All birds are naturally subject to infection by parasitic worms or Helminths. The majority of Helminths infect the digestive tract but some are also found in other organs, such as the brain, trachea, and eye.
Not all Helminth infections cause obvious clinical diseases.
The roundworms (or nematodes) and the flatworms (including cestodes and trematodes) are the two major classes of Helminth.
Roundworms are the most common intestinal worm of commercial poultry and cause the most economic impact. They are called roundworms because they are spindle-shaped and non-segmented with a smooth cuticle or skin. Some may have transverse grooves. With few exceptions, roundworms have separate males and females. Roundworms can have either a direct or an indirect lifecycle.
Roundworms that have a direct lifecycle pass through four developmental stages before becoming adults. Mature roundworms living in the infected bird produce eggs (1) that pass in the droppings, where they embryonated (2) in the environment, and when ingested, they hatch (3) in the proventriculus of the host and undergo the larval growth (4) to maturity.
Flatworms that infect poultry include tapeworms (cestodes) and flukes(trematodes). Tapeworms are white or yellowish ribbon-like segmented flatworms. Tapeworms grow by forming new segments (called proglottids) just behind their head (scolex). Each segment contains both male and female sexual organs. The tapeworm attaches itself to the wall of the intestine with the scolex. As it grows, segments on the tail end mature and break off, passing out of the intestine with the droppings. The excreted segments are filled with “eggs”, which are actually first-stage larvae.
Tapeworms have an indirect lifecycle. Insects and other arthropods eat the excreted segments and become intermediate hosts for the parasite. Many tapeworms require a specific intermediate host. Poultry becomes infected by eating infected intermediate hosts.
Flukes are flattened unsegmented, leaf-like parasites. Flukes are hermaphroditic (each individual has both male and female sex organs) and all trematodes that infect poultry have an indirect lifecycle. Their lifecycles vary in complexity and can involve up to four hosts. More than 500 species have been found in birds but only a few are known to cause disease.