Study sheds light on Crimean-Congo haemorrhagic fever risks in livestock

BULGARIA – A new multinational study has identified the key factors driving the spread of Crimean-Congo haemorrhagic fever virus (CCHFV) in sheep, offering fresh insights into how the deadly tick-borne disease circulates in livestock and the implications for human health.

The research, led by The Pirbright Institute and published in Emerging Infectious Diseases, is based on sequential observational studies and a multi-site randomised trial carried out in Bulgaria, a region where the virus is endemic. 

The findings highlight the importance of identifying high-risk areas, implementing tick control measures, and developing targeted interventions to limit transmission.

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

While humans are accidental hosts, often infected through tick bites or contact with infected animal blood and tissue, livestock are central to the virus’s ecology. Infected animals rarely show clinical symptoms, and because their viraemia is short-lived, infections often go undetected.

The Pirbright-led team found that human exposure risk, while generally limited, was higher in livestock hotspots, raising occupational concerns for farmers and agricultural workers. 

The study showed that tick control measures significantly reduced infection rates among sheep, underlining their importance as a frontline defence against spread.

“Our work sheds new light on the complex dynamics of CCHFV exposure in natural hosts in the field over time,” said Dr Georgina Limon-Vega, Group Leader in Applied Epidemiology at Pirbright. 

“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.”

Hotspots, seasonality, and control measures

The research revealed that high-risk hotspots exist within broader endemic regions and that transmission risk varies seasonally, with the highest infection rates recorded during peak tick activity in late summer. Interestingly, factors such as animal age and sex were not strongly linked to infection risk.

The study also highlighted that the immune response dynamics to CCHFV antigens varied over time, complicating the picture of livestock exposure.

Dr Simon Gubbins, Group Leader in Transmission Biology at Pirbright, said: “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.”

Global implications amid climate change

As CCHFV spreads into new territories due to climate change, global travel, and livestock movement, the study’s conclusions carry significant weight for international public health.

Prof. Bryan Charleston MRCVS FRS, Director of The Pirbright Institute, emphasised the broader relevance of the findings.

“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,” he noted.

The researchers called for further investigations into the role of microclimates in livestock sheds on tick survival, particularly during winter, as well as studies on maternal antibodies that could provide early-life protection for lambs.

Collaborative approach

The project brought together expertise from several institutions, including 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.

By combining local field studies with international research expertise, the team delivered one of the first randomised controlled evaluations of control measures against CCHFV in livestock. 

The study sets the stage for improved regional and global disease preparedness strategies at the intersection of animal and human health.

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