Human Co-Infections between Borrelia burgdorferi s.l. and Other Ixodes-Borne Microorganisms: A Systematic Review

When it comes to tick-borne diseases, co-infections are often mentioned. This concept includes several entities. On the one hand, tick vectors or vertebrate reservoir host can harbor several microorganisms that can be pathogenic for humans. On the other hand, human co-infections can also be understood in different ways, ranging from seropositivity without clinical symptoms to co-disease, i.e., the simultaneous clinical expression of infections by two tick-borne microorganisms. The latter, although regularly speculated, is not often reported. Hence, we conducted a systematic review on co-infections between B. burgdorferi s.l., the etiological agent of Lyme borreliosis, and other microorganisms potentially transmitted to humans by Ixodes spp. ticks. A total of 68 relevant articles were included, presenting 655 cases of possible co-infections. Most cases of co-infections corresponded to patients with one tick-borne disease and presenting antibody against another tick-borne microorganism. Co-disease was particularly frequent in two situations: patients with clinical symptoms of high fever and erythema migrans (EM), and patients with neurological symptoms linked to the TBEv or a neuroborreliosis. No impact on severity was evidenced. Further studies are needed to better appreciate the frequency and the impact of co-infections between several tick-borne microorganisms.