Vector-borne diseases: a concern for cattle health
The common factor linking bluetongue, epizootic haemorrhagic disease, besnoitiosis, etc. is that they are all caused by pathogens transmitted to ruminants by arthropods such as ticks, midges or mosquitoes. ANSES has funded a review of studies carried out in metropolitan France, which highlighted the many gaps in current knowledge about the vectors found on cattle farms and the pathogens they can transmit. Such knowledge is nevertheless essential given current global changes, which are having an impact on vector distribution and therefore on the epidemiology of vector-borne diseases in livestock.
Vector-borne diseases transmitted to livestock animals are a major economic issue. Vector transmission of pathogens, whether through blood-sucking insects or ticks, accelerates their geographical spread. This has been the case with epizootic haemorrhagic disease, detected in mainland France in 2023, and besnoitiosis, a disease transmitted by blood-sucking insects that was historically present in the south of France but is now spreading northwards. Understanding the distribution and living conditions of arthropod vectors is essential to anticipate the risks of their spread and rapidly take appropriate prevention and management measures.
Despite these issues, no overview of the presence of arthropod vectors on French livestock farms or of the pathogens they can potentially transmit to animals had previously been available. To address this lack, ANSES funded a literature review conducted by the Toulouse National Veterinary School (ENVT). "This literature review focused on cattle, because the way they are reared means that they are affected by a greater number of vector-borne diseases than other domestic animal species. In addition, the economic and health stakes are high: with over 17 million animals, France has the largest cattle population in the European Union", explained Johanna Fite, Project Officer for Vectors at ANSES.
A wide variety of vectors found on farms
Blood-sucking arthropods represent a twofold health issue: they can transmit pathogens, but their bites can also cause allergies and itching, endangering animal welfare and even leading to anaemia.
The literature review identified 13 families of blood-sucking arthropods likely to have an impact on cattle health. They include biting midges of the genus Culicoides, which are the main vectors of pathogens for cattle. In particular, they can transmit the viruses responsible for bluetongue and epizootic haemorrhagic disease.
Ticks, blood-sucking flies, fleas, horseflies, lice and mosquitoes are other potential vectors. Some of these families contain several hundred different species.
This review also provided an analysis of the collection methods used and the knowledge acquired, whether on the vectors, their geographical distribution, their abundance or the risks to animal health.
Research needed on the factors determining the presence of vectors
As part of this review, the scientists examined 290 publications. More than 85% of them provided no details about the type of farm (dairy, meat or mixed production, livestock breed) on which the research had been carried out. Information on breeding practices, such as farm size and access to the outdoors, was also insufficient. Moreover, most of the scientific studies were conducted over a short period of time, meaning that it was not possible to analyse the impact of climate or weather parameters. "This lack of information is a major obstacle to identifying the factors that determine the presence of arthropod vectors and the risk of disease transmission on cattle farms", said Elsa Quillery, Scientific Expert Appraisal Coordinator at ANSES.
More entomologists needed in France
Another finding of the publication was that the sharp fall in the number of entomologists specialising in species of veterinary and medical interest has led to a shortage of research projects to identify the vectors and assess the risks to population health. Most studies describing blood-sucking arthropods found on French livestock farms were carried out before the 1980s.
Better characterisation of the factors determining the presence of vectors on farms is nevertheless essential to anticipate and prevent the risks of pathogen transmission to animals and, in some cases, to humans.