engleski

AVIAN INFLUENZA: A CONTINUOUS THREAT TO POULTRY PRODUCTION

Avian influenza (AI) continues to cause serious economic losses in the chicken and turkey industries world-wide. The AI viruses have low and high pathogenicity for chickens. The latter cause so-called highly pathogenic avian influenza with up to 100% morbidity and mortality. Although viruses of low pathogenicity for chickens produce no or only mild disease in controlled infection, the field disease can vary from inapparent or only mild respiratory disease with mild egg depression up to high morbidity and mortality occasionally as high as 50-70%. The severity of the field disease caused by AI viruses of low pathogenicity depends on species and age of host, deficiency conditions, environmental factors and especially on concurrent bacterial infections. There is no practical treatment for avian influenza infection, although amantadine and rimantadine can alleviate symptoms and decrease morbidity and mortality. However, treated birds are still infected and shed virus and there is rapid emergence of amantadine and rimantadine-resistant viruses that kill both treated and contact infected birds. On the other hand, antibiotic treatments are used to reduce the effects of concurrent/opportunistic mycoplasmal and bacterial infections. This sounds more reasonable knowing that some bacteria can produce enzymes, which enable viruses of low and moderate pathogenicity to replicate and spread to a greater extent in the host. Prevention and control of influenza virus infection has to be focused on preventing of initial introduction of the virus and controlling spread if it is introduced. The latter means strict separation of susceptible and infected birds. Particular care must be taken on manure, equipment and personnel movement. Education of the poultry industry regarding how the viruses are introduced and spread and how it can be prevented is one of the major tasks of prevention and control of the disease. Vaccination may be an useful tool and inactivated vaccines are available. Vaccines of the same subtype should be used, or even better to make vaccine from the local isolate. The birds can be vaccinated as young as day-old, and even 18-day-old embryos can be vaccinated. Main disadvantages of vaccination are: 1) serologic surveillance is impeded in vaccinated flocks and 2) vaccination protects the birds from clinical signs and death but it does not prevent the virus from replicating and it can lead to a greater perpetuation of infection and possible mutation of the virus. It is likely that recombinant fowl poxvirus vaccines will reduce these problems. It is important to stress that vaccination by itself, without other measures of prevention and control, can not solve the problem of avian influenza infection.

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