Salmon poisoning disease (Neorickettsia helminthoeca)

Signalment: 3-year-old male Labrador Retriever

History / Physical exam: 3 day history of lethargy, with profuse watery diarrhea just prior to presentation. The diarrhea contained frank blood.
On physical examination, the patient was mentally obtunded and was febrile (T=105°F). The patient was 5-7% dehydrated, with skin tenting and tacky mucous membranes. Generalized lymphadenomegaly was palpated, with lymph nodes approximately 3cm in diameter and firm.

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Microscopic description (cytology):  One slide from a lymph node is examined that is highly cellular, and has a clear background with a mild amount of blood, and occasional ruptured cells with bare nuclei and cytoplasmic fragments. Nucleated cells are distributed individually, comprising mostly of a heterogeneous population of lymphocytes, dominated by small mature lymphocytes and plasma cells, but with an expanded population of intermediate to large reactive variants. Also present is a population of reactive macrophages. These cells have abundant pale purple cytoplasm and ovoid nuclei that are eccentrically placed. Within the cytoplasm are numerous, blue/grey organisms seen individually and in aggregates. The organisms are curvilinear, and approximately 1-2μm in length. Similar organisms can be seen in the background of the slide.

Microscopic interpretation: Salmon poisoning disease (Neorickettsia helminthoeca)

Discussion: Salmon poisoning disease (SPD) is caused by the Rickettsial organism Neorickettsia helminthoeca. Infection occurs after ingestion of raw or undercooked fish, which contains cysts of the trematode Nanophyetes salmincola, which is the vector for N helminthoeca.1 The disease is seen in the Pacific Northwest of the US including Northern California, Oregon and Washington, as well as Southern British Columbia, mirroring the distribution of the aquatic snail Oxytrema silicula – an intermediate host of N. salmincola. A similar disease has also been reported in Brazil.2

  1. helminthoeca replicates in macrophages, and causes granulomatous inflammatory disease in the lymph nodes, spleen and gastrointestinal tract. Common clinical signs include vomiting and diarrhea (which may contain frank blood), fever, lymphadenomegaly, and splenomegaly.1

Diagnosis can be reached several ways. Rickettisal organisms may be seen within macrophages typically within lymphoid organs, both with cytology and histopathology. PCR testing may also be used; however, false negative results are possible.1 N. salminocola ova frequently are seen in the feces of infected patients, and while the presence of these ova does not confirm SPD, this is an important finding when interpreted with clinical signs and other diagnostic tests.

Prognosis for dogs with SPD is good with appropriate therapy; however, mortality can reach 90% in untreated patients.3

REFERENCES

  1. Sykes JE, Marks SL, Mapes S, et al. Salmon poisoning disease in dogs: 29 cases. J Vet Intern Med 2010;24(3):504-513.
  2. Headley SA, Kano FS, Scorpio DG, et al. Neorickettsia helminthoeca in Brazilian dogs: A cytopathological, histopathological, and immunohistochemical study. Clin Microbiol Infect 2009;15:21-23.
  3. Johns JL. Salmon poisoning in dogs: a satisfying diagnosis. Vet J 2011;187(2):149-150.

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