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.
Preventing 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.