Do tick-associated viruses impose a fitness cost on their tick hosts?

Alice C.C. Lau¹, Mambo Silvia Wanjiru², Hiroshi Shimoda^2,3, Ai Takano^1,2
1 Laboratory of Epidemiology, Department of Veterinary Medicine, Joint Faculty of Veterinary Medicine, Yamaguchi University
² Joint Graduate School of Veterinary Medicine, Yamaguchi University
³ Laboratory of Veterinary Microbiology, Department of Veterinary Medicine, Joint Faculty of Veterinary Medicine, Yamaguchi University

Presenter and correspondence: alicelau@yamaguchi-u.ac.jp

Ticks harbor diverse microorganisms, including endosymbiotic bacteria that support their biology, yet the diversity and impact of tick-associated viruses remain less understood. This study investigated two tick-associated viruses: Kabuto Mountain virus (KAMV, nonpathogenic) and Langat virus (LGTV, mildly pathogenic to humans). Nymphal Amblyomma testudinarium and Haemaphysalis flava were experimentally infected, and viral RNA levels, survival, and blood-feeding on BALB/cJ mice were assessed. Viral RNA levels were measured post-inoculation and on day 28, coinciding with blood feeding. The LGTV RNA copy number was ~10⁵ copies/tick immediately after inoculation, remaining relatively unchanged in A. testudinarium (~10⁵ copies/tick) but increasing to ~10⁶ copies/tick in H. flava by day 28. In contrast, KAMV RNA ranged from ~10¹–10² copies/tick post-inoculation, increasing markedly in H. flava (~10⁵ copies/tick) but only reaching ~10³–10⁴ copies/tick in A. testudinarium on day 28. An initial feeding experiment showed delayed engorgement in LGTV-infected A. testudinarium (Day 7–11) compared to Controls and KAMV-infected ticks (Day 6–8), but this was not observed in a repeated experiment with a larger sample size (n=40/group), where engorgement duration was similar across all groups (Day 5–7). Engorged ticks were measured for idiosoma length (L) and width (W): Control L = 5.3–7.5 mm, W = 4.4–6.3 mm; KAMV L = 5.3–7.3 mm, W = 4.5–6.1 mm; LGTV L = 4.5–7.1 mm, W = 3.7–6.4 mm, with LGTV-infected ticks tending to be smaller. Feeding success remained high across groups, although slightly lower in KAMV-infected ticks (Control: 97.5%, LGTV: 97.1%, KAMV: 92.5%). In H. flava, infection with either virus reduced body size post-engorgement and after molting. Viral RNA persisted after molting, except in male tick infected with KAMV. These results suggest some viral infections may impose a fitness cost, as indicated in H. flava, whereas effects in A. testudinarium are less clear. Studies are ongoing to validate these preliminary observations and to better understand how tick-associated viruses interact with their arthropod hosts.
Keywords: Amblyomma testudinarium, blood feeding, Haemaphysalis flava, KAMV, LGTV, nymphs, ticks