This is the first report genetically linking a Bartonella species isolated from a human to a Bartonella species isolated from an indigenous North American rodent. The ability to accurately identify the strains of infectious agents that cause disease is central to epidemiological surveillance and public health decisions. Analyses of a diverse array of Bartonella strains that have been recovered from a wide range of wild mammals provide a useful tool for identification of potential animal sources of human cases of Bartonella infection. The finding that Bartonella strains obtained from ground squirrels in Nevada are identical to the strain isolated from the patient provides evidence for the transmission of Bartonella species from rodents to humans.
Comparisons of sequences from seven genes have been used for phylogenetic analyses of Bartonella
species, but the present knowledge about Bartonella
taxonomy is supported mostly by the sequence information of three genes, namely the 16S rRNA, citrate synthase (gltA
), and 60-kDa heatshock protein (groEL
) genes (4
). DNA sequencing of these target genes, as well as of other Bartonella
genus- and species-specific genes, should prove valuable in providing information on the regional diversity and classification of rodent isolates and the molecular epidemiology of future human bartonellosis cases.
The finding of Bartonella strains in California ground squirrels which are identical or very similar to B. washoensis is particularly interesting from an epidemiological perspective because these rodents are often closely associated with areas of human habitation, recreational areas, and agricultural lands, making it likely that humans will come into contact with infected animals. Both sites where squirrels infected with B. washoensis were identified lie within urban areas of the city of Reno. These sites offer excellent habitats that support large colonies of California ground squirrels, as well as being very popular recreational areas. Washoe County is a good example of an area experiencing rapid growth as residential development occurs in previously undisturbed areas. The amount of human interaction with California ground squirrels is likely to increase.
The role of California ground squirrels in the maintenance and transmission of plague (Yersinia pestis) is well documented. While much remains to be determined concerning the incidence of disease, mode of transmission, and possible vector species associated with B. washoensis, it appears likely that contact between people and California ground squirrels remains a significant link in the epidemiology of the disease.
Investigators were unable to determine how the abovementioned human patient with myocarditis became infected with B. washoensis.
While the patient denied handling California ground squirrels, making this route of acquisition unlikely, his residence is located in a rural setting with habitat suitable for populations of different rodent species. Other more well-known species of Bartonella
are believed to be transmitted, at least in part, by arthropod vectors. For example, B. bacilliformis
, the etiologic agent of Carrion's Disease, is transmitted by sand flies, and B. quintana
, which causes trench fever, can be transmitted by human body lice (5
). Recently, cat fleas (Ctenocephalides felis
) have been reported to be capable of transmitting B. henselae
to cats, perhaps through the contamination of cat skin with flea feces (8
). Ticks also have been reported recently to be naturally infected with various species of Bartonella
, although transmission of these agents by ticks has yet to be demonstrated (6
The implication of cat fleas as vectors of B. henselae
is noteworthy because S. beecheyi
are often heavily infested with Oropsylla montana
, a flea species that readily feeds on humans and generally is considered to be the most important vector of human plague in the United States (2
). These squirrels also often support heavy infestations of another flea species, Hoplospyllus anomalus
, that is known to feed at least occasionally on humans (20
). Humans are most likely to be exposed to the bites of ground squirrel fleas when the normal hosts of these insects die as a result of injury, predation, or disease. In the western United States, humans are most at risk of plague exposure when plague epizootics cause high mortality among S. beecheyi
or other major hosts of O. montana
and S. lateralis
), thereby forcing infectious fleas to seek alternative hosts, including humans. If California ground squirrel fleas transmit B. washoensis
to humans, then it is possible that the risk of human exposure to this bacteria will increase significantly following the occurrence of plague epizootics. Although it is possible that other arthropods can transmit B. washoensis
to humans, it should be noted that ticks are rarely encountered on California ground squirrels from the Reno area.
The identification of this new human pathogen in a common peridomestic rodent of the far western United States adds public health significance to rodent sanitation. If fleas or other arthropods are identified as vectors, then it also will be important to monitor flea densities and apply appropriate flea control when the risk of flea bite exposure is high, especially in areas such as parks and campgrounds that have high public usage. Further investigation is needed to define the role of ectoparasites in the route of transmission. It is also important to continue efforts to identify additional human cases of B. washoensis infection in order to better evaluate the extent and importance of this disease agent.