Infectious Disease and Microbiology – Nanophyetus salmincola

Overview

Nanophyetus salmincola is a small intestinal trematode (fluke) that causes nanophyetiasis, a foodborne parasitic infection acquired by eating raw or inadequately cooked infected fish, particularly salmonids.

Human infections have been reported primarily from the Pacific Northwest of the United States. Infection involves the gastrointestinal tract and is often mild, although patients may develop nonspecific gastrointestinal symptoms.

Classification

Genus: Nanophyetus

Species: N. salmincola

Type: Trematode helminth (fluke)

Disease: Nanophyetiasis

Microbiologic Characteristics

N. salmincola is a:

• Trematode

• Foodborne intestinal parasite

• Small fluke that develops in the gastrointestinal tract after ingestion of infective larvae

The infective stage for humans is the:

Metacercaria

which is present in infected fish.

Epidemiology

Human infection with N. salmincola has been reported particularly from:

Northwestern United States

The parasite is associated with freshwater and anadromous fish, especially:

Salmonids

Transmission

Humans acquire infection by consuming:

Raw or inadequately cooked infected fish

The source particularly emphasizes:

• Raw salmon

• Incompletely cooked salmon

• Smoked salmon that has not been adequately processed to kill the parasite

Life Cycle

The life cycle involves aquatic intermediate hosts.

Eggs reach freshwater

↓

Freshwater snail serves as first intermediate host

↓

Larval development

↓

Cercariae leave snail

↓

Fish becomes second intermediate host

↓

Metacercariae develop in fish tissues

↓

Human eats raw or undercooked infected fish

↓

Adult flukes develop in intestine

↓

Eggs are passed in stool

High-Yield Transmission Pattern

Pacific Northwest

• ●

Raw or undercooked salmon

• ●

Intestinal fluke infection

→ Think Nanophyetus salmincola

Nanophyetiasis

Human infection is called:

Nanophyetiasis

The adult parasites inhabit the:

Small intestine

and may produce gastrointestinal symptoms.

Clinical Manifestations

Symptoms are generally nonspecific and may include:

• Abdominal discomfort

• Abdominal pain

• Diarrhea

• Nausea

• Other gastrointestinal complaints

The severity of symptoms may depend partly on the parasite burden.

Asymptomatic Infection

Some infections may produce:

Few or no symptoms

Therefore, diagnosis may occasionally occur after eggs are identified during stool examination rather than because of a distinctive clinical syndrome.

Important Salmon Association

The most useful epidemiologic clue is:

Consumption of raw or inadequately cooked salmon or related fish

This exposure is particularly important in patients from or traveling through the:

Pacific Northwest

Diagnosis

Diagnosis is made by:

Parasitologic examination of stool specimens

The characteristic finding is:

N. salmincola eggs

Egg Morphology

The source describes eggs measuring approximately:

64–97 μm long × 43–55 μm wide

Recognition of the eggs in stool establishes the parasitologic diagnosis.

Diagnostic Pattern

Nonspecific gastrointestinal symptoms

• ●

Raw salmon exposure

• ●

Pacific Northwest

• ●

Trematode eggs in stool

→ Nanophyetus salmincola

Treatment

The source recommends:

Praziquantel

at a total dose of:

60 mg/kg/day

divided into:

3 doses for 1 day

Praziquantel is therefore the principal antiparasitic treatment described for human nanophyetiasis.

Treatment Principle

Confirmed intestinal N. salmincola infection

↓

Praziquantel

↓

Eradication of intestinal trematodes

↓

Resolution of gastrointestinal manifestations

Prevention

The primary preventive measure is:

Avoid eating raw or inadequately cooked infected fish

particularly salmon and related species from endemic areas.

Food Safety

Risk can be reduced through:

• Thorough cooking of salmon

• Avoiding raw infected fish

• Avoiding inadequately processed fish

• Recognizing that some traditional smoking processes may not reliably eliminate parasites

Nanophyetus vs. Heterophyes vs. Metagonimus

Nanophyetus salmincola

→ Intestinal trematode

→ Raw salmon/fish

→ Pacific Northwest association

→ Nonspecific gastrointestinal disease

Heterophyes heterophyes

→ Minute intestinal fluke

→ Raw/undercooked fish

→ Middle East and parts of Asia

→ Usually mild intestinal disease

Metagonimus yokogawai

→ Small intestinal fluke

→ Raw freshwater fish

→ Primarily East Asian distribution

→ Diarrhea and abdominal discomfort

All three are foodborne intestinal trematodes, but geographic and fish-exposure histories help distinguish them.

Important Veterinary Distinction

N. salmincola has an additional well-known association in veterinary medicine: the fluke can carry a bacterial pathogen responsible for salmon poisoning disease in dogs.

This veterinary syndrome should not be confused with ordinary human nanophyetiasis.

For human infection, the primary concern is the:

Intestinal trematode itself

High-Yield Clinical Pattern

Patient from the Pacific Northwest

• ●

Raw, undercooked, or inadequately smoked salmon

• ●

Nonspecific gastrointestinal symptoms

• ●

Trematode eggs in stool

→ Think Nanophyetus salmincola

→ Nanophyetiasis

Exam Essentials

Organism: Nanophyetus salmincola

Type: Trematode helminth

Disease: Nanophyetiasis

Major geographic association: Pacific Northwest United States

Transmission: Eating raw or inadequately cooked infected fish

Classic food association: Salmon

Infective stage: Metacercaria in fish

First intermediate host: Freshwater snail

Second intermediate host: Fish

Site of human infection: Small intestine

Clinical disease: Nonspecific gastrointestinal symptoms

Diagnosis: Eggs in stool

Egg size in source: 64–97 × 43–55 μm

Treatment: Praziquantel 60 mg/kg/day divided into 3 doses for 1 day

Prevention: Thoroughly cook fish and avoid raw or inadequately processed salmon

Key clinical pearl: Nanophyetus salmincola is a foodborne intestinal trematode classically associated with the Pacific Northwest and consumption of raw or inadequately cooked salmon. Patients typically have nonspecific gastrointestinal symptoms, diagnosis is made by finding eggs in stool, and praziquantel is the treatment of choice.