Salmon poisoning disease (SPD) is caused by a rickettsial organism, Neorickettsia helminthoeca, that is carried by the trematode Nanophyetus salmincola, which encysts in freshwater fish, most commonly salmonids. We reported two dogs from the United States West Coast that had similar clinical signs, hematologic and biochemistry findings. They were both diagnosed with salmon poisoning disease. Lymph node cytology showed morula formation, suggestive of N. helminthoeca organisms in macrophages, while the parasitological fecal test found ova of N. salmincola. The dogs were treated early and showed complete remission of clinical signs within a few days. Lymph node cytology and fecal parasitology are quick and low-cost tests that can be performed whenever SPD is suspected. SPD should be considered as a differential diagnosis for a canine patient with clinical signs of vomiting, diarrhea, lethargy, and lymphadenomegaly; laboratory findings of thrombocytopenia and hypoalbuminemia; and potential exposure to raw fish from the West Coast of the US or Southern Brazil. The earlier the diagnosis and treatment, the greater the chance of survival.
Keywords: Neorickettsia helminthoeca, Nanophyetus salmincola, lymph node cytology, Rickettsia, morula
Salmon poisoning disease (SPD) has been reported along the western coast of North America, from northern California through Washington and Vancouver Island (Pastenkos et al., 2020). Neorickettsia helminthoeca was also diagnosed in dogs from Southern Brazil due to pathological findings comparable to those of SPD, this being the first description of this disease occurring outside the conventional area of N. helminthoeca (Headley et al., 2006). Definitive diagnosis of SPD in these Brazilian dogs was by molecular (Headley et al., 2006) and immunohistochemical diagnostic methods (Headley et al., 2009; Headley et al., 2020).
SPD is caused by a rickettsial organism, N. helminthoeca, that belongs to the family Anaplasmataceae. Neorickettsia helminthoeca is carried within all stages of the trematode Nanophyetus salmincola, which encysts in freshwater fish, most commonly salmonids (Diniz, 2020). The Pacific giant salamander is also a known intermediate host. Most dogs become infected after ingestion of undercooked freshwater fish (Headley et al., 2011). However, dogs can also be infected by swimming in lakes and rivers with cercaria contamination (Diniz, 2020). An incubation period of 5-7 days follows the ingestion of parasitized salmon (Headley et al., 2011). After ingestion of the trematode-infested fish by a definitive host, the flukes mature and adhere to the intestine's mucosa, where the rickettsiae enter the epithelial cells of the intestinal villi (Greiman et al., 2016). The rickettsiae then disseminate via the lymphatic system, resulting in characteristic clinical signs of swollen lymph nodes, depression, anorexia, weight loss, dehydration, fever, vomiting, and diarrhea (Headley et al., 2011). N. helminthoeca replicates within macrophages. This results in the granulomatous inflammatory response within the stomach, lymph nodes, intestines, and spleen (Diniz, 2020).
Currently, serologic assays forN. helminthoecaare not available from commercial diagnostic laboratories (Diniz, 2020). Polymerase chain reaction (PCR) assays are available for the specific detection ofN. helminthoecaDNA (Greiman et al., 2016). Due to the high mortality of SPF, clinicians should not wait for a PCR confirmation before starting appropriate antibiotic therapy (Sykes et al., 2010). Therefore, a definitive and quicker diagnosis of SPD usually is obtained via either cytologic identification of rickettsiae in lymph node aspirates or identification of trematode ova in feces (Headley et al., 2011; Johns et al., 2006; Pastenkos et al., 2020).
Survival is directly related to early diagnosis and appropriate antibiotic therapy and support care. If left untreated, the illness progresses quickly, which can cause severe hypotension, mucosal pallor, cardiac arrhythmias, and death. Most untreated animals die within 6-10 days of the onset of clinical signs. In contrast to other pathogens in the family Anaplasmataceae, infection withN. helminthoecagenerates protective immunity against the same strain; however, alternate strains can still cause illness (Diniz, 2020; Headley et al., 2011; Sykes et al., 2010).
The aim of this study is to report clinical and laboratory changes in two dogs diagnosed, via lymph node cytology and fecal parasitological examination, with SPD. We present two successful cases, in which patients were diagnosed and treated early.
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https://pmc.ncbi.nlm.nih.gov/articles/PMC9718139/