D415 - Rickettsia rickettsii

Description:

Rocky Mountain spotted fever (RMSF) is a tickborne, rickettsial disease that affects dogs and people. Deciduous forests, increased humidity, and warmer temperatures are factors associated with the high prevalence of this tick-transmitted disease in these areas. Rickettsia rickettsii is an obligate intracellular parasite from the family Rickettsiaceae, and is responsible for RMSF. Rickettsia rickettsii can enter the body after a bite from an infected tick. Rickettsiae are disseminated via the circulatory system and invade and replicate in endothelial cells of smaller arteries and venules. Both clinical and subclinical illnesses have been reported in dogs with naturally occurring and experimentally induced RMSF. Naturally acquired immunity has an important role in protection against clinical illness. Immunogenic contact with Rickettsia rickettsii induces a protective response in experimental dogs to re-infection for up to 3 years. These factors may explain the propensity of infection in dogs younger than 2 years. Clinically, infected dogs commonly display fever within 2 to 3 days following tick attachment. Early cutaneous lesions in some dogs consist of edema and hyperemia of the lips, penile sheath, scrotum, pinna, other extremities, and rarely the ventral abdomen. Infected dogs may make a rapid complete recovery if they are mildly affected or if anti-microbial therapy is instituted early. Permanent, organ damage, particularly resulting in neurological dysfunction may occur within 1 to 2 weeks of onset of clinical signs in severely affected dogs that survive the acute stages of illness. Dogs may die in the acute stages of illness as a result of hemorrhagic diathesis or from thrombosis of vital organs. Cardiovascular, neurologic, and renal damage are the most consistent causes of death or permanent organ dysfunction.

Diagnosis:

Although Rickettsia rickettsii can be inoculated into a tissue culture and grown for identification purposes, this method is rarely used because of its hazards to laboratory personnel. Serological methods of detection have also been used in the past, however, anti-rechettsial antibodies can be found in healthy canine populations of endemic areas. This observation may be a consequence of subclinical infections or the result of exposure to other non-pathogenic Rickettsia species. Therefore, any antibody reactivity to Rickettsia rickettsii should also be considered group-reactive for other spotted fever group members, which are likely not involved in disease pathogenesis because Rickettsia richettsii is the only group member in the western hemisphere known to infect people and dogs. A new PCR-based test for Rickettsia rickettsii has made it possible to detect DNA from low numbers of rickettsiae in whole blood or tissue specimens and for comparison of isolates. This PCR assay therefore allows for detection of clinical or subclinical Rickettsia rickettsii infection. Amplification of a unique region from Rickettsia rickettsii’s genome facilitates accurate and specific, acute-phase diagnosis of RMSF.

Treatment:

The antibiotics used to treat RMSF include tetracycline, chloramphenicol, doxycycline and enrofloxacin. Doses for tetracycline are 22-30 mg/kg every 8 hours for 7 days. Chloramphenicol treatment requires 15-30 mg/kg every 8 hours for 7 days. Doxycycline treatment requires 10-20 mg/kg every 12 hours for 7 days, while enrofloxacin treatment requires 3 mg/kg every 12 hours for 7 days (Greene C. – Infectious diseases of the dog and cat. 1998, p 160-161).

Sample:

1. Whole blood (3 ml) in a lavender (EDTA) top tube.
2. Ocular swabs.

Special Handling:

Store samples at 4°C until pick up or shipment.

Test Code:

D415