Tick-borne encephalitis virus, Borrelia burgdorferi sensu lato, Borrelia miyamotoi, Anaplasma phagocytophilum and Candidatus Neoehrlichia mikurensis in Ixodes ricinus ticks collected from recreational islands in southern Norway

Abstract

The aim of this study was to determine the occurrence of tick-borne pathogens of medical importance in questing ticks collected from five recreationally used islands along the Norwegian coastline. Furthermore, since coinfection may affect the disease severity, this study aimed to determine the extent of coinfection in individual ticks or co-localization of tick-borne pathogens. In all, 4158 questing Ixodes ricinus ticks were analyzed. For detection of tick-borne encephalitis virus (TBEV), nymphs (3690) were analyzed in pools of ten. To detect Borrelia burgdorferi sensu lato, B. miyamotoi, Anaplasma phagocytophilum and Candidatus Neoehrlichia mikurensis, 468 nymphs were analyzed individually. A total of five nymph pools was infected with TBEV, giving an overall prevalence of 0.14%. In the individually analyzed ticks, B. burgdorferi s. l. (15.6%), Candidatus N. mikurensis (11%), A. phagocytophilum (1.4%) and B. miyamotoi (0.9%) were detected. Coinfection was found in 3.3% of the ticks, and the only dual infection observed was with B. afzelii and Candidatus N. mikurensis. This association was significantly higher than what would occur by random chance.

Introduction

The main tick vector for human and animal disease in Europe is Ixodes ricinus. Norway is part of the northern border of the geographical range of I. ricinus, and ticks are mainly found along the coastal regions from Østfold County in the southeast to Brønnøysund in Nordland County in the north (Mehl, 1983; Hvidsten et al., 2015; Soleng et al., 2018).

Ixodes ricinus maintains a diverse array of pathogens in enzootic cycles. Tick-borne encephalitis virus (TBEV) is the causative agent of tick-borne encephalitis (TBE), which is considered the most serious viral tick-borne human disease in Europe (Süss, 2011). The European TBEV subtype is present in Norway (Andreassen et al., 2012), however, the incidence of TBE is low; from 1994 to 2017, there have been 142 reported TBE cases infected in Norway according to Norwegian Surveillance System for Communicable Diseases (MSIS, 2018).

Ixodes ricinus may also transmit B. burgdorferi s. l., which is widely distributed throughout the tick infested areas of Norway (Kjelland et al., 2010a; Soleng and Kjelland, 2013). This group includes the causative agents of Lyme borreliosis (LB), the most important human tick-borne disease in Europe in terms of disease incidence and public attention (Franke et al., 2013). In 2016, 333 cases of disseminated LB infection were reported in Norway, a national incidence of 6.3/100.000 inhabitants (MSIS, 2018). In the southernmost parts where the tick population density is higher, the occurrence of Borrelia infection is higher. For instance, in Vest-Agder County, where one of the islands in the present study is located, the incidence is 18.5/100.000 inhabitants. However, early localized infection as erythema migrans skin lesion is not notifiable in Norway, hence the total frequency of infection is unknown. Recently, B. miyamotoi, the only tick-borne member of the relapsing fever borreliae detected in I. ricinus, was reported in Norway (Kjelland et al., 2015; Quarsten et al., 2015).

Tick-borne fever (TBF) is caused by Anaplasma phagocytophilum, and is a major scourge of the sheep industry in Norway. It has been estimated that more than 300 000 lambs, 15% of lambs on summer pasture, are infected annually (Stuen and Bergström, 2001). In humans, clinical manifestations range from a mild self-limited febrile illness, to a life-threatening collapse of the immune system (Bakken and Dumler, 2015). Human infection has been reported in Norway, but the epidemiological importance of A. phagocytophilum in this country is unknown (Kristiansen et al., 2001; Hjetland et al., 2015).

Candidatus Neoehrlichia mikurensis (Candidatus N. mikurensis) is an emerging tick-borne pathogen belonging to the Rickettsiales. The first case of human disease caused by the pathogen was reported in 2010 from Sweden (Welinder-Olsson et al., 2010). Recently, neoerlichiosis was also described in one patient from Norway (Frivik et al., 2017). Neoehrlichiosis is primarily a disease of immunosuppressed patients, who experience recurring fevers accompanied by a variety of other symptoms including musculoskeletal pain and deep-vein thrombosis (Grankvist et al., 2014).

As tick-borne pathogens often occur in the same area, wildlife and humans are frequently infected by multiple pathogens, or several genotypes of a single pathogen, simultaneously (Diuk-Wasser et al., 2016). The risk of coinfection with multiple pathogens after a tick bite differs by geographic location, depending on the prevalence of pathogens in the ticks and their host animals. However, the prevalence of coinfecting human pathogens among Ixodes ticks remains unknown in the majority of geographic locations. The aim of this study was to investigate the prevalence of multiple tick-borne pathogens in public-use recreational sites at five island locations in Norway.