Larvae have been shown to remain viable for more than three years encapsulated in earthworm muscles. Other invertebrates may also serve as paratenic hosts, including terrestrial snails and slugs. The parasitic phase involves substantial migration in the definitive host to reach the predilection site. Young birds are most severely affected with migration of larvae and adults through the lungs causing a severe pneumonia. Lymphoid nodules form at the point of attachment of the worms in the bronchi and trachea. Adult worms also appear to feed on blood. Worms in the bronchi and trachea provoke a hemorrhagic tracheitis and bronchitis, forming large quantities of mucus, plugging the air passages and, in severe cases, causing asphyxiation. Pheasants appear to be particularly susceptible to infections resulting in mortality rates as high as 25% during outbreaks. The rapidly growing worms soon obstruct the lumen of the trachea, causing suffocation. Turkey poults, baby chicks and pheasant chicks are most susceptible to infection. Turkey poults usually develop gapeworm signs earlier and begin to die sooner after infection than young chickens. Lesions are usually found in the trachea of turkeys and pheasants but seldom if ever in the tracheas of young chickens and guinea fowl. The male worm, in the form of lesions, remains permanently attached to the tracheal wall throughout the duration of its life. The female worms apparently detach and reattach from time to time in order to obtain a more abundant supply of food.
Treatment of the soil or litter to kill intermediate hosts may be beneficial. Insecticides suitable for litter treatment include carbaryl, tetrachlorvinphos (stirofos). However, treatment is usually done only between grow-outs. Extreme care should be taken to ensure that feed and water are not contaminated. Treatment of range soil to kill ova is only partially successful. Changing litter can reduce infections, but treating floors with oil is not very effective. Raising different species or different ages of birds together or in close proximity is a dangerous procedure as regards parasitism. Adult turkeys, which are carriers of gapeworms, can transmit the disease to young chicks or pheasants, although older chickens are almost resistant to infection. Treatment
Flubendazole (Flubenvet) is the only licensed anthelmintic for use in poultry and game birds. Continuous medication of pen-reared birds has been recommended, but is not economical and increases the possibility of drug resistance. Several other compounds have been shown effective against S. trachea under experimental conditions. Methyl 5-benzoyl-2-benzimidazole was 100% efficacious when fed prophylactically to turkey poults. 5-Isopropoxycarbonylamino-2-(4-thizolyl)-benzimidazole was found to be more efficacious than thiabendazole or disophenol. The level of control with three treatments of cambendazole on days 3–4, 6–7, and 16-17 post-infection was 94.9% in chickens and 99.1% in turkeys. Levamisole (Ergamisol), fed at a level of 0.04% for 2 days or 2 g/gal drinking water for 1 day each month, has proven effective in game birds. Fenbendazole (Panacur) at 20 mg/kg for 3–4 days is also effective. Ivermectin injections may be effective in treating resistant strains. Sources

https://web.archive.org/web/20100703145151/http://cal.vet.upenn.edu/projects/merial/Strongls/strong_4.htm
https://web.archive.org/web/20110717194656/http://www.vetsweb.com/diseases/syngamus-trachea-d75.html#effects