Avian Influenza
Carol J. Cardona, Extension Poultry Veterinarian, University of California, Davis
Avian influenza is caused by type A influenza virus. The symptoms can vary from amild disease with little or no mortality to a highly fatal, rapidly spreading epidemic (highlypathogenic avian influenza) depending on the infecting virus strain, host factors, andenvironmental stressors.
Hosts
More avian influenza viruses have been isolated from ducks than any other speciesalthough most free-flying birds may also be infected including shorebirds, gulls and otherseabirds. Waterfowl are more resistant to avian influenza than are domestic poultry. Virusesthat cause no obvious disease in waterfowl can be highly pathogenic (rapidly fatal) in domesticpoultry. Among domestic poultry species, turkeys are more commonly infected than arechickens.
Transmission
Waterfowl act as a reservoir of avian influenza virus by carrying the virus in theirintestinal tract and shedding it in their feces. Avian influenza viruses are spread to susceptiblebirds through inhalation of influenza particles in nasal and respiratory secretions and fromcontact with the feces of infected birds.
Signs of disease
Signs of avian influenza are extremely variable. In some flocks the only evidence of theinfection is seroconversion i.e., the birds develop a detectable antibody titer to AI. AI can alsobe manifest as respiratory, enteric, reproductive or nervous system disease. Decreased foodconsumption and drops in egg production are among some of the earliest and most predictablesigns of disease.
Signs including coughing, sneezing, ruffled feathers, swollen heads, nervous signs likedepression, and diarrhea may occur together or singly. In some cases, birds die rapidly withoutclinical signs of disease.
Prevention and control
Wild birds and their excreta should be considered a major source of avian influenza. Preventing direct contact with free-flying birds and protecting domestic poultry from contactwith the feces of wild birds is an important way to prevent avian influenza.
Live bird markets have been an important source of avian influenza, especially on the Eastcoast of the U.S. It is important to avoid live markets, educate employees about the dangersposed by these markets, and prevent the spread of disease from these markets to your flock bypreventing any contact.
Infected birds shed virus in saliva, nasal secretions and feces in the first two weeks ofinfection. Four weeks after infection, virus can no longer be detected. Hence, prevention is bestaccomplished by preventing contact between newly infected and susceptible birds. Biosecurityis a first line of defense (see
Biosecurity for Poultry Flocks, J. Jeffrey, UC Davis ExtensionPoultry Veterinarian. Avianinfluenza can be spread from infected birds through the transfer of feces especially oncontaminated equipment and clothing. Controlling the traffic between infected and uninfectedbirds is essential.
-->Cleaning and disinfection
Influenza viruses are very sensitive to most detergents and disinfectants. They arereadily inactivated by heating and drying. However, flu viruses are well-protected frominactivation by organic material and infectious virus can be recovered from manure for up to 105days. Complete removal of all organic material is part of any effective disinfection procedure.
Contaminated houses are heated for several days to inactivate virus. Organic material isremoved followed by complete cleaning and disinfection of all surfaces. Contaminated litter andmanure is problematic and should be composted or buried to ensure that it does not spreadinfectious virus.
Frequently asked questions
1. Are the flu viruses of human and birds the same?
In most cases, the influenza viruses that infect birds do not infect humans and vice versa. However, in Hong Kong in 1997, a unique avian influenza virus infected both chickens andhumans. This is the only time an avian influenza virus has ever been transmitted directly frombirds to humans. This appears to have been a totally unique occurrence. The World HealthOrganization continuously monitors human influenza viruses isolated from cases all over theworld. No avian viruses have been found infecting humans since 1997.
2. What are the risks of getting avian influenza from waterfowl?
Avian influenza virus infections are widespread in wild birds, especially ducks. Migratingwaterfowl are a significant source of avian influenza viruses especially in the major flyways. Turkeys on open ranges in Minnesota, a state in the major flyway for migrating ducks,frequently experience avian influenza problems. But the prevalence of avian influenza in turkeyshas been high in some years and minimal in others. The reason why influenza viruses come andgo is not known. The risk to susceptible birds from contact with waterfowl must be consideredvery high although it may vary from year to year for unknown reasons.
3. Why can't I prevent infection by vaccinating my flocks?
Vaccines effectively prevent clinical signs of influenza infections in many species includingpoultry. However, the vaccines are not cross-protective for the 15 virus subtypes that can infectpoultry. Since there is no way to predict which type will infect a flock, vaccines are generallynot practical to prevent infections.
4. What should I do if I suspect avian influenza in my birds?
You should contact your veterinarian if you observe any of the signs of avian influenza,especially if they are accompanied by a drop in feed consumption and/or a significant drop in eggproduction. Because the signs of avian influenza are so variable, it is important to get the help ofan expert for diagnosis.