A.S. Santos, F. Amaro, M.M. Santos-Silva, R. De Sousa, M.L. Mathias, M.G. Ramalhinho, M.S. Nuncio, M.J. Alves, F. Bacellar, J.S. Dumler, Centro de Estudos de Vectores e Doenças Infecciosas, Instituto Nacional de Saúde Dr. Ricardo Jorge, Lisboa, Portugal

Vector Borne Zoonotic Dis. 2009 Dec;9(6):663-9

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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VBORNET comment: 2010-01-24
The paper proposes to investigate the potential role of Portugese rodent species in the maintenance of A. phagocytophylum enzootic cycles. In this country, A. phagocytophylum has been previously detected in two Ixodes species and an active infection has been recently reported in a horse, as well as presence of antibodies against this agent in Ixodes-exposed patients. 3.6% of M. spretus present IFA-positive results, suggesting the exposure of this species to the pathogen. Seropositive animals were detected in the whole country and the highest seroprevalences were observed in the areas where Ixodes is present. However, although all the other rodent species were found negative, no significant difference exists between these ones and M. spretus; this result denies the hypothesis of peculiar species sensitivity. Moreover, the rodents that were positive in IFA were not positive in PCR, suggesting that M. spretus may not be a reservoir of A. phagocytophylum. Other explanations are also proposed such as the low level of bacteria resulting in the lack of detection using PCR or the possibility of cross-reactions of the serological test with agents that share antigenic similarities with A. phagocytophylum. This final discussion from authors shows the high importance to continue investigations and resolve such issues.