Human and animal influenza
Influenza is a disease common to man and a limited number of lower animal species mainly horses, pigs, domestic and wild birds, wild aquatic mammals such as seals and whales, minks and farmed carnivores. There are 3 types of influenza viruses A, B and C. Types B and C are human viruses mainly affecting young children and causing a mild disease. Type A virus is the important type as far as cross-species infections are concerned.
Influenza type A is distributed worldwide and usually causes a mild respiratory disease in humans and animals. Human influenza epidemics due to new epidemic strains occur at regular intervals of 2 to 3 years and affect mainly elderly people. However, influenza is a potentially devastating disease in both humans and animals thereby very important for both human and veterinary medicine.
Pandemics are major epidemics characterized by the rapid spread of a novel type of virus to all areas of the world resulting in an unusually high number of illnesses and deaths in humans in most age groups. Three pandemics of human influenza have affected the world population (1918, 1957 and 1968). The most infamous pandemic was “Spanish Flu” which is thought to have killed at least 40 million people in 1918-1919.
Birds, especially aquatic birds represent a vast reservoir of type A influenza viruses. These viruses have the capacity to spread to many lower mammalian species and sometimes cause high morbidity and mortality. A small number of cases of animal influenza in humans has been described in the past. In these cases the virus originated from pigs, seals, ducks and chicken.
In birds, highly pathogenic avian influenza is an extremely contagious and aggressive disease that causes rapid systemic illness and death in susceptible birds. Domestic chickens and turkeys are most severely affected; mortality in these birds often exceeds 50%. From 1959 to 2003 only 21 outbreaks occurred worldwide, mainly in the Americas and Europe. Although all had serious consequences for the poultry industry, most remained geographically circumscribed.
After its first detection in 1996 in Guangdong, a province of China, the disease broke out in Hong Kong in 1997. In the beginning of 2003 mortality in wild and domestic birds in East and South-East Asia extended; an epidemic started. From the recent findings it can be assumed that interspecies transmission of influenza A viruses occurs more frequently than we think, mainly from birds to mammalian species. 41 human deaths have been confirmed in this region since the start of the epidemic till 28 January 2005.
Although the outbreaks in poultry have weakened economies and jeopardized food security, the greatest concern for human health is the risk that present conditions could give rise to an influenza pandemic.