Normally, if the virus spreads through the flock, all of the birds are affected. Many times the IB virus may spread through the flock without producing obvious clinical signs of disease except a mild cough. However, the virus may trigger a serious, long-lasting respiratory disease if Mycoplasma gallisepticum or Mycoplasma synoviae is present. The disease can be even more severe if complicated by E. coli infection. Some strains of the virus, referred to as nephropathogenic IB viruses, infect the kidneys and cause permanent renal damage.
Infected chickens excrete watery droppings, resulting in wet litter.
Urates are common and can be identified easily in the droppings and in the kidneys and ureters at necropsy. The kidneys of affected birds are pale, mottled, and can be 2 to 3 times their normal size. Even though mortality in uncomplicated IB outbreaks can be relatively low, infection with nephropathogenic strains may cause high mortality. The commercial egg-laying chicken has a level of productivity second to none. However, an outbreak of IB in a developing pullet flock may have devastating effects on future productivity.