From "Tick-borne infection: What starts as a tiny tick bite may have a
serious outcome
Vol95/No 5; pp 131-139/April 1994/Postgraduate Medicine
Rocky Mountain Spotted Fever
Dermacentor variabilis (the dog tick), Dermacentor andersoni (the mountain
wood tick), and to a lesser extent other ticks transmit Rickettsia
rickettsii, which is a small gram-negative bacterium that is the agent of
Rocky Mountain spotted fever. After an incubation period of 2 to 14 days
following a tick bite or contact of infected {*filter*} with mucous membrane
(eg, the conjuctiva) or an open sore (eg, a bleeding hangnail), Rickettsia
begin to multiply in the endothelial cells, eventually creating a
generalized vasculitis. Rickettsia infection can affect virtually any
organ and patients of all ages. As few as 10 organisms can produce this
deadly disease.
Peak prevalence of Rocky Mountain spotted fever is in the summer months,
when people are most active outdoors. Cases have been reported from
almost every state. Even New York City had 3 cases in 1989, whereas North
and South Carolina, Missouri, and Oklahoma reported a combined total of
273 cases.
History taking is a key to recognition of Rocky Mountain spotted fever.
Summer trips to high-risk states are particular attention-getters.
Employment (eg, forest management); leisure actitivites (eg, camping); pet
ownership (especially of dogs); a previous finding of ticks, even in the
distant past; and a current finding of ticks on clothing or the body may
provide clues that alert an astute physician to the possibility of Rocky
Mountain spotted fever. Of patients found to have the disease, 84% report
exposure to a dog and 56% report a tick bite.
SIGNS AND SYMPTOMS. The classic triad found in patients with Rocky
Mountain spotted fever consists of fever, retrobulbar headache, and rash.
However, many infections are subclinical, and reported symptoms and signs
vary widely.
Neurologic signs usually dominate the clinical picture. Most patients
report feeling feverish and having headaches, often accompanied by
myalgias and arthralgias. Almost all patients have a rash, which usually
develops after about 4 days (range, 0 to 14 days) of symptoms. The rash -
blanching pink macules 2 to 5 mm in diameter - initially presents around
the wrists or ankles and spreads to include the arms, legs, and trunk.
The palms and soles may be involved. The rash progresses to
non-blanching, palpable purpura or darkens into coalescent macules.
LABORATORY FINDINGS. Laboratory abnormalitites found with Rocky Mountain
spotted fever also vary. The hemoglobin level may be as low as 7 g/dL and
hematocrit 20%. The white {*filter*} cell count may be as high as 32,000/mm3
or as low as 3,800/mm3. Low platelet counts ( in the extremely dangerous
range of 2,000 to 3,000/mm3) have been reported. Because of serum leakage
from damaged capillaries and resultant thirst with water drinking, the
serum sodium level may fall to as low as 110 mmol/L; in fact, up to 70% of
patients have hyponatremia. PaO2 may be reduced because of pulmonary
congestion.
Other levels are often high: lactate dehydrogenase may reach 5,000 U/L,
bilirubin 16 mg/dL, and creatine kinase 40,000U/L. Prothrombin time is
often prolonged.
Testing of cerebrospinal fluid often reveals sterile leukocytosis with
either polymorthonuclear cells or lymphocytes predominating.
Abnormalitites on electroencephalograms, electrocardiograms, and chest
roentgenograms are common.
DIAGNOSIS. Diagnosis is usually clinical, because a mean of 10 days is
required before diagnostic titers of antibodies are found. However, high
titers may be found as early as 3 to 4 days after symptoms occur. A
fourfold or greater increase in titer on paired immunofluorescence
antibody testing (2 weeks apart) is diagnostic. Because specificity and
sensitivity of titers of Proteus Ox antigens are poor, indirect
immunofluorescence antibody testing is the method of choice.
Direct fluorescent antibody staining of biopsy specimens from active skin
lesions may confirm the diagnosis in 4 to 6 hours. Unfortunately, this
technique is not widely available.
DISEASE COURSE WITH DELAYED OR NO TREATMENT. Early recognition of the
disease is critical. The later the diagnosis is made, the greater the
difficulty in controlling the infection. In fulminant Rocky Mountain
spotted fever, death can occur in 3 to 5 days. Mortality rates are high
in patients who have glucose-6-phosphate dehydrogenase deficiency.
Overall, the case-fatality in patients with RMSF was 5.2% in 1990. The
mortality rate was 6.2% when antibiotic therapy was delayed more than 3
days, but 1.3% if treatment was started within 3 days. Because dark skin
masks the rash, black patients have a higher mortality rate than white
patients.
Untreated RMSF lasts 2 to 3 weeks and carries a 25% mortality rate.
Respiratroy, renal, hepatic, and cardiac failure accounts for most deaths.
Other fatal complications (eg, disseminated intravascular coagulation,
gastrointestinal hemorrhage with perforation occur infrequently).
Permanent sequelae include skin necrosis and scarring of gangrenous
digits, earlobes, tip of the nose, or {*filter*}. Permanent neurologic
deficits (eg, neuropathy, major paresis, cognitive dysfunction) develop in
some patients.
TREATMENT. The mainstay of outpatient therapy is {*filter*}tetracycline
hydrochloride, 25 to 50 mg/kg per day (maximum, 2 grams) divided into four
doses, for 7 to 14 days. {*filter*}doxycycline (Doryx, Vibramycin), 100 mg
twice a day for 7 to 14 days, is equally effective. Treatment may be
discontinued 3 days after the patient becomes afebrile. Because
tetracyclines stain developing teeth, their use is not normally
recommended for patients under 8 years of age. However, since one course
of tetracycline therapy stains the teeth less than one shade (as defined
by the American Dental Association), young children are sometimes given
limited doses of tetracycline to avoid exposure to chloramphenicol
(Chloromycetin), the only other agent that is effective against RMSF.
Chloramphenicol is usually used only in severe infection because of the
risks of idiosyncratic aplastic anemia, which can occur months after use,
and bone marrow suppression, which is more reversible. The dose for the
first 24 hours is 100 mg/kg (maximum, 4 g) divided into four doses. In
all but the most severely ill patients, the dose is then reduced to 50
mg/kg per day. The therapeutic chloramphenicol level is 15 to 25
micrograms/mL. Because both tetracycline and chloramphenicol are
bacteriostatic {*filter*} for Rickettsia infection, host defenses play a major
role in ultimate cure.
Rocky Mountain Spotted Fever