(Herring worm disease) A common parasitic infection from fish. The parasites are widely distributed. Human disease occurs where people eat raw or lightly smoked or salted saltwater fish or squid (e.g., in Japan, the Netherlands, Scandinavia, and Central America). The causative agents are Anisakis, Phocanema and Contracaecum (Nematoda). There is no vaccine.

RESERVOIR AND MODE OF TRANSMISSION:

Definitive hosts are marine mammals such as dolphins or seals. These pass the parasite's eggs in their feces. The eggs hatch and produce larvae which infect the first intermediate host, usually a crustacean. A fish may be the second intermediate host. Humans are aberrant hosts infected by eating fish.

INCUBATION PERIOD:

Humans and animals. A few hours to a few weeks.

CLINICAL FEATURES:

Humans. There may be fever, abdominal pain, vomiting, hematemesis, coughing, pseudoappendicitis, and possibly symptoms associated with intestinal perforation.
Animals. Fish fail to thrive if heavily infected.

PATHOLOGY:

Humans.  The larvae usually remain in the intestine causing few lesions. However, they sometimes invade the stomach wall causing hematemesis and may lodge in the mesenteric veins or in the viscera where they induce eosinophilic granulomas and abscesses. Larvae may migrate up the esophagus to the oropharynx. There is a low grade eosinophilia.
Animals.  In fish, atrophy of the liver occurs and sometimes fatal infection of the heart. Visceral adhesions and muscle damage can be severe.

DIAGNOSIS:

Humans. Stools may show occult blood. Mild leukocytosis and eosinophilia may be present. ELISA and RAST serologic tests may be tried but are not reliable in chronic disease. In acute infection, the larvae sometimes can be seen and removed endoscopically from the stomach. X-rays of the stomach may show a localized edematous, ulcerated area with an irregularly thickened wall, decreased peristalsis, and rigidity. Double contrast technique may show the threadlike larvae. Small bowel x-rays may show thickened mucosa and segments of stenosis with proximal dilation. Ultrasound examination of gastric and intestinal lesions may also be useful. In the chronic stage, x-rays and endoscopy of the stomach-but not of the bowels-may be helpful. The diagnosis is often made only at laparotomy with surgical removal of the parasite.
Animals. Demonstrate the parasite in tissues of fish.

PROGNOSIS:

Humans. The condition is rarely fatal.
Animals. Wide distribution of the parasites and consequent disease results in difficulty in maintaining marine vertebrates in laboratories.

PREVENTION:

Humans. Avoid raw or undercooked fish. Freezing fish kills larvae. Eviscerate fish immediately after catching.
Animals. Impractical.

TREATMENT:

Humans. Physical removal of larvae by gastroscopy or surgery.
Animals. Not appropriate.