Influenza is an acute infectious disease caused by influenza viruses of genus Orthomyxovirus in family Orthomyxoviridae. The name Influenza is derived from an Italian phrase which means influenced. There are 3 types of influenza viruses distinguished by their capsid proteins as Type A, B and C. It is characterized by a short incubation period followed by fever, cough, coryza and headache. It occurs in endemic, epidemic and pandemic forms. Pandemics of the disease have been recorded periodically in the world. Noted among them are pandemics of 1781, 1889, 1918 and 1957. During the 1918 pandemic, 20 million people were believed to have died from the disease in the world. The 1957 epidemic was equally extensive although the mortality was much less.
Epidemiology: Influenza is an international disease and occurs in all countries. It occurs in several forms. Epidemics usually occur at an interval of 2-3 years in case of influenza A and 4-6 years in case of influenza B. Epidemics of influenza A viruses are usually more widespread and more severe than those of influenza B. Influenza C virus usually produces inapparent infections.The incidence is highest in the age group of 5- 9 years. It gradually declines with increasing age. Mortality is common among very young and very old. Epidemics are common from early fall to late spring.Of the 3 influenza viruses, type A occurs in man, animals and birds. It causes infection in pigs, horses, turkey and other birds. Infection in some of these animals preceded human infection. It is an indication of the zoonotic nature of the infection.
Avian influenza (AI), also known as ‘fowl plague’ is also caused by type A strains of the influenza viru.
Clinical features: Infection occurs mainly as droplet. In man the disease is characterized by a febrile illness lasting for 3-4 days and accompanied with cough and respiratory symptoms. The disease may be complicated by secondary bacterial infections. The infection may remain subclinical or latent in human as well as in the animal population.Clinical features in animal are mainly those of respiratory system involvement (for details see Chapter 19). Death in case of HPAI is usually by multiple visceral organ failure as the virus may affect the respiratory, digestive and nervous system.
Prevention and control: Prevention can be done by vaccination and strict sanitary practices. Continuous antigenic drifting of the virus poses difficulties for involving suitable immunizing means against the infection. Immunization against one type or subtype does not afford complete protection against others. Although vaccine is available, the duration of immunity is very short.
Recently a crude bird flu vaccine for poultry against the highly pathogenic avian influenza has been developed at the High Security Animal Disease laboratory (HSADL), ICAR, Bhopal. Control of LPAI is also very important as it may mutate at any time to HPAI. Biosecurity to any infectious disease should be the first line of defense. Improved surveillance to estimate the spread of disease, immediate destruction and disposal of infected and exposed birds, strict quarantine and movement control, total ban on marketing of slaughter birds from infected farms must be enforced and decontamination to remove and reduce the virus are the important preventive measures to reduce the risk of transmission of influenza from birds.