Gastrointestinal Imaging

Ascariasis

infection by the Nematode roundworm Ascaris lumbricoides. The infection occurs through ingestion of contaminated food. On ingestion the larvae migrate from the intestine to the lungs, where they cause a pneumonitis, and then to the trachea, oesophagus and intestine, where they mature. From the intestines they can migrate to the bile ducts and the gallbladder and even into the pancreatic duct. Ascariasis probably infects 25% of the world population but in the tropics the infection rate may be as high as 90% in some areas. The worm is most frequently found in children but may be seen also in adults. The symptoms may be fever, bronchitis and pneumonitis during the acute phase. During the later stage the worms may cause symptoms of intestinal and bile duct obstruction.

Biliary involvement

Obstructive jaundice occurs when the worm obstructs the common bile duct. As the worms disintegrate thousands of worms can be released and will cause acute and chronic suppurative cholangitis. In mild cases ascaridic cholangitis resolves with granuloma or stone formation but extension to the portal and hepatic veins may lead to pylephlebitis. Liver abscesses can also occur.

Sonographically round worms in the common duct and the gallbladder can be seen as an accumulation of nonshadowing echogenic linear structures that may be mobile. In living worms the worms alimentary canal may be visible. If scanned transversally, a "target" sign, i.e. a round hyperechoic central dot, separated by a hypoechoic area from a peripheral hyperechoic ring may be seen. Although not primarily used for the detection of ascariasis CT may incidentally visualize the worms in the bile ducts as soft tissue round structures within dilated bile ducts. On contrast cholangiography (Fig.1) and on MRCP tubular smoothly lined radiolucencies will reveal the presence of the parasites.

Intestinal involvement

Most of the worms inhabit the small bowel, the majority being located in the ileum. Complications of intestinal ascariasis include obstruction, perforation and peritonitis, and acute appendicitis. The worms may be demonstrated on plain films of the abdomen as a coiled, inhomogeneous rounded soft tissue structure with unsharp borders when the worms are outlined by bowel gas.

Ultrasound is able to visualize the worms in the small bowel based on the same features as those described in the biliary tract. On barium study longitudinal or tortuous smoothly lined multiple filling defects are seen in the small bowel (Fig.2). Ingestion of barium by the ascaris results in a typical radiographic feature: a very thin radiopaque line, representing the worms alimentary canal, is located centrally within the elongated radiolucency which corresponds to the parasite itself (Fig.3). Sometimes the elongated filling defects are coiled up and will produce a larger polylobular filling defect.

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Fig.1

Ascariasis of the bile ducts. Note the tubular radiolucency (arrows) in the hepatic duct. There is a considerable dilatation of the hepatic and the common bile duct.
Ascariasis, Fig.1
Ascariasis, Fig.2
Ascariasis, Fig.3