Murine typhus, also known as endemic typhus, is one of the most widely distributed flea borne diseases. The causative agent of murine typhus is Rickettsia typhi, previously known as R. mooseri. It is a relatively mild febrile illness with 6 to 14 days of incubation period [1–3]. It is considered less pathogenic than R. rickettsii and R. prowazekii (in terms of mortality rate), but R. typhi is virulent enough to cause severe infection in the elderly population . The major reservoir of the pathogens is the rat (Rattus rattus and R. norvegicus) with the rat flea (Xenopsylla cheopis) as the main vector. Fleas are infected by transovarian transmission or acquire the contagion while feeding on an infected animal . R. typhi is transmitted to the human body or vertebrate host by infected fleabites, or contamination of the broken skin, respiratory tract or conjunctivae of the host with infected feces or tissues during and after flea feeding [2, 3].
The flea once acquiring the infection remains infective for life. Interestingly, neither flea nor rat is harmed by the pathogens . Although humans are infected mainly via rat fleas, murine typhus exists endemically in many places where rat and rat fleas are absent . In the United States, the reported cases of murine typhus are focused in south and central Texas, Los Angeles and Orange County, California, where rats and rat fleas are rarely documented. The cat flea/opossum cycle may be one of the possibilities responsible for the disease.
The clinical symptoms of infection with R. typhi in humans are fever, headache, and myalgia. The fever lasts about 12 days in adults with temperature ranges between 102-104F . In severe cases the pathogen can cause meningoencephalitis, interstitial pneumonia and disseminated vascular lesions .
Many researchers have reported the response of animals to different doses of Rickettsia typhi in order to develop effective therapy and to study the pathology of infected animals. The purpose of this study is to develop dose-response models and to compare the responses in term of age, route of infection and time post inoculation.