Rocky Mountain Spotted Fever

Rocky Mountain Spotted Fever (RMSF) and the Spotted Fever Group (SFG) rickettsias

Few individuals in Wisconsin have experienced RMSF and most have probably been exposed elsewhere.  However in Minnesota, although RMSF is considered rare, the Minnesota Department of Health (MDH) receives a few reports every year in patients with no travel history.  It is possible that RMSF is rarely transmitted by local ticks in both Minnesota and Wisconsin.

The bacteria that causes RMSF is associated with the wood tick (also known as the American dog tick, Dermacentor variabilis), the Rocky Mountain wood tick (D. andersoni; does not occur in Wisconsin) and the brown dog tick (Rhipicephalus sanguineus, exceedingly rare in Wisconsin) in other parts of the USA but we have not detected the bacterium (Rickettsia rickettsii)  in wood ticks in Wisconsin.  In 2013, we surveyed wood ticks from the Mountain area (Oconto county in eastern Wisconsin).  We estimate that Rickettsia montanensis, a  spotted fever group species which has not yet been shown to cause human illness, was present in 5-15% of the ticks.  It is important to discriminate between these two closely related bacterial species when assessing the results of tick testing.  One can cause very serious disease, the other does not.

Rickettsia rickettsii bacteria–the causative agent of Rocky Mountain spotted fever inside tick hemolymph cells.
Public domain

What is Rocky Spotted Mountain Fever?

Rocky Mountain Spotted Fever or RMSF is a tick-borne disease disease caused by a bacteria (Rickettsia rickettsii).  RMSF can be transmitted to humans by the bite of an infected American dog tick (Dermacentor variabilis), Rocky Mountain wood tick (Dermacentor andersoni) or brown dog tick (Rhipicephalus sanguineus) in most of the United States  (Dantas-Torres, 2007). RMSF is considered one of the most severe and tick-borne rickettsial infections with up to 20% of untreated and 5% of treated cases having fatal outcomes (Chapman et al. 2006). American dog ticks (Dermacentor variabilis) are found primarily in the eastern and Midwestern states but can be found in some areas in California. Rocky Mountain wood ticks (Dermacentor andersoni) are known to transmit RMSF in the Rocky Mountain regions and Canada. The brown dog tick (Rhipicephalus sanguineus) is thought to be the primary vector of R. rickettsii in Mexico (Dantas-Torres 2007) and was recently connected to an outbreak in eastern Arizona (Demma et al. 2005). 

Domestic dogs may also develop clinical illness with Rocky Mountain spotted fever and there have been reported cases where dogs have developed the disease concurrently with other human household members (Paddock et al. 2002; Elchos & Goddard 2003).

In the late 1800’s, Edward E. Maxey provided the first clinical description of what was initially named, “spotted fever of Idaho” based on the unique clinical presentation of a continuously moderate to high fever with a characteristic red, spotted (petechial) rash that has a centripetal or inward pattern of spread beginning at the extremities, moving toward the trunk (Maxey 1899) and does not usually appear until at least 6 days after the onset of symptoms. Maxey’s description became the first report of RMSF to be published in medical literature (Dantas-Torres 2007). In 1906 Dermacentor spp. were implicated in the transmission of RMSF, which remained unnamed at the time (King 1906; Ricketts 1906). Howard T. Ricketts was the first to isolate R. rickettsii and from 1906 to 1910 demonstrated that it was maintained among ticks and mammals in an enzootic or natural cycle (Ricketts 1906; Ricketts & Gomez 1908). Ricketts was also able to show that infected ticks could transmit the disease transovarially to their offspring (Ricketts 1909). In 1919, scientist S. Burt Wolbach confirmed that ticks carried the bacterium and noted the intracellular nature of the bacteria (Wolbach 1919). In 1922, the name Rickettsia rickettsii was later proposed in honor of Howard T. Ricketts (Bengston 1947). 

Incidence and Prevalence in Wisconsin- Rocky Mountain Spotted Fever is rare in Wisconsin. 

The majority of cases in the U.S. occur in the southeastern states. Only 63 cases were reported in Wisconsin between 2009 and 2015. The majority of these infections were acquired when Wisconsin residents traveled outside of Wisconsin and we do not have evidence that ticks in the state are transmitting the RMSF bacterium. 

However in Minnesota, although RMSF is considered rare, the Minnesota Department of Health (MDH) receives a few reports every year in patients with no travel history. This suggests the possibility that local ticks may be transmitting Rickettsia rickettsii.

A Dakota County child who contracted the infection in Minnesota died of RMSF in 2009. –

Therefore, it remains possible that Wisconsin could have endemic (locally-acquired) transmission of RMSF. 

Seasonality and Geography:

Cases of RMSF can occur during any month of the year, yet the majority of reported cases have an illness onset during the summer months and a peak in cases typically occurs in the months of June and July (Openshaw et al. 2010).  Seasonality may vary by region of the country due to variations in temperatures and climates throughout the U.S. and the primary tick vectors involved in transmission of R. rickettsii.

The disease is more common in the eastern, southern and western states. RMSF cases have been reported throughout most of the contiguous United States, yet confirmed cases reported from 2000-2007 in five states (North Carolina, Oklahoma, Arkansas, Tennessee, and Missouri) accounted for 64% of all RMSF cases during this time period (Openshaw et al. 2010). 

Who gets RMSF?

Persons at increased risk:

The frequency of reported cases of Rocky Mountain spotted fever is highest among males, American Indians, and people aged 50-69.  Individuals with frequent exposure to dogs and who reside near wooded areas or areas with high grass may be at increased risk of infection. Children ages 0-9, adults over 70 years of age and American Indians have an increased risk of fatal outcome from infection with RMSF (Openshaw et al. 2010). 

What are the Clinical Signs and Symptoms of RMSF?

Early clinical presentation of RMSF can be challenging for healthcare providers to distinguish from other infectious and non-infectious causes due to the non-specific symptoms in the early stage of infection. Clinical illness with RMSF frequently begins as a sudden onset of fever, headache and malaise. Most patients with RMSF seek medical care during the first few days of symptoms–most often before a rash appears. Several visits may occur before the correct diagnosis of RMSF is made and treatment begins. 

The first symptoms of Rocky Mountain spotted fever typically begin an average of 7 days after the bite of a an infected tick–but may occur anywhere from 2-14 days afterwards (Dantas-Torres 2007). A tick bite is usually painless and less than half of people who develop RMSF recall being bitten by a tick (Chapman et al. 2006; Hazin et al. 2009). 

Initial infection with RMSF is frequently followed several days later by a skin rash, (often referred to a maculopapular rash which progresses into a papular or petechial rash). A classic case of RMSF involves a rash that first appears 2-5 days after the onset of fever as small, flat, pink, non-itchy spots (macules) on the ankles, wrists or forearms and spreads to include the trunk and may include the palms and soles. The classic spotted or petechial rash is often not apparent until the fifth or sixth day of illness. Often the rash may vary from this description and is difficult to discern in darker skinned individuals.

A rash may not develop or be atypical in approximately 10-20% of cases of RMSF. People who fail to develop a rash, or develop an atypical rash, are at increased risk of being misdiagnosed (CDC 2010; Chapman et al. 2006). The rash on the palms and soles of the feet is not characteristic for infection with RMSF and can occur in other illnesses or reactions caused by drug hypersensitivities (Chapman et al. 2006).

The rash commonly occurs earlier in children than in adults and may be observed in up to 90% of children with RMSF infection (Chapman et al. 2006).

Other symptoms of infection with RMSF may include (CDC 2010; Hazin et al. 2009; Dantas Torres 2007; Chapman et al. 2006):

  • Nausea
  • Vomiting or Diarrhea
  • Abdominal pain (which may mimic appendicitis or other causes of acute abdominal pain) 
  • Myalgia (deep muscle pain) 
  • Lack of appetite 
  • Altered mental status
  • Conjunctival injection (red eyes)
  • Photophobia (sensitivity to light)
Common Laboratory Findings:
  • Thrombocytopenia (low platelet count)
  • Elevated liver enzymes
  • Hyponatremia (level of sodium in your blood is abnormally low)
It is important to remember that few people with the RMSF will develop all recognized symptoms or clinical laboratory features and may vary greatly from person to person.

RMSF is a very serious illness and delays in correct diagnosis and/or treatment can lead to fatal outcome –even in previously healthy people. The progression and outcome of the disease can vary greatly from person to person. Patients who are treated early may not develop severe complications from infection and recover quickly, while other individuals who experience a more severe course may require hospitalization or intensive care and intravenous antibiotic therapy (CDC 2010; Chapman et al. 2006)

How is RMSF Diagnosed?

Diagnosis of RMSF by a physician or medical professional is based on symptoms, a thorough medical history and history of a tick bite or exposure to tick-infested habitat or previous travel to known regions where RMSF occurs. The unique appearance and characteristics of the rash are also important. 

Laboratory Detection: 

R. rickettsii infects the endothelial cells which line the blood vessels and usually are not found in large numbers circulating in the blood therefore direct detection methods such as blood smears, culture or polymerase chain reaction (PCR) to detect R. rickettsii are not typically reliable methods unless the patient has progressed to a severe stage of infection (CDC 2010; Dantas Torres 2007; Chapman et al. 2010). 

If the patient has a rash, immunohistochemical (IHC) staining or polymerase chain reaction (PCR) can be performed on a skin biopsy from the rash site (CDC 2010; Chapman et al. 2006). These tests can be very sensitive when used in tissue specimens collected during early stages of infection before antibiotic treatment has been initiated. This method may be useful for diagnosis of RMSF in patients where diagnostic antibody titers have not yet developed sufficiently for detection by serologic tests.

The preferred or gold standard serologic test for diagnosis of RMSF is an indirect detection method–the immunofluorescence assay or IFA– for detection of antibodies to R. rickettsii developed by the body’s immune system in response to infection with RMSF (CDC 2010; Dantas Torres 2007).

These serologic tests can be used to detect the production of antibodies by the body’s immune system but are only reliable approximately 7-10 days after illness onset since the body’s immune system takes time to produce the antibodies in response to infection with the RMSF bacteria. Antibodies may not be detectable for the first week of illness in up to 85% of patients and it is important to remember that a negative serologic test result during this time period does not rule out infection with RMSF (CDC 2010; Dantas Torres 2007). Two-tiered testing on paired serum specimens taken early (acute) and late (convalescent) during the disease course are preferred for evaluation of a significant “4-fold” or greater increase in the antibody titer (CDC 2010; Chapman et al. 2006).

How is RMSF Treated?

Doxycycline is the first line treatment for adults and children of all ages and should be initiated immediately whenever infection with Rocky Mountain spotted fever is suspected.

The use of the antibiotic doxycycline to treat suspected RMSF in children is standard practice recommended by both CDC and the American Academy of Pediatrics (AAP) Committee on Infectious Diseases. Use of antibiotics other than doxycycline in children has been shown to increase the risk of patient death (CDC 2010; Chapman et al. 2006).

In cases of severe allergy to doxycycline or in pregnant women with a mild clinical presentation of RMSF, the antibiotic chloramphenicol may be considered as an alternative although the risk for fatal outcome is elevated in patients who are treated with chloramphenicol compared to those treated with doxycycline (CDC 2010; Hazin et al. 2009; Chapman et al. 2006).

The diagnosis of RMSF must be made based on clinical signs and symptoms, and can later be confirmed using specialized confirmatory laboratory tests discussed above.

Treatment should never be delayed pending the receipt of laboratory test results, or be withheld on the basis of an initial negative finding for R. rickettsii.

Use of antibiotics other than doxycycline is associated with a higher risk of fatal outcome. Treatment is most effective at preventing death if doxycycline is started in the first 5 days of symptoms. Therefore, treatment must be based on clinical suspicion alone and should always begin before laboratory results arrive or symptoms of severe disease, such as rash (petechiae), develop in all suspected cases of RMSF.

Prevention of RMSF:

Avoiding exposure to known tick habitats or taking proper precautions when spending time outdoors where ticks may be present such as walking on mowed or cleared trails,  avoiding areas with overgrown grasses or brush, wearing long pants, long-sleeved shirts, and socks when outdoors, creating a barrier to ticks by tucking pants into socks and shirt into pants, applying appropriate repellents to skin and/or clothing and showering and checking your entire body for ticks shortly after spending time outdoors are some of the most effective methods for preventing tick-borne diseases.

Regularly and thoroughly inspecting domestic pets for ticks or the use of flea and tick repellents is also especially important if the animal spends time outdoors. 


Bengtson IA. (1947). Classification of the Rickettsiae of Rocky Mountain spotted fever and of endemic (Murine) typhus. J Bacteriol; 53: 325–27.
Page last reviewed and updated November 4, 2010
Chapman AS. et al. (2006). Centers for Disease Control and Prevention. Diagnosis and management of tickborne rickettsial diseases: Rocky Mountain spotted fever, ehrlichioses, and anaplasmosis — United States: a practical guide for physicians and other health-care and public health professionals. MMWR;55(No. RR-4).

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Minnesota Department of Health-

Wisconsin Department of Health Services

CDC- Rocky Mountain Spotted Fever