8) is the main strain causing the disease. Historically, EIV-A/ equine 1 has caused disease but for more than 20 years, it has not been reported. A major outbreak of equine influenza caused by Influenza A- Equi-2 virus (H3N8) was reported in India during 1987 and it re-emerged in 2008 after a gap of 21 years.
Epidemiology and transmission: The disease is transmitted by direct contact and inhalation of virus with infected materials. Fomite transfer, viz, veterinary clothing, equipment, vehicle from infected shed to healthy shed spread the disease. Aerosol spread occurs up to a distance of 35 meters. Housing of infected horses with healthy horses leads to rapid spread of this disease. Frequent coughing by infected horses produce infectious aerosols spreading the infection to healthy animals resulting in a highly explosive outbreak.
Clinical signs: The disease starts with fever (38.5-410C, 101-1060C). The main symptom is dry and hacking cough in the beginning that become moist later. The temperature lasts for 1-3 weeks. Watery nasal discharge may be seen in the early stage. Secondary bacterial complications are common, which lead to mucopurulent nasal discharge, persistent fever and abnormal lung sound.
Diagnosis: Diagnosis of EIV-A infection is based on virus isolation from nasal swabs, and/or by serology. Demonstration of seroconversion a 3- to 4- fold increase in haemagglutination inhibition (HI) antibody titre in paired serum samples collected on the day of onset of infection and then on 14-21 days provides confirmation of diagnosis. In single radial haemolysis test, two-fold increases in antibody titre in paired serum samples gives confirmatory diagnosis of EIV-A infection in horses. The EIV-A antigen can also be detected in nasal swab of infected horses by enzyme- linked immunosorbent assay (ELISA) and by polymerase chain reaction (PCR).
Prevention and control: There is no specific treatment. Affected animals should be given easily digestible food, housed in well-ventilated stalls and protected from cold wind. . Horses should be vaccinated with EIV-A killed vaccine. However, the immunity of the vaccine is of short duration. Hence, horses should be given booster immunization at interval of 4-6 months. At the onset of the disease, identify the infected horses and get the disease confirmed by laboratory testing. Segregate the infected horses. No new horse should be introduced in the stud farm during an outbreak and horses should also not to be allowed to leave until the outbreak is over.