Coinfection of Bovine ephemeral fever virus and 'Candidatus Mycoplasma haemobos' in cattle in Central China: implications for tick-borne transmission and viral divergence.

Background: In central China, free-range cattle face high risks of exposure to arthropod-borne pathogens like Bovine ephemeral fever virus (BEFV) and 'Candidatus Mycoplasma haemobos' ('Ca. M. haemobos'). Despite their co-endemicity, the role of ticks in BEFV transmission remains unexplored in China, and potential interactions between these pathogens are unknown. This study aimed to (1) investigate the coinfection of BEFV and 'Ca. M. haemobos' in cattle fever outbreaks, (2) evaluate ticks as potential vectors for both pathogens, and (3) characterize viral evolution.

Results: From August to September 2024, blood, tissue, and tick samples were collected from 61 clinically ill cattle (Group 1) and 17 asymptomatic cattle (Group 2) in the same farms, and 18 healthy cattle (Group 3) in six different farms in central China. Ticks were morphologically and molecularly identified. Pathogens were screened by PCR and sequencing analysis. The serum was subjected to virus neutralization assays. In Group 1, BEFV and 'Ca. M. haemobos' were detected in 86.9% and 67.2% of cattle, respectively, with a coinfection rate of 44.3% (27/61). All deceased cattle (8/8) tested positive for BEFV, and 75% were coinfected. Ticks (Haemaphysalis longicornis, Rhipicephalus microplus, Rhipicephalus sanguineus sensu lato) from Group 1 carried BEFV (46.8%, 11.3%, and 18.8%) and 'Ca. M. haemobos' (34.8%, 35.5%, and 31.3%). Engorged H. longicornis exhibited higher BEFV infection rates than unfed ticks (p < 0.001). In Group 2, the two pathogens were only detected in H. longicornis ticks with positive rates significantly lower than those in ticks in Group 1 (p < 0.05). In Group 3, all samples were negative for the pathogens. Moreover, the BEFV strain HN2437 in this study showed divergence from global lineages, suggesting unique evolution. Neutralizing antibodies against BEFV were only found in cattle in Group 1 with the seroprevalence rate of 26.2% (16/61).

Conclusions: This study provides the first evidence of ticks as potential BEFV carriers in China and highlights that coinfection with 'Ca. M. haemobos' may be a factor in severe disease outcomes. The distinct evolutionary profile of HN2437 strain underscores the need for evaluating effect of vaccine in use.