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BBB - Nanophyetus spp.

Bad Bug Book:
Foodborne Pathogenic Microorganisms and Natural Toxins Handbook
Nanophyetus spp.

A new version of the Bad Bug Book was released in 2012, below is a previous version.


1. Name of the Organism:

Nanophyetus spp.

Nanophyetus salmincola or N. schikhobalowi are the names, respectively, of the North American and Russian troglotrematoid trematodes (or flukes). These are parasitic flatworms.

2. Nature of Acute Disease:

Nanophyetiasis is the name of the human disease caused by these flukes. At least one newspaper referred to the disease as "fish flu." N. salmincola is responsible for the transmission of Neorickettsia helminthoeca, which causes an illness in dogs that may be serious or even fatal.

3. Nature of Disease:

Knowledge of nanophyetiasis is limited. The first reported cases are characterized by an increase of bowel movements or diarrhea, usually accompanied by increased numbers of circulating eosinophils, abdominal discomfort and nausea. A few patients reported weight loss and fatigue, and some were asymptomatic. The rickettsia, though fatal to 80% of untreated dogs, is not known to infect humans.

4. Diagnosis of Human Illness:

Detection of operculate eggs of the characteristic size and shape in the feces is indicative of nanophyetiasis. The eggs are difficult to distinguish from those of Diphyllobothrium latum.

5. Associated Foods:

There have been no reported outbreaks of nanophyetiasis in North America; the only scientific reports are of 20 individual cases referred to in one Oregon clinic. A report in the popular press indicates that the frequency is significantly higher. It is significant that two cases occurred in New Orleans well outside the endemic area. In Russia's endemic area the infection rate is reported to be greater than 90% and the size of the endemic area is growing.

6. Relative Frequency of Disease:

Nanophyetiasis is transmitted by the larval stage (metacercaria) of a worm that encysts in the flesh of freshwater fishes. In anadromous fish, the parasite's cysts can survive the period spent at sea. Although the metacercaria encysts in many species of fish, North American cases were all associated with salmonids. Raw, underprocessed, and smoked salmon and steelhead were implicated in the cases to date.

7. Course of Disease and Complications:

Mebendazole was ineffective as a treatment; patients kept shedding eggs, and symptoms gradually decreased over 2 months or more. Treatment with two doses of bithionol or three doses of niclosamide resulted in the resolution of symptoms and disappearance of eggs in the feces. These drugs are available in the U.S. from the Centers for Disease Control's Parasitic Drug Service.

8. Target Populations:

Consumers of raw or underprocessed freshwater or anadromous fish, especially salmonids.

9. Food Analysis:

There are no tested methods for detection of Nanophyetus spp. in fishes. Candling with the aid of a dissecting microscope, or pepsin HCl digestion should detect heavily infected fish.

10. Selected Outbreaks:

For more information on recent outbreaks see the Morbidity and Mortality Weekly Reports from CDC.

11. Education and Background Resources:

Literature references can be found at the links below.

Loci index for genome Digenea (Fluke family)

Available from the GenBank Taxonomy database, which contains the names of all organisms that are represented in the genetic databases with at least one nucleotide or protein sequence.

12. Molecular Structural Data:

None currently available.

13. FDA Regulations or Activity:

FDA has no specific regulation or activity regarding these trematodes. As pathogens, however, they should not be live in fish consumed raw or semiraw.