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RICKETTSIAL DISEASES (RICKETTSIAS). Morphology and biological properties of rickettsias. Rickettsias are small bacteria that are obligate intracellular parasites and—except for Q fever—are transmitted to humans by arthropods.
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Morphology and biological properties of rickettsias • Rickettsias are small bacteria that are obligate intracellular parasites and—except for Q fever—are transmitted to humans by arthropods. • They are small, pleomorphic coccobacilli maintained in the environment through a cycle involving mammalian reservoirs and insect vectors.
Rickettsiae grow in different parts of the cell. • Those of the typhus group are usually found in the cytoplasm; • those of the spotted fever group, in the nucleus. • Coxiellae grow only in cytoplasmic vacuoles. It has been suggested that rickettsiae grow best when the metabolism of the host cells is low.
Rickettsia akari causes rickettsial pox in humans, mice and rats. • Rickettsia australis causes queensland tick typhus in humans, small marsupials, rats. • Rickettsia canadensis causes new typhus in humans and rabbits. • Rickettsia conorii causes boutonneuse fever in humans and dogs and small feral mammals. • Rickettsia prowazeki causes epidemic typhus in humans and possibly cattle, sheep and goats. • Rickettsia rickettsii causes spotted fever in humans and many feral animals, especially rodents and in dogs and birds. • Rickettsia sibirica causes Siberian tick typhus in humans and many feral mammals, especially rodents. • Rickettsia typhi causes murine typhus in humans and the brown rat.
Rickettsias are grouped on the basis of clinical features, epidemiological aspects, and other characteristics • Scrub typhus group • The causative agent of SCRUB TYPHUS formerly known as R. tsutsugamushi has been reclassified into the genus Orientia.
Spotted fever group • R. rickettsii (Western hemisphere) • Rocky Montain spotted fever • R. akari (USA, former Soviet Union) • Rickettsialpox • R. conorii (Mediterranean countries, Africa, Southwest Asia, India) • Boutonneus fever • R. siberica (Siberia, Mongolia, northern China) • Siberian tick typhus • R. australis (Australia) • R. japonica (Japan) • Oriental spotted fever • R. africae (South Africa) • African Tick Bite Fever
Typhus group • R. prowazekii (Worldwide) • Epidemic, recrudescent and sporadic typhus • R. typhi (Worldwide) • Murine (endemic) typhus
Epidemiology • The life cycles of different rickettsiae vary: • (1) Ricketisia prowazekii has a lire cycle limited to humans and to the human louse (Pediculus humanus carporis and Pediculus humanus capitis). The agent by fecal excretion on the surface of the skin of another person. Whenever a louse bites, it defecates at the same time. The scratching of the area of the bite allows the rickettsiae excreted in the feces to penetrate the skin. As a result of the infection the louse dies, but the organisms remain viable for some lime in the dried feces of the louse
Epidemiology • (2) Rickettsia typhi has its reservoir in the rat, in which the infection is inapparent and long-lasting. Rat fleas carry the rickettsiae from rat to rat and sometimes from rat to humans, who develop endemic typhus. Cat fleas can serve as vectors. In endemic typhus, the flea cannot transmit the rickettsiae transovarially. • (3) R. tsutsugamushi has its true reservoir in the mites that infest rodents. Rickettsiae can persist in rats for over a year after infection. Mites transmit the infection transovarially. Occasionally, infected mites or rat fleas bite humans, and scrub typhus results. The rickettsiae persist in the mite-rat-mite cycle in the scrub or secondary jungle vegetation that has replaced virgin jungle in areas of partial cultivation. Such areas may become infested with rats and trombiculid mites.
Epidemiology • (4) R. rickettsii may be found in healthy wood ticks (Dermacentor andersoni) and is passed transovarially. Vertebrates such as rodents, deer and humans are occasionally bitten by infected ticks m the western USA. In order to be infectious, the tick carrying the rickettsiae must be engorged with blood, for this in creases the number of rickettsiae in the tick. Thus, there is a delay of 45-90 minutes between the time of the attachment of the tick and its becoming infective. In the eastern USA Rocky Mountain spotted fever is transmitted by the dog tick Dermacentor variabilis. Dogs are hosts to dog ticks and may serve as a reservoir for tick infection. Small rodents are another reservoir. Most cases of Rocky Mountain spotted fever in the USA now occur in the eastern and southeastern regions. • (5) R. akari has its vector in bloodsucking mites of the species Allodermanyssus sanguineus. These mites may be found on the mice (Mus musculus) trapped in apartment houses in the USA where ricketsialpox has occurred. Transovarial transmission of the rickettsiae occurs in the mite. Thus, the mite may act as a true reservoir, as well as a vector. R. akari has also been isolated in Korea.
Epidemiology • (6) C. burnetii is found in ticks, which transmit the agent to sheep, goats and cattle. Workers in slaughterhouses and in plants that process wool and cattle hides have contracted the disease as a result of handling infected animal tissues. Occasionally, the source is a parturient cat. C. burnetii is transmitted by the respiratory pathway rather than through the skin. There may be a chronic infection of the udder of the cow. In such cases the rickettsiae are excreted in the milk and rarely may be transmitted to humans by the ingestion of unpasteurized milk.
Rocky Mountain Spotted Fever(tick-borne typhus fever) • Rocky Mountain spotted fever (RMSF) is a disease caused by a microorganism transmitted to humans by the bite of an infected American dog tick (Dermacentor variabilis) and by several other tick species.
Etiology • Rickettsiae comprise a group of microorganisms that phylogenetically occupy a position between bacteria and viruses. • The species Rickettsia rickettsii • The genus Rickettsia • The family Rickettsiaceae, • The order Rickettsiales. They are obligate intracellular gram- negative coccobacillary forms that multiply within eukaryotic cells. Rickettsiae do not stain well with Gram stain, but they take on a characteristic red color when stained by the Giemsa or Gimenez stain. They have typical gram- negative cell walls and lack flagella.
Epidemiology • RMSF is spread by the bite of an infected tick (the American dog tick, the lone-star tick or the wood tick) or by contamination of the skin with the tick's body fluids or feces. • Person to person and direct animal to human transmission of RMSF does not occur. • Children and young adults are most frequently affected/ • The reservoir for the agent appears is a broad array of small mammals. • Dog ticks are often found to be carrying the agent.
The symptoms of RMSF The incubation period is 3 to 14 days after the bite of an infected tick. • High fever • Severe headache, • Fatigue, • Deep muscle pain, • Chills • Rash begins on the legs or arms, may include the soles of the feet or palms of the hands and may spread rapidly to the trunk or rest of the body.
Pain in the large muscle of the calf is very common, and may be particularly severe. • The patient may be somewhat confused and delirious. Without treatment, these symptoms may last two weeks or more.
Other symptoms that may occur are as follows: • breathing difficulties as the lungs are affected • heart rhythms abnormal • kidney failure in very ill patients • liver function decrease • nausea, vomiting, abdominal pain, and diarrhea • brain inflammation (encephalitis) in about 25 percent of RMSF patients (Brain injury can result in seizures, changes in consciousness, actual coma, loss of coordination, imbalance on walking, muscle spasms, loss of bladder control, and various degrees of paralysis.) • the clotting system impaired and blood evident in the stools or vomit
Laboratory diagnostics • Immunofluorescent detection of the microorganism in biopsies • Detection of antigens or antibody to the agent in the blood (serology) • Weil-Felix test • Thrombocytopenia and elevated white cell count in second week.
The treatment for RMSF • Antibiotics: tetracycline or chloramphenicol. • Very ill patients need to be hospitalized in an intensive care unit. Depending on the types of complications a particular patient experiences, a variety of treatments may be necessary, including intravenous fluids, blood transfusions, anti-seizure medications, kidney dialysis, and mechanical ventilation (a breathing machine).
The prevention • Avoid tick infested areas, especially during the warmer months. • Wear light colored clothing so ticks can be easily seen. Wear a long sleeved shirt, hat, long pants, and tuck your pant legs into your socks. • Walk in the center of trails to avoid overhanging grass and brush. • Check your body every few hours for ticks when you spend a lot of time outdoors in tick infested areas. Ticks are most often found on the thigh, arms, underarms and legs. Ticks can be very small (no bigger than a pinhead). Look carefully for new "freckles". • Use insect repellents containing DEET on your skin or permethrin on clothing. Be sure to follow the directions on the container and wash off repellents when going indoors. • Remove attached ticks immediately. • A vaccine is not available!
