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Bacteriology at UW- Madison |
Introduction to the Rickettsiae
The rickettsiae are small (0.3-0.5 x 0.8-2.0 um), Gram-negative, aerobic, coccobacilli that are obligate intracellular parasites of eukaryotic cells. They may reside in the cytoplasm or within the nucleus of the cell that they invade. They divide by binary fission and they metabolize host-derived glutamate via aerobic respiration and the citric acid (TCA) cycle. They have typical Gram-negative cell walls, amd they lack flagella. The rickettsiae frequently have a close relationship with arthropod vectors that may transmit the organism to mammalian hosts. The rickettsiae have very small genomes of about 1.0-1.5 million bases.
Rickettsiae must be grown in the laboratory by co-cultivation with eukaryotic cells, and they have not been grown by in axenic culture. The basis of their obligate relationship with eukaryotic cells has been explained by rickettsial possesion of "leaky membranes" that require the osmolarity and nutritional environment supplied by an intracellular habitat.
The rickettsiae, in spite of their small size and obligate intracellular habitat, are a group of alphaproteobacteria, which inlcude many well-known organisms such as Acetobacter, Rhodobacter, Rhizobium and Agrobacterium. Very few of the alphaproteobacteria are pathogens of humans. Brucella, Bartonella, Rickettsia, and a related intracellular parasite, Ehrlichia, are the main exceptions.
Taxonomy
The genus Rickettsia is included in the bacterial tribe Rickettsieae,
family Rickettsiaceae, and order Rickettsiales. This
genus
includes many other species of bacteria associated with human disease,
including those in the spotted fever group and the typhus group
(figure1).
Figure 1. Taxonomic classification of the order Rickettsiales
The rickettsiae can be subdivided into two or three major groups, depending on your taxonomic point of view (1. spotted fever; 2. typhus; and 3. scrub typhus groups) based on clinical characteristics of disease and phylogenetic relationships.
Spotted Fever Group (SFG)
Rickettsia rickettsii is the cause of Rocky Mountain spotted
fever (RMSF) and is the prototype bacterium in the spotted fever
group
of rickettsiae. Rickettsia rickettsii is found in the
Americas
and is transmitted to humans through the bite of infected ticks. The
bacterium
infects human vascular endothelial cells, producing an inflammatory
response.
The pathogenesis of RMSF is discussed in some detail below.
Other spotted fever group rickettsiae that produce human rickettsioses include R. conorii, R. mongolotimonae and R. slovaca (bouteonneuse fever and similar illnesses), R. akari (rickettsial pox), R. japonica (Japanese spotted fever), R.sibirica (North Asian tick typhus), R. africae (African tick bite fever), R. helvetica (perimyocarditis), R. australis(Queensland tick typhus) and R. honei (Flinders Island spotted fever). The spotted fever rickettsiae have been found on every continent except Antarctica.
Typhus Group (TG)
Rickettsia prowazekii is the cause of epidemic or louse-borne
typhus and is the prototypical bacterium from the typhus group of
rickettsiae.
R. prowazekii infects human vascular endothelial cells, producing
widespread
vasculitis. In contrast to RMSF, louse-borne typhus tends to occur in
the
winter. Infection usually is transmitted from person to person by the
body
louse and, therefore, tends to manifest under conditions of crowding
and
poor hygiene. The southern flying squirrel is apparently the reservoir
in the United States, but the vector involved in transmission from the
flying squirrel to humans is unknown. The disease has a worldwide
distribution.
Other rickettsiae in the typhus group include R. typhi and R. felis. Murine typhus is caused by transmission of R. typhi from rats, cats and opossums to humans via a flea vector. Murine typhus is found worldwide and is endemic to areas of Texas and southern California in the United States. Although R. felis is phylogenetically more closely related to the spotted fever group of rickettsiae than the typhus group, it shares antigens with R. typhi and produces a murine typhus-like illness. Rickettsia felis has been detected in cat fleas and opossums.
Scrub Typhus Group (STG)
Orientia (Rickettsia) tsutsugamushi is the cause of scrub
typhus.
Originally called Rickettsia tsutsugamushi, this organism was
given
its own genus designation because it is phylogenetically distinct from
the other rickettsiae, though closely related. Orientia
tsutsugamushi
is
transmitted to humans by the bite of trombiculid mites (chiggers),
which
are the vector and host. Scrub typhus occurs throughout much of Asia
and
Australia.
Virulence of Rickettsiae
Adherence to the Host Cell
Rickettsiae are inoculated into the dermis of the skin by a tick bite
or through damaged skin from the feces of lice or fleas. The bacteria
spread
through the bloodstream and infect the endothelium. Adherence to the
host
cell is the first step of rickettsial pathogenesis. The adhesins are
presumed
to be outer membrane proteins. The outer membrane protein OmpA has been
implicated in adherence of R. rickettsii because antibodies to
OmpA
habe been shown to block adherence.
The host cell receptor for any Rickettsia has yet to be identified. Although the main target cells of Rickettsia in vivo are endothelial cells, rickettsiae can infect virtually every cell line in vitro. Thus, either the receptor for Rickettsia is ubiquitous among cells, or rickettsiae can bind to different receptors.
Invasion of Host Cells
Upon attaching to the host cell membrane, rickettsiae are phagocytosed
by the host cell. The rickettsiae are believed to induce host
cell
phagocytosis because they can enter cells that normally do not
phagocytose
particles. Once phagocytosed by the host cell, rickettsiae are observed
to quickly escape from the phagosome membrane and enter the cyoplasm.
The
mechanism of escape from the phagosome membrane is not well understood,
but it is thought to be mediated by a rickettsial enzyme, phospholipase
A2.
Movement within and Release from the Host Cell
Observations in cell culture systems suggest that the mechanisms of
intracellular movement and destruction of the host cells differ among
the
spotted fever group and typhus group rickettsiae.
Typhus group rickettsiae are released from host cells by lysis of the cells. After infection with R. prowazekii or R. typhi, the rickettsiae continue to multiply until the cell is packed with organisms and then bursts. Phospholipase A2 may be involved in cell llysis. Typhus group rickettsia-infected host cells have a normal ultrastructural appearance.
Spotted fever group rickettsiae seldom accumulate in large numbers and do not lyse the host cells. They escape from the cell by stimulating polymerization of host cell-derived actin tails, which propel them through the cytoplasm and into tips of membranous extrusions, from which they emerge. Infected cells exhibit signs of membrane damage associated with an influx of water, but the means by which rickettsiae damage host cell membranes is uncertain. There is evidence to suggest a role for free radicals of oxygen, phospholipase, and a protease. The protein responsible for the actin-based movement in spotted fever group rickettsiae has yet to be identified, but it is apparently different than the proteins responsible for actin polymerization by Listeria monocytogenes and Shigella flexneri.
Diseases
Rickettsial diseases vary in clinical severity according to the virulence of the Rickettsia and host factors, such as age, male gender, alcoholism, and other underlying diseases. The most virulent rickettsiae are R. rickettsii and R. prowazekii, which kill a significant portion of infected persons, unless the diseases are treated sufficiently early in the course of infection with an effective antimicrobial agent, usually doxycycline.
All rickettsial infections begin with introduction of the organisms into the skin, either through a tick bite or cutaneous abrasions contaminated by flea or louse feces. Rickettsiae enter dermal cells including endothelium and proliferate locally intracellularly with endothelial cell-to-cell spread for most SFG rickettsioses resulting in an eschar or tache noire, a zone of dermal and epidermal necrosis approximately 1 cm in diameter with a surrounding zone of erythema. Eschars do not occur in epidemic and murine typhus and are rarely observed in Rocky Mountain spotted fever.
SFG rickettsioses often manifest regional lymphadenopathy in the drainage of the eschar, suggesting that rickettsiae may spread via lymphatic vessels from the tick bite inoculation site early in the infection. Rickettsiae spread throughout the body and infect mainly endothelial cells, establishing many foci of contiguous infected blood vessel-lining cells. Injury in these local sites causes vascular damage manifesting as rash, interstitial pneumonia, encephalitis, interstitial nephritis, and interstitial myocarditis, as well as lesions in the liver, gastrointestinal wall, pancreas, and potemtially any vascularized tissue of the body.
The most important pathophysiologic effect is increased vascular permeability with consequent edema, loss of blood volume, hypoalbuminemia, decreased osmotic pressure, and hypotension. These effects can be life threatening resulting in pulmonary edema and adult respiratory distress syndrome, shock, or acute tubular necrosis.
Rocky Mountain Spotted Fever
Rocky Mountain spotted fever is the most severe and most frequently reported rickettsial disease in the United States. In the pre-antibiotic era, 20-25% of previously healthy, infected persons died of the illness. Today, even with antimicrobial agents that are highly effective, 3-5% of persons die mainly because of late or mis-diagnosed infection and delayed or ineffective antimicrobialtreatment.
The disease is caused by Rickettsia rickettsii, a species of bacteria that are spread to humans by Ixodes ticks. The onset of disease follows an infective bite by a week (range 2-14 days), beginning with fever, severe 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.
The reasons are that up to 40% of patients are unaware of a tick bite, which is painless and may go unnoticed or be forgotten, and that the rash does not usually appear until 3-5 days after onset of illness. To further confound the diagnosis, symptoms such as nausea, vomiting, diarrhea, abdominal pain, and cough may suggest other diagnoses such as enterocolitis, acute surgical abdomen, or pneumonia. The rash typically appears on the ankles and wrists as faint pink 1-5 mm macules that represent a focus of vascular infection and surrounding vasodilation. These lesions may progress to become maculopapular, owing to the leakage of edema fluid from the affected blood vessels, with the development of a hemorrhage (petechia) in the center of the lesions.
Figure 2. Characteristic
spotted rash of late-stage Rocky Mountain spotted fever on legs of a
patient.
(CDC)
Rocky Mountain spotted fever was first recognized in 1896 in the Snake River Valley of Idaho and was originally called "black measles" because of the characteristic rash. It was a dreaded and frequently fatal disease that affected hundreds of people in this area. By the early 1900s, the geographic distribution of the disease in United States was recgnized as far north as Washington and Montana and as far south as California, Arizona, and New Mexico.
Howard Ricketts was the first to establish the identity of the infectious organism that causes Rocky Mountain spotted fever in . He and others characterized the basic epidemiologic features of the disease, including the role of tick vectors. Their studies found that Rocky Mountain spotted fever is caused by Rickettsia rickettsii, and invo alves a complex cycle between ticks and mammals. Humans are accidental hosts, but are not involved in the natural transmission cycle of the pathogen.
The name Rocky Mountain spotted fever is somewhat of a misnomer. Beginning in the 1930s, it became clear that this disease occurred in many areas of the United States other than the Rocky Mountain region. It is now recognized that this disease is broadly distributed throughout the continental United States, as well as southern Canada, Central America, Mexico, and parts of South America. Between 1981 and 1996, this disease was reported from every U.S. state except Hawaii, Vermont, Maine, and Alaska.
Rocky Mountain spotted fever remains a serious and potentially life-threatening infectious disease today. Despite the availability of effective treatment and advances in medical care, approximately 3- 5% of individuals who become ill with Rocky Mountain spotted fever die from the infection. However, effective antibiotic therapy has dramatically reduced the number of deaths caused by Rocky Mountain spotted fever. Hefore the discovery of tetracycline and chloramphenicol in the late 1940s, as many as 30% of indyviduals infected with R. rickettsii died.
Figure 3. Discovery of
chloramphenicol and tetracycline antibiotics in the 1940s led to a
sharp
decline in RMSF-related mortality. Today, doxycycline is the drug of
choice
for treatment of RMSF. (CDC)
Rickettsia rickettsii, is a very small bacterium that must live inside the cells of its hosts. Consequently, they are difficult to see in tissues by using routine histologic stains and generally require the use of special staining methods (Figure 4).
Figure 4. Gimenez stain
of tick hemolymph cells infected with R. rickettsii. (CDC)
In humans, Rickettsia rickettsii live and multiply primarily within cells that line small- to medium-sized blood vessels. Spotted fever group rickettsiae can grow in the nucleus or in the cytoplasm of the host cell. Once inside the host the rickettsiae multiply, resulting in damage and death of these cells. This causes blood to leak through tiny holes in vessel walls into adjacent tissues. This process causes the rash that is traditionally associated with Rocky Mountain spotted fever and causes damage to organs and tissues.
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 feces.
Only members of the tick family Ixodidae (hard ticks) are naturally infected with Rickettsia rickettsii. These ticks have four stages in their life cycle: egg, larva, nymph, and adult. After the eggs hatch, each stage must feed once to develop into the next stage. Both male and female ticks will bite.
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, 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 spermatazoa during mating.. In this manner generations or each life stage of infected ticks are maintained. Once infected, the tick can carry the rickettsie 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. Generally, about 1 -3% of the tick population carries R. rickettsii, even in areas where the majority of human cases are reported.
Major Tick Vectors in the United States
There are two major vectors of R. rickettsii in the United
States,
the American dog tick and the Rocky Mountain wood tick.
The American dog tick (Dermacentor variabilis) is widely
distributed
east of the Rocky Mountains and also occurs in limited areas along the
Pacific Coast. Dogs and medium-sized mammals are the preferred hosts of
adult D. variabilis, although it feeds on other large mammals,
including
humans. This tick is the most commonly identified species responsible
for
transmitting R. rickettsii to humans.
Figure 5. American dog tick
(Dermacentorvariabilis).
(CDC)
Figure 6. Approximate
distribution of the American dog tick. (CDC)
The Rocky Mountain wood tick (Dermacentor andersoni) is 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.
Figure 7. Rocky Mountain wood
tick (Dermacentor andersoni). (CDC)
Figure 8. Approximate
distribution of the Rocky Mountain wood tick. (CDC)
Other tick species have been shown to be naturally infected with R. rickettsii but these species are likely to play only a minor role in the ecology of R. rickettsii.
Epidemiology
Rocky Mountain spotted fever has been a reportable disease in the
United
States since the 1920s. In the last 50 years, approximately 250-1200
cases
of Rocky Mountain spotted fever have been reported annually, although
it
is likely that many more cases go unreported.
Figure 9. Reported cases
of Rocky Mountain spotted fever in the United States,1942-1996. CDC
compiles
the number of cases reported by the state health departments.
(CDC)
Over 90% of patients with Rocky Mountain spotted fever are infected during April through September. This period is the season for increased numbers of adult and nymphal Dermacentor ticks. A history of tick bite or exposure to tick-infested habitats is reported in approximately 60% of all cases of Rocky Mountain spotted fever.
Figure 10. Seasonal
distribution
of reported cases of Rocky Mountain spotted fever, 1993-1996. (CDC)
Over half of Rocky Mountain spotted fever infections are reported from the south-Atlantic region of the United States (Delaware, Maryland, Washington D.C., Virginia, West Virginia, North Carolina, South Carolina, Georgia, and Florida). Infection also occurs in other parts of the United States, namely the Pacific region (Washington, Oregon, and California) and west south-central (Arkansas, Louisiana, Oklahoma, and Texas) region.
The states with the highest incidences of Rocky Mountain spotted fever are North Carolina and Oklahoma. These two states combined accounted for 35% of the total number of U.S. cases reported to CDC during 1993 through 1996. Although Rocky Mountain spotted fever was first identified in the Rocky Mountain states, actually less than 3% of the U.S. cases were reported from that area during the same interval (1993-1996).
Figure 11. Number of reported cases of Rocky Mountain spotted fever by state and region, 1994-1998. (CDC)
Certain induvudyuals are at higher risk of disease. The frequency of
reported cases of Rocky Mountain spotted fever is highest among males,
Caucasians, and children. Two-thirds of the Rocky Mountain spotted
fever
cases occur in children under the age of 15 years, with the peak age
being
5 to 9 years old (see Figure 12). Individuals with frequent
exposure
to dogs and who reside near wooded areas or areas with high grass may
also
be at increased risk of infection.
Figure 12. Average annual incidence of Rocky Mountain spotted fever by age group, 1993-1996. (CDC)
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 5-10 days 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 fever, nausea, vomiting, severe headache, muscle pain, and lack of appetite. Later signs and symptoms include rash, abdominal pain, joint pain and diarrhea.
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 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 (Figure 13). 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, and this type of rash occurs in only 35% to 60% of patients with Rocky Mountain spotted fever (Figure 14). 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 rash.
Figure 13. Early
(macular)
rash on sole of foot. (CDC)
Figure 14. Late
(petechial)
rash on palm and forearm. (CDC)
Rocky 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.
Long-term health 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 hospitalizations.
Laboratory Diagnosis
There is no widely available laboratory assay that provides rapid
confirmation
of early Rocky Mountain spotted fever. Treatment decisions must
be
based on epidemiologic and clinical clues, and should never be delayed
while waiting for confirmation by laboratory results.
Serologic assays are the most widely available and frequently used methods for confirming cases of Rocky Mountain spotted fever. The indirect immunofluorescence assay (IFA) is generally considered the reference standard in Rocky Mountain spotted fever serology and is the test currently used by CDC and most state public health laboratories (Figure 15).
Figure 15. IFA reaction of a
positive human serum on Rickettsia rickettsii grown in chicken
yolk
sacs, 400X. (CDC)
IFA can be used to detect either IgG or IgM antibodies. Blood samples taken early (acute) and late (convalescent) in the disease are the preferred specimens for evaluation. Most patients demonstrate increased IgM titers by the end of the first week of illness. Diagnostic levels of IgG antibody generally do not appear until 7-10 days after the onset of illness. It is important to consider the amount of time it takes for antibodies to appear when ordering laboratory tests, especially because most patients visit their physician relatively early in the course of the illness, before diagnostic antibody levels may be present. The value of testing two sequential serum or plasma samples together to show a rising antibody level is considerably more important in confirming acute infection with rickettsial agents because antibody titers may persist in some patients for years after the original exposure.
Another approach to Rocky Mountain spotted fever diagnostics is immunostaining. This method is used by taking a skin biopsy of the rash from an infected patient prior to therapy or within the first 48 hours after antibiotic therapy has been started. Because rickettsiae are focally distributed in lesions of Rocky Mountain spotted fever, this test may not always detect the agent. Even in laboratories with expertise in performing this test, the sensitivity is only about 70% on biopsied tissues. This assay may also be used to test tissues obtained at autopsy and has been used to confirm Rocky Mountain spotted fever in otherwise unexplained deaths (Figure 16). Immunostaining for spotted fever group rickettsiae is offered by the CDC, a few state health departments, and some university-based hospitals and commercial laboratories in the United States.
Figure 16. Red
structures
indicate immunohistological staining of Rickettsia rickettsii
in
endothelial cells of a blood vessel from a patient with fatal
RMSF.
(CDC)
Treatment
Appropriate antibiotic treatment should be initiated immediately when
there is a suspicion of Rocky Mountain spotted fever on the basis of
clinical
and epidemiologic findings. Treatment should not be delayed until
laboratory
confirmation is obtained.
If the patient is treated within the first 4-5 days of the disease, fever generally subsides within 24-72 hours after treatment with an appropriate antibiotic (usually a tetracycline). In fact, failure to respond to a tetracycline antibiotic argues against a diagnosis of RMSF. Severely ill patients may require longer periods before their fever resolves, especially if they have experienced damage to multiple organ systems. Prophylactic 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 (100 mg every 12 hours for adults or 4 mg/kg body weight per day in two divided doses for children under 45 kg [100 lbs]) 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 unnequivocal 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.
Tetracyclines are usually not the preferred drug for use in pregnant women because of risks associated with malformation of teeth and bones in unborn children. 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 including aplastic anemia, and may require careful monitoring of blood levels.
Prevention and Control
Limiting exposure to ticks is the most effective way to reduce the
likelihood of Rocky Mountain spotted fever infection. In persons
exposed
to tick-infested habitats, prompt careful inspection and removal of
crawling
or attached ticks is an important method of preventing disease. It may
take several hours of attachment before organisms are transmitted from
the tick to the host.
Currently, no licensed vaccine is available for Rocky Mountain spotted fever.
It is unreasonable to assume that a person can completely eliminate activities that may result in tick exposure. Therefore, prevention measures should be aimed at personal protection. CDC recommends the following prevention measures:
-Wear light-colored clothing to allow you to see ticks that are crawling on your clothing.
-Tuck your pants legs into your socks so that ticks cannot crawl up the inside of your pants legs.
-Apply repellants to discourage tick attachment. Repellents containing permethrin can be sprayed on boots and clothing, and will last for several days. Repellents containing DEET(diethyltoluamide) can be applied to the skin, but will last only a few hours before reapplication is necessary. Use DEET with caution on children. Application of large amounts of DEET on children has been associated with adverse reactions.
-Conduct a body check upon return from potentially tick-infested areas by searching yourentire body for ticks. Use a hand-held or full-length mirror to view all parts of your body.
-Remove any tick you find on your body. Parents should check their children for ticks, especially in the hair, when returning from potentially tick-infested areas. Additionally, ticks may be carried into the household on clothing and pets. Both should be examined carefully.
Tick Control
Strategies to reduce populations of vector ticks through area-wide
application of acaricides (chemicals that will kill ticks and mites)
and
control of tick habitats (e.g., leaf litter and brush) have been
effective
in small-scale trials. New methods being developed include applying
acaricides
to rodents by using baited tubes, boxes, and feeding stations in areas
where these pathogens are endemic. Biological control with fungi,
parasitic
nematodes, and parasitic wasps may play alternate roles in integrated
tick
control efforts. Community-based, integrated, tick-management
strategies
may prove to be an effective public health response to reduce the
incidence
of tick-borne infections. However, limiting exposure to ticks is
currently
the most effective method of prevention.
Boutonneuse Fever and African Tick-bite Fever
Boutonneuse fever and its agent were first described in North Africa in 1910, and variants of R. conorii have been identified in South Africa, Kenya, Somalia, Israel, Morocco, Ethiopia, Russia, India and Pakistan. In parts of Africa, tick-transmitted diseases caused by R. conorii and R. africae overlap geographically. Although their clinical manifestations also overlap, their are differences sufficient to distinguish two different disease agents. Generally milder than boutonneuse fever, African tick bite fever has a lower incidence of rash, which is more often vesicular and sparse, a higher incidence of eschars that are frequently multiple, and more prominent regional lymphadenopathy. Each of these diseases has been diagnosed in the United States after patients return from vacation abroad, particularly from African safaris.
Rickettsialpox
R. akari has been recognized mainly in the urban United States as an agent maintained in a mite-mouse cycle with humans as an accidental host. The organism may, however, have a broader host range and geographic distribution.
A papule appears at the site of mite feeding in the skin during the incubation period, and over 2-7 days, evolves into an eschar. Later fever, chills, malaise, headache, and myalgia develop, followed after 2-6 days by a macular rash that becomes maculopapular and then vesicular before crusting and healing. Fatalities have not been reported.
Cat Flea Typhus
Despite the widespread geographic distribution and prevalence of R. felisin cat fleas, there have been only a handful of clinical investigations of undertaken to diagnose cat flea typhus.
Among eight reported cases of human infection with R. felis (five diagnosed by polymerase chain reaction [PCR] and three by differential antibody titers), all had fever and constitutional symptoms. The majority manifested rash, headache, and central nervous system (CNS) involvement, and variable proportions suffered nausea, vomiting, diarrhea, abdominal pain, myalgia and conjunctivitis. The actual spectrum of illness of this infection requires further clinical studies.
Typhus Fever
Rickettsia prowazekii infections occur in three situations: louse-transmitted epidemics, reactivation of a longstanding latent infection, and zoonotic infection transmitted from flying squirrels by their ectoparasites. Onset of disease is characterized by fever, chills, headache, and myalgia. Macules of 2-6 mm usually appear first on the trunk on day 5 and later spread to the extremities. Rales, conjunctival injection, and delirium are frequent manifestations. Reactivated typhus is a milder version wih the same signs and symptoms. Flying squirrel-associated typhus has also been described as less severe; whether this is due to antimicrobial treatment or less virulent strains of rickettsiae is unclear.
Murine Typhus
Flea-borne R. typhi infections cause extreme discomfort but are seldom fatal healthy young individuals. The difficulty in detecting a rash in darkly pigmented skin was evident in a study finding only 20% of experimentally infected African-American volunteers had rashes, compared to 80% of Caucasian volunteers. The infection can follow a mild course in children with as many as half suffering only fever at night, but necessitates intensive care unit support in 10% of hospitalized adult patients. Pneumonitis or meningoencephalitis can be the major manifestation in some patients.
Treatment of Rickettsioses
Doxycycline is the drug of choice for the treatment of infections caused by Rickettsia except in cases of pregnancy and tetracycline hypersensitivity. some studies have shown that doxycycline is superior to chloramphenicol for the treatment of Rocky Mountain spotted fever as it is associated with a lower case fatality rate and a lower hospitalization rate. Several fluoroquinolones, azithromycin, and clarithromycin, have been used successfully to treat boutonneuse fever but are not recommended for more pathogenic rickettsioses. It should be emphasized that rickettsiae are highly resistant to most antibiotics. Most fatal cases of Rocky Mountain spotted fever have received substantial courses of antimicrobial treatment, including beta lactams, aminoglycosides, and erythromycin. Sulfonamide antimicrobials actually appear to exacerbate the severity of rickettsial infections.
Immunity
Rickettsial infection stimulates an early innate immune response
with
activation of natural killer cells and production of gamma interferon
(gammaIFN),
which act in concert to dampen rickettsial growth. Acquired immunity
develops
with clonal expansion of CD4 and CD8 T lymphocytes as well as
antibody-producing
B cells. Clearance of intraendothelial rickettsiae is achieved by
rickettsicidal
effects due to cytokine activation of the infected endothelial cells
themselves.
Cell mefiated immunuty (CMI) plays an important role as expected in
infection
by an intracellular parasite, but antibodies (including those directed
at epitopes of OmpA and OmpB) also play a role in protective immunity.
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