Rickettsial infections are caused by bacterial organisms (Rickettsiae), which are found throughout the world. The genus Rickettsia is included in the bacterial tribe Rickettsiae, family Rickettsiaceae, and order Rickettsiales.
Family Rickettsiaceae comprise a group of microorganisms that phylogenetically occupy a position between bacteria and viruses. Rickettsiae are small, non-flagellate, gram negative pleomorphic cocco-bacilli adapted to obligate intracellular parasitism and transmitted by arthropod vectors.
The diseases caused by Rickettsia species are often collectively referred to as rickettsioses. Rickettsiae live in small insects such as ticks, fleas, mites, and lice and can infect mammals (including humans), but cannot survive for any significant period outside their host.
Among returned travelers, rickettsial diseases have been estimated to be the fourth most common cause of fever, with symptoms such as rash, abdominal pain, and a dry, black/dark scab at the site of the infecting bite.
The incubation period ranges from 2-14 days. The disease begins with fever, myalgia and headache. Headache is usually quite severe, young children may not complain of pain at all. Onset of nausea, vomiting, pain abdomen, diarrhea and abdominal tenderness occur in substantial number of patients suggesting gastroenteritis or an acute surgical abdomen.
Tick-borne rickettsioses are often acquired in rural settings in risk areas; however, urban transmission also occurs. Most rickettsial infections are transmitted to humans by the bite or feces of fleas, ticks, mites, or lice. Even the smallest forms of ticks (such as larvae) can transmit the infection to humans, meaning that many infected individuals do not recall a tick bite before symptoms appear.
Rickettsiae microorganisms appear to exert their pathologic effects by adhering to and then invading the endothelial lining of the vasculature within the various organs affected.
Infected ticks and mites transmit rickettsiae primarily through their infected salivary glands directly into the bite. In lice and fleas, rickettsiae do not infect the salivary glands but multiply in the cells of the gut wall and are excreted in the faeces in large amounts, contaminating the skin area around the bite.
The most important pathophysiologic effect is increased vascular permeability with consequent edema, loss of blood volume, hypoalbuminemia, decreased osmotic pressure, and hypotension.
Rickettsial infections
