Haemaphysalis longicornis ticks as a competent vector for 'Candidatus Mycoplasma haemobos' in experimental transmission studies.

'Candidatus Mycoplasma haemobos' ('Ca. M. haemobos') is an emerging hemotrophic mycoplasma affecting cattle and other animals worldwide. While arthropod vectors have been suspected in its transmission, experimental evidence confirming vector competence remains limited. We investigated the ability of the hard tick Haemaphysalis longicornis (H. longicornis) to acquire, maintain, and transmit 'Ca. M. haemobos' under laboratory conditions. Ticks were exposed to the pathogen via microinjection or by feeding on infected BALB/c mice. Transovarial and transstadial transmission were assessed using quantitative PCR detection across tick life stages. All developmental stages (larvae, nymphs, adults) acquired the pathogen after feeding on infected mice, with infection rates in larvae and nymphs increasing significantly over 14 days. Microinjected adults maintained high pathogen loads for up to 28 days. Transovarial transmission was demonstrated, with 68.4 % of egg pools and 84.9 % of larval pools testing positive. Transstadial transmission was also observed, with pathogen persistence and significant replication during the larval-to-nymph molt. In transmission trials, larvae, nymphs, and adults successfully infected mice, with efficiencies of 80.0 %, 70.0 %, and 80.0 %, respectively. Adult ticks induced significantly higher bacteremia in hosts compared to larvae. This study provides the first experimental evidence that H. longicornis is a competent vector for 'Ca. M. haemobos', capable of both transovarial and transstadial transmission, and maintaining the pathogen throughout its life cycle. These findings underscore the role of H. longicornis in the ecology and transmission of 'Ca. M. haemobos', with significant implications for disease control in endemic regions.