Deer ticks are well known to spread dangerous bacteria, and in 2009 two U.S. States, Minnesota and Wisconsin in particular saw the emergence of a previously unknown species of the tick-borne Ehrlichia bacterium that was responsible for mysterious infections in the region, previously thought to only exist overseas.
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 98% sequence homology between the two species.
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 Bobbi S. Pritt, MD, of the Mayo Clinic in Rochester, Minnesota.
Pritt and colleagues had collected more than 4,000 blood samples from patients with suspected ehrlichiosis or anaplasmosis (a closely related infection) across the U.S. in 2009, including 1,519 from Wisconsin and Minnesota. None of the samples from the two states were positive for other strains 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.
Patients contributing these samples, two of whom were organ transplant recipients taking immunosuppressants however, 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.
Pritt and colleagues also tested 697 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. Eureka.
The most common vector for pathogenic Ehrlichia species in the U.S. is a different tick species, Amblyomma americanum. That species in particular however, cannot withstand the Upper Midwestern winters -- perhaps one reason that ehrlichiosis is uncommon in the region.
The scientists 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. 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."
The prevalence of lyme disease (LD) in the northeast and upper mid-west is due to the presence of large numbers of the deer tick's preferred hosts, white-footed mice and deer, and their proximity to humans. White-footed mice serve as the principal carriers on which many larval and juvenile ticks feed and become infected with the LD spirochete. An infected tick can then transmit infection the next time it feeds on another host, namely...you.
Written by Sy Kraft