Rocky Mountain Spotted Fever (RMSF) in dogs

People share many things with their dogs one of these is the illness Rocky Mountain Spotted Fever (RMSF). A tick borne fever, caused by the intracellular pathogen Rickettsia rickettsii, RMSF is endemic through much of the United States. While recognized as a human pathogen since the 1930’s R. rickettsii infections in dogs were not discovered until the 1970’s

Small rodents such as chipmunks form the reservoir of the disease. The hard ticks responsible for the transmission of RMSF to dogs and humans catch the infection, as larvae or nymphs, from biting these rodents. Transmission within a tick population occurs during mating and from pregnant ticks to their offspring (transovarial spread).

Members of the Dermacentor genus of hard tick are the prime vectors of RMSF with Dermacentor variabilis (American dog tick) responsible for the majority of cases in the eastern states and Dermacentor andersoni (wood tick) those in the western states. Other hard ticks, such as Amblyomma americanum and Rhipicephalus sanguineous, act as vectors for occasional cases of RMSF. Between one and three percent of ticks carry the infection. Most cases of RMSF occur between March and October as this is when ticks are most active.

The most common sign of RMSF in dogs is a fever of within the range of 102.6 - 104.9 F (39.2 – 40.5), usually appearing about five days after a tick bite. This incubation period may range from two to fourteen days. The rash from which the disease gets its name may not always be seen in infected dogs. Other signs in dogs include edema or swelling of extremities including the ears, pinpoint hemorrhages or petechiae on the mucosal surfaces. Dogs often develop ocular lesions with signs such as uveitis (inflammation of the middle layer of the eye) and retinal hemorrhages. Cases in which the dog exhibits joint swelling, myalgia (painful muscles), dyspnea (rapid breathing) or neurological signs such as ataxia (lack of coordination) or altered mental state tend to be more severe and have a poorer prognosis.

Laboratory findings from a blood sample taken from an infected dog include a low platelet count, abnormal white cell count and anemia. The biochemical analysis, of such a blood sample, reveals a raised glucose level and raised liver enzymes. Detecting antibodies to rickesttsial infection particularly the specific IgM confirms the diagnosis.

Treatment must be instigated immediately RMSF infection is suspected. Delaying treatment until the serological confirmation of infection can have tragic consequences, as one to ten percent of cases are fatal.

RMSF responds well to antibiotic treatment. The recommended antibiotics for use in dogs are doxycycline or tetracycline. As these are unsuitable for use in young puppies or pregnant bitches, chloramphenicol makes an effective alternative. The dosage of antibiotic used depends on the body weight of the dog.

Prevention of RMSF deepens on careful removal of ticks as soon as they are seen. Anti-tick collars are of use in endemic areas. Tick collars contain Amitraz a chemical extremely harmful to felines so must not be use on a pet cat.

Reference Sources:

University of Georgia College of Veterinary Medicine

Merck Veterinary Manual