Scientifically, this term is now applied only to diseases caused by a myxovirus.

The World Health Organisation (WHO) was much exercised as to what happens to the virus of human influenza between epidemics. It has long been known that there is a relationship between this disease and swine influenza. The human influenza virus (type A) was isolated from the parasitic pig lung-worm. Larvae of these lung-worms are harboured by earthworms — the only known intermediate hosts — which live for as long as 10 years.

Between epidemics, the virus is not found in the tissues of the pigs. However, earthworms taken from infected pig farms seem to carry inapparent viruses, and these can develop, in pigs eating the worms, into normal viruses capable of being isolated from the respiratory system. The question, therefore, arises whether the pigs are, in fact, the virus reservoirs, rather than being secondarily infected by the human virus. (See also SWINE INFLUENZA.)

There is evidence that influenza viruses of mammals and birds play an important part in the emergence of new viruses which cause out-breaks of illness in man in several continents.

The recovery from pigs in Taiwan in 1970 of influenza virus indistinguishable from that causing type A Hong Kong influenza epidemics in man in 1968 provided the first direct evidence of the inter-species transfer of influenza viruses. Pigs experimentally inoculated with that virus transmitted it to pen-mates. Moreover, the Taiwan virus taken from pigs readily infected human volunteers, who developed antibodies effective against virus from both pigs and people.

It is now suggested that the Hong Kong human influenza virus did not arise by mutation from a pre-existing human strain, but that it probably arose from the mixed infection in a mammal or bird with an animal influenza virus and a human type A Asian strain. The animal virus may have provided certain subunits or components; the other subunits having come from a human strain.

For influenza in the horse, see under EQUINE INFLUENZA.

Pneumonia in calves may be caused by a virus of influenza-type.

In the dog, parainfluenza virus SV5 has been isolated in the USA and the UK from dogs with upper respiratory disease. (See ‘KENNEL COUGH’.)

Avian strains of type A influenza virus cause a number of diseases in hens, ducks, turkeys, etc. During 1980 and 1981, 9 subtypes of influenza A virus were isolated from birds in Britain, usually as a result of investigations of disease or death. However, these viruses were shown to be of low virulence for chickens. Highly pathogenic avian influenza virus has caused serious outbreaks among turkeys.

Avian influenza is a NOTIFIABLE DISEASE throughout the EU. Waterfowl are the main reservoir host for both avian and mammalian strains but they are not themselves much affected by the disease. The slaughter of all chickens in Hong Kong in 1997 was justified as the strain of virus present transmitted fairly easily to the human population. (See AVIAN INFLUENZA.)