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A nested polymerase chain reaction-restriction fragment length polymorphism (nPCR-RFLP) assay based on the amplification of the Anaplasma spp. highly conserved msp5 gene and posterior digestion with HindIII endonuclease was developed and evaluated in field samples. Results were compared using an nPCR specific for Anaplasma marginale (nPCR-Am) based on the msp1β gene and an nPCR specific for A. centrale (nPCR-Ac) based on the msp2 operon (msp2-o) gene. Amplicons dilutions of msp1β and msp5 of A. marginale and msp2-o and msp5 of A. centrale and dilutions of parasited erythrocytes (PE) with A. marginale and A. centrale were used to determine the detection limits. The results were 20 DNA copies/reaction and 30 PE for A. marginale and A. centrale by nPCR-RLFP and nPCR-Am/Ac. A mix of msp5-Am and msp5-Ac was used to evaluate the interference of msp5 from one species for the detection of the other. Co-amplification of the DNA from both species was observed up to a 1:7 ratio of one species to the other. Field samples positive for Anaplasma spp. antibodies (n = 260) from 32 herds were evaluated. Strength of agreement between results by nPCR-RFLP and nPCR-Am or nPCR-Ac was 78% (κ = 0.44) and 94% (κ = 0.85), respectively. Thirty-four samples were positive for A. marginale by nPCR-RFLP but negative by nPCR-Am. msp1β amplicons of 10 samples from 5 herds with discrepancies between nPCR-Am and nPCR-RFLP results were cloned and sequenced. The analysis of the msp1β sequence showed several mutations in the target region of the internal forward primer that would explain the failure in the amplification. Only 10 of the 20 coinfections identified by nPCR-Ac/nPCR-Am were detected by nPCR-RFLP. nPCR-RFLP is a sensitive, low-cost and accessible molecular method for low-complexity laboratories. More studies are needed to establish in which circumstances coinfections can be underestimated.

The aim of this study was to evaluate the influence of the long-acting oxytetracycline (OTC) treatment on A. marginale genotypes of the isolate S1P, by analyzing the msp1α genotype based on a microsatellite (ms) and tandem repeat sequences (TRS) located at the 5´ end of the gene. DNA samples were obtained from a longitudinal study of chemosterilization; 10 2-year-old steers were experimentally infected with blood from a splenectomized calf inoculated with the A. marginale isolate S1P. All the steers had received a first dose of 20 mg kg−1 OTC to treat acute disease, and once recovered all steers received a sterilizing treatment based on three doses of 20 mg kg−1 OTC 7 days apart. Blood from two steers not sterilized by the treatment was inoculated into two splenectomized calves (receptors) 104 days after treatment. DNA samples (S) used for msp1α amplification were obtained from i) the donor calf (S0), ii) 10 steers during acute disease (S1), after the first antibiotic treatment (S2), and after the chemosterilization procedure (S3 and S4), and iii) two receptor calves (S5). Thirty clones from the donor calf and at least 5 clones from the other DNA samples were analyzed. The genotype E/αββββГ msp1α identified in the donor calf and steers, before OTC treatment, was not detected either in steers that continued infected after the sterilizing treatment or in the receptor calves, in which only genotype C/EϕFF msp1α was observed. These results highlight the existence of A. marginale genotypes with different sensitivity to OTC and the importance of other variables to successfully sterilize the carriers.