Avian influenza (AI)
One of only two 'List A' diseases of poultry targeted for emergency disease control measures by OIE, avian influenza is a highly contagious viral disease affecting several species of birds as well as pet birds and wild birds. The AI viruses are divided in 2 groups based on their ability to cause disease. Highly pathogenic avian influenza (HPAI) virus spreads rapidly, may cause serious disease and result in high mortality rates (>75% within 48 hours). The low pathogenic avian influenza (LPAI) can causes mild disease that may go undetected as no clinical signs are noticed in some species of birds. Wild species of birds usually do not develop clinical disease, but some AI viruses cause severe illness or death in chickens, turkeys and guinea fowl. AI viruses of low to moderate pathogenicity are identified regularly in the domestic poultry populations. AIV is reintroduced into domestic poultry by migratory waterfowl, which are carriers of the virus.
The virus belongs to family Orthomyxoviridae that is divided into 3 types- influenza A (multiple species), influenza B (humans), influenza C (humans and swine). The viral genome is segmented having 8 segments of ssRNA genome with lipid envelop. AI viruses are subdivided into serotypes based on their hemagglutinin (H) and neuraminidase (N) surface antigens. The present knowledge indicates there are 16 'H' and 9 'N' antigens, the combination of which results in different strains (serotypes) of the virus. Some of the highly virulent strains are evolved from milder strains following repeated passages in chicken. The AIV was found to mutate at an extremely high rate as it serially infects poultry. Chickens are not the normal host for avian influenza, so the virus strain picked up by them from other species of birds has a tendency to mutate and become more pathogenic. Avian subtypes include H1 to H16 and N1 to N9 whereas human subtypes include H1N1, H3N2, H1N2 and H2N2. The subtypes H5N1, H9N2, H7N7 and H7N3 can be transmitted from bird to humans.
HPAI was earlier known as fowl plague. It first appeared in Italy more than 100 years ago (around 1878). A major epidemic occurred in the United States in 1983-84 that was eradicated within 2 years at a cost of more than 70 million dollars by destroying approximately 17 million birds. The current outbreaks of HPAI, which began in South- East Asia in mid-2003, are the largest and most severe on record. In the history of this disease never before so many countries have been simultaneously affected, resulting in the loss of so many birds. In India, the outbreaks of HPAI have been recorded in Maharashtra, Gujarat and Madhya Pradesh in 2006, Manipur in 2007, West Bengal (WB) and Tripura in early 2008 and Asom, WB (re-occurrence) and Sikkim in later part of 2008 and continued in 2009. The causative agent, the H5N1 virus, has proved to be especially tenacious. By first quarter of the 2009, outbreaks of AI in countries neighboring to India i.e. Bangladesh (2007), Myanmar (2006), Nepal (2009) and Pakistan (H7N3-1994 and H5N1-2007) have also been reported. Despite the death or destruction of an estimated 150 million birds, the virus is now considered endemic in many parts of Indonesia, Viet Nam, Cambodia, China, Thailand etc. Control of the disease in poultry is expected to take several years.
Infected birds shed the virus in fecal and oculo-nasal discharges. Even though recovered flocks shed less virus than clinically ill flocks, recovered flocks will intermittently shed the virus hence should be considered infected for life. Waterfowl (wild and domesticated) are the primary natural reservoir of influenza viruses. Wild waterfowl usually do not show clinical signs, but they can excrete the virus for long periods of time. In addition, waterfowl can be infected with more than one type of influenza virus. Detection is further complicated by the fact that they often do not develop a detectable antibody response after exposure to the virus.
Influenza virus has been recovered from water and organic material from lakes and ponds utilized by infected ducks. Co-mingling of these birds under backyard system is a factor in some outbreaks.
The AIV can remain viable for long periods of time at moderate temperatures, and can survive indefinitely in frozen material. As a result, the disease can be spread through improper disposal of infected carcasses, manure, or poultry by-products. The disease also can be easily spread by people and equipment contaminated with avian influenza virus. AIV can be transmitted on contaminated shoes, clothing, crates, egg flats, egg cases, vehicles, and other equipment. Insects and rodents may mechanically carry the virus from infected to susceptible poultry. AIV has been isolated from turkey eggs suggesting vertical transmission, although typically the virus kills the embryo. There is little or no evidence of egg-borne infection in poultry, however, egg-shell surfaces can be a means of transmission when contaminated with the influenza virus.
Symptoms and lesions: The severity of the disease ranges from inapparent (mild) to rapidly fatal. Lethal strains of the virus can strike so quickly, particularly in young chickens, that there may be no clinical signs other than sudden death. Clinical signs vary greatly and depend on many factors including the age and species of poultry affected, husbandry practices, and the inherent pathogenicity of the AIV strain. They may include ruffled feathers, loss of appetite, depression and droopiness, sudden drop in egg production, soft-shelled eggs, cyanosis (purplish-blue coloring) of wattles and comb, edema and swelling of head, eyelids, comb, wattles and hocks, diarrhoea, respiratory distress, blood-tinged discharge from nostrils, incoordination, including loss of ability to walk and stand etc.
Gross lesions may include pin-point hemorrhages (most easily seen on the feet and shanks), engorgement of blood vessels, hemorrhage in the trachea, proventriculus, beneath the lining of the gizzard and throughout the intestines, clear straw-colored fluid in the subcutaneous tissues, swelling and hemorrhages over muscles along the breast bone as well as in the heart, gizzard fat, and abdominal fat. The lining of the gizzard may be easily removed. Young broilers may show signs of severe dehydration with other lesions less pronounced or absent entirely.
Diagnosis: Typical history, signs and PM lesions may be suggestive of avian influenza. Confirmation of a diagnosis is done by molecular detection of the specific gene sequences, serologic testing and virus isolation and identification. Because HPAI strains are considered to be exotic in India, they are compulsorily reportable to Animal Husbandry Commissioner, Government of India. The BSL-4 Laboratory of ICAR i.e. High Security Animal Disease Laboratory (IVRI) at Bhopal is the OIE-recognized Reference Laboratory for HPAI in India. The confirmation of the disease and assessment of virulence level is done by RT-PCR, real-time PCR, virus isolation and controlled laboratory challenge of experimental chickens. The serological diagnosis is based on AGID, HI tests and ELISA.
Prevention and control: Good husbandry, proper nutrition and biosecurity are the key factors to keep away AIV infection. It must be remembered that recovered flocks continue to shed the virus intermittently. All buildings should be cleaned and disinfected after an infected flock is culled and disposed off. The poultry litter or manure should be composted before application to cultivated lands. A vaccination program, in conjunction with strict quarantine, has been used in a few countries to control mild forms of the disease in commercial chicken and turkey flocks. With the more lethal forms of the disease, however, strict quarantine and rapid depopulation of infected flocks remains the only effective methods of stopping HPAI. The success of such a program depends, of course, on the full cooperation and support of the poultry and allied agencies.
AI viruses have frequently been isolated from clinically normal, imported exotic birds. These infected birds are a potential threat to cage birds, wild birds, and poultry. With the realization that there is a reservoir of AIV in wild waterfowl, every effort must be made to prevent direct or indirect contact between domestic poultry and wild waterfowl. Persons handling wild game birds (especially waterfowl) must change clothes completely and bathe prior to entering poultry houses. Live-bird markets are a reservoir of infection. Such markets serve as a focal point for gathering and housing many species of bird. These facilities are rarely cleaned or disinfected.