Rickettsial Diseases

Rickettsia are microorganisms hosted by ticks, mites, lice, and fleas, which occasionally infect animals and humans. Unlike most bacteria, rickettsia live inside cells.

Tickborne rickettsial diseases (TBRD) are clinically similar, yet distinct illnesses. In the United States, these diseases include:

  • Rocky Mountain spotted fever (RMSF). Dogs are susceptible to RMSF, and they might frequently develop the disease concurrently with other household members.
  • Human monocytotropic (or monocytic) ehrlichiosis (HME) is caused by Ehrlichia chaffeensis and is transmitted to humans by the lone star tick and possibly other ticks. The white-tailed deer is a major host of all stages of lone star ticks and is an important natural reservoir for Ehrlichia chaffeensis.
  • Human granulocytotropic (or granulocytic) anaplasmosis (HGA, formerly known as human granulocytotropic ehrlichiosis or HGE), is caused by Anaplasma phagocytophilum which is transmitted by the blacklegged tick (Ixodes scapularis). Deer, elk, and wild rodents are thought to be hosts. The disease that may require hospitalization and treatment in intensive care units.
  • Ehrlichia ewingii infection. Dogs can be infected and transmit the pathogen to humans.

Despite the availability of low-cost and effective antibiotic therapy, TBRD continue to cause severe illness and death in otherwise healthy adults and children. The greatest challenge to clinicians is diagnosing these infections early in their clinical course, when antibiotic therapy is most effective

Rickettsia infect cells, causing rash and life-threatening damage to the brain, lungs, and other organs. Rickettsia are not evident in blood smears, which makes the diagnosis difficult. Clinicians usually query patients concerning similar illnesses among family members, close coworkers, or community residents, and even among household dogs.




Wild rodents are hosts of fleas, ticks and other parasites

Initial symptoms commonly include a sudden onset of fever, chills, and headache, commonly associated with malaise and myalgia. In adults, photophobia might be observed. Headache is nearly always reported by adults who seek medical care and can be severe. Patients also might report nausea, vomiting, and anorexia early in the course of their illness, especially with RMSF (35) and HME in children. Diarrhea might occasionally occur. Other frequently observed signs and symptoms in children with either RMSF or HME are abdominal pain, altered mental status, and conjunctival injection.

Brown Norway rat

Appropriate antibiotic treatment is usually initiated immediately when a clinician suspects that the diagnosis could be RMSF, HME, HGA, or E. ewingii infection, based on clinical, laboratory, or epidemiologic findings. Delay in treatment can lead to severe disease and fatal outcome for TBRD. Because each of the agents causing TBRD is susceptible to tetracycline-class antibiotics, these drugs, particularly doxycycline, are considered the therapy of choice in nearly all clinical situations.

The following is a summary of prevention:

  • Avoid tick bites, which is key to the prevention of TBRD.
  • Limit exposure to tick habitats, including grassy and wooded areas.
  • Inspect the body carefully for ticks after being in a tick habitat.
  • Remove attached ticks immediately by grasping with tweezers close to skin and pulling gently with steady pressure.




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