Issue Description Rocky Mountain spotted fever is the most
lethal and most frequently reported rickettsial illness in the United
States. It has been diagnosed throughout the Americas. The disease is
caused by Rickettsia rickettsii, a species of bacterium that is spread
to humans and dogs by ixodid (hard) ticks. Initial signs and symptoms
of the disease include sudden onset of fever, headache, and muscle
pain, followed by development of rash. The disease can be difficult to
diagnose in the early stages, and without prompt and appropriate
treatment it can be fatal. Other Names Rocky Mountain Spotted Fever, Fiebre Manchada, Tobia Fever, Sao Paulo Fever, Febre Maculosa
Natural History Rocky Mountain spotted fever, like all
rickettsial infections, is classified as a zoonosis. Zoonoses are
diseases of animals that can be transmitted to humans. Many zoonotic
diseases require a vector (e.g., a mosquito, tick, or mite) in order
to be transmitted from the animal host to the human host. In the case
of Rocky Mountain spotted fever, ticks are the natural hosts, serving
as both reservoirs and vectors of R. rickettsii. Ticks transmit the
organism to vertebrates primarily by their bite. Less commonly,
infections may occur following exposure to crushed tick tissues,
fluids, or tick feces.
The life cycle of Dermacentor
variabilis and Dermacentor andersoni ticks (Family Ixodidae)A female
tick can transmit R. rickettsii to her eggs in a process called
transovarial transmission. Ticks can also become infected with R.
rickettsii while feeding on blood from the host in either the larval
or nymphal stage. After the tick develops into the next stage, the R.
rickettsii may be transmitted to the second host during the feeding
process. Furthermore, male ticks may transfer R. rickettsii to female
ticks through body fluids or spermatozoa during the mating process.
These types of transmission represent how generations or life stages
of infected ticks are maintained. Once infected, the tick can carry
the pathogen for life.
Rickettsiae are transmitted to a
vertebrate host through saliva while a tick is feeding. It usually
takes several hours of attachment and feeding before the rickettsiae
are transmitted to the host. The risk of exposure to a tick carrying
R. rickettsii is low. In general, about 1%-3% of the tick population
carries R. rickettsii, even in areas where the majority of human cases
are reported. There are two major vectors of R. rickettsii in the
United States, the American dog tick and the Rocky Mountain wood tick.
American dog ticks (Dermacentor variabilis) are widely distributed
east of the Rocky Mountains and also occur in limited areas on the
Pacific Coast. Dogs and medium-sized mammals are the preferred hosts
of adult D. variabilis, although it feeds readily on other large
mammals, including humans. This tick is the most commonly identified
species responsible for transmitting R. rickettsii to humans. Rocky
Mountain wood ticks (Dermacentor andersoni) are found in the Rocky
Mountain states and in southwestern Canada. The life cycle of this
tick may require up to 2 to 3 years for completion. Adult ticks feed
primarily on large mammals. Larvae and nymphs feed on small rodents.
Other tick species have been shown to be naturally infected with R.
rickettsii or serve as experimental vectors in the laboratory.
However, these species are likely to play only a minor role in the
ecology of R. rickettsii.
There are only 800 cases reported
in the U.S. a year and only 20% find the tick.
Signs and Symptoms Rocky Mountain spotted fever can be very
difficult to diagnose in its early stages, even among experienced
physicians who are familiar with the disease.
Patients infected with R. rickettsii generally visit a physician in
the first week of their illness, following an incubation period of
about one to two weeks after a tick bite. The early clinical
presentation of Rocky Mountain spotted fever is nonspecific and may
resemble a variety of other infectious and non-infectious diseases.
Initial symptoms may include:
lack of appetite
Later signs and symptoms include:
The classic triad of findings for this disease
are fever, rash, and history of tick bite. However, this combination
is often not identified when the patient initially presents for care.
The rash has a centripetal,or "inward" pattern of spread, meaning it
begins at the extremities and courses towards the trunk.
The rash first appears 2-5 days after the onset of fever and is often
not present or may be very subtle when the patient is initially seen
by a physician. Younger patients usually develop the rash earlier than
older patients. Most often it begins as small, flat, pink, non-itchy
spots (macules) on the wrists, forearms, and ankles. These spots turn
pale when pressure is applied and eventually become raised on the
skin. The characteristic red, spotted (petechial) rash of Rocky
Mountain spotted fever is usually not seen until the sixth day or
later after onset of symptoms, but this type of rash occurs in only
35% to 60% of patients with Rocky Mountain spotted fever. The rash
involves the palms or soles in as many as 50% to 80% of patients;
however, this distribution may not occur until later in the course of
the disease. As many as 10% to 15% of patients may never develop a
Abnormal laboratory findings seen in patients with
Rocky Mountain spotted fever may include thrombocytopenia,
hyponatremia, or elevated liver enzyme levels.
Mountain spotted fever can be a very severe illness and patients often
require hospitalization. Because R. rickettsii infects the cells
lining blood vessels throughout the body, severe manifestations of
this disease may involve the respiratory system, central nervous
system, gastrointestinal system, or renal system. Host factors
associated with severe or fatal Rocky Mountain spotted fever include
advanced age, male sex, African-American race, chronic alcohol abuse,
and glucose-6-phosphate dehydrogenase (G6PD) deficiency. Deficiency of
G6PD is a sex-linked genetic condition affecting approximately 12% of
the U.S. African-American male population; deficiency of this enzyme
is associated with a high proportion of severe cases of Rocky Mountain
spotted fever. This is a rare clinical course that is often fatal
within 5 days of onset of illness.
problems following acute Rocky Mountain spotted fever infection
include partial paralysis of the lower extremities, gangrene requiring
amputation of fingers, toes, or arms or legs, hearing loss, loss of
bowel or bladder control, movement disorders, and language disorders.
These complications are most frequent in persons recovering from
severe, life-threatening disease, often following lengthy
Treatment Appropriate antibiotic treatment is initiated
immediately when there is a suspicion of Rocky Mountain spotted fever
on the basis of clinical and epidemiological findings. Treatment
should not be delayed until laboratory confirmation is obtained. In
fact, failure to respond to a tetracycline antibiotic argues against a
diagnosis of Rocky Mountain spotted fever. Severely ill patients may
require longer periods before their fever resolves, especially if they
have experienced damage to multiple organ systems. Preventive therapy
in non-ill patients who have had recent tick bites is not recommended
and may, in fact, only delay the onset of disease.
Doxycycline (For adults, 100 mg every 12 hours. For children under 45
kg [100 lb], 4 mg/kg body weight per day in two divided doses) is the
drug of choice for patients with Rocky Mountain spotted fever. Therapy
is continued for at least 3 days after fever subsides and until there
is unequivocal evidence of clinical improvement, generally for a
minimum total course of 5 to 10 days. Severe or complicated disease
may require longer treatment courses. Doxycycline is also the
preferred drug for patients with ehrlichiosis, another
tick-transmitted infection with signs and symptoms that may resemble
Rocky Mountain spotted fever.
Chloramphenicol is an alternative drug that can be used to treat Rocky
Mountain spotted fever; however, this drug may be associated with a
wide range of side effects and may require careful monitoring of blood
levels (as it can cause aplastic anemia).