Rocky Mountain Spotted Fever (RMSF) is a potentially life-threatening disease that dogs can develop from being bitten by ticks. The actual organism that causes Rocky Mountain Spotted Fever is Rickettsia rickettsii. The infective organisms get into dogs (and people and other mammals) when they are bitten by ticks that have them in their bloodstream. The "bugs" go from the saliva of the blood-sucking ticks into the mammals that they are feeding upon, and this is what causes the infection. Rocky Mountain Spotted Fever has been reported in North America, Central America and South America. It is considered to be one of the most significant tick-borne diseases of humans in the United States, although the number of cases in people is dropping. People can become infected incidentally, if they come into contact with the blood of infected ticks, especially when they are pulling ticks off of their dogs.
Most cases of RMSF in this country are seen in children and young adults along the eastern seaboard (especially North and South Carolina), in the Mississippi River valley (in the Southeast), in the Midwest and in the south-central states. RMSF has been confirmed in other areas, as well. Only a small percentage of RMSF cases are now reported in the Rocky Mountains, where the disease was first discovered at the turn of the 20th century.
Causes of Rocky Mountain Spotted Fever
Rocky Mountain Spotted Fever (RMSF) is caused by a tiny parasite, Rickettsia rickettsii, which infects the cells that line small blood vessels. Most cases of RMSF in our pets show up between late March and the end of September. This is considered to be the height of the tick season. Rodents are the main reservoirs for Rickettsia rickettsii, which means that these organisms live inside rodents, in their blood, but usually do not cause any illness to their "hosts." Dogs get Rocky Mountain Spotted Fever when an adult tick bites an infected reservoir animal – usually a rodent - and then later feeds on an uninfected dog, passing the microorganism into the bite wound through its saliva.
The main carrier (vector) of Rickettsia rickettsii in the western United States, from the Cascades to the Rocky Mountains, is the wood tick, which is technically called Dermacentor andersoni. East of the Rockies, the American dog tick, Dermacentor variabilis, is responsible for transmitting RMSF to domestic dogs. Rocky Mountain Spotted Fever can also be transferred to dogs through blood transfusions, if the blood was taken from an infected dog.
Adult Dermacentor ticks that carry Rickettsia rickettsii must bite and remain attached to a dog for many hours in order for the dog to become infected with the microorganism. How long the ticks must be attached for infection to transfer is the subject of some debate; reported estimates range from 5 hours to 48 hours, or more.
After an incubation period of between 2 and 14 days, the tiny organisms that cause RMSF invade, lodge and multiply in the lining of the dog's small blood vessels (the "vascular endothelium"). This causes bleeding (hemorrhage), loss of the fluid component of blood (plasma), low blood pressure (hypotension) and loss of platelets. Platelets are essential to the ability of blood to clot. When platelet numbers go down, the affected animal's blood cannot clot properly, which can lead to serious bleeding disorders. Left untreated, RMSF can cause a very serious condition called disseminated intravascular coagulation (DIC), organ swelling, shock and even sudden death.
Prevention of Rocky Mountain Spotted Fever
Effective control of the tick population is the best way to prevent dogs from getting Rocky Mountain Spotted Fever. Ticks must be attached to a dog for many hours before they can transmit the tiny organism, Rickettsia rickettsii, that causes Rocky Mountain Spotted Fever. So, ticks should always be physically removed from dogs as soon as possible. Special tick-removal devices are commercially available, although tweezers can also be used. Organisms in the blood of ticks can be dangerous to people. Gloves should be worn when removing ticks. Before disposing of a tick, it may be worth a call to a veterinarian to see whether the tick should be taken to the clinic for identification and determination of whether it is carrying any infectious diseases. It is best to dispose of ticks in a closed container, such as a jar or a sealed plastic bag, which contains a bit of rubbing alcohol or liquid dish detergent. The container should be discarded in an outdoor garbage receptacle. Believe it or not, ticks can survive and infect other animals after being flushed down a toilet, so that is not the best route of disposal.
A number of topical products that can protect dogs from being bitten by ticks are available over-the-counter and from veterinarians. These come in dips, sprays, shampoos, collars and other forms. They typically contain dichlorvos, dioxathion, propoxur, carbaryl or chlorfenvinphos. Many topical flea preventatives are also effective against ticks. Discuss these products with a veterinarian to determine their safety for use on your dog, especially if puppies or young children are present in the household. Outdoor tick control should include regular cutting of tall grass, brush and weeds, because thick vegetation is a prime habitat for adult ticks. Pet-safe outdoor insecticides are also commercially available. Areas known to be tick-infested should be avoided, especially during high tick season.
Dogs with Rocky Mountain Spotted Fever often have vague, nondescript symptoms when they first start getting sick. Their symptoms can mimic those of many other diseases.
How Rocky Mountain Spotted Fever Affects Dogs
Some dogs infected with the organism that causes Rocky Mountain Spotted Fever (RMSF) never become observably ill. Others, however, develop a high fever, body aches, tissue and organ swelling and a great deal of discomfort. Dogs with severe cases of this disease are extremely ill and require immediate veterinary attention. RMSF can be fatal.
Symptoms of Rocky Mountain Spotted Fever
Dogs infected with Rickettsia rickettsii may have subclinical disease, which means that they show no outward signs of illness even though they are infected with the microscopic organism. They also may develop clinical disease, which means that they do have symptoms that their owners can see. Unfortunately, the signs of RMSF can be quite nondescript and often mimic the signs of other diseases. During high tick season, dogs usually develop acute symptoms, including one or more of the following:
Fever (often high; usually develops within 2 or 3 days after a prolonged tick bite)
Lack of appetite (inappetence; anorexia)
Eye redness (conjunctivitis; scleral injection)
Difficulty breathing (dyspnea; respiratory distress)
Swollen limbs (limb edema)
Swollen tissues (edema of the face, lips, scrotum, prepuce, ears, other tissues)
Muscle pain (especially in the neck region)
Stiff, unstable gait (uncoordination; ataxia; balance problems)
Rash around the area of a tick bite (not especially common)
Enlarged regional lymph nodes (lymphadenopathy)
Enlarged spleen (splenomegaly)
Damage to kidneys, brain, heart and/or skin
Nosebleeds (epistaxis; spontaneous bleeding; may be associated with sneezing)
Blood in the urine (hematuria)
Blood in the feces (hematochezia)
Digested blood in the feces (melena)
Behavioral changes; altered mental state
Abnormal heart rhythms (arrhythmias)
These symptoms can mimic those of canine Lyme disease, distemper or ehrlichiosis, among many other disorders.
Dogs at Increased Risk
Purebred dogs reportedly are more likely to develop symptoms of RMSF than are mixed-breed pets. German Shepherd Dogs are predisposed to this illness. The reason for this association is unclear, but it does suggest a potential genetic contribution to the cause of the disease. Dogs that are free-roaming, or that spend significant time outdoors in tick-infested areas, such as dogs used for hunting, tracking or field trial competitions, as well as dogs receiving blood transfusions, also are at increased risk.
How Rocky Mountain Spotted Fever is Diagnosed
Rocky Mountain Spotted Fever (RMSF) should be high on the differential list for any dog that shows general signs of illness accompanied by a history of tick bites, especially between late March and late September. The veterinarian will take a complete history of the dog's medical background and will perform a thorough physical examination. This initial data base usually will also include routine blood work (a complete blood count [CBC] and a serum biochemistry profile), together with a urinalysis. Dogs with RMSF often have low blood platelet levels, mild anemia (low red blood cell numbers) and slightly low white blood cell levels (leukopenia), especially early in the course of the infection. As the disease progresses, white blood cell numbers tend to increase. Cholesterol levels are almost always elevated in dogs with Rocky Mountain Spotted Fever. The reason for this association still remains a mystery. The urinalysis of affected dogs usually discloses protein and blood in the urine.
The diagnosis of Rocky Mountain Spotted Fever can be confirmed by specialized blood tests that detect antibodies to Rickettsia rickettsii, which is the infectious microorganism that causes this disease. These advanced diagnostic techniques require sending blood samples to a specialized veterinary pathology laboratory. These tests should be done at the time the dog presents with signs of illness, and then repeated several weeks later. Biopsies of affected tissues or organs can also be taken and submitted to a diagnostic laboratory. Tissues or blood can be cultured, although this is not commonly done and requires a high biosafety-level laboratory. The dog's attending veterinarian can explain these tests and other diagnostic protocols to the owner in greater detail.
Rocky Mountain Spotted Fever can be virtually indistinguishable from infection with Ehrlichia canis, which is transmitted by the brown dog tick, Rhipicephalus sanguineus. RMSF and ehrlichiosis can be differentiated by specialized tests, called serology, done on blood samples.
If a dog is suspected of being infected with Rickettsia rickettsii, especially if ticks are found attached to its body, the veterinarian probably will prescribe oral antibiotics based on a presumptive diagnosis of Rocky Mountain Spotted Fever (RMSF). Most authorities feel that clinical suspicion of RMSF justifies treatment. Current antibiotics that are effective against the microorganisms that cause RMSF include tetracycline, oxytetracycline, doxycycline, chloramphenicol and enrofloxacin, among others. Each of these drugs has potential side effects, which should be discussed with the attending veterinarian before they are used. For example, tetracycline can cause permanent tooth discoloration in puppies less than 6 months of age, and can also lead to kidney damage. Enrofloxacin should be avoided in young puppies, because it can cause damage to developing cartilage. Discuss any treatment protocols with your dog's veterinarian.
Authorities recommend starting antibiotic treatment for suspected cases of RMSF, even if the diagnosis is not 100% confirmed, because delaying treatment greatly increases the risk of death. Most dogs with RMSF respond quickly to antibiotic treatment – often in as little as one or two days. Steroids, such as prednisone or prednisolone, may be prescribed to reduce the inflammation that commonly accompanies RMSF. The initial treatment is usually done on an inpatient basis at the hospital, until the dog has stabilized. This is especially true if the animal is dehydrated. If the dog is severely anemic from blood loss, it may require a blood transfusion.
Supportive care and anti-tick preventative treatments can greatly improve the dog's comfort and reduce its risk of becoming reinfected. Of course, any attached ticks must be removed, and the areas of tick bite wounds should be cleaned and disinfected. Supportive care for infected dogs may include aggressive administration of intravenous fluids to correct dehydration.
The prognosis for dogs with Rocky Mountain Spotted Fever is good, as long as the infection is caught and treated in a timely fashion. Most dogs respond to treatment within a matter of hours. Unfortunately, dogs whose disease has progressed to include central nervous system signs, such as seizures, disorientation, behavioral changes and/or gait abnormalities, have a guarded to grave prognosis and may die suddenly within hours of the onset of their neurological symptoms.