Ticks, Human Babesiosis and Climate Change

Abstract

The effects of current and future global warming on the distribution and activity of the primary ixodid vectors of human babesiosis (caused by Babesia divergens, B. venatorum and B. microti) are discussed. There is clear evidence that the distributions of both Ixodes ricinus, the vector in Europe, and I. scapularis in North America have been impacted by the changing climate, with increasing temperatures resulting in the northwards expansion of tick populations and the occurrence of I. ricinus at higher altitudes. Ixodes persulcatus, which replaces I. ricinus in Eurasia and temperate Asia, is presumed to be the babesiosis vector in China and Japan, but this tick species has not yet been confirmed as the vector of either human or animal babesiosis. There is no definite evidence, as yet, of global warming having an effect on the occurrence of human babesiosis, but models suggest that it is only a matter of time before cases occur further north than they do at present.

Keywords: Babesia divergens; Babesia microti; Ixodes ricinus; Ixodes scapularis; climate; global warming.

Figure 1

Factors determining the abundance and spread of Ixodes spp. Modified from Lindgren et al., 2000 [6].

Figure 2

(A) Maps of values of the basic reproduction number (R₀) of Ixodes scapularis in North America, estimated from ANUSPLIN observed temperature (1971–2000: upper panel), and projected climate obtained from the climate model CRCM4.2.3 following the SRES A2 greenhouse gas emission scenario for 2011–2040 (middle panel) and 2041–2070 (bottom panel). The colour scale indicates R₀ values. Temperature conditions that result in an R₀ of >1 permit survival of I. scapularis populations. Reproduced from Ogden et al., 2014 [38]. (B) Risk maps for the occurrence of Ixodes scapularis in Canada in response to increasing temperatures associated with climate change. The methods used to generate these maps are described by Ogden et al., 2008 [40].

Figure 3

Climate change prediction of Ixodes ricinus distribution in Scandinavia based on the length of the vegetation growth period, IPCC 2000 high emission scenario. Modified with permission from Jaenson and Lindgren, 2011 [41].

Figure 4

Surveillance for Ixodes scapularis populations in central and eastern Canada conducted from 2009 to 2015. Regions where I. scapularis populations have been identified by field surveillance are shown as red hatched areas. In 2004 there were only four known I. scapularis populations in locations shown by the red arrows. Tick populations have been identified in surveillance programs for Lyme disease (blue circles show municipalities where human Lyme disease cases have been identified). Infections due to Babesia microti are not yet nationally notifiable (reproduced with permission from Gasmi et al., 2017 [57]).

Figure 5

Vegetation growing season length (VGSL) in days in 1961–1990 and 1991–2015 correlated with the presence of Ixodes ricinus in northern Norway. The VGSL threshold for tick establishment was estimated to be approximately 170 days. Modified with permission from Hvidsten et al., 2020 [59].