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Pathology of Rocky Mountain Spotted Fever

Dr Sampurna Roy MD





Rocky Mountain spotted fever is one of the severest infectious disease, with a mortality in previously healthy persons of 20% before the advent of antimicrobial therapy.  


Rocky Mountain spotted fever is the most important rickettsiosis in the United States from the aspects of morbidity and mortality.


Most cases occur in the spring and summer when the ticks are active.


However, contrary to its name, cases occur throughout the U.S.A. as well as in Central and South America with the majority of cases occurring in the Southeastern states. Article


Rickettsia rickettsii are released from the salivary glands of  a feeding Dermacentor variabilis, Dermacentor andersoni, Rhipicephalus sanguineus, or Amblyomma cajennense tick and are injected into the feeding blood pool in the host’s skin. 


Clinical presentation:  After an incubation period of 2 to 12 days the patient develops severe headache, fever, and frequently nausea, vomiting, or abdominal pain.


A maculopapular rash appears on the wrists and ankles 2 to 5 days later, usually spreads to involve the trunk, palms, and may become petecheal.


Delay or absence of rash and frequent lack of a history of tick bite make misdiagnosis and fatality a genuine problem.


In severe cases the patient may manifest signs of encephalitis, noncardiogenic pulmonary edema, skin necrosis, coagulopathy with bleeding, acute renal failure failure, jaundice, hypovolemic shock.


In fatal cases death usually ensues 8 to 15 days after onset of symptoms.


There is a fulminant form of Rocky Mountain spotted fever, observed most often in glucose-6-phosphate  dehydrogenase deficient black males in which the patient may die before the fifth day of illness.


Early treatment with tetracycline or chloramphenicol cures most patients, however, late diagnosis and inappropriate treatment result in an overall mortality of 3% to 8%.

Pathological presentation:  

Rickettsiae spread via the blood stream, enter endothelial cells, proliferate within the cytoplasm and nuclei of endothelial and vascular smooth muscle cells of the microcirculation of virtually all organs, and directly injure  the foci of infected cells.

The consequence is systemic vascular damage that is the pathologic basis for the rash, interstitial pneumonia, interstitial myocarditis, meningoencephalitis, hepatic portal triaditis, and interstitial nephritis.

Microscopically, the vasculitis consists of swollen or necrotic endothelial cells, intramural and perivascular infiltration, predominantly by macrophages and T-lymphocytes with  few polymorphonuclear leukocytes, focal extravasation of erythrocytes and occasionally, usually non-occlusive, eccentric thrombi in the foci of rickettsial infection.

In the skin these foci are located principally in the dermis. Cutaneous histopathology of Rocky Mountain spotted fever is caused by endothelial damage by the rickettsial  organisms which elicit an initial lymphohistiocytic small vessel vasculitis with progression to Leukocytoclastic vasculitis. The vasculitis in Rocky Mountain spotted fever is thus considered to be a form of septic vasculitis.

Visit: Vascular Diseases: Various forms of Vasculitis

In the brain the lesions assume a characteristic appearance, so-called typhus nodules, found most frequently in the  brain stem.

These  perivascular accumulations of mononuclear cells, which measure 100 to 180 micrometer in diameter,  indicate a probable rickettsial infection though they are not pathognomonic.

Other neuropathologic lesions include microinfarcts of white matter and a mild mononuclear cell-rich leptomeningitis.

Lungs are congested and heavy.

Microscopic pulmonary lesions include mononuclear interstitial pneumonia and interstitial and alveolar edema and hemorrhages.

The heart is grossly normal except for epicardial petechiae, but it usually manifests a mild mononuclear interstitial myocarditis on microscopic examination.

The hepatic portal triaditis and multifocal perivascular interstitial nephritis correspond to foci of infection of hepatic portal blood vessels and the renal microcirculation near the corticomedullary junction, respectively.

Erythrophagocytosis occurs in Kupffer cells and macrophages within sinus of lymph nodes.

In fulminant Rocky Mountain spotted fever there are more thrombi and fewer intramural and perivascular leukocytes in foci of vascular injury. 

Disseminated vascular foci of rickettsial infection and microvascular injury result in leakage of intravascular fluid into the interstitial space with consequent edema and hypovoluemia.

Consumption of platelets and coagulation factors in thrombi at the sites of injury can cause thrombocytopenia and in very severe cases more severe coagulopathy.

Focal lesions in the skin are the cause of rash. Vasodilatation and petechiae are the basis of the cutaneous erythematous macules and central “spots” respectively.   

Increased vascular permeability of the infected pulmonary microcirculation may result in noncardiogenic pulmonary edema.

Myocardial injury is not a significant factor.

Central nervous system lesions are the cause of coma, seizures, multifocal, neurologic signs, and probably cardio-respiratory arrest.

Jaundice correlates with hemolysis and portal triadal inflammation.

Acute renal failure results from hypovolemic prerenal azotemia or, in more severe cases, acute tubular necrosis.

Vasculitis in the gastrointestinal tract is the apparent pathologic basis for nausea, vomiting, and abdominal pain and tenderness.


Thrombosis and activation of the kallikrein-kinin pathways may exacerbate the disease and the acute-phase response presumably mediated by interleukin-1 is observed. 

Rickettsia rickettsii does not seem to produce a toxin.

Direct injury of infected cells by rickettsiae has been documented.

The pathogenic mechanisms of the host cell injury have not been elucidated though cell membrane injury by phospholipase A and protease enzymes has been proposed. 

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Further reading

Atypical Rocky Mountain spotted fever with polyarticular arthritis.

Why sulfonamides are contraindicated in Rocky Mountain spotted fever.

Cutaneous histopathology of Rocky Mountain spotted fever.

Rocky Mountain Spotted Fever in a patient treated with anti-TNF-alpha inhibitors.

Spotted fever group Rickettsia in small rodents from areas of low endemicity for Brazilian spotted fever in the eastern region of Minas Gerais State, Brazil.

Adult Onset Still's Disease and Rocky Mountain Spotted Fever.

Absence of Rickettsia rickettsii and occurrence of other spotted fever group rickettsiae in ticks from Tennessee.

Outbreak of Rocky Mountain spotted fever in Córdoba, Colombia.

Rocky Mountain spotted fever: a warm weather problem.

Rocky Mountain spotted fever. Gastrointestinal and pancreatic lesions and rickettsial infection.

The liver in Rocky Mountain spotted fever.



Dr Sampurna Roy  MD

Consultant  Histopathologist (Kolkata - India)  ; Dermpath-India  ;  Soft Tissue Tumour Online ;Cardiac Path Online ;

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