A longitudinal study on the incidence of mortality of infectious diseases of commercial layer birds in Bangladesh.

A 20-month longitudinal field study was undertaken during the period from January 2010 to August 2011 to determine the incidence of mortality due to infectious diseases affecting commercial layer birds in 8 upazilas (an administrative unit) of 5 different districts in Bangladesh. Diagnosis of different diseases was made based on the flock history, age of birds, clinical signs, characteristic gross and microscopic lesions, and isolation and identification of the organisms. During the study period, 4710 birds were found dead as a result of disease occurrence. The incidence rate (true incidence rate) of mortality for the study period was 0.0171 per bird-months at risk. The incidences of mortality of almost all the infectious diseases were significantly higher in rainy followed by summer seasons. Particularly, mortality rate of ND and FC was significantly higher in rainy and summer seasons compared to winter and autumn seasons. And higher mortality rate of IBD, salmonellosis, IB, colibacillosis and MD was found in rainy than other three seasons. The highest mortality was recorded in birds below 8 weeks of age followed by birds aged 21 weeks and above. The mortality due to IBD was significantly higher (0.006) in the young birds (<8 weeks of age) than older birds. On the other hand, mortality rate of ND was significantly higher (0.003) in older birds (>8 weeks of age). Statistically no significant difference (p>0.05) was observed in the mortality rate of colibacillosis between different age groups. The proportional mortality due to infectious diseases was 54.2% (including single or mixed infections). Of the overall mortality, 13.4% was attributed to ND, 9.53% to IBD, 6.69% to MD, 4.33% to IB, 4.23% to salmonellosis, 3.23% to FC, 3.31% to colibacillosis, 1.1% to aspergillosis and 45.8% to non-infectious causes. The findings indicated that infectious diseases appear to be a major constraint of commercial layer birds in Bangladesh.