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Proventricular or Stomach Worm
The proventricular or stomach worm (Dispharynx nasuta) is a parasite found in the lining of the proventriculus and gizzard. Adult worms are short, thick, white, coiled nematode parasites found in the glandular and muscular stomachs of numerous passerine and gallinaceous birds. Some authors believe this parasite is the most important helminth parasite of the ruffed grouse and at one time it was thought to cause "grouse disease" in the northeastern United States.
D. nasuta has a cosmopolitan distribution and is found in numerous species of birds. The common crow, blue jay, catbird, American robin, numerous other songbirds, and all upland game birds can be infected with the proventricular worm. Ruffed grouse and American woodcock have been reported to be pathologically affected by infections with the parasite. Ruffed grouse parasitized by D. nasuta have been reported from several states and provinces, including Michigan. The infections appear to be scattered through North America with ruffed grouse in some isolated areas being heavily parasitized.
The highest prevalence and intensities of infection are in the young of the year. D. nasuta is the first nematode acquired by turkeys in areas of high incidence and young birds are infected by 3 to 4 days of age. Large numbers of worms (200+) may be present in grouse by fall. Water levels in the bird's habitat are significant, as there is an inverse relationship between water levels and infectivity. Therefore, grouse may be more likely to acquire a D. nasuta infection in dry areas where the availability of sowbugs is greater.
Transmission and Development
Adult female D. nasuta lay their eggs in the proventriculus and embryonated eggs are passed in the feces. The eggs are eaten by suitable hosts. Wood lice or sowbugs (Armadillidum vulgare) and (Porcellio scaber) are known intermediate hosts and there are probably others. Shortly after ingestion of the eggs, larvae are found in the tissues of the wood lice. The first-stage larvae develop to the infective third-stage larvae in 26 days. The third-stage larvae are capable of surviving in the intermediate host for 6 months, thereby ensuring their availability to birds in the spring and summer.
There is no further development by the D. nasuta larvae until they are ingested by a susceptible definitive host. Twenty-seven days after ingestion, the third-stage larvae attain sexual maturity. The adult worms reside in the proventriculus and in the later stages of the infection may spread to the muscular walls of the gizzard.
Diagnosis of a D. nasuta infection can be made by egg or adult worm identification. The eggs passed in the feces are embryonated, thick shelled, and small, measuring 40 by 20 microns. The adult worms can be found in the proventriculus or gizzard during postmortem examination. The adult worms are white, thick, coiled, and measure from 5 to 7mm (male) to 10mm (female) in length. Both sexes have 4 wavy cuticular bands originating at the base of the lips and extending for a short distance posteriorly.
There is no known treatment for this parasite.
Birds infected with D. nasuta may be emaciated, sluggish in flight, or easily caught by predators. It is believed that the birds are weakened by the worm infection and their disease resistance is lowered, increasing their susceptibility to other diseases. Pathological changes caused by the proventricular worm have been reported in pigeons, American woodcock, and ruffed grouse. In grouse, lesions severe enough to warrant consideration as a primary pathogen have been found in 33% of the birds examined from certain areas.
Adult D. nasuta may be emaciated, sluggish in flight, or easily caught by predators. It is believed that the birds are weakened by the worm infection and their disease resistance is lowered lending them susceptible to other diseases. Pathological changes caused by the proventricular worm have been reported in pigeons, American woodcock, and ruffed grouse. In grouse, lesions severe enough to warrant consideration as a primary pathogen have been found in 33% of the birds examined from certain areas.
Adult D. nasuta attach by the anterior end to the mucous and epithelial cells of the proventriculus. Initially, ulcerations are formed in the proventriculus at the site of attachment. Usually if 10 or more adult worms are present, a proliferative proventriculitis with necrosis and sloughing of mucosal surfaces is seen. The lumen of the proventriculus becomes filled with thick, white, slimy mucous and sloughed stomach epithelial tissue while the tissues below the lining of the proventriculus undergo hyperplasia. These factors may combine to entirely occlude the lumen of the proventriculus thereby preventing the passage of food, resulting in death due to starvation. The adult worms are usually found lying beneath and in the proliferating tissue. The extensive destruction of the glandular tissue and muscular layers of the proventriculus may lead to a flaccid condition and a great (3-4 times normal size) enlargement of this organ. If the destruction is severe enough, a perforation of the proventriculus may occur, resulting in a peritonitis.
In the past this parasite has been said to be the most important factor concerning ruffed grouse scarcity in some places. D. nasuta may cause high mortality in areas where the parasite occurs in high numbers.
In most instances a light to moderate infection is seen and the birds do not suffer from a blockage of the proventriculus by the inflammatory products.
The worm is of no public health significance as it is removed from the carcass when the infected bird is eviscerated.
For questions about wildlife diseases, please contact the Michigan DNR Wildlife Disease Laboratory.