In periods of especially wet weather, when earth worms are abundant on the ground surface, birds can become infected when eating contaminated worms. Blackhead organisms can also infect a common internal parasite, called the cecal worm. This worm is very small and rarely causes much harm to the host. Mature cecal worms produce many microscopic eggs which are shed in the droppings. The Blackhead organism can penetrate and live inside the cecal worm egg.
Birds picking up the infected egg also infect themselves with Blackhead.
As with many parasite eggs, the cecal worm egg is highly resistant to heat, cold, and most disinfectants. Therefore, a contaminated premise remains contaminated for a long time. The signs associated with this illness are not specific. Affected birds develop droppings which often have a yellow color. This, however, is not specific for Blackhead since it may be seen with a wide variety of diseases in birds.