Rocky Mountain Spotted Fever

Dima Youssef

James W. Myers

INTRODUCTION

Rickettsia rickettsii affects the endothelial cells of small vessels. This infectious vasculitis causes microvascular injury and subsequent fluid leakage.
Gram-negative coccobacilli of the Rickettsiaceae family.
Obligate intracellular organisms.
Incidence—two to four cases per million.

EPIDEMIOLOGY

Incidence—four cases per million.
Occurs in the Western hemisphere. Oklahoma, Tennessee, and North Carolina are the states that usually have the highest incidence. Note that RMSF has been described in nontraditional locations like Arizona and New York City on occasion.
Humans become infected from the bite of an infected tick.
- Dermacentor variabilis (American dog tick) in most of the USA
- Dermacentor andersoni (Rocky Mountain wood tick) in the Rockies and Canada
- Rhipicephalus sanguineus (brown dog tick) in Mexico and Arizona
- Amblyomma cajennense (Cayenne tick) in Central and S. America
- Amblyomma areolatum (yellow dog tick) in Brazil
Most cases occur during the months of April through September.

RISK FACTORS

Males > females
Residence in a wooded area, with exposure to ticks or dogs with ticks.
Children from 5 to 10 years old are at highest risk.

CLINICAL PRESENTATION

The incubation period ranges from 2 to 14 days.
Classic triad—fever, headache, and rash. May not be present on admission.
No eschar. Bite is painless. Many do not recall a bite.
Rash typically appears on the second or third day of illness.
Rash begins as macules, then becomes petechial.
Spreads from ankles and wrists to palms and soles and then the chest.
No nuchal rigidity
Can sometimes present with myalgias, abdominal findings mimicking appendicitis or cholecystitis, periorbital edema, conjuctival effusion, hand or foot edema, myocarditis, and hepatosplenomegaly.
WBC count is often normal but can see thrombocytopenia.
Mortality is 20% untreated and 5% treated (see Tables 23-1 and 23-2).

DIAGNOSIS

The diagnosis of RMSF must be entertained before the onset of rash if possible as delays in therapy are associated with a much worse outcome.
Laboratory diagnosis of RMSF may be achieved by isolation of R. rickettsii from the blood.
Serology (Table 23-2).
- Indirect fluorescent antibody (IFA) is 95% sensitive but takes at least 7 to 10 days to appear. A negative test early on does not rule out the diagnosis. Convalescent serology in 4 weeks with a fourfold increase is often the best way to make the diagnosis.
- Greater than 1:64 is diagnostic.
Polymerase chain reaction (PCR) amplification is not very useful at this time.
Skin Biopsy
Immunohistochemical staining is 100% specific but 70% sensitive.

DIFFERENTIAL DIAGNOSIS

Meningococcemia, other Rickettsia, and viral illnesses are the main entities to be considered. Treatment with ceftriaxone in addition to doxycycline is often prescribed until the diagnosis of either is more secure.

DIAGNOSTIC LABORATORIES

ARUP	(Rocky Mountain Spotted Fever) Antibodies, IgG and IgM by IF 00050371 Test code	http://www.arupconsult.com/index.html
Specialty Labs	7896: Rickettsia rickettsii IgG and IgM Antibodies	http://www.specialtylabs.com/
Mayo Clinic Labs	Unit Code 84343: Rickettsia Antibody Panel, Spotted Fever and Typhus Fever Groups, Serum	http://www.mayomedical-laboratories.com/
Centers for Disease control (CDC)^a	Skin biopsy diagnosis	http://www.cdc.gov/rmsf/ 1-800-CDC-INFO (1-800-232-4636) Click to access CDC50_34_SpecimenSubmission.pdf
^aSerologic tests for RMSF are available at commercial labs, state public health laboratories, and CDC. Early serologic tests (within 1 week of illness onset) frequently are negative, and testing of acute and convalescent phase serum samples is recommended to confirm diagnosis. Nucleic acid detection (e.g., by using PCR assay), immunohistochemical staining of formalin-fixed tissues, and cell culture of biopsy or autopsy specimens also can be used for diagnosis and are available at specialized research laboratories and CDC.

Table 23-1 Tick Diseases of Africa

Name	Vector	Eschar/Rash	Clinical Features
Rickettsia conorii subsp. conorii Mediterranean Spotted Fever	Rhipicephalus sanguineus, R. simus, Haemaphysalis leachi	Rash occurs in 97%. Single eschar	Cases generally sporadic. Single eschar. Case fatality ratio, approximately 2.5%.
Rickettsia africae	Amblyomma hebraeum, A. variegatum, A. lepidum	Eschars are often multiple in 54%. Maculopapular rash in 49%. May be vesicular in 24%.	Disease occurs in predominantly rural settings and is associated with international travelers returning from safari, hunting, camping, or adventure races. Outbreaks and clustered cases common (74%).
African Spotted Fever			Symptoms include fever (88%) and lymphadenopathy (43%) No fatalities reported.
Rickettsia sibirica subsp. mongolitimonae Lymphangitis-associated rickettsiosis	Hyalomma truncatum	Symptoms include eschar (75%), rash (63%).	Few described cases in South Africa. Patients have lymphangitis in (25%) of cases.
Rickettsia slovaca	Dermacentor marginatus	Fever and rash rare Typical eschar on the scalp with cervical lymphadenopathy; illness mild.	Tick-borne lymphadenopathy “TIBOLA” (1997), Dermacentor-borne necrosis and lymphadenopathy “DEBONEL”
Rickettsia helvetica	Ixodes ricinus	Rash and eschar seldom occur	Has been linked to perimyocarditis and sarcoidosis
Rickettsia aeschlimannii	Hyalomma marginatum marginatum, H. marginatum rupifes, Rhipicephalus appendiculatus	Symptoms include eschar and maculopapular rash	Few cases described in patients from Morocco and South Africa.
Rickettsia monacencis	Ixodes ricinus	Fever and maculopapular rash
Rickettsia raoultii	Dermacentor marginatus	TIBOLA/DEBONEL	Typical eschar on the scalp with cervical lymphadenopathy