Mycobacterium microti (vole tuberculosis) infections in small wild mammals were first described more than 60 years ago in several populations in Great Britain. Few studies of vole tuberculosis have been undertaken since then, and little is known about the relationship between M. microti isolates originating from different populations or at different times or of the prevalence of this infection in wild rodent populations, despite human cases of M. microti infections being increasingly reported. In this study, field voles (Microtus agrestis), bank voles (Clethrionomys glareolus), and wood mice (Apodemus sylvaticus) were found to be infected, with up to 8% having external tuberculous signs, in wild populations in Northumberland and Cheshire, England. Spoligotyping applied directly to the clinical material simultaneously detected and typed M. microti bacteria in skin lesions, lymph glands, and internal abcesses. IS6110 restriction fragment length polymorphism typing of cultured bacteria was used to compare these isolates with previously isolated strains from both animals and humans. This demonstrated that although the current rodent isolates were distinct from those isolated from voles in the 1930s in Great Britain, they had a high degree of similarity to these strains and were distinct from the M. microti isolates from humans, a pig, and a ferret from The Netherlands. Thus, M. microti infection seems to be widespread in wild rodent populations, but more studies are needed to understand how M. microti might be transmitted from animals to humans and to determine better the zoonotic risk posed.Tuberculosis (TB) in small wild mammals, namely, field voles (Microtus agrestis), bank voles (Clethrionomys glareolus), wood mice (Apodemus sylvaticus), and shrews (Sorex araneus), was first reported by Wells and Oxon in 1937 (25). The causative agent was named Mycobacterium tuberculosis subsp. muris (4) and later Mycobacterium microti (or vole tuberculosis), a member of the M. tuberculosis complex (24). The other members of this complex are M. tuberculosis, Mycobacterium bovis (including the attenuated BCG vaccine strains), Mycobacterium africanum, and the recently described subspecies Mycobacterium canetti (22,23). M. microti has a characteristic pleomorphic microscopic morphology, with sickle-shaped, spiral, or S-like forms being seen in fresh material. This typical curved appearance is, however, usually lost during in vitro culture (18). It is difficult to distinguish M. microti from other members of the M. tuberculosis complex on the basis of biochemical properties, but they are readily identified by PCR-based spoligotyping and/or IS6110 restriction fragment length polymorphism (RFLP) typing (23). The latter requires large numbers of cultivated bacteria but facilitates more detailed phylogenetic comparisons.Well's early studies showed a varying prevalence of M. microti of 9 to 31% in field voles, depending on the place and season of capture, and overall prevalences of 11% in bank voles, 2% in wood mice, and 1.5% in shrews (...