Rocky Mountain Spotted Fever (RMSF)
Dr. Anne Marie Manning
Rocky Mountain spotted fever was first identified in the Rocky Mountain states; however, it is found throughout the United States and parts of Canada, Mexico and Central and South America. Within the United States, most cases (67 percent) are reported along the East Coast and particularly south of Virginia. Respiratory signs include discharge from the eyes and nose, coughing and an elevated respiratory rate.
Rocky Mountain spotted fever is a disease carried and transmitted by infected ticks. The ticks responsible for carrying RMSF are Dermatocentor variabilis (American dog tick) and Dermatocentor andersoni (Rocky mountain wood tick). These ticks serve as the natural host, reservoir and vector for RMSF. Most infections occur in the spring and summer, as this is when the ticks are at their peak activity.
Human exposure occurs when an infected tick is removed from the pet using bare hands and infected blood from the crushed tick contacts and enters the body through abraded skin.
Ticks transmit the rickettsial organism responsible for RMSF to the dog during feeding. The tick must be attached for a minimum of 5 to 20 hours before the disease can be transmitted. Ticks that are removed prior to feeding cannot transmit the disease. Once the organism penetrates the pet's skin, it enters the blood and lymph and spreads throughout the system to other organs. Fever usually develops within 2 to 3 days of infection and the temperature is commonly over 104 F. The fever is usually accompanied by generalized, non-specific aches and pains, shifting leg lameness, swollen joints, stiff gait, depression, and decreased appetite.
Gastrointestinal signs include vomiting and diarrhea.
Vasculitis – inflammation of the blood vessels – may occur resulting in petecchiae (pinpoint hemorrhages under the skin), and edema (fluid under the skin). Edema typically occurs on the pet's muzzle, ears and scrotum making these areas appear swollen. Low platelets may cause nose bleeds and bloody urine.
RMSF may cause inflammation in the eyes resulting in squinting, excessive blinking, and signs that look similar to conjunctivitis.
Approximately one third of dogs infected with RMSF exhibit neurological symptoms such as partial paralysis involving 2 to 4 limbs, ataxia (wobbly gait), head tilt, hearing loss, seizures and coma. These symptoms look similar to those caused by encephalitis.
Related Symptoms or Diseases
Because Rocky Mountain spotted fever affects so many body systems, it can be confused with many other diseases that cause similar symptoms. Fortunately, the finding of high titers for RMSF make this disease distinguishable from those listed below.
Ehrlichia and Lyme disease are also carried and transmitted by ticks. Ehrlichia is caused by the rickettsial organism Ehrlichia canis, and Lyme disease is caused by the spirochete (spiral shaped organism) Borrelia burgdorferi. Lyme disease and Ehrlichia cause symptoms almost identical to RMSF. Veterinarians often submit blood for a "tick panel" when presented with a pet exhibiting signs of fever, lameness, enlarged lymph nodes, decreased appetite and other symptoms typical of rickettsial disease. The panel includes titers for each of the three diseases (Lyme, Ehrlichia, and RMSF) to determine which of the three is present.
Immune mediated arthropathy is a generalized arthritis that results when the patient's immune system attacks the joints causing inflammation, fever and lameness that may shift from leg to leg. The symptoms of immune mediated arthropathy look very similar to RMSF, as does analysis of joint fluid from pets with either disease. A positive titer for RMSF is often the only way to distinguish between the two diseases.
Immune mediated thrombocytopenia (ITP) is a disease that causes low platelets in the blood stream, which may then cause petecchiae. Because RMSF may cause both petecchiae and low platelets, the diseases can appear very similar. The platelet count is usually much lower with ITP and a fever is usually absent.
Canine distemper virus may cause depression, decreased appetite, coughing, increased respiratory rate, and neurological signs. A positive titer for RMSF helps distinguish between the two diseases.
Meningoencephalitis is inflammation of the tissue that surrounds the brain and the spinal cord. This disease results from immune mediated, viral or bacterial causes and is often associated with a fever, generalized stiffness and pain and possibly seizures. This disease is distinguished from RMSF on the basis of cerebrospinal fluid analysis and a positive RMSF titer.
Pneumonia may cause a cough, increased respiratory rate and fever.
Gastroenteritis results in decreased appetite, vomiting, diarrhea and occasionally a fever.
Enterovirus infection is gastroenteritis resulting from a viral infection such as parvovirus or coronavirus.
Lymphosarcoma is a form of cancer that can affect any organ within the body resulting in depression, decreased appetite, vomiting, diarrhea and a fever. Lymphosarcoma can also invade the bone marrow where platelets are made. If the bone marrow is severely affected, the platelet count may drop and petecchiae may appear on the skin.
Systemic lupus erythematosus (SLE) is an immune mediated disease that may affect all the organs of the body causing generalized illness, fever, and lameness. A positive RMSF titer helps distinguish RMSF from SLE.
Bacterial sepsis is a generalized or systemic bacterial infection. Sepsis causes a fever, depression, decreased appetite, vomiting and diarrhea. Identification of a source of bacterial infection, such as an abscess, pneumonia, infected uterus, and a positive blood culture help identify bacterial sepsis.