Avian Influenza (Bird Flue) in Poultry
Dr. Mujeeb Ather
Avian Influenza may play a role in the evolution of new Human strains by contributing viral genes to human strains via genetic resortment. Antigentic and genetic evidence supports the suggestion that the HA gene in a virus responsible for the 1968 pandemic in human originated from a virus circulating in duck.
Direct transmission viruses between birds and humans do not generally occur. Further more there is evidence to suggest that the HINI Viruses present in pigs, turkey and ducks may be involved in interspecies transmission. So a swine-avian-human connection could have public health significance. Recent havoc in Hongkong has attention of the world.
Influenza is a disease caused by type A influenza virus, a member of the Orthomyxoviridae family. Influenza A viruses are responsible for major disease problem in birds as well as in humans and lower mammals.
Influenza viruses have caused considerable economic losses. The U.S. government expended over $60 millions 1983-84 to eradicate a highly pathogenic H5N2 virus in poultry in the Pennsylvania-Virginia-New Jersey outbreak. This includes the cost of diagnosis, quarantine, flock disposal, cleaning, decontamination, epidemilogic investigation and other regulatory procedures and payments to the flock owners. In Australia during the 1985 on one chicken farm and outbreak costed over $2 millions to eradicate.
In contrast to domestic and confined birds free flying do not experience significant diseases problems due to influenza viruses but they act as carrier. Influenza viruses are readily recovered from migratory waterfowl particularly ducks and this serve as a source of viruses for other species including humans, lower mammals and birds and that such a high rate of infection provides the opportunity for the maintenance and emergence of new and potentially higher pathogenic strains through the process of mutation and or genetic resortment.
Avian influenza viruses are distributed throughout the world in many domestic birds, including, turkeys, chickens, guineafowl, quails, pheasants, geese and ducks. Migratory waterfowl, particularly ducks have yielded more viruses than any other group
while domestic turkeys and chicken have experienced the most substantial diseases problems due to influenza. Influenza viruses have also been isolated from caged birds, including mynas, parakeets, parrots, finches and hawks.
Influenza is reported in several other countries including Belgium , Scotland , Italy , Soviet Union , Australia , Hongkong , France Israel and Pakistan . Not yet reported in India .
The antigenic subtypes of viruses involved in various outbreaks in the world are:
Avain influenza viruses with H5 and H7 antigens are more virulent with high pathogenicity while H3 serotypes can cause only mild disease with respiratory signs in turkey.
The disease can spread by import of birds/chicks from infected countries and can spread by cull birds/live birds transport, transport of infected eggs/egg trays/infected feed/feed bags etc.
RESISTANCE TO CHEMICAL AND PHYSICAL AGENTS:
Avain influenza viruses are relatively sensitive to inactivation by lipid solvents such as detergents. Infecting is also rapidly destroyed by formalin, b propiolactone, oxidizing agents, dilute acid etc. they are also inactivated by heat, extremes of pH and dryness.
TRANSMISSION AND CARRIERS:
Infected birds excrete virus from the respiratory tract, conjunctiva and faeces. Thus likely modes of transmission include both direct contact between infected and susceptible birds and indirect contact between infected and susceptible birds and indirect contact including aerosols (droplets) or exposure to virus-contaminated fowls. Since infected birds excrete high levels of virus in their faecal, spreading readily accomplished by virtually anything contaminated with faecal materials. E.g. Birds and mammals, feed, water, equipment, clothes, vehicles and insects. Thereby viruses are readily transported to other areas by people and equipment shared in production or live-bird marketing. There is no evidence of vertical transmission.
Incubation period: Few hours to 14 days
Morbidity and mortality ranges from mild form to 100%. The signs of disease are extremely variable and depend on the species affected, age, sex, concurrent infection, virus, environment factors etc. signs may reflect abnormalities of the respiratory, enteric, reproductive or nervous systems. The signs most commonly reported include pronounced depression, decreased feed consumption, increased broodiness and decreased egg production with soft shelled eggs, increased mortality, mild to severe respiratory signs including coughing, sneezing, rales and excessive laceration, hurdling, ruffled feathers, oedema of head and face, cyanosis unfeathered skin, in co-ordination and other abnormalities of the nervous system. Hemorrhages of shank's skin or vesicles filled with clear fluid or blood are absorbed on combs and wattles.
A diagnosis of influenza A virus infection is conclusively demonstrated by the isolation and identification of the virus, but detection of antibodies to the virus is a very useful indirect diagnostic tool.
It is necessary to collect (from necropsied birds) tracheal and cloacal swabs as well as lungs, pancreas and spleen tissue in brain heart infusion broth and use it for inoculation of embryonated eggs (9 to 10 days old) by allantoic route for isolation of the virus. The allantoic fluids from the infected embryos can be used to detection of haemagglutination activity which can be inhibited by use of a battery of specific reference A.I. antisera for detection of H antigens.
Other tests like ELISA, Nucleotide probing, Radial diffusion hameolysis tests and use of moniclonal antibodies can be done at reference laboratories with the help of Global organizations including FAO/WHO/OIE etc.
PREVENTION AND CONTROL
Methods for prevention and control of influenza virus infection center on preventing the initial introduction of the virus and controlling spread if it is introduced. One critical aspect in reaching the goal of prevention and control is the education of the poultry industry regarding how the viruses are introduced, how they spread and how such events can be prevented. Biosecurity is the first line of defense. Study group formulated by minister of agriculture, Govt. of India on poultry diseases including IBD, IBH and Avian Influenza, March, 1996 has advised Govt. of India on vaccination strategies and other measures to contract Avian Influenza in India in further they include.
Inactivated influenza virus vaccines have been used in a variety of species and their effectiveness in alleviating clinical signs and mortality is well documented. Birds are susceptible to infections with influenza viruses belonging to any of the 15 HA subtypes and there is no way to predict their exposure to any particular one. It is not practical to practice preventive vaccination against all possible subtypes. On the other hand, once an outbreak occurs and the subtype of the virus is identified, vaccination may be a useful tool.
No vaccines specifically designed to contain or prevent highly pathogenic influenza are currently available. However is U.S.A. there has been wide spread use of experimental inactivated vaccines produced under special license on a commercial basis. National veternary services laboratory have type A influenza viruses of each subtype for use as seed virus in the preparation of inactivated vaccines.
Other approaches besides inactivated vaccine include the use of genetic engineering to isolate HA genes, particularly H5 and H7 and place them into viral vectors such as fowl pox vaccinia virus and retrovirus. Another approach is to inoculate DNA directly in to chickens.
An interesting application of avian influenza viruses has been their use as gene donors for making live attenuated vaccines for potential use in humans, whether these vaccines will be used is not yet known.
|Source : IPSACON-2005|