The recent detection of Anaplasma phagocytophilum in Portugal stimulated further research on the agent's enzootic cycle, which usually involves rodents. Thus a total 322 rodents belonging to five species, including 30 Apodemus sylvaticus (wood mouse), 65 Mus musculus (house mouse), 194 M. spretus (algerian mouse), 5 Rattus norvegicus (brown rat) and 28 R. rattus (black rat), were studied by indirect immunofluorescent assay (IFA) and/or polymerase chain reaction (PCR) for A. phagocytophilum exposure in four sampling areas of mainland and two areas of Madeira Island, Portugal. Overall, 3.6% (7/194) of M. spretus presented with IFA-positive results. Seropositive mice were detected in all three mainland sampling areas where this species was captured, with prevalence of 5.2% (5/96) and 5.0% (1/20) for the Ixodes-areas of Arrábida and Mafra, and 1.3% (1/78) for Mértola, a difference that was not statistically significant (p > 0.05). The majority of IFA-positive mice were detected in spring when considering either Arrábida alone (p = 0.026) or all M. spretus sampling areas together (p = 0.021), although the significance of this association was not evident after Bonferroni correction. Nevertheless, neither the seropositive M. spretus, nor additional samples of 10% seronegative rodents from mainland, and 16% of rodents collected in Madeira Island showed evidence of A. phagocytophilum active infections when spleen and/or lung samples were tested by PCR. Either the M. spretus results represents residual antibodies from past A. phagocytophilum infections, present infections with limited bacteremia, or cross-reactions with closely related agents deserves more investigation.
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