Salmon Poisoning Disease and Elokomin Fluke Fever
(Neorickettsia spp infection)
Salmon poisoning disease (SPD) is an acute, infectious disease of canids, in which the infective agent is transmitted through the various stages of a fluke in a snail-fish-dog life cycle. The name of the disease is misleading, because no toxin is involved. Elokomin fluke fever (EFF) is an acute infectious disease of canids, ferrets, bears, and raccoons that resembles SPD but has a wider host range. In people, Neorickettsia sennetsu causes a disease known as Sennetsu ehrlichiosis, and in horses N risticii causes a disease known as Potomac horse fever; these have not been reported as a cause of illness in dogs except for one report of N risticii in Illinois.
SPD is caused by N helminthoeca and is sometimes complicated by a second agent, N elokominica, which causes EFF. The vector for these Neorickettsia agents is a small fluke, Nanophyetus salmincola. Dogs and other animals become infected by ingesting trout, salmon, or Pacific giant salamanders that contain the encysted metacercaria stage of the rickettsia-infected fluke. In the dog’s intestine, the metacercarial flukes excyst, embed in the duodenal mucosa, become gravid adults, and transmit the rickettsiae to monocytes-macrophages. The fluke infection itself produces little or no clinical disease. A recent report of SPD in two captive Malayan sun bears underscores the need to consider this etiology in non-native exotic species with compatible exposure and clinical histories.
The life cycle of Neorickettsia helminthoeca is maintained by the release of infected fluke ova in the feces of the mammalian host. Infected miracidia develop from these ova and infect the snails Juga plicifera and Juga silicula to form infected rediae. Rediae develop into infected cercariae that are released from the snail, penetrate the salmon or trout, and develop into encysted metacercariae infected with Neorickettsia. The cycle is completed when mammals eat the fish, and infected metacercariae become infected gravid adults and pass Neorickettsia to fluke eggs. Although Neorickettsia infection of dogs is not required for the life cycle of Neorickettsia, mammalian infection is required to maintain the trematode life cycle. Transmission by cage-to-cage contact, rectal thermometers, or aerosols is rare.
There are no age, sex, or breed predilections; 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 is also seen inland along the rivers of fish migration. SPD also has been reported in southern California and Brazil. The snail is the primary factor for geographic limitation, but dogs fed undercooked or raw fish from the supermarket may have developed SPD.
In SPD, signs appear suddenly, usually 5–7 days after eating infected fish, but may be delayed as long as 33 days, and persist for 7–10 days before culminating in death in up to 90% of untreated animals. Body temperature peaks at 104°–107.6°F (40°–42°C) 1–2 days later, then gradually declines for 4–8 days and returns to normal. Frequently, animals are hypothermic before death. Fever is accompanied by depression and complete anorexia in virtually all cases. Persistent vomiting usually occurs by day 4 or 5. Vomiting occurs in most cases, and diarrhea, which develops by day 5–7, often contains blood and may be severe. Dehydration and extreme weight loss occur. When severe, the GI signs are clinically indistinguishable from those of canine parvoviral infection. Generalized lymphadenopathy develops in ~60% of cases. Nasal or conjunctival exudate may be present and mimic signs of distemper. Neutrophilia is common, but a marked, absolute leukopenia with a degenerative left shift may occur. Thrombocytopenia is reported in 94% of the cases. Serum chemistry values are normal.
Clinically, EFF is a milder infection than SPD. Severe GI signs are less commonly seen in EFF infections, and lymphadenopathy may be a more pronounced finding. Case fatality rates with EFF are lower, at ~10% of untreated cases.
Infection appears to chiefly affect the lymphoid tissues and intestines. There is enlargement of the GI lymph follicles, lymph nodes, tonsils, thymus, and to some extent, the spleen, with microscopic necrosis, hemorrhage, and hyperplasia. Remarkable abdominal or mesenteric lymphadenomegaly may be seen. A variable but often severe nonhemorrhagic enteritis is seen throughout the intestine with SPD but is less commonly seen with EFF. Microscopic foci of necrosis also appear apart from the follicles. Nonsuppurative meningitis or meningoencephalitis has been identified in some dogs.
Fluke ova are found on fecal examination in ~92% of cases, which supports the diagnosis. The ova are oval, yellowish brown, rough-surfaced, and ~87–97 × 35–55 μm, with an indistinct operculum and a small, blunt point on the opposite end. During the first day or two, few ova may be passed. Intracellular organisms have been demonstrated by Romanowsky staining on lymph node aspirates in ~70% of cases. PCR testing to detect DNA-specific N helminthoeca (or Neorickettsia genus) is recommended for accurate diagnosis. Serologic testing using the N helminthoeca organism has been developed. Other causes of fever of unknown origin, generalized lymphadenopathy, vomiting, and diarrhea are differential diagnoses. When diarrhea and exudative conjunctivitis occur, distemper should be considered.
Currently, the only means of prevention is to restrict the ingestion of uncooked salmon, trout, steelhead, and similar freshwater fish. In animals that recover, a profound humoral immune response persists, but there is no cross-resistance between N helminthoeca and N elokominica. Sulfonamides are not effective and may exacerbate the clinical disease. Recommended treatment is parenteral oxytetracycline (7 mg/kg, IV, tid for 5 days) or doxycycline (10 mg/kg, bid for 7 days). Oral tetracycline or doxycycline is contraindicated because of impinging GI signs. Animals usually succumb because of dehydration, electrolyte and acid-base imbalances, and anemia. Therefore, general supportive therapy to maintain hydration and acid-base balance, while meeting nutritional requirements and controlling diarrhea, is often essential. Judicious use of whole blood transfusions may be helpful.