Ultrasound Findings:  Adrenal glands may be slightly small for the patient’s size, the right measuring 0.59 cm in short axis and the left measuring 0.51 cm. The medial iliac lymph nodes are slightly large, the left measuring 0.6 cm in short axis and the right measuring 0.94 cm. Lymph nodes maintain normal echotexture and echogenicity. There is a mildly to moderately enlarged hypoechoic lymph node (1.3 cm short axis measurement) in the right cranial abdomen, bordered by hyperechoic fat. There are similar appearing enlarged jejunal lymph nodes measuring up to 1.3 cm in short axis. There is diarrhea and there is fluid occasionally within the small intestines.

Conclusion

1) Diarrhea, impression of enteritis.

2) Slightly small adrenal glands may be normal variation for the patient.

2) Mild-to-moderate lymphadenopathy.

Recommendations

1) Lymph node aspirates are indicated.

2) Fecal analysis is also warranted.

3) Consider assessment of baseline cortisol levels to rule out Addison’s disease.

Lymph Node Cytology Results:

INTERPRETATION: Lymph node: Mild to moderate reactive lymphoid hyperplasia with mild neutrophilic and moderate macrophagic inflammation with intralesional structures suggestive of rickettsial infection.

COMMENTS: The history and the cytological findings of macrophages with stippled to aggregated basophilic structures consistent with rickettsial origin, is consistent with Neorickettsia helminthoeca or Neorickettsia elokominica infection. Further confirmation can be achieved by PCR testing and fecal examination for characteristic trematode ova.

Discussion 

Salmon poisoning is a disease seen regionally and most commonly here in the Pacific Northwest. It develops when dogs ingest raw or partially-cooked fish or Pacific Giant Salamanders that contain a fluke.

The Nanophyetus salmincola fluke (also known as a trematode) contains a rickettsial organism called Neorickettsia helminthoeca. Two different intermediate hosts are needed to complete the trematode life cycle. Fluke eggs are released in the feces of the mammalian host.  These infect snails, which are the first intermediate hosts. Miracidia then develop in the snail and leave as free-swimming N. salmincola larvae (i.e. cercariae). Cercariae then penetrate a second intermediate host, which can include salmonid fish, some species of non-salmonid fish, and the Pacific giant salamander. Once the dog eats the raw fish, the larval flukes reach the dog’s intestinal tract where they release Neorickettsia helminthoeca which then systemically spread to the liver, lungs, brain, and lymphoid tissue.

The incubation period is typically 5-7 days from the time of infection, although delayed onset as long as 33 days has been reported. Clinical signs typically include fever, gastrointestinal (vomiting, diarrhea +/- blood), respiratory, and neurologic manifestations of disease. Lymphadenopathy also often occurs.

Diagnosis: Fecal flotation combined with clinical history is helpful to achieve a diagnosis (combination of flotation and sedimentation examinations provide the best chance of parasite isolation). Ova can be detected as early as 5-8 days after infection. However, it’s important to remember that the disease itself is caused by Neorickettsia helminthoeca, so identification of the Nanophyetus salmincola fluke alone does not prove the rickettsial infection. Lymph node cytology and histopathology may provide a more definitive diagnosis, if necessary. Intra-cytoplasmic rickettsial bodies can be seen in histiocytes or macrophages. Organisms appear as variably-sized, small, basophilic to dark granules.  Organisms may also be found in splenic aspirates.

Treatment: Treatment is aimed at treating the rickettsial infection and supportive care for clinical signs. This typically includes antibiotics, fluid therapy, antiemetics, antidiarrheals, and anticestodals.

Prognosis: as long as appropriate treatment is started promptly, most dogs respond well clinically and resolution of clinical signs can occur as early as 24-72 hours of starting therapy. However, if left untreated, death can occur 7-10 days after the onset of clinical signs. In one report, overall mortality rate was 14%. In another study of 43 dogs that were treated with antibiotics, 39 survived.

Preventive Measures: Dogs should not be allowed to eat raw or smoked-raw fish (salmon, trout, steelhead), or Pacific Giant Salamanders. Extra precautions should be taken during salmon spawning season in the fall (with mid-September to mid-October being peak season). Dogs that survive salmon poisoning will be immune to re-infection with the same strain. However, infection with an alternate strain can occur because there is no cross-protection.