A study has revealed crucial insights into the transmission patterns and risk factors of Crimean-Congo haemorrhagic fever virus (CCHFV) in sheep, a natural host of the virus. 

Led by The Pirbright Institute and based on sequential observational studies and a multi site randomised trial conducted in Bulgaria, the multinational collaboration underlines the importance of preventive measures in reducing virus spread and the need to identify high-risk areas for targeted intervention.

Crimean-Congo haemorrhagic fever (CCHF) is a severe, tick-borne disease prioritised by the World Health Organization due to its high fatality rate and epidemic potential. 

While humans are accidental hosts, livestock play a key role in the disease's epidemiology. Infected animals typically show no clinical signs, and their short-lived viraemia often goes unnoticed.

Writing in Emerging Infectious Diseases, the researchers show how human exposure, though limited, is higher in areas identified as livestock hotspots, suggesting occupational risk for farmers, and that tick control measures significantly reduce the force of infection.

The researchers also found:

  • High-risk hotspots are embedded within broader endemic regions.
  • Animal characteristics, including age and sex, were not strongly associated with infection risk.
  • Transmission risk varies seasonally, with higher infection rates observed during peak tick activity in late summer.
  • The dynamics of CCHFV NP and Gc IgG vary over time and seroreactivity responses among the two antigens are not correlated.

Dr Georgina Limon-Vega, Group Leader in Applied Epidemiology at Pirbright, said “Our work sheds new light on the complex dynamics of CCHFV exposure in natural hosts in the field over time. We show that localised hotspots within endemic areas can sustain higher levels of transmission, and that interventions like tick control can significantly reduce infection rates.”

Associated scientists
Close up of Hylomma tick on white background
Hyalomma ticks are the primary vectors for Crimean-Congo hemorrhagic fever (CCHF)
Small flock of sheep standing
Sheep are natural hosts of CCHFV.

As CCHFV continues to spread into new regions due to climate change, global travel, and livestock movement, the findings have broad implications for international public health preparedness.

“Our work also sheds light on the effectiveness of control measures in real-world settings. Results from the field studies provide key epidemiological parameters that can inform surveillance, vaccine development, and tick control strategies,” observed Dr Simon Gubbins, Group Leader in Transmission Biology. 

Prof. Bryan Charleston MRCVS FRS, Institute Director, added: “Given the challenges of conducting experimental trials with high-containment pathogens, natural host studies like ours offer a practical alternative to understanding disease dynamics and evaluating interventions.”

The Pirbright researchers say further studies to assess the role of microclimate in animal sheds for tick survival, especially over winter, as well as studies to better understand the role of maternal antibodies in early-life protection for lambs could pave the way for more effective regional and global control strategies.

Conducted in Bulgaria, a region endemic for CCHFV, the study analysed serological data from multiple farms over several seasons. It is one of the first studies to use a randomized controlled design to evaluate the impact of control measures on the force of infection of CCHFV in livestock.

The study involved the Bulgarian Food Safety Agency, Bulgaria’s National Center of Infectious and Parasitic Diseases, the UK Health Security Agency, the University of Oxford, Oregon State University, and the London School of Hygiene & Tropical Medicine. 

Read the paper

Limon G., Tchakarova S.R., Ludi A. et al. (2025) Drivers of Crimean-Congo Hemorrhagic Fever in Natural Host and Effects of Control Measures, Bulgaria. Emerg Infect Dis 31 https://doi.org/10.3201/eid3109.241952

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