These worms are 5 to 8cm long and live in the middle part of a bird's intestine. Bad infestations of large roundworms in poultry usually occur when birds are kept in intensive conditions on dirty litter or in runs that have been used for many years.
Large roundworm infestations occur directly by birds eating eggs that have been passed out in droppings. These eggs have to be infective which takes 10 days. Once eaten, eggs hatch in the bird's proventriculus and the larvae move on to part of the intestine called the lumen. A week later, the larvae get into the mucosa, part of the intestine wall and start to cause damage. Occasionally, large roundworms crawl up the oviduct and can appear inside eggs.
Photos courtesy of Elanco. Roundworms inside the intestines shown after a post mortem.
Loss of egg production and pale yolk colour and anaemia are the biggest signs of a large roundworm infection as well as a loss in weight gain for growing birds. Birds can look depressed and eventually if the infestation is heavy, they can die.
As with all internal parasites, there are many treatments that will kill these worms. Worming Chickens provides some ideas for worming poultry. Flubenvet is a licensed in feed wormer that can be used with most poultry.