engleski

APPEARANCE AND CURRENT SITUATION OF vvIBD IN CROATIA

Infectious bursal disease (IBD) was first reported in Croatia in 1971 as an acute disease, but in following decades the disease changed its nature to a mainly subclinical disease. Until the mid-nineties, mortality due to IBD occurred very rarely, mainly in layers, and did not exceed 1%. An epidemic of very virulent IBD (vvIBD), obviously related to recent epidemics in other European countries, emerged in Croatia in late 1995. It is still unclear how vvIBD circumvented Croatia for several years despite the extensive poultry trade with neighbouring and other European countries already affected with this disease. The disease appeared in the north-western part of Croatia, close to Slovenian and Hungarian borders and during the following year it spread throughout the entire country, attacking mostly farm reared chickens. The moderate levels of maternally derived antibodies were insufficient to protect progeny against infection with this virus. Also, mild live vaccines, which were in wide use, were not able to protect the chickens against the disease. The clinical signs were observed in very young birds, even those as young as seven days. Later on, the onset of infection appeared more often at the age of 3 to 5 weeks. Specific mortality due to field infection normally amounted to 10% in broilers, but could rise as high as 20% or more, while mortality in layers reached 60%. These data were comparable to results obtained by the infection of IBD-antibody-free birds with 104,0 EID50 of field isolate of the virus. Mortality of both infected 40-day-old broilers and 28-day-old layers totalled 10% and 50% respectively. Apart from typical gross lesions, swelling of the bursa was not so obvious. Thirteen outbreaks of vvIBD were recorded in farm reared chickens during the period October-December 1995, while in 1996, 1997 and 1998, 84, 98 and 79 outbreaks were recorded, respectively. The first four months of this year are showing decreased outbreak rate of vvIBD in farm reared chickens in Croatia. Since 1997 the vvIBD spread widely to backyard chickens and small flocks ranging from fifty to several hundred chickens, mainly broilers. Such small flocks are, recently, very common in Croatia, mainly during the spring and early summer, thus the disease in such type of flocks has seasonal appearance. It is very difficult to estimate, even roughly, the incidence of the vvIBD in these flocks because only some of the flockowners submit birds for diagnosis or report the disease. Moreover, these flocks are usually IBD non-vaccinated, so they are serious source of field virus for farm reared chickens. The emergence of vvIBD in Croatia forced us, almost overnight, to switch from mild to intermediate live IBD vaccines and timing of vaccination based on the level of maternally derived antibodies as well as the use of two vaccinations instead of single vaccination as practised earlier. So called hot vaccines have not been used in Croatia. Area of proposed future research activity: Evaluation of diagnostic methods and tools for rapid diagnosis and differentiation of vaccinal and field IBD viruses. collecting and exchange of virus and serum samples, evaluation of the interactions between IBD and opportunist infections as well as vaccinations against other diseases.

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