Rift Valley fever infections in humans are more likely to originate from mosquitoes than from cattle
October 27, 2020
A group of international researchers captured the dynamics of a Rift Valley fever infection in Mayotte and found that infections in humans are caused by mosquito bites rather than exposure to infected cattle.
Rift Valley Fever (RVF) is a viral disease that is responsible for severe epidemics and occurs mainly in Africa. It is transmitted from cattle to humans. Although the World Health Organization (WHO) R&D approach was classified as a priority disease in 2015, little research has been done into the dynamics of its transmission.
In a multidisciplinary collaboration, researchers and health professionals from INSERM, the French Public Health Authority, 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 RVF -Epidemic 2018-2019 in Mayotte.
This study, published in the journal PNAS, showed that mosquito transmission to humans was higher than through direct contact with infected cattle. It also showed that vaccinating 20 percent of the animals could reduce the number of cases in humans by 30 percent.
In light of the ongoing COVID-19 pandemic, research into emerging diseases and zoonoses – infectious diseases that can be transmitted from animals to humans – has never been more important.
Rift Valley Fever (RVF) is a viral zoonosis that occurs primarily in some African regions, Mayotte and the Arabian Peninsula. It primarily affects farm animals and causes waves of miscarriages and widespread mortality in the youngest animals. Humans can become infected through direct contact with the body fluids of contaminated animals or through the bites of infected mosquitoes. No human-to-human contamination has been reported to date. 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 characterized by ocular and meningeal disorders, and life-threatening hemorrhagic fever.
RVF, which is a major public health concern in some countries, was identified by the WHO in 2015 as a priority disease that involves the accelerated development of control agents. As research into farm animal vaccines advances, the potential impact of vaccination on the dynamics of the epidemic has never been assessed.
The team, coordinated by INSERM researcher Raphaëlle Métras and her colleague Marion Subiros from the French Health Department, worked with CIRAD to study the 2018-2019 RVF epidemic in Mayotte. A large amount of high quality RVF monitoring data has been collected since 2008 thanks to the implementation of two monitoring systems – one for animals (supported by the Mayotte Veterinary Service and CIRAD) and the other for humans. These data concern the seroprevalence of farm animals and the epidemiology of humans (number of cases in humans, socio-demographic characteristics, criteria for exposure to diseases and geolocation).
As part of their study, the researchers and their colleagues developed a mathematical model that integrates this data collected jointly by both systems to reproduce the transmission dynamics of the virus during the 2018-2019 epidemic. One of the goals was to get more information about how the virus is transmitted from infected animals to humans.
For the first time in the context of an RVF epidemic, the team showed that mosquito transmission to humans was higher than through direct contact with infected cattle. Assuming 30 percent of Mayotte’s population are farmers, up to 55 percent of infections in humans would have been caused by mosquito bites, compared with 45 percent from exposure to farm animals. This is the first study to provide figures on the distribution of mosquito bite transmission compared to direct contact transmission. The results of the model show that vaccinating 20 percent of animals can reduce the number of cases in humans by 30 percent. Early mass livestock vaccination campaigns would therefore be an essential measure to reduce the incidence of the disease in humans.
In a context where zoonotic epidemics occur one after the other, this study shows the importance of implementing a “One Health” approach, which is a systemic and unified approach to human, animal and environmental health on a local, national basis and tracked globally. “The health emergency associated with the COVID-19 pandemic must force us to rethink how we see the connections between human, animal and environmental health. Our research underscores the importance and added value of a multidisciplinary and integrated quantitative approach by One Health in combating zoonoses. It also provides opportunities to improve surveillance and research into emerging infectious diseases, ”conclude Raphaëlle Métras from INSERM, Marion Subiros from the French Health Service and Eric Cardinale from CIRAD.
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. Rift Valley fever virus spread estimate to humans during the Mayotte epidemic 2018-2019. PNAS