Other Flea Borne Rickettsial Agents

In addition to the causative agent of murine typhus, several other rickettsial agents may be transmitted to humans by fleas. One of these, Coxiella burnetii, the agent of Qfever, can be transmitted not only by fleas but also by other blood-feeding arthropods, infected mammalian tissues, infective fomites, or aerosol. Sylvatic epidemic typhus (sporadic epidemic typhus) is a curious but potentially serious disease that occasionally is diagnosed in humans in the United States. The agent of this disease is R, prowazekii, which causes classic epidemic typhus transmitted to humans by the body louse, P. h. humanus (see Chapter 4). However, flying squirrels (Glaucomys volans), rather than humans, are the reservoir hosts of sylvatic epidemic typhus. Flying-squirrel fleas, especially the widespread squirrel flea Orchopeas howardi, and lice also harbor the causative rickettsiae. The exact mode of transmission to humans is unknown, but because ectoparasites of flying squirrels rarely feed on humans, it is hypothesized that under certain conditions infective rickettsiae in the feces of fleas and lice become aerosolized and may be inhaled by humans. Infected flying squirrels are not adversely affected. These squirrels sometimes are closely associated with humans through their predilection for constructing nests in attics or eaves of houses. McDade (1987) provides additional discussion of this rickettsial zoonosis.

A recently discovered rickettsial agent is R. felis (formerly named the "ELB agent" for EL Laboratories in Soquel, CA. This rickettsia has been found in Virginia opossums and cat fleas (which are common ectoparasites of this opossum) in the United States and has been shown to have caused infection in humans that are serologically positive for infection with R. typhi, the etiologic agent of murine typhus. Definitive demonstration of infection by either R. felis or R. typhi involves PCR amplification of specific nucleotide primers. Thus, it is likely that some human infections serologically attributed to R. typhi are actually caused by R felis. R. felis is also of interest because it is transmitted transovarially in fleas, and fleas rather than mammals may be the reservoirs for the organism. Azad et al. (1997) provide further information on R. felis infections. In addition to these rickettsiae, some other fleas are known to harbor symbiotic rickettsiae about which little is currently known.

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