Infectious Disease and Microbiology - Anisakis Species

Basics

Anisakis is a genus of parasitic nematodes, or roundworms, that can infect humans after ingestion of raw or inadequately prepared marine seafood. Human infection is called anisakiasis.

Several Anisakis species can cause disease. Humans are accidental hosts, and the larvae do not usually mature into adult worms in the human gastrointestinal tract. Instead, disease results from larval penetration of the stomach or intestinal wall and the associated inflammatory response.

Microbiologic Characteristics

Anisakis species are nematode helminths.

The infective form for humans is the larval stage, which may be present in marine fish and cephalopods.

The larvae are usually several centimeters long and may be directly visualized during endoscopy.

Transmission

Humans acquire anisakiasis by eating:

• Raw saltwater fish

• Undercooked saltwater fish

• Raw squid

• Raw or inadequately treated octopus

• Smoked seafood that has not been adequately processed

• Marinated or salted seafood in which viable larvae remain

The important principle is that preservation methods such as salting, smoking, or marinating do not always reliably kill the larvae.

Epidemiology

Anisakiasis is particularly associated with regions where consumption of raw or lightly prepared seafood is common.

It has been reported frequently in:

• Japan

• Scandinavian countries

• The Netherlands

• Pacific coastal regions of Latin America

The number of recognized cases has also increased in the United States and Europe as raw seafood dishes have become more widely consumed.

Incubation Period

The timing of symptoms depends on the site of infection.

Gastric anisakiasis

Symptoms may begin within a few hours after ingestion of infected seafood.

Intestinal anisakiasis

Symptoms involving the small or large intestine may develop more slowly, usually over:

Several days to weeks

This difference in timing is clinically useful.

Gastric Anisakiasis

Clinical Presentation

Gastric anisakiasis occurs when an infective larva penetrates the gastric mucosa.

Typical symptoms include:

• Sudden or severe epigastric abdominal pain

• Nausea

• Vomiting

• Abdominal discomfort

Symptoms often begin relatively soon after eating raw or inadequately cooked seafood.

High-Yield Pattern

Raw seafood ingestion

• ●

Severe upper abdominal pain within hours

• ●

Nausea or vomiting

→ Think gastric anisakiasis.

Intestinal Anisakiasis

If the larva passes beyond the stomach and penetrates the intestinal wall, patients may develop intestinal disease.

Possible manifestations include:

• Lower abdominal pain

• Nausea

• Abdominal distention

• Localized tenderness

• Signs of bowel obstruction

• Inflammatory mass-like lesions

Because symptoms may develop days after the original meal, the connection to raw seafood exposure can easily be missed.

Intestinal Obstruction

Inflammation and edema around an embedded larva may narrow the intestinal lumen and produce partial or complete obstruction.

Clinical findings may include:

• Cramping abdominal pain

• Vomiting

• Abdominal distention

• Failure to pass stool or flatus

Severe cases may resemble appendicitis, Crohn disease, or other surgical abdominal disorders.

Oropharyngeal Disease

Occasionally, larvae may attach to the oropharynx shortly after contaminated seafood is eaten.

Patients may experience:

• Throat irritation

• Foreign-body sensation

• Cough

• Nausea

• Occasionally visible larval movement

Direct removal can be curative.

Allergic Manifestations

Anisakis can also provoke significant hypersensitivity reactions.

Some patients develop:

• Urticaria

• Angioedema

• Wheezing

• Anaphylaxis

These allergic manifestations may occur with or without obvious gastrointestinal invasion.

Therefore, sudden allergic symptoms after consumption of raw marine fish should raise consideration of Anisakis exposure.

Diagnosis

Diagnosis is based on a combination of:

• Recent ingestion of raw or inadequately treated marine seafood

• Compatible gastrointestinal symptoms

• Direct visualization of the larva

The most definitive finding is identification of an Anisakis larva attached to or penetrating gastrointestinal tissue.

Endoscopy

Upper gastrointestinal endoscopy is particularly valuable in gastric anisakiasis.

The larva may be seen:

• Attached to the gastric mucosa

• Partially penetrating the gastric wall

• Moving within the stomach

Direct visualization often establishes the diagnosis immediately.

Parasite Appearance

The larva is typically approximately a few centimeters in length.

The original description notes a larva of about 2 cm invading the oropharynx or stomach.

When visible, the organism can often be removed directly during endoscopy.

Laboratory Findings

Routine laboratory abnormalities are not always present.

Possible findings include:

• Peripheral eosinophilia

• Mild leukocytosis

• Elevated inflammatory markers

Eosinophilia may be absent early and therefore should not be required for diagnosis.

Imaging

In intestinal disease, CT imaging may show:

• Bowel-wall thickening

• Localized edema

• Inflammatory changes

• Ascites

• Evidence of obstruction

These findings are nonspecific but may support the diagnosis in a patient with appropriate dietary exposure.

Differential Diagnosis

Gastric anisakiasis may resemble:

• Peptic ulcer disease

• Gastritis

• Pancreatitis

• Gallbladder disease

• Food poisoning

Intestinal anisakiasis may resemble:

• Appendicitis

• Small-bowel obstruction

• Crohn disease

• Diverticulitis

• Intestinal tumor

• Other parasitic infections

A careful dietary history is therefore crucial.

Treatment

Endoscopic Removal

For gastric anisakiasis, the preferred treatment is:

Endoscopic removal of the larva

Removal usually produces rapid symptomatic improvement because the main cause of inflammation is eliminated.

Surgical Treatment

Surgical intervention may be necessary when patients develop:

• Intestinal obstruction

• Perforation

• Severe inflammatory mass

• Persistent disease not amenable to endoscopic treatment

Surgery allows both removal of the affected tissue and definitive diagnosis.

Albendazole

Albendazole has been used in some cases when endoscopic or surgical removal is not possible.

There may be clinical benefit, but the evidence is less robust than for mechanical removal.

Therefore, treatment should be individualized according to the location and severity of disease.

Supportive Care

Supportive treatment may include:

• Analgesics

• Antiemetics

• Intravenous or oral fluids

• Bowel rest in selected intestinal cases

• Management of obstruction if present

If an allergic reaction occurs, treatment should follow standard management for urticaria, angioedema, or anaphylaxis.

Prevention

Prevention depends on killing larvae before seafood is eaten.

Important measures include:

• Thorough cooking of marine fish and squid

• Proper commercial freezing procedures

• Avoiding raw or inadequately frozen seafood

• Recognizing that marinating, salting, or smoking alone may not reliably eliminate viable larvae

Safe food preparation is the most effective preventive strategy.

High-Yield Gastric Pattern

Raw fish or squid

• ●

Severe epigastric pain within hours

• ●

Larva seen on endoscopy

→ Gastric anisakiasis

High-Yield Intestinal Pattern

Raw seafood exposure

• ●

Delayed lower abdominal pain

• ●

Inflammatory bowel thickening or obstruction

→ Intestinal anisakiasis

High-Yield Allergy Pattern

Raw marine seafood

• ●

Urticaria, angioedema, or anaphylaxis

± abdominal symptoms

→ Consider Anisakis-associated hypersensitivity.

Exam Essentials

Organism:

→ Nematode helminth

Genus:

→ Anisakis

Major source:

→ Raw or inadequately prepared saltwater fish, squid, or octopus

Important geographic associations:

→ Japan, Scandinavia, the Netherlands, and Pacific Latin America

Gastric incubation:

→ A few hours

Intestinal symptoms:

→ Days to weeks

Gastric disease:

→ Epigastric pain, nausea, vomiting

Intestinal disease:

→ Lower abdominal pain and possible obstruction

Classic diagnosis:

→ Direct visualization of larva

Best treatment for gastric disease:

→ Endoscopic removal

Complicated intestinal disease:

→ Surgical treatment may be required

Possible medical therapy:

→ Albendazole

Important extra manifestation:

→ Allergic reactions, including anaphylaxis

Key distinction

Rapid upper abdominal symptoms after raw seafood → gastric anisakiasis

Delayed lower abdominal symptoms or obstruction → intestinal anisakiasis