A previously unknown species of the tick-borne Ehrlichia bacterium was responsible for mysterious infections affecting four people in Wisconsin and Minnesota in 2009, researchers reported in the Aug. 4 issue of the New England Journal of Medicine.
Culturing, serological testing, and DNA analysis confirmed that the four individuals, all of whom recovered, were infected with a still unnamed Ehrlichia species distinct from E. chaffeensis and E. ewingii, the most common pathogens responsible for ehrlichiosis in the U.S., according to Dr. Bobbi S. Pritt of the Mayo Clinic in Rochester, Minn. and colleagues.
Currently designated as Ehrlichia Wisconsin HM543746, the new species appears most closely related to E. muris, which until recently was thought to be confined to eastern Europe and parts of Asia. Pritt and colleagues found the two species to be 98 percent genetically similar.
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However, another research group recently reported that E. muris, or a very similar bacterium, was present in Wisconsin deer ticks collected in the 1990s.
Pritt and colleagues said the discovery is significant because ehrlichiosis is rare in Wisconsin and Minnesota and the new species would not have been identified with conventional tests.
The four patients highlighted in the NEJM report had not been singled out for special investigation. Instead, they were discovered in the course of a larger study of suspected ehrlichiosis cases nationwide.
Pritt and colleagues had collected more than 4,000 blood samples from patients with suspected ehrlichiosis or anaplasmosis (a closely related infection) across the United States in 2009, including 1,519 from Wisconsin and Minnesota.
None of the samples from the two states were positive for E. chaffeensis or E. ewingii, although 163 contained DNA for Anaplasma phagocytophilum, which causes anaplasmosis.
But molecular testing showed that four of the samples were anomalous, prompting additional investigations that led to the new discovery.
The four patients contributing these samples -- two of whom were organ transplant recipients taking immunosuppressants -- had presented in the summer and early fall of 2009 with fever, headache, and fatigue; one patient also had nausea and vomiting. Blood tests showed low levels of lymphocytes in all patients and low platelet counts in three.
Because the symptoms and the patients' previous travel had suggested possible tick-borne infections, they were treated with doxycycline or ceftazidime and all of them recovered.
Organisms were cultured from their blood, with DNA sequences that matched those obtained from the clinical specimens.
Besides analyzing the clinical samples, Pritt and colleagues also tested 697 Ixodes scapularis ticks from the two states. They found that 17 (including seven nymphs and nine adults) carried organisms with the same sequences as the four patients.
That finding, too, was significant, Pritt and colleagues wrote, because the most common vector for pathogenic Ehrlichia species in the U.S. is a different tick species, Amblyomma americanum.
That species, however, cannot withstand the Upper Midwestern winters -- perhaps one reason that ehrlichiosis is uncommon in the region.
I. scapularis, on the other hand, is "abundant" there, according to Pritt and colleagues.
They cautioned that their findings did not conclusively implicate it as the vector for the new Ehrlichia species. "Extended investigation and tick surveillance are required to understand the distribution of this agent in Wisconsin and Minnesota," they noted.
In the meantime, Pritt and colleagues wrote, "physicians should be aware that a novel pathogenic Ehrlichia agent is present in Minnesota and Wisconsin and that organism-specific PCR [polymerase chain reaction] and serologic testing can be used to identify the cause of suspected infections."