Rickettsia is a unique genus classified between a virus and a bacteria. The organism is unique in that it has a cell wall and exists only in living cells like a virus, but is susceptible to antibiotics like a bacteria. Only the species Rochalimea quintana is not able to multiply inside living host cells. For now, rickettsiae is considered to be a small obligate parasitic bacterium with widely varying shapes. There are two other genera sharing the tribe Rickettsieaeas with Rickettsia. These are Rochalimaea and Coxiella. There are sixteen known species of Rickettsia that cause diseases in humans. Most are readily inactivated when they leave the host. Therefore, their transmission usually depends on direct contact or vapor transmission. With the exception of Q fever, all other rickettsial diseases require an arthropod vector (fleas, mites, ticks, and lice) for successful transmission between hosts.
The vector, whether flea, louse, tick, or mite, can travel only a short distance in search of a new host. Thus, most diseases are associated with crowded conditions and poor hygiene. The vector itself will die within one to three weeks, but, during that time, rickettsiae will proliferate in its digestive tract and be excreted in its feces. When an infected vector bites a human, it leaves its feces at the site, and itching forces the parasite into the bloodstream of the host. Those recovering from a rickettsial disease can carry the microorganism in their tissues for the remainder of their lives.
A few of the rickettsial diseases are listed along with their common vector:
- Boutoneuse fever (Rickettsia conorii) is carried by ticks.Ehrlichiosis (Ehrlichia chaffeenis, E. canis) is a rickettsia-like disease carried by ticks.
- Q fever (Coxiella burnetii) is carried by ticks.
- Rickettsial pox (R. akari) is carried by mouse mites.
- Rocky mountain spotted fever (R. rickettsii) is carried by certain ticks. R. sibirica, R. conorii, R. australis are also of the spotted fever group.
- Trench fever (Rochalimaea quintana) is carried by lice.
- Typhus is transmitted by body lice, rat fleas, or mites of various rodents. There are many types of typus which should not be confused with typhoid fever, a more serious disease, caused by Salmonella typhi. A few of the types of typhus include: epidemic or classic typhus (R. prowazekii), endemic or murine typhus (R. typhi, R. mooseri), and scrub typhus (R. tsutsugamushi).
- Bartonella quintana was once known as Rochalimea quintana and Rickettsia quintana. Spread by body, head, and pubic lice, the organism causes disease common to the homeless and those in such places as refugee camps. The disease is known by various names including “shinbone fever,” “five-day fever,” “Wolhynia fever,” “Quintana fever,” and “His-Werner disease.”
Rocky Mountain spotted fever is rare, but it still causes some panic. In the US, there are about 500 to 1,000 cases every year, with a mortality rate of about 7%, if antibiotic treatment is not started immediately. It seems to be limited to the Western Hemisphere, with the Eastern Hemisphere having other types of tick fevers. The microbe depends on certain Ixodidae, or hard-backed ticks, for its survival. Since ticks have their own specialty, ricksiettia do not inhabit the “deer” ticks which cause Lyme disease.
The earliest reports of Rocky Mountain spotted fever came from doctors in the Idaho Snake River region, who called it the “black measles.” But, in 1902, a doctor in Great Falls, Montana, identified the association between the disease and tick bites. Between 1906 and 1909, a young pathologist, Howard T. Ricketts of the University of Chicago, began to study this association in earnest. When money promised from the governmentt of Montana did not reach him, Ricketts left for Mexico to study typhus. It killed him in 1910 at the age of 39. In his honor, the Rickettsia organism was officially given his name in 1916.
Rocky Mountain spotted fever has an incubation period of three to fourteen days. Victims will develop a high fever, which can last several weeks. Three days later, a rash will begin to appear on the extremeties (wrists, ankles, soles of the feet and palms of the hands, and forearms). Within a few hours, the rash spreads to the rest of the body. The signs and symptoms result from inflammation and swelling of the small blood vessels that can stop or reduce the blood flow, causing some leakage in the surrounding tissues. This leakage produces spots or rashes that are characteristic to rickettsial diseases. Headache, chills, and fever are present because of generalized inflammation. This is often followed by stupor, delirium, and shock caused by alterations in blood flow to the brain and other vital organs. R. rickettsii often requires fresh blood to become activated, meaning that the tick must stay attached to a human for several hours before the disease can be transmitted. There are many ways recommended for tick removal, including using a light of some kind or petroleum jelly to cause it to back out, or a drop of alcohol – whatever works best. Just be sure to examine all the areas of the body after a day in the woods.
Tetracyclines and chloramphenicol are the drugs of choice; and antibiotics must be taken immediately, even before the test results are back. Antibiotics taken three or more days after the initial illness begins, take longer to kick in. Recovery is slow, but one attack does brings lifelong immunity. If left untreated, the disease can progress rapidly to pneumonia, shock, brain or heart damage, and sometimes death. Before the advent of antibiotics in the late 1940’s, such fevers claimed about 20% of the infected population.
Generally, the tick-borne diseases found in the Eastern Hemisphere are milder than those found in the West. It is still considered to be of epidemic proportions in Russia and Poland, as well as in Mexico and Northern Africa. Ehrlichia chaffeenis is a rickettsial-like bacterium that was first identified in 1986 in the southern and mid-Atlantic US states. It is known to cause systemic infections with fever, headache, low WBC’s, and occasionally meningitis.
Typhus can produce two types of illnesses that are rickettsial diseases. Epidemic typhus is caused by Rickettsia prowazekii carried by body lice from person to person. Endemic typhus is caused by Rickettsia typhi, brought to humans by rats that carry the same fleas that can transport bubonic plague (Yersinia pestis). Both stem from filthy overcrowded living conditions. The body louse (Pediculus humanus) feeds on infected blood, then jumps to other hosts, where it defecates live R. prowazekii organisms that get scratched into the skin. In order to stop the disease, the lice must first be eradicated.
The first recorded outbreak of typhus was in a Sicilian monastery in 1083. From there, it swept through Europe. Natives in the New World died in great numbers by this disease – as well as several others. The worst epidemic of typhus occurred in Russia about the time of the Bolshevik Revolution (between 1917 and 1921) when twenty-five million cases and three million deaths were recorded. Having swept the world at one time or another, typhus continued to plague concentrated populations until DDT brought an end to it – and creating a whole new set of problems. Today, antibiotics prove effective – so far. After a bout of typhus, there may develop a further mild case called Brill-Zinsser disease (Recrudescent typhus). These people could be the focus of any new epidemic in crowded environment where body lice are present. Virulence of the disease varies with death rates as high as 70% with epidemic typhus or less than 1% with rickettsial pox.
There are many stories of armies being virtually wiped out with epidemics of typhus. Often it was not military might and strategy that won the war, but rather who best survived an epidemic of typhus. In 1489, for example, only 3,000 Moorish troops died in battle, while 17,000 died of typhus. In 1528, just as the French were in a position to win at Naples, typhus killed 30,000 of their troops, resulting in their defeat. Napolean’s army of 200,000 was reduced significantly when 180,000 died from typhus during his campaign to conquer Moscow. One of the last major outbreaks to have occurred was in Southeastern Europe during WWI. Between 1915 and 1918, typhus was responsible for 1/5th to 1/3rd of all illnesses in the British forces, and for about 1/5th of the German and Austrian armies. An epidemic in Russia a short time later was the last major outbreak, where an estimated thirty million people contracted the disease and three million died.
Q fever is acquired via bites, inhalation, direct contact with infected animals tissues or fluids, or the ingestion of contaminated food or milk. The “Q” comes from “Query” fever because the cause was not known for some time. Rickettsia burnetii is now called Coxiella burnetii, and is considered to be more stable than other rickettsia. It is also able to survive for long periods outside the host, allowing transmission by indirect means. The organism can be transmitted through bites, as well as being spread in saliva, urine, and milk. High concentrations are present in the placenta and amniotic fluid of infected animals. These rickettsia remain viable on drying, and can be carried on dust particles through the air, especially from barns or other indoor livestock areas. The airborne transmission results in pneumonitis, while the other transmission methods produce a disease similar to influenza. Some strains are associated with serious infections that cause scarring of the liver and heart valves. Another difference is that there is no rash with Q fever. Plus, it is a milder disease than the spotted fevers. Q fever is found throughout the world, and occurs most frequently in people working directly with livestock or meat-producing industries.
Piscirickettsia salmonis, recognized in 1989, is the first rickettsial pathogen of fish to be fully characterized. This impact is becoming increasingly apparent as growing awareness has led to the discovery of rickettsial diseases among diverse species of fish from different geographic locations and aquatic environments. The source, reservoir, and mode of transmission of these agents, as well as appropriate methods of disease prevention and control still has to be established. The earliest report of a rickettsialike organism in fish occurred in 1939 during examination of diseased Tetrodon fahaka from the Nile River in Egypt.
