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The causative agent of RMSF in dogs is Rickettsia
rickettsii; while the agent causing canine ehrlichiosis is Ehrlichia canis.
Dogs in the United States have been reported to be naturally infected with both of these
organisms based on seropositivity data in surveys conducted in many states including
Connecticut, Kentucky, Maryland, Massachusetts, Mississippi, New York, North Carolina,
Ohio, Pennsylvania, and Virginia. Despite the endemicity of RMSF in people in the eastern
United States, a corresponding disease syndrome has not been well-defined in the closely
associated pet dog population. In fact, ehrlichiosis has been the primary rickettsial
disease that has been recognized in dogs in the United States.
Clinical signs including listlessness, depression, fever, and anorexia are among the most common clinical signs of both diseases. Generalized lymphadenopathy, neurologic abnormalities, abdominal or paralumbar hyperesthesia, diarrhea, and vomiting are found in both diseases, but more commonly in dogs with RMSF. Petechial or ecchymotic hemorrhages or other evidences of hemorrhagic diathesis have been observed in 18% and 22% of dogs with RMSF and ehrlichiosis, respectively. Certain features such as stiffness of gait, scleral congestion and polydipsia, and polyuria are only seen in dogs with RMSF. On the other hand, anterior uveitis, pale mucosa, and weight loss are peculiar to ehrlichiosis. Edema of the extremities has been noted in 21% and 33% of dogs with RMSF and ehrlichiosis, respectively.
Depression is a consistent neurologic finding in dogs with either disease. Other signs of neurologic involvement have been observed in approximately one third of dogs having either disease. Paraparesis or tetraparesis and ataxia reflecting upper motor neuron dysfunction are the most commonly observed findings. Acute central or peripheral vestibular dysfunction is another frequent neurologic disorder, as was generalized or localized hyperesthesia. Anisocoria and signs of cerebellar dysfunction are more common in ehrlichiosis. Seizures, cranial nerve deficits, and coma have been observed only in dogs with RMSF. Fundic lesions have been commonly found in dogs with either disease. Those dogs with RMSF tend to have focal retinal hemorrhage or chorioretinal exudate; those with ehrlichiosis often have more severe hemorrhage and exudation, with retinal detachment.
Response to treatment in most dogs with RMSF, treated with tetracycline (22 mg/kg, TID), is rapid. Most dogs without neurologic deficits improve within 24 to 48 hours. Chloramphenicol (15 mg/kg, TID) has been used in pups less than 12 weeks old because of the risk of staining their dental enamel with tetracycline. Dogs with neurologic deficits can have more prolonged recovery periods.
The response to treatment with tetracycline (22 mg/kg, TID) in dogs with ehrlichiosis has been more variable. Prolonged recovery periods have been noted in dogs with chronic ehrlichiosis and severe bone marrow damage that resulted in pancytopenia.
Further Reading Topics:
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