Rocky Mountain Spotted Fever (RMSF), also called tick typhus, Tobia fever, fiebre manchada, and São Paulo fever, depending on the country, is a disease caused by the infective parasitic organism Rickettsia rickettsia. This organism is passed to dogs (and humans, as well as other mammals) when they are bitten by an infected Dermacentor tick. Cases of RMSF have been reported in North, Central, and South America. This is the most common of all tick-borne diseases in the Americas.
The first case ever diagnosed was in the Rocky Mountains, at the beginning of the 1900s. The disease is hard to diagnose in the early stages, and many cases are fatal. Up until the late 1940s, almost a third of all humans died from the disease; it is almost certain that all dogs that contracted the disease during this time frame died from its effects.
A tick becomes infected by R. rickettsia when it bites an infected animal. Male ticks can transfer the organism to female ticks during mating, and female ticks can pass the organism to her eggs. Once infected, the tick is infected for the rest of its life, and acts as a vector for that entire time.
There are two types of ticks that will act as vectors to R. rickettsia in the US and Canada: the American dog tick (Dermacentor variabilis) and the Rocky Mountain wood tick (Dermacentor andersoni).
Rickettsiae get transmitted from the tick to its host (the dog) via saliva during feeding. For a tick to transfer the bacteria, it must be attached to its host for approximately 24 hours, which makes risk of exposure low for humans, but higher for dogs whose owners don’t inspect them daily for ticks. Most cases of infection occur during the months of March through October.
Once passed to the host, the parasite enters into an incubation period of between two and 14 days, after which time it infects the vascular endothelium (the cells that line the small blood vessels). The parasite then multiplies and can cause hemorrhage, fluid loss, low blood pressure, and platelet loss. Platelet loss leads to improper clotting ability, which can, in turn, cause severe bleeding problems. When left untreated, Rocky Mountain Spotted Fever can cause organ swelling, disseminated intravascular coagulation, shock, and sudden death.
The only way to prevent RMSF is to control the tick population. This can be done in several ways: the two most common involve inspecting the dog every time it comes in from outside and manually removing any attached ticks, and by applying topical tick preventatives to the dog. Dogs treated with anti-tick chemicals also need to be inspected for attached ticks. Not all topical treatments are safe for all dogs, so speak to your vet before deciding. Another way to control the tick population is by keeping grass, weeds, and brush cut short. Outdoor insecticides may be applied in tick-prone areas. Any area that is known to be infested by ticks should be avoided, particularly during tick season.
Humans should always wear gloves when removing a tick. All ticks removed from a dog should be preserved in a small container or alcohol or sealed bag and taken to the vet for analysis. Otherwise, the tick should be burned or disposed of; infected ticks that get flushed down the toilet may survive to infect another animal.
Some dogs may never show symptoms of illness. Others have clinical signs of the disease. These symptoms tend to be very general and may mimic other diseases. This makes the disease somewhat hard to diagnose. Any dog that exhibits one or more of the following symptoms during high tick season should be suspected of having RMSF and tested accordingly: depression, lethargy, listlessness, weakness, fever, eye redness, weight loss, exercise intolerance, difficulty breathing, lack of appetite, blood in the urine, stiff gait, muscle pain, swollen limbs or tissues, cough, enlarged spleen, blood in the feces, nosebleeds, stupor, abnormal heart rhythms, shock, coma, behavioural changes, enlarged lymph nodes.
Dogs at Risk
For some reason, pure-bred dogs are more at risk than mixed-breed dogs. GSDs are particularly prone to RMSF, which indicates a possible genetic component. Free-roaming dogs, hunting or working dogs, and dogs that receive blood transfusions are also at higher risk of developing Rocky Mountain Spotted Fever.
Testing and Diagnosis
First, the vet will ask about any known tick bites, the dog’s health history, and will perform a complete physical exam. This exam should include complete blood count, serum biochemistry profile, and urinalysis. Indicators of RMSF are anemia, low blood platelet levels, and slightly low numbers of white blood cells in dogs in early stages of the infection. Dogs in later stages of the disease will have elevated WBC counts. Infected dogs also tend to have elevated cholesterol levels, and blood and protein present in the urine.
There are specialized blood tests that can definitively diagnose RMSF. These tests look for the R. rickettsia antibodies. These tests must be performed at a veterinary pathology lab, and should be done immediately upon initial suspicion of the disease, as well as a few weeks later.
Treatment and Prognosis
Dogs with RMSF, or suspected of having RMSF (particularly if a tick or ticks are found attached) are prescribed an oral antibiotic like oxytratracycline, chloramphenicol, tetracycline, or enrofloxacin. Each has side effects, so treatment options should be discussed on an individual basis. Supportive care such as IV fluid replacement may be necessary.
Prognosis is good in most dogs, particularly if the infection is caught and treated early on. Dogs with more advanced cases have a guarded to grave prognosis since the central nervous system may be involved. These dogs may die within hours of neurological symptoms.