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Kathi Smith, RVT
Definition: RMSF is a tick-transmitted rickettsial disease of dogs, and people. Untreated, this disease can be fatal. It is found on the eastcoast, Midwest and plains states. With the mortality rate in humans at approximately 5% each year, dogs may serve as a “sentinel species” for human. Understanding the epidemology, transmission, clinical disease, and treatment of canine RMSF may help with the human awareness of the disease in prevalent areas.
Etiologic Agent involved with RMSF is Rickettsia rickettsii and is the most pathogenic of the rickettsial species identified with the spotted fever group (SFG). The vectors include the American dog tick (Dermacentor variabilis) found in the eastern United States, and the wood tick (Dermacentor andersoni) found in areas from the Rocky Mountains to the Cascades. These are 3 host ticks; the permanent hosts are humans, dogs, and cats. The ticks can also serve as reservoirs and hosts although rodents are the primary reservoir. Ticks usually acquire the organism through feeding on infected animals, although vertical transmission can occur. Attachment to the host for a minimum of 5-20 hours is necessary for the tick to transmit infection. However, Iatrogenic transmission can also occur through contaminated blood transfusions.
Clinical Signs can be observed in natural and experimentally induced infections of RMSF and are classified by the subclinical and acute stages. In the subclinical stage, dogs are usually asymptomatic with possible mild laboratory abnormalities.
The acute stage can have presentation of clinical signs as early as 2-3 days after tick attachment and the incubation period can run from 2-14 days.
Clinical signs can include:
· Fever, anorexia, vomiting, diarrhea, generalized lymphadenopathy, myalgia, arthralgia · Cutaneous lesions affecting the scrotum, mammary glands, nose, lips, distal extremities · Ocular hemorrhagic lesions, including, subconjunctival hemorrhage, retinal petechiae, focal areas of retinal edema · Pulmonary edema may cause tachypnea, dyspnea, coughing, and exercise intolerance · Neurologic signs such as para/tetraparesis, vestibular disease, and seizures are associated with central nervous system edema · Cardiac arrhythmias due to myocarditis can result in sudden death · Permanent organ damage (renal, cardiovascular, neurological) and death from meningoencephalitis, shock, cardiovascular collapse and oliguria can occur in severely affected dogs.
Hematological and Biochemical Abnormalities typically associated with RMSF can include:
· Thromocytopenia, leukopenia or leukocytosis, anemia · Mild increase in serum glucose · Increased serum Alk Phos, ALT, AST, hypercholesterolemia · Hypoalbuminemia, hyponatremia, hypokalemia, hypochloremia, metabolic acidosis, azotemia
Diagnosis for RMSF is generally done through serologic testing. The microscopic immunofluorescence (Micro-IF), ELISA and latex agglutination (LA) tests are better suited to test canine sera. Serum titers however, usually do not rise until after 2-3 weeks post inoculation, which may be later than the onset of clinical signs. It is best to get paired serum samples from patients with acute disease.
· Serum titers > 1:64 = low + > 1:128 = + · Confirmation must be obtained with a four fold rise in titers from the acute and convalescent serum samples · High titers can persist for up to 1 year after successful treatment or recovery from the disease
Treatment for RMSF should be initiated immediately after obtaining samples for diagnostic testing, due to the fact that the diagnosis of RMSF is made long after the onset of clinical signs.
· Doxycycline 5 mg/kg PO q 12 hours for 14-21 days 5 mg/kg IV q 12 hours until definitive diagnosis is made for dogs with acute onset of neuro signs, pyrexia, and other systemic signs in areas endemic for RMSF · Tetracycline 22 mg/kg PO q 8 hours for 14-21 days (give on an empty stomach) · Enrofloxacin 3 mg/kg PO or SQ q 12 hours for 10-14 days · Supportive Care IV fluid therapy (used with caution due to possible exacerbation of pulmonary and cerebral edema)
Prognosis is good if the proper treatment is initiated immediately. Responses can occur within hours of appropriate antibiotic administration. The mortality rate is high if treatment has been delayed until the disease is in the advanced stages, ineffective antibiotics have been used or if presented with severe CNS signs.
Prevention is through avoidance of tick-infested areas, removal of ticks from dogs (gloves should be worn) and control with the use of sprays, spot-ons and collars. Extra care should be taken when placing IV catheters, obtaining blood samples, or performing laboratory testing, as transmission on RMSF is possible through contaminated blood. Zoonotic warning labels should be used on patient cages, laboratory samples and any other situation where possible human contact can occur.
Dog Trekker – Travel Tips