Influenza

Influenza is caused by a family of viruses divided into three genera, type A, type B and type C. Type B and C viruses usually only infect humans. The type A viruses include avian, swine, equine and human influenza viruses.

Type A Influenza

There are many subtypes of the type A influenza virus. The subtypes are identified by the presence of proteins on the surface of the virus. The two proteins of importance are hemagglutinin (H) and neuraminidase (N). There are 16 different H proteins (H1-H16) and 9 different N proteins (N1-N9) giving rise to 144 possible combinations or subtypes of the virus. The virus is named, based on the H and N subtype identified on its surface, eg. H5N1, H1N1 etc. Within each subtype there are also numerous strains that vary in respect to the species infected and the severity of disease.

The avian influenza viruses are further classified as highly pathogenic (HPAI) or low pathogenic (LPAI) based on certain genetic characteristics or on the severity of the disease in poultry with HPAI being more severe than LPAI. To date, only the H5 and H7 subtypes have caused HPAI in poultry. HPAI does not currently exist in the United States. When HPAI is identified in poultry in the United States, the birds are destroyed and the disease is eradicated.

Type A influenza viruses characteristically change frequently. New strains and subtypes are constantly evolving and are the cause of periodic epidemics and pandemics in man and animals.

Avian influenza is caused by type A influenza viruses and is found in a wide variety of wild and domestic birds. The natural habitat for the virus is wild water fowl in which signs of disease are rarely seen. The avian influenza viruses mainly infect birds, but can also cause disease in horses, swine, mink, cats, marine mammals and humans.

Swine influenza is caused by type A influenza viruses and is mainly found in pigs. Infrequent infections of man and turkeys are known to occur. Similarly, equine influenza is caused by type A influenza and mainly infects horses, donkeys and other Equidae. Human type A influenza causes disease in humans and ferrets and may occasionally infect other species such as swine, dogs, cattle and birds.

The various strains of type A influenza viruses tend to be host specific and do not easily infect other species. However, the occurrence of different host adapted viruses within the same host does occasionally occur and can result in mixing of the viruses to create a new virus with different disease and transmissibility characteristics. This is one way that a new pandemic virus can evolve.

Clinical Signs

Signs of infection vary with the strain, species and individual animal. In poultry, HPAI can cause serious disease with death loss of up to 90-100% of a flock. Typical symptoms include; discharges from the nose and eyes, swollen red to purple wattles and combs, sneezing, lack of coordination, darkened areas on the legs, loss of appetite, decreased egg production and appearance of deformed or shell-less eggs. In contrast, the presence of a LPAI may cause no signs of disease in infected birds.

In swine, signs of infection include: fever, depression, lack of appetite, coughing, difficulty breathing, discharges from the eyes, and occasional abortions may occur. Animals usually recover in 3-7 days. Certain virus strains in swine may cause very minimal to no signs of disease.

Horses infected with equine influenza will typically have a fever and develop a deep cough. Nasal discharge, depression and lack of appetite may also occur. In absence of secondary infections, recovery usually occurs in 1-2 weeks.

Prevention

Transmission of influenza viruses occurs through contact with infected fecal material or infected nasal or oral secretions. In all species, the following activities will help decrease the risk of spreading influenza virus to uninfected animals:

  • Isolate newly acquired birds/animals on their arrival to the farm
  • Isolate birds/animals returning from fairs and exhibitions
  • Maintain high sanitation standards on your farm
  • Control the presence of rodents, insects and wild birds
  • Minimize contact with all wildlife
  • Control access to your farm by neighbors, service people and strangers
  • Properly clean and disinfect any shared equipment
  • After visits to farms, markets, exhibitions, feed stores, etc. and before caring for your animals, wash hands and change clothing and footwear
  • Wash hands and change or disinfect clothing and footwear after contact with sick birds/animals
  • Provide adequate nutrition and shelter to your animals
  • Maintain vaccination status as recommended by your veterinarian
  • Report unusual signs of disease or high mortality to your veterinarian or regulatory authorities

Treatment

Animals with influenza are usually treated with supportive care and rest. Antibiotics are occasionally used to control secondary infections. Antiviral drugs are generally not recommended for use in animals.

Further Information

Additional information on influenza in man and animals can be accessed at the following web sites: