Salmon Poisoning Disease in Dogs

Salmon poisoning disease (SPD) is caused by the intracellular bacterium Neorickettsia helminthoeca. Dogs become infected by eating raw or undercooked fish, most commonly salmonids, that contain the trematode Nanophyetus salmincola, which is itself the host of N helminthoeca (see fish kidney image).

Salmon poisoning disease, kidney, fish

Image

Courtesy of Dr. Yasuko Rikihisa.

After being ingested by a dog, the encysted trematodes (metacercariae) mature and feed on the host's intestinal mucosa (see small intestine photomicrograph), infecting the dog with N helminthoeca in the process. Approximately 60–250 days after ingestion, the trematode's miracidia are shed in the dog's feces, which, after they mature, will infect the trematode's intermediate host, Oxytrema silicula (an aquatic snail). O silicula is commonly eaten by fish, completing the life cycle of the trematode.

Salmon poisoning disease, photomicrograph, small intestine, dog

Image

Courtesy of Dr. Yasuko Rikihisa.

When the fish is ingested by a dog, N helminthoeca will infect the host's macrophages (see N helminthoeca image), causing granulomatous inflammation of the intestinal tract, lymph nodes, and spleen. The marked GI signs of SPD are not due to trematode damage to the intestinal tract; rather they are due to the marked inflammation caused by N helminthoeca.

Neorickettsia helminthoeca, photomicrograph, dog

Image

Courtesy of Dr. Yasuko Rikihisa.

There are no age, sex, or breed predilections for SPD; however, the disease prevalence is higher when the availability of trematode-infected fish is greater. Infected fish are found in the Pacific Ocean from San Francisco to the coast of Alaska, but SPD is more prevalent from northern California to Puget Sound. It also occurs inland along the rivers of fish migration.

SPD also has been reported in southern California and Brazil.

Clinical signs of salmon poisoning disease appear suddenly, approximately 7 days after infected fish is ingested; however, signs can be delayed up to 33 days. Fever is noted in > 70% of dogs, but patients in decompensated shock can present with hypothermia.

Vomiting and diarrhea are reported in most SPD cases, and the presence of melena or frank blood in feces is common. Dehydration and extreme weight loss occur. Generalized lymphadenopathy develops in > 70% of cases. Nasal or conjunctival exudate might be present, mimicking signs of distemper.

Thrombocytopenia is the most common finding on CBCs; it is found in up to 90% of SPD cases. The severe loss of electrolytes and nutrients by the GI tract results in varying degrees of hypokalemia, hyponatremia, hypocalcemia, panhypoproteinemia, and hypocholesterolemia.

Lesions of Salmon Poisoning Disease

SPD appears to affect mainly the lymphoid tissues and intestines. There is enlargement of the GI lymph follicles, lymph nodes (see gross pathology image), tonsils, thymus, and, to some extent, spleen, with microscopic necrosis, hemorrhage, and hyperplasia. Marked abdominal or mesenteric lymphadenomegaly can be evident.

Salmon poisoning disease, gross pathology, dog

Image

Courtesy of Dr. Yasuko Rikihisa.

A variable but often severe nonhemorrhagic enteritis occurs throughout the small intestine in cases of SPD. Microscopic foci of necrosis also appear apart from the follicles. Nonsuppurative meningitis or meningoencephalitis has been identified in some dogs.

• PCR assay

• Fecal examination

• Cytology

When diarrhea, exudative conjunctivitis, and neurological signs occur in young, unvaccinated dogs, distemper should be considered as a diagnosis alternative to salmon poisoning disease. A combination of fecal flotation and sedimentation tests is the recommended approach to detecting Nanophyetus salmincola eggs in the feces of infected dogs, because this technique is highly sensitive. The presence of N salmincola eggs in a dog with compatible clinical signs is sufficient for SPD diagnosis.

N salmincola ova are oval, yellowish brown, rough-surfaced, and approximately 87–97 mcm × 35–55 mcm, with an indistinct operculum and a small, blunt point on the opposite end. If testing is done early in the disease (< 5 days after infection), trematode eggs might not be present in feces.

Cytological preparations of the lymph nodes or the spleen in dogs with SPD typically reveal histiocytic inflammation with intramacrophagic coccus- to coccobacillus-shaped organisms measuring 0.3 mcm in diameter.

PCR assays are commercially available for detection of Neorickettsia helminthoeca in blood, lymph node, or splenic samples. No commercial serological tests are currently available for the diagnosis of SPD.

• Tetracycline antimicrobials

Doxycycline (5 mg/kg, PO, every 12 hours) or tetracycline (22 mg/kg, PO, every 8 hours) for a minimum of 7 days is recommended as treatment for salmon poisoning disease. Clinical signs typically improve within 24 hours after antimicrobial therapy is initiated (1).

For SPD patients with severe vomiting, IV doxycycline (5–10 mg/kg every 12–24 hours), if available, can be administered. To help improve GI signs, as well as to limit shedding of trematode eggs, patients should also be treated with praziquantel (10–30 mg/kg, PO, every 24 hours for 2 days) (1).

For SPD patients with severe GI signs, supportive care involving aggressive fluid resuscitation and antiemetics is indicated.

Currently, the only means to prevent SPD is to restrict the ingestion of fish. Although animals that recover from SPD develop a robust antibody response, the protection appears to be strain specific. Consequently, repeated SPD episodes can occur in the same dog.

• Headley SA, Scorpio DG, Vidotto O, Dumler JS. Neorickettsia helminthoeca and salmon poisoning disease: a review. Vet J. 2011;187:165-173.

• Also see pet owner content regarding salmon poisoning disease in dogs.

1. Sykes JE. Salmon poisoning disease. In Sykes JE, ed. Canine and Feline Infectious Diseases. Elsevier; 2014:311-319.