Bacteria from the genus Rickettsia give rise to rickettsial diseases. These bacteria are transmitted from infected mammals to humans via arthropod vectors. Once the bacteria are in the body, they infect the cells lining blood vessels and cause cell death. This results in complications relating to the blood. There are numerous types of rickettsial diseases caused by different species of these bacteria. Common symptoms of rickettsial diseases include fever, headache, depression, and fatigue. In some cases, a rash forms on the body, either around the site of infection, or on random areas.
Rickettsial diseases occur worldwide. Some diseases remain limited to certain geographic regions, while others are present on almost all continents. The distribution of the arthropod that carries the infectious bacteria determines the distribution of the disease. Rickettsial diseases are generally treated with a course of antibiotics. However, delayed administration of treatment can lead to more serious illness, and even death. While no vaccine is available to prevent contracting rickettsial diseases, prevention is achieved by avoiding contact with arthropods. This involves using repellents or wearing protective clothing.
Rickettsial diseases are caused by bacteria from the genus Rickettsia. These bacteria are named after Howard Taylor Ricketts (1871–1910) who died from typhus, one type of rickettsial disease. Rickettsial disease should not to be confused with another disease called rickets, which is caused by a deficiency of vitamin D.
Rickettsial bacteria cause illness in hosts by infecting the cells and causing cell destruction or death. They tend to infect vascular cells, that is, cells lining the blood vessels, and thus cell death leads to increased permeability of these vessels. This causes changes in blood volume, concentration, and pressure, which is debilitating for the host.
Rickettsial bacteria are present in arthropods and mammals. Transmission usually occurs when an arthropod feeds on an infected mammal, sometimes becoming the intermediate host, and then feeds on a human. Humans may also become infected if they come in direct contact with the blood or feces of an infected arthropod. One rickettsial disease, Q fever, is transmitted not by arthropods, but by airborne droplets containing the bacteria.
There are many different types of rickettsial diseases. The main types are grouped into the spotted fever group, the typhus group, or with scrub typhus. Most of the diseases share similar symptoms. In general, acute symptoms of fever, headache, depression, and fatigue occur within two weeks of exposure to a bacterium. A rash also often appears a few days after the onset of fever. This rash can appear on various regions of the body, as in Rocky Mountain spotted fever, or may occur specifically as skin lesions that develop at the site of the arthropod bite. Other complications of infection include blood vessel damage and organ damage, but these symptoms depend on the severity and type of infection.
Rickettsial diseases occur worldwide. However, not all rickettsial diseases are present in all countries. Endemic (naturally occurring at a steady rate) and epidemic typhus occur worldwide, whereas North Asian tick typhus, Queensland tick typhus, and scrub typhus have particular distributions that limit them to certain locations. The distribution of rickettsial diseases is determined by the distribution of their arthropod vectors.
However, cases of specific rickettsial diseases sometimes occur in areas not known to harbor the bacterium. This is a consequence of travel. Travelers infected in one country may not exhibit symptoms until they are in another country due to the long incubation period for these bacteria.
WORDS TO KNOW
ARTHROPOD: A member of the largest single animal phylum, consisting of organisms with segmented bodies, jointed legs or wings, and exoskeletons.
INTERMEDIATE HOST: An organism infected by a parasite while the parasite is in a developmental form, not sexually mature.
VECTOR: Any agent, living or otherwise, that carries and transmits parasites and diseases. Also, an organism or chemical used to transport a gene into a new host cell.
In the United States, endemic rickettsial diseases include Rocky Mountain spotted fever, rickettsial pox, and cat-flea transmitted infection. Rickettsial diseases tend to be less prevalent during modern times than when they were first discovered. This is most likely a result of both improved prevention methods and the introduction of effective treatments. However, despite effective treatments, some rickettsial diseases can still be fatal, including Rocky Mountain spotted fever, which has an approximate mortality rate of 4%, or epidemic typhus, which has a mortality rate of approximately 10% in young adults and 60–70% in older patients. In Rocky Mountain spotted fever a late diagnosis can lead to more serious complications, which contributes to higher mortality rates. In the case of epidemic typhus, increased age can decrease a patient's chance of survival.
Effective treatment of rickettsial disease is usually involves a course of antibiotics. The best results are achieved when treatment is administered within the first week of illness. The longer the illness goes without treatment, the less chance there is of a good recovery. Tetracycline antibiotics effectively destroy rickettsial bacteria and thus derivatives from this group are most often used for treatment. The preferred tetracycline is doxycycline, since it has few negative side effects when used for short periods in low doses. Another form of tetracycline used is chloramphenicol. For patients older than 18 years of age, fluoroquinolones may be used, and have been shown to be effective against some forms of rickettsial bacteria.
Antibiotic treatment usually takes less than a week. Fever generally disappears 1–3 days after treatment begins. If the fever does not begin to subside, misdiagnosis is likely. Once a patient no longer has a fever, treatment is stopped. Other treatment, based on treating the complications caused by the bacteria, such as hypotension, coagulation, and fluid leakage, is usually given with the antibiotics. This treatment usually lasts about two weeks.
Rickettsial diseases cannot yet be prevented by vaccination. Research is underway to determine a possible vaccination for certain infections. Currently, the best prevention method is avoidance or elimination of arthropod vectors in order to decrease the chance of being bitten. Avoidance measures include using repellents when outdoors, avoiding long grass or woodlands, wearing clothing that completely covers the arms and legs, wearing boots, and thoroughly checking the body after walking through arthropod-inhabited areas. Often, quick removal of ticks that have become attached to the body can prevent infection. Elimination of arthropod vectors involves using pesticides in arthropod-inhabited areas.
One of the greatest issues surrounding rickettsial diseases is the impact that late diagnosis has on recovery. The treatments currently used for cases of rickettsial disease usually result in a successful recovery. However, the later treatment is given, the less chance there is of a good recovery. Late diagnosis may occur when patients do not visit their doctor, or when medical personnel misdiagnose the disease. Some types of rickettsial disease, such as Rocky Mountain spotted fever, sometimes feature a characteristic red rash. In the absence of this rash, the symptoms of this infection are similar to a multitude of other infections. Therefore, Rocky Mountain spotted fever may be misdiagnosed and the wrong treatment administered. Despite effective treatment for this disease, the mortality rate still stands at almost 4%, and this mortality rate is largely due to misdiagnosis and, thus, delayed administration of proper treatment.
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"Rickettsial Disease." Infectious Diseases: In Context. . Encyclopedia.com. (January 23, 2019). https://www.encyclopedia.com/media/educational-magazines/rickettsial-disease
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