Vector Borne Diseases Part 7: Rocky Mountain Spotted Fever

RMSF is caused by the organism Rickettisa rickettsii and is spread by the American dog tick and the wood tick.  The name of this disease is somewhat of a misnomer.  Although it can be found in both North and South America the most common areas are the eastern seaboard, Mississippi River valley, and south central US, and it is not as common in the Rocky Mountains.  Spots, or skin rash actually only occur in only about 50% of human patients and is even less common in our canine patients.  Fever, however, does occur in dogs...but that alone is certainly not diagnostic!

In contrast to some of the other tick borne diseases transmission requires only five hours of attachment so is much faster and requires more immediate tick removal.  Incubation period can be as short at two days from time time of the tick bite.  This organism causes multisystemic disease leading to vasculitis, hypotension, shock, bleeding, enlarged spleen and lymph nodes, joint pain, neurologic signs, redness of the eyes, difficulty breathing, cough, and swelling of the limbs and face.  Other clinical signs include depression, anorexia, stiff gait and bloody nose.

Untreated the course of the disease is 2-4 weeks and can result in death.

 Routine blood tests often show a decreased platelet count, mild anemia, increases in liver and kidney values and decreased proteins.  There are many specialized laboratory tests than can be used to aid in diagnosis, this is not an organism that can be visualized in the bloodstream.

Symptomatic treatment is aimed at correcting the dehydration, shock, anemia and bleeding disorders and can be comprised of fluids, blood or plasma transfusions.  Treatment of the organism is most commonly done with doxycycline and response is often rapid.  Prognosis is good early in the course of the disease but is much more guarded later on, especially if neurologic signs have begun.

Fortunately this is a relatively uncommon disease.  Current estimates indicate only 2% of ticks are infected with the organism.  Humans can also become infected via ticks.  Cats are not affected.