Lyme Disease and Human Granulocytic Anaplasmosis Coinfection: Impact of Case Definition on Coinfection Rates and Illness Severity

Background. Lyme disease is transmitted by the bite of the Ixodes scapularis tick, which can also transmit Anaplasma phagocytophilum, the cause of human granulocytic anaplasmosis (HGA). Conflicting data exist on the frequency of coinfection and on whether Lyme-HGA coinfected patients have more symptoms than patients with Lyme disease alone. Methods. Blood culture and serology were used to detect HGA infection in patients with early Lyme disease who presented with erythema migrans. The rate of coinfection was determined using different definitions. The clinical and laboratory features of Lyme-HGA coinfection were compared with that of the individual infections. Results. Among 311 patients with erythema migrans, the frequency of coinfection with HGA varied from 2.3% to 10.0%, depending on the definition used (P< .001). Only 1 of 4 groups with presumed coinfection had significantly more symptoms than patients with Lyme disease alone P< .05. High fever and cytopenia were less common in Lyme-HGA coinfection than in patients with HGA alone. Conclusion. The results of this study indicate that how HGA is defined in patients with early Lyme disease has an impact on the apparent rate of coinfection and the severity of illness. The findings also suggest that HGA may be less severe than is usually believed, suggesting the existence of referral bias in testing patients preferentially who present with high fever or cytopenia.