Evaluation of Disease Causality of Rare Ixodes ricinus-Borne Infections in Europe

Abstract

In Europe, Ixodes ricinus ticks transmit pathogens such as Borrelia burgdorferi sensu lato and tick-borne encephalitis virus (TBEV). In addition, there is evidence for transmission to humans from I. ricinus of Anaplasma phagocytophilum, Babesia divergens, Babesia microti, Babesia venatorum, Borrelia miyamotoi, Neoehrlichia mikurensis, Rickettsia helvetica and Rickettsia monacensis. However, whether infection with these potential tick-borne pathogens results in human disease has not been fully demonstrated for all of these tick-borne microorganisms. To evaluate the available evidence for a causative relation between infection and disease, the current study analyses European case reports published from 2008 to 2018, supplemented with information derived from epidemiological and experimental studies. The evidence for human disease causality in Europe found in this review appeared to be strongest for A. phagocytophilum and B. divergens. Nonetheless, some knowledge gaps still exist. Importantly, comprehensive evidence for pathogenicity is lacking for the remaining tick-borne microorganisms. Such evidence could be gathered best through prospective studies, for example, studies enrolling patients with a fever after a tick bite, the development of specific new serological tools, isolation of these microorganisms from ticks and patients and propagation in vitro, and through experimental studies.

Emergence and Epidemiology of Bovine Babesiosis Due to Babesia divergens on a Northern German Beef Production Farm

“…Additionally, detection of B. microti indicates abundance of rodents, which are preferred hosts of I. ricinus larvae. Like B. divergens, B. microti as well as B. venatorum are of zoonotic importance ( 24 ), although clinical B. microti infections of humans have so far only been acquired in the Americas ( 3 ), suggesting that European B. microti strains may be less pathogenic. Here, B. microti and B. venatorum were detected at similar prevalences in adult and nymphal ticks (0.88%/0.77% and 0.44%/0.77%, respectively) compared to studies from other European countries ( 25 – 27 ).…”

“…In cattle, Babesia bigemina and Babesia bovis are of worldwide economic importance ( 1 ), whereas in European countries located north of the Alps, Babesia divergens is the main cause of bovine babesiosis ( 2 ). Furthermore, B. divergens —amongst other Babesia spp.—may cause potentially fatal disease in humans, primarily in splenectomized or immunologically compromised people, but also in immunocompetent individuals ( 3 ).…”

Tripartite Interactions among Ixodiphagus hookeri, Ixodes ricinus and Deer: Differential Interference with Transmission Cycles of Tick-Borne Pathogens

“…These increases are partially explained by geographical spread of its vector, Ixodes ricinus [4,5]. Other tick-borne diseases (TBDs), such as anaplasmosis and neoehrlichiosis, are also emerging [6,7]. Understanding which factors drive the population densities of ticks and the transmission cycles of tick-borne pathogens are important steps in assessing disease risk and formulating possible intervention strategies.…”

Zoonotic Tick-Borne Pathogens in Temperate and Cold Regions of Europe—A Review on the Prevalence in Domestic Animals

“…However, clinically symptomatic human B. microti infections reported in Europe were mostly acquired in the Americas, so it is unclear whether European B. microti strains are human-pathogenic (154). Furthermore, Babesia venatorum, a parasite of deer, has been recognized as a human pathogen in immunocompromised patients in Europe (154). Regarding domestic animals, only cattle are affected by and act as reservoirs for a Babesia spp.…”