Neorickettsiosis

Neorickettsiosis is a tick-borne, intracellular bacterial infection caused by organisms of the genus Neorickettsia, primarily Neorickettsia risticii (the agent of Potomac horse fever) and Neorickettsia helminthoeca (the agent of salmon poisoning disease in dogs). These obligate intracellular bacteria infect monocytes, macrophages, and endothelial cells, leading to systemic inflammatory responses. In dogs, the most common presentation is salmon poisoning disease, characterized by acute gastroenteritis, fever, and lymphadenopathy, typically following ingestion of raw salmon or trout infected with metacercariae containing the bacterium. The disease is endemic in the Pacific Northwest of the United States and parts of Canada. Neorickettsiosis can also affect horses, causing fever, colitis, and laminitis. The disease is treatable with tetracyclines, but without prompt therapy, mortality can be high.

The primary causative agents are Neorickettsia risticii (formerly Ehrlichia risticii) and Neorickettsia helminthoeca. These are gram-negative, obligate intracellular bacteria belonging to the family Anaplasmataceae. They are transmitted to dogs via ingestion of raw or undercooked salmonid fish (salmon, trout, steelhead) that harbor metacercariae of the fluke Nanophyetus salmincola. The fluke is the vector; the bacterium infects the fluke's tissues, and when the dog ingests the infected fish, the fluke releases the bacteria in the small intestine. The bacteria then invade intestinal epithelial cells, macrophages, and monocytes, spreading via the bloodstream to lymph nodes, spleen, liver, lungs, and other organs. In horses, Neorickettsia risticii is transmitted by ingestion of aquatic insects (caddisflies, mayflies) that carry infected metacercariae. The bacteria are shed in feces and can survive in the environment. Virulence factors include the ability to survive within phagocytes, evade immune responses, and induce pro-inflammatory cytokine release, leading to systemic inflammation and tissue damage.

Neorickettsiosis is most commonly reported in dogs in the Pacific Northwest of the United States (Washington, Oregon, northern California, Idaho) and British Columbia, Canada. The disease is seasonal, with peak incidence in late spring through early fall, correlating with salmon runs and increased outdoor activity. Dogs of any breed, age, or sex can be affected, but those with access to rivers and streams where salmon spawn are at higher risk. There is no breed or sex predisposition. In horses, Potomac horse fever is seen in the eastern United States, particularly in the Potomac River Valley, but also in other regions. The disease is sporadic and often associated with proximity to water sources. The incidence is low, but outbreaks can occur. The disease is not directly contagious between animals; transmission requires the intermediate host (fluke).

After ingestion of infected fish, the metacercariae excyst in the small intestine, releasing Neorickettsia organisms. The bacteria invade intestinal epithelial cells and underlying lymphoid tissue, then spread to regional lymph nodes and the bloodstream. They are phagocytosed by macrophages and monocytes, where they survive and replicate within phagosomes, avoiding lysosomal degradation. This intracellular survival leads to activation of the innate immune system, with release of pro-inflammatory cytokines (TNF-α, IL-1, IL-6) and chemokines, causing fever, systemic inflammation, and vascular endothelial damage. The bacteria also infect endothelial cells, leading to vasculitis and increased vascular permeability, contributing to edema and organ dysfunction. In the gastrointestinal tract, the infection causes severe enteritis, with necrosis of intestinal crypts, ulceration, and hemorrhage, leading to protein-losing enteropathy and malabsorption. Systemic dissemination results in lymphadenopathy, splenomegaly, hepatitis, nephritis, and pneumonitis. The immune response may also contribute to tissue damage via immune complex deposition and oxidative stress. In horses, the pathophysiology is similar, with colitis and endotoxemia being prominent features.

The primary predisposing factor is ingestion of raw or undercooked salmonid fish that are infected with Nanophyetus salmincola metacercariae. Dogs that are allowed to roam freely near rivers or streams during salmon spawning season are at highest risk. Hunting dogs, working dogs, and those with a history of eating raw fish are more likely to be exposed. There is no genetic predisposition, but immunosuppressed animals may be more susceptible to severe disease. Concurrent infections (e.g., other tick-borne diseases) may exacerbate clinical signs. Environmental factors such as flooding or changes in water temperature can affect the distribution of the fluke and increase exposure risk.

Clinical signs in dogs typically appear 5-7 days after ingestion of infected fish. The disease is often peracute to acute. Early signs include fever (often >103°F/39.4°C), lethargy, anorexia, and vomiting. Within 24-48 hours, diarrhea develops, which may be watery, mucoid, or hemorrhagic. Abdominal pain is common. Lymphadenopathy (especially mandibular, prescapular, and popliteal) is a hallmark finding. Other signs include dehydration, weight loss, and depression. In severe cases, septic shock, disseminated intravascular coagulation (DIC), and acute respiratory distress syndrome (ARDS) may develop. Chronic cases may show persistent diarrhea, weight loss, and poor condition. In horses, signs include fever, depression, anorexia, and acute colitis with profuse watery diarrhea, which can lead to severe dehydration, endotoxemia, and laminitis. Abortion may occur in pregnant mares.

Differential diagnoses for neorickettsiosis in dogs include: 1) Canine parvovirus infection (parvoviral enteritis) – presents with acute vomiting, hemorrhagic diarrhea, and leukopenia; diagnosis via fecal antigen test or PCR. 2) Canine distemper – respiratory and neurological signs, but can have gastrointestinal signs; diagnosis via PCR or serology. 3) Salmon poisoning disease must be differentiated from other causes of acute gastroenteritis such as dietary indiscretion, foreign body, or toxin exposure. 4) Other rickettsial diseases (e.g., Ehrlichiosis, Anaplasmosis) – may have similar systemic signs but are tick-borne and not associated with fish ingestion; diagnosis via serology or PCR. 5) Bacterial enteritis (e.g., Salmonella, Campylobacter) – culture or PCR of feces. 6) Inflammatory bowel disease – chronic, but can have acute flares; biopsy for definitive diagnosis. 7) Intestinal intussusception – imaging (ultrasound) can differentiate. 8) Pancreatitis – elevated pancreatic lipase, imaging. 9) Hemorrhagic gastroenteritis (HGE) – often in small breed dogs, no fever, and rapid recovery with supportive care. 10) Leptospirosis – renal and hepatic signs, but can have gastrointestinal signs; serology or PCR. In horses, differentials include other causes of acute colitis (e.g., salmonellosis, clostridiosis, antibiotic-associated colitis, sand enteropathy).

The diagnostic approach begins with a thorough history, especially recent exposure to raw fish or salmon. Physical examination may reveal fever, dehydration, and lymphadenopathy. Initial laboratory tests include a complete blood count (CBC), serum biochemistry, and urinalysis. Hematology often shows leukocytosis with a left shift, or leukopenia in severe cases, and thrombocytopenia. Biochemistry may reveal azotemia, elevated liver enzymes, and hypoalbuminemia. Fecal examination may reveal Nanophyetus salmincola eggs, but this is not definitive for neorickettsiosis. Definitive diagnosis is made by detection of Neorickettsia organisms in lymph node aspirates or buffy coat smears using cytology (morulae may be seen), or by PCR on blood, lymph node aspirates, or feces. Serology (IFA) can detect antibodies, but acute and convalescent titers are needed for confirmation. In horses, diagnosis is similar, with PCR on blood or feces, and serology. A presumptive diagnosis can be made based on clinical signs and history, and treatment should be initiated immediately if the disease is suspected, as delay can be fatal.

Hematology: In dogs, the CBC may show leukocytosis (neutrophilia with left shift) or leukopenia (in severe cases), thrombocytopenia, and mild anemia. In horses, leukopenia and neutropenia are common. Serum biochemistry: Common findings include azotemia (prerenal or renal), elevated liver enzymes (ALT, AST), hypoalbuminemia, and electrolyte imbalances (hyponatremia, hypokalemia, metabolic acidosis). In horses, severe dehydration and electrolyte loss due to diarrhea are prominent. Urinalysis: May show proteinuria, casts, and hematuria if renal involvement. Blood gas analysis: Metabolic acidosis due to diarrhea and lactic acidosis. Specific biomarkers: C-reactive protein (CRP) may be elevated. Serology: IFA titers for Neorickettsia spp. may be positive, but acute and convalescent titers are needed. PCR: Detection of Neorickettsia DNA in blood, lymph node aspirates, or feces is highly sensitive and specific. Cytology: Lymph node aspirates may show macrophages containing morulae (intracytoplasmic inclusions).

Radiography: Thoracic radiographs may show interstitial or alveolar patterns if pneumonia is present. Abdominal radiographs may reveal gas-filled loops of bowel, but are often unremarkable. Ultrasonography: Abdominal ultrasound may show thickened small intestinal walls, mesenteric lymphadenopathy, and free abdominal fluid. In horses, transabdominal ultrasound may reveal thickened large colon walls and increased peritoneal fluid. Computed Tomography (CT) and Magnetic Resonance Imaging (MRI) are not typically used for diagnosis but may be helpful in complicated cases to assess organ involvement. Endoscopy: Upper GI endoscopy may reveal erythema, erosions, or ulcerations in the stomach and duodenum. Echocardiography: Not typically indicated unless cardiac involvement is suspected.

Cytology: Fine needle aspirates of enlarged lymph nodes may reveal macrophages containing intracytoplasmic morulae (basophilic inclusions). Buffy coat smears may also show morulae in monocytes. Histopathology: On biopsy of the small intestine, there is severe necrotizing enteritis with crypt necrosis, ulceration, and infiltration of macrophages and neutrophils. Lymph nodes show reactive hyperplasia with histiocytic infiltration. The bacteria can be visualized with special stains (e.g., Giemsa, Steiner silver stain) or immunohistochemistry. In horses, colonic biopsy shows similar necrotizing colitis.

Treatment should be initiated immediately based on clinical suspicion. The drug of choice is doxycycline (5-10 mg/kg PO q12h for 7-14 days) or oxytetracycline (7-10 mg/kg IV q12h). Tetracycline (20 mg/kg PO q8h) can also be used. In severe cases, intravenous fluids are essential to correct dehydration and electrolyte imbalances. Colloids may be needed for hypoproteinemia. Anti-emetics (e.g., maropitant 1 mg/kg SC q24h) and gastroprotectants (e.g., famotidine 0.5-1 mg/kg IV/PO q12h) are indicated. For diarrhea, probiotics may be beneficial. In cases of septic shock, aggressive fluid resuscitation and vasopressors (e.g., norepinephrine CRI) may be required. In horses, treatment includes doxycycline (10 mg/kg PO q12h) or oxytetracycline (6.6 mg/kg IV q12h), along with fluid therapy, anti-inflammatory drugs (e.g., flunixin meglumine 1.1 mg/kg IV q12h), and supportive care for laminitis.

With prompt and appropriate antibiotic therapy, the prognosis is good, with recovery rates exceeding 90% in dogs. However, if treatment is delayed, mortality can be high (up to 90% in untreated cases). In horses, the prognosis is guarded, with mortality rates of 10-30% even with treatment. Negative prognostic indicators include severe dehydration, hypoproteinemia, endotoxemia, and laminitis. Early diagnosis and treatment are critical for a favorable outcome.

After initiation of treatment, patients should be monitored closely for clinical improvement. Recheck examinations should be performed at 7-14 days after starting antibiotics to assess response. Serial blood work (CBC, biochemistry) should be repeated to monitor for resolution of abnormalities. PCR testing can be repeated to confirm clearance of the organism, but this is not always necessary if clinical signs resolve. In horses, monitoring for laminitis is essential, and hoof care may be needed. Long-term follow-up is generally not required if the patient recovers fully, but owners should be advised to prevent future exposure to raw fish.

Pearls: 1) Always ask about recent exposure to raw fish in dogs presenting with acute fever, vomiting, and diarrhea, especially in endemic areas. 2) Lymphadenopathy is a key clinical finding that should raise suspicion for neorickettsiosis. 3) Doxycycline is the treatment of choice and should be started immediately if the disease is suspected, even before diagnostic confirmation. 4) Cytology of lymph node aspirates can provide a rapid diagnosis if morulae are seen. Pitfalls: 1) Failure to consider the disease in non-endemic areas can lead to misdiagnosis. 2) Delaying treatment while waiting for diagnostic test results can be fatal. 3) Using inappropriate antibiotics (e.g., penicillins) will not be effective. 4) Overlooking concurrent infections or complications such as DIC or sepsis.

Based on Plumb's Veterinary Drug Handbook: Doxycycline: Dogs: 5-10 mg/kg PO q12h for 7-14 days. Horses: 10 mg/kg PO q12h. Oxytetracycline: Dogs: 7-10 mg/kg IV q12h. Horses: 6.6 mg/kg IV q12h. Tetracycline: Dogs: 20 mg/kg PO q8h. Supportive care: IV fluids (e.g., Lactated Ringer's solution) at rates to correct dehydration (e.g., 60-90 ml/kg/day in dogs, adjust based on losses). Anti-emetics: Maropitant (Cerenia) 1 mg/kg SC q24h. Gastroprotectants: Famotidine 0.5-1 mg/kg IV/PO q12h. In cases of septic shock, consider vasopressors (e.g., norepinephrine 0.05-0.5 mcg/kg/min CRI). For horses, flunixin meglumine 1.1 mg/kg IV q12h for anti-inflammatory effects. Note: Tetracyclines should be used with caution in young animals due to potential tooth discoloration. Doxycycline is preferred in renal impairment.

Key studies: 1) A study by Madigan et al. (1995) demonstrated the efficacy of doxycycline in treating Potomac horse fever. 2) A retrospective study by Sykes et al. (2010) on salmon poisoning disease in dogs showed a high recovery rate with tetracycline therapy. 3) ACVIM consensus guidelines on rickettsial diseases (2019) recommend doxycycline as first-line therapy. 4) Research by Pusterla et al. (2003) on the pathogenesis of Neorickettsia risticii in horses highlighted the role of endothelial cell infection. 5) A study by Greig et al. (1996) evaluated the use of PCR for diagnosis of neorickettsiosis. These studies support the current diagnostic and therapeutic approaches.