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Rift valley fever infections in humans more likely to come from mosquitoes than cattle

27 October 2020

A group of international researchers have mapped the dynamics of a rift valley fever infection in Mayotte and found that human infections were more likely to come from mosquito bites than through contact with infected livestock.

A viral disease responsible for major epidemics, occurring mainly in Africa, Rift Valley fever (RVF) is transmitted from livestock to humans. Despite being listed as a priority emerging disease by the World Health Organization (WHO) R&D Blueprint programme in 2015, there had until now been little research into the dynamics of its transmission.

As part of a multidisciplinary collaboration, researchers and public health professionals from INSERM, the French Public Health Agency, CIRAD and the London School of Hygiene and Tropical Medicine, with the support of the REACTing consortium, have developed a mathematical model to study the dynamics of the 2018-2019 RVF epidemic in Mayotte.

 

This research, which has been published in the journal PNAS, showed that there was a higher level of transmission to humans from mosquitoes than through direct contact with infected livestock. It also revealed that vaccinating 20 percent of livestock could cut the number of cases in humans by 30 percent.

Against the background of the continued COVID-19 pandemic, research into emerging diseases and zoonoses – infectious diseases transmissible from animals to humans, has never been so important.

Rift Valley fever (RVF) is a viral zoonotic disease occurring primarily in some African regions, Mayotte and the Arabian Peninsula. It mainly affects livestock, causing waves of miscarriage and widespread mortality in the youngest animals. Humans can be infected through direct contact with the body fluids of contaminated animals or through the bites of infected mosquitoes. To date, no human-to-human contamination has been reported. While the majority of patients develop asymptomatic or benign forms, in rare cases (1 to 3 percent of patients) the disease can take a more severe turn, characterised by ocular and meningeal disorders, and life-threatening haemorrhagic fever.

A major public health concern in some countries, RVF was listed by the WHO as a priority emerging disease in 2015, involving the accelerated development of means of control. While research into vaccines for livestock is making progress, the potential impact of vaccination on the dynamics of the epidemic had never previously been evaluated.

The team, coordinated by INSERM researcher Raphaëlle Métras and her French Public Health Agency colleague Marion Subiros, in collaboration with CIRAD, looked at the 2018-2019 RVF epidemic in Mayotte. Since 2008, thanks to the implementation of two surveillance systems – one concerning animals (supported by the Veterinary Services in Mayotte and by CIRAD), the other humans – a large amount of high-quality RVF surveillance data has been collected. These data concern livestock seroprevalence and human epidemiology (number of human cases, sociodemographic characteristics, disease exposure criteria and geolocation).

As part of their study, the researchers and their colleagues developed a mathematical model integrating these data collected jointly by the two systems in order to reproduce the transmission dynamic of the virus during the 2018-2019 epidemic. One of the objectives was to obtain more information on how the virus passes from infected animals to humans.

The team showed for the first time in the context of an RVF epidemic that there was a higher level of transmission to humans from mosquitoes than through direct contact with infected livestock. If it is assumed that 30 percent of the population of Mayotte are farmers, up to 55 percent of human infections would have been caused by mosquito bites, versus 45 percent through exposure to livestock. This is the first study to provide figures on the distribution of transmission through mosquito bites versus transmission through direct contact. The results produced by the model show that vaccinating 20 percent of livestock could reduce the number of human cases by 30 percent. Early mass vaccination livestock campaigns would therefore be an essential measure when it comes to reducing the incidence of the disease in humans.

In a context in which zoonotic disease epidemics are emerging in succession, this research illustrates the importance of implementing a “One Health” approach, taking a systemic and unified approach to public, animal and environmental health at local, national and global levels. “The health emergency associated with the COVID-19 pandemic must force us to rethink how we see the links between human, animal and environmental health. Our research highlights the importance and added value of a multidisciplinary and integrated One Health quantitative approach in fighting zoonotic diseases. It also provides avenues for improving the surveillance of and research into emerging infectious diseases ,” INSERM's Raphaëlle Métras, Marion Subiros from the French Public Health Agency and CIRAD's Eric Cardinale conclude.

Reference

Raphaëlle Métras, W. John Edmunds, Chouanibou Youssouffi, Laure Dommergues, Guillaume Fournié, Anton Camacho, Sebastian Funk, Eric Cardinale, Gilles Le Godais, Soihibou Combo, Laurent Filleul, Hassani Youssouf, and Marion Subiros. 2020. Estimation of Rift Valley fever virus spillover to humans during the Mayotte 2018–2019 epidemic. PNAS



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