Anisakiasis: causes, symptoms, diagnosis, and treatment

Anisakiasis is a zoonotic disease caused by third-stage larvae of Anisakis parasitizing the gastrointestinal tract. Humans are infected by ingestion of raw or undercooked fishes containing live larvae. The clinical manifestations in the acute stage are gastrointestinal symptoms such as severe abdominal pain, nausea, vomiting, and diarrhea, and eosinophilia; while the disease is characterized by gastric or intestinal eosinophilic granuloma in the chronic stage, and may be complicated by intestinal obstruction, intestinal perforation, and peritonitis.


Causes

The larvae of Anisakis are yellowish white, 1 - 3cm long, cylindrical. Male adults are 31 - 90mm long, and female adults are 63 - 100mm long. Adults live in the stomachs of marine mammals such as whales, dolphins, seals, and sea lions. The eggs are 50.7μm x 53μm in size, and are discharged into the sea through the feces of hosts. The eggs are hatched and the hatched larvae develop into freely living first-stage larvae. When the seawater temperature is comfortable (about 10℃), the first-stage larvae develop into 230μm long second-stage larvae. When the second-stage larvae are swallowed by crustaceans in the sea (first intermediate host) such as krill, the larvae penetrate the body cavity and develop into third-stage larvae in the blood cavity of the hosts. Third-stage larvae are slender, about 30mm x 1mm in size, with lateral funiculus, with Y-shaped transection, with 3 lips and one tooth. When fishes and mollusks (second intermediate host) ingest crustaceans containing larvae, the larvae penetrate the digestive tract and parasitize internal organs and muscles of the hosts. After the fishes containing third-stage larvae are swallowed by marine mammals (definitive host), the larvae penetrate the gastric mucosa, where they grow, develop into female and male adults, and mate and lay eggs, completing their life cycle.

Humans are not suitable hosts for Anisakis. The third-stage larvae can parasitize the human digestive tract, and can cause visceral larva migrans. However, the larvae cannot develop into adults in the human body, and generally die within 2 - 3 weeks. Anisakis has certain resistance to salt, alcohol, vinegar, and radiation, but its resistance to temperature is extremely weak. The larvae cannot survive for 2 hours at 60°C and -20°C.

The route of transmission is the oral ingestion of Anisakis larvae. The infection is caused by ingestion of raw or undercooked fishes and mollusks containing live Anisakis larvae.


Signs and Symptoms

The incubation period is generally 2 - 20 hours, mostly within 12 hours. The incubation period is 1 - 5 days in intestinal anisakiasis.

Anisakiasis can be divided into gastric anisakiasis, intestinal anisakiasis, and parenteral anisakiasis.

Gastric anisakiasis

The disease is the most common, and can be divided into acute and chronic gastric anisakiasis. Acute gastric anisakiasis occurs 4 - 6 hours after ingestion of the infected fish, and is mainly manifested by stomachache, nausea, and vomiting. Chronic gastric anisakiasis is mainly characterized by intractable epigastric pain, nausea, and vomiting, as well as increased eosinophils in the peripheral blood, and can last for several weeks or even two years. Larvae parasitizing gastric body and gastric angles account for more than 85%, and often cause gastric filling defects, thickened gastric folds, gastric deformation, and mild gastric mucosal bleeding and erosion. Third-stage larvae of Anisakis can be detected in gastroscopy.

Intestinal anisakiasis

The lesions include duodenum, jejunum, ileum, cecum, appendix, and rectum. The condition usually occurs 1 - 5 days after ingestion of raw or undercooked fish, and is manifested by severe lower abdomen pain, nausea, vomiting, abdominal distension, and low-grade fever, followed by diarrhea, tarry mucous stools, and right lower quadarant and periumbilical tenderness. Sometimes, there are urticaria, pale yellow ascites, and significantly increased white blood cells, but eosinophilia is less common. There may be intestinal perforation, peritonitis, or localized intestinal necrosis.

Parenteral anisakiasis

Anisakis larvae enter directly into the mucous membrane of the throat after ingestion of raw or undercooked fish, causing esophageal anisakiasis, manifested by itchy throat, nausea, cough, and expectoration. Patients may be coughing up larvae. White Anisakis can be found in the lower esophagus in fiber endoscopy, and the symptoms are relieved after the Anisakis are removed. Anisakis larvae can also migrate to the liver, pancreas, greater omentum, mesenterium, ovarium, subcutaneous abdominal wall, groins, and oral mucosa, causing peritonitis, eosinophilic granuloma, and subcutaneous masses, often erroneously diagnosed as malignant tumors.


Diagnosis

If there are a history of ingestion of raw or undercooked fish, clinical presentations, and Anisakis found in endoscopy, the disease can be diagnosed.


Treatment

The preferred treatment is to remove the larvae in endoscopy, and the medication is albendazole 400mg orally twice a day for 3 - 5 days.

Once complications such as intestinal perforation, peritonitis, or intestinal obstruction are found, immediate surgical treatment is required.