To determine whether Borrelia burgdorferi sensu lato (s.l.) influences tick survival under thermohygrometric stress, Ixodes ricinus (L.) (Acari: Ixodidae) questing ticks were tested under various relative humidities (13, 32, 51.5, 61, and 89% RH) at two different temperatures (12.5 and 25°C) and investigated for Borrelia infection. Survival rate of females was highest (77.6%), followed by males (51.6%), and nymphs (43.2%). The thermohygrometric factor that most importantly determined survival was saturation deficit (SD). As SD increased, tick survival rate decreased in all stages. Among the 1,500 ticks tested for B. burgdorferi s.l., 34.8% (n = 522) were infected. Adult infection rate (39.6%) was higher than that of nymphs (25.5%). Infection load in real-time polymerase chain reaction ranged from 1 to 1.2 million spirochetes per tick. B. afzelii (39.7%), B. burgdorferi sensu stricto (12.1%), B. garinii (37.9%), B. myamotoi (3.6%), and B. valaisiana (23.8%) were recorded. B. garinii infected significantly less nymphs than adults whereas B. afzelii displayed the opposite trend. Survival rate of nymphal and adult I. ricinus was significantly enhanced by infection by B. burgdorferi s.l. (χ2: nymph, P = 0.008; adult, P = 0.021). In adults, a negative effect of infection on tick survival was observed when spirochete load overcame a threshold estimated at 160,000 spirochetes per tick but not in nymphs. Moreover, ticks infected by B. afzelii survived better than other ticks (infected by other genospecies or not). The results here indicate that infection by B. burgdorferi s.l., and more specifically infection by B. afzelii, confers survival advantages to l. ricinus under challenging thermohygrometric conditions.
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