Human cyclic thrombocytopenia and Anaplasma spp. infection

To the Editor: Cyclic thrombocytopenia (CTP) is an uncommon condition characterized by regularly oscillating platelet counts that manifests as periodic thrombocytopenia of variable severity (1). While some cases have been associated with myeloproliferative disorders, the etiology is mostly unknown (1). To date, less than 100 patients have been reported in the literature. However, we have seen three patients during the past several years at our institution in La Crosse (Wisconsin). The pertinent clinical features of these patients are summarized in Table 1. Interestingly, a similar disorder known as canine CTP occurs in dogs infected with Anaplasma platys, a species closely related to the agent causing human granulocytic ehrlichiosis (HGE), A. phagocytophila (2). Thrombocytopenia occurs in most patients during the acute phase of HGE and is partly immune mediated (3). While our region is not highly endemic for HGE (4), we have documented cases apparently acquired locally (5). To test the possibility that CTP in our patients is associated with HGE, we evaluated fresh blood samples from each patient for evidence of current or past infection with A. phagocytophila. The Gundersen Lutheran Research Committee and Institutional Review Board approved the protocol and informed consent was obtained from each patient prior to obtaining a blood sample. None of the patients were treated with antibiotics within the previous 6 months. To provide direct confirmation of a current infection, we examined blood smears for characteristic morulae (Wright stain) and evaluated whole blood by using a polymerase chain reaction (PCR) procedure described previously (5). The primers detect a target sequence within the 16S rRNA gene that is also conserved 100% in other Anaplasma spp. including A. platys (6). Briefly, a mixture containing each primer, buffer, dNTPs, MgCl2, and AmpliTaq was combined with DNA extracted from the blood, amplified with a thermal cycler, and detected by electrophoresis and staining. Positive (DNA extracted from a patient infected with A. phagocytophila) and negative (water) controls were included. However, the patients were not actively infected with A. phagocytophila or any other closely related Anaplasma spp. , because we did not detect morulae or amplify Anaplasma spp. DNA. In addition, we evaluated the blood serum for indirect serologic evidence of a past infection by using an immunofluorescence antibody (IFA) assay (7). A. phagocytophila-infected HL-60 cells were fixed to a Teflon-coated slide and incubated with the patient serum. Bound antibodies were then detected using FITC-labeled goat anti-human IgM, IgG, and IgA sera. The procedure would have detected antibodies produced against numerous Anaplasma spp. , but we found no significant levels of these antibodies. In conclusion, although canine CTP is strongly correlated with A. platys infection, our study does not support a relationship between A. phagocytophila or other Anaplasma spp. infection and CTP in the three patients seen at our institution. Because of the limited number of patients in our study, it is premature to generalize our findings to all CTP patients. Additional studies into the relationship between Anaplasma spp. infection and CTP by other investigators are necessary.