Rocky Mountain spotted fever in dogs

Rocky Mountain spotted fever in dogs

Rocky Mountain spotted fever (RMSF) is a rickettsial disease that can infect both dogs and humans. This disease is spread by ticks and caused by a small gram-negative obligate intracellular parasite named Rickettsia rickettsii. In many cases, the dog will be healthy again within 2 weeks, but seeking veterinary care is recommended if you suspect that your dog suffers from Rocky Mountain spotted fever because severe RMSF can be deadly if left untreated.

Rocky Mountain spotted fever transmission to dogs

Rocky Mountain spotted fever is transmitted to dogs via a number of hard-bodied ticks. In the Eastern United states, the most common spreader is the American dog tick (Dermacentor variabilis). In eastern United States, the culprit is usually the Wood tick (Dermacentor andersoni). A majority of the Rocky Mountain spotted fever cases in dogs are caused by ticks from the genus Dermacentor, but Amblyomma americanum and Rhipicephalus sanguineous have also been shown to carry Rickettsia rickettsii.

Rocky Mountain spotted fever symptoms

When a dog has been infected with Rocky Mountain spotted fever, the Rickettsia rickettsii organism will undergo an incubation period which can vary in time form just 2 days to over 2 weeks. After the incubation period, the organism will enter the circulatory system of the dog, and proceed to invade the endothelial cells of the venules and capillaries. In these parts of the dog's body, the organisms can start multiplying rapidly. Eventually, this can lead to vasculitis, oedema, vascular collapse, shock and haemorrhage. Brain, heart, kidneys and skin are usually the most damaged parts of the dog's body. 

Common RMSF symptoms in dogs:
Fever in the 102.6-104.9º F range (typically 5 days after being bitten by a tick)
Petechiae and ecchymotic hemorrhages on exposed mucosal surfaces in the dog
Vasculitis that leads to oedema in the extremities of the dog, e.g. swollen ears, prepuce and scrotum

Not so common RMSF symptoms in dogs:
A red rash (typically 12 days after being bitten by a tick)
Joint swelling
Meningoencephalitis that causes neurological symptoms, e.g. vestibular ataxia     

When can I suspect RMSF?

Rocky Mountain spotted fever is most common during the period of the year when ticks are active, which means March to October for most parts of the United States. A majority of the infected dogs are less than 3 years old and have been living in or visiting habitats where ticks can be found.

Rocky Mountain spotted fever prevention

In most cases, the tick must stay attached to the dog for 5-20 hours in order to transmit Rocky Mountain spotted fever. Checking your dog for ticks several times a day and removing ticks as soon as you notice them is therefore a good habit in order to prevent Rocky Mountain spotted fever.

Topical agents, so called anti-tick treatments, can be purchased and used on your dog to discourage ticks. Fiprinol and Permethrin are two examples of compounds that have been shown to be effective against ticks. Another alternative is getting an anti-tick collar for your dog.

Some owners chose to limit the dog's access to tick infested habitats, at least during the warm season when ticks are active.

There is no Rocky Mountain spotted fever vaccine.

Rocky Mountain spotted fever treatment for dogs

Since it can take a long time before a sample sent for laboratory testing come back with a definitive diagnosis in the case of suspected RMSF, most vets commence Rocky Mountain spotted fever treatment without waiting for the test result. If the dog has Rocky Mountain spotted fever, the result of antibiotic treatment will usually start showing within 24-48 hours. It should be noted that if the dog is suffering from an advanced form of RMSF it might not respond to antibiotics treatment.

In addition to antibiotics, many dogs will need supportive care. Fluid therapy should however be used with caution due to the risk of pulmonary and cerebral oedema. 

Two of the most commonly used Rocky Mountain spotted fever treatments for dogs are tetracycline and doxycycline. Tetracycline is normally administered three times per day, 22-30 mg per kg body weight. Doxycycline is normally given only twice a day, and no more than 10-20 mg per kg body weight.

If a puppy younger than 6 months is infected with Rocky Mountain spotted fever, most veterinarians use chloramphenicol instead. 15-30 mg of chloramphenicol per kg body weight administered three times per day has proven effective for the treatment of Rocky Mountain spotted fever in puppies. The problem with tetracycline is that it can cause dental staining in young animals, including foetuses. Chloramphenicol is therefore also the drug of choice for pregnant bitches.

In older dogs, 3 mg /kg body weight of enrofloxacin given twice a day is an effective treatment for Rocky Mountain spotted fever, but it should only be used in older dogs to prevent cartilage damage.

If the disease has caused anterior uveitis or hyphema, the dog can be given topical corticosteroids. Retinal hemorrhage, chorioretinitis and other serious lesions can require systemic corticosteroid administration.

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