Rocky Mountain Spotted Fever (RMSF) in Dogs

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Rocky Mountain spotted fever (RMSF) is an infectious, tick-borne disease caused by the organism Rickettsia rickettsii. Ticks infected with Rickettsia rickettsii transmit the disease when they feed on a host (dog, human, other large mammal). A tick must be attached to the host for 5 to 20 hours before it can transmit the disease.

Rocky Mountain spotted fever occurs most often in the spring and summer when ticks are most active. Dogs at increased risk include those younger than four years of age and large breed dogs primarily because these dogs are more likely to be outdoors. Purebred dogs are more likely to develop the symptoms of RMSF after infection than are non-purebred dogs and German shepherd dogs may be at higher risk than other breeds.

With an active RMSF infection, any organ in the body may be affected.

What To Watch For

  • Fever, usually over 104 degrees Fahrenheit
  • Depression
  • Decreased appetite
  • Lymphadenopathy, or generalized enlargement of lymph nodes
  • Petecchiae, or pinpoint hemorrhages under the skin
  • Uveitis, which is inflammation of the interior of the eye
  • Swollen or painful joints

    Diagnosis

    Veterinary care should include diagnostic tests to determine the underlying cause and help guide subsequent treatment recommendations. These tests may include:

  • Complete blood count
  • Biochemistry profile
  • Urinalysis
  • Coagulation tests
  • Indirect fluorescent antibody (IFA) testing
  • Coomb's test
  • Antinuclear antibody test (ANA)

    Treatment

  • Depending on the severity of infection and clinical symptoms, affected pets may require hospitalization for several days.

  • Tetracycline antibiotics (oxytetracycline, doxycycline) or chloramphenicol are the antibiotics of choice to treat rickettsial diseases.

  • Intravenous fluids are administered to pets that are actively vomiting, dehydrated from vomiting or have evidence of kidney insufficiency due to RMSF.

  • Colloids are administered to prevent edema in pets with low blood protein levels.

  • Administration of plasma may be necessary in pets with bleeding abnormalities or with exceptionally low blood protein levels.

  • Nutritional support may be necessary in pets that have not eaten for several days or have protracted vomiting.

    Home Care and Prevention

    Administer antibiotics as directed by your veterinarian. Antibiotic therapy is usually continued for a 2 to 3 week period. It is important to finish all medications as directed because infection may reoccur if therapy is discontinued too soon.

    If you are having difficulty administering the medication, or the antibiotic causes nausea or vomiting in your pet, contact your veterinarian as soon as possible.

    There is no vaccine available for protection against RMSF. Pets that have been infected are immune to re-infection for 9 to 12 months. You should inspect your pet for ticks regularly when the pet comes in from outdoors, particularly in the spring and summer when ticks are most active. Remove ticks from pets as soon as they are detected. Ticks infected with Rickettsia rickettsii must be attached for 5 to 20 hours before they can transmit the organism to your pet. If the tick is removed before this time, transmission cannot occur.

    Take care to prevent your own exposure when removing ticks from your pet. Wear gloves or use tweezers to prevent fluid from crushed ticks from contacting abraded areas on your skin. If your yard is heavily infested with ticks, consider an environmental spray to control tick numbers.

    Use oral or topically applied tick preventative medication to deter and kill ticks before they can harm your pet.

  • 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.

    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.

    Symptoms

  • Respiratory signs include discharge from the eyes and nose, coughing and an elevated respiratory rate.

  • 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.

  • Diagnosis In-depth

    In addition to taking an extensive medical history including questions regarding travel and exposure to ticks, your veterinarian will perform a thorough physical exam and may recommend the following diagnostic tests.

  • Complete blood count (CBC) to evaluate red blood cells, white blood cells and platelets. Platelets are often decreased and anemia may be present although this is relatively uncommon. White blood cell counts are often decreased initially and then rise later in the course of infection. A shift toward immature white cells may also be observed.

  • The biochemistry profile for showing increases in liver enzymes, blood sugar and cholesterol levels as well as a decrease in the blood protein albumin. Occasionally, decreases are seen in blood sodium and chloride levels due to vomiting and diarrhea. Evidence of kidney insufficiency such as elevations in creatinine and blood urea nitrogen (BUN) levels may be present.

  • A urinalysis to show proteinuria (protein in the urine) or hematuria (blood in the urine).

  • A coagulation profile to identify potential bleeding or coagulation problems that may occur as a result of low platelets or vasculitis. Tests included in a coagulation panel are the prothrombin time (PT), activated partial thromboplastin time (APTT), fibrinogen levels, fibrin degradation products (FDPs), and a platelet count.

  • A Coomb's test or ANA (antinuclear antibody test) to rule out the presence of an immune mediated disorder that could produce symptoms similar to those of RMSF.

  • An indirect fluorescent antibody (IFA) test to measure antibody levels or titers to RMSF. This is the most important test for identification and diagnosis of RMSF. A blood sample is drawn during suspected infection and a convalescent sample is drawn 4 weeks later to confirm infection. Titers greater than 1:128 suggest recent infection. In the eastern United States a single titer greater than 1:512 is required to confirm active infection.

    Treatment In-depth

  • Most pets with RMSF require hospitalization for initial treatment although length of hospitalization depends on the severity of infection and clinical symptoms and may vary from 24 hours to several days.

  • An intravenous catheter is placed in pets that are dehydrated or debilitated, or in pets that are unable to take medications orally due to vomiting.

  • Intravenous fluids are administered to pets that are dehydrated, vomiting, or are showing signs of kidney insufficiency. The presence of a fever greater than 104 F also warrants the administration of intravenous fluids. Anti-emetics, or anti-vomiting medications, may be added to IV fluids to control vomiting.

  • Antibiotics such as oxytetracycline, doxycycline or chloramphenicol are administered to treat infection with RMSF. Doxycycline is the preferred antibiotic and is usually administered orally but may be administered intravenously if the pet is vomiting. A favorable response to antibiotic administration is usually seen within 24 to 48 hours of their initiation.

  • Colloids such as hydroxyethyl starch (hetastarch) or dextrans may be administered to pets with low protein levels. Colloids are solutions that mimic proteins and help prevent edema formation due to low proteins.

  • Blood products such as plasma may be administered to pets that have evidence of bleeding disorders or exceptionally low protein levels.

  • Rocky Mountain spotted fever can cause rapid weight loss so nutritional support is important, particularly in those pets that have not eaten for several days or have protracted vomiting. Nutrition may be administered intravenously, called parenteral nutrition, to those pets that are vomiting or via a feeding tube that reaches the stomach in those pets that are not vomiting but refuse to eat despite encouragement.

    Follow-up

    At home, administer all medication prescribed by your veterinarian. Treatment of RMSF often requires a two to three week course of doxycycline. Do not stop medication before instructed or infection may reoccur.

    Doxycycline may cause stomach upset in some pets. If you have trouble administering the medication or your pet is nauseated or vomiting as a result of the doxycycline, call your veterinarian as soon as possible.

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