Detection of a novel Orthobunyavirus associated with acute fever, lethargy, reduced milk yield and diarrhoea in cows

During the heatwave at the beginning of summer, several cases of acute diarrhoea in cows were reported on farms in eastern France, particularly in the Doubs, Ain, Savoie and Haute-Savoie departments. Initial investigations focused on the very high temperatures and the nutritional and metabolic disturbances they might cause in the animals. At the same time, a second hypothesis was investigated: that the symptoms could be caused by viral enteritis or a febrile syndrome. However, targeted tests carried out by the departmental laboratories were negative, providing no evidence to support this hypothesis.

Reports from the field subsequently showed that the acute diarrhoea was in fact associated with a broader syndrome involving fever, lethargy and reduced milk yield. Since then, the number of cases has continued to rise, with cases reported in Switzerland, Germany and other parts of France, including Normandy. At the end of July, the departmental laboratories contacted ENVT and sent samples of plasma and faeces collected from infected cows.

The emergence of a new virus

ENVT then carried out a metagenomic analysis. This approach involves analysing and sequencing all the DNA and RNA fragments present in the samples. The resulting sequences are then compared with existing databases containing all known sequences from fungi, viruses and bacteria, to identify the sequences they most closely resemble.

Bioinformatic analysis revealed Orthobunyavirus sequences closely related to Schallenberg, Shamonda and Akabane viruses. These viruses infect ruminants and are transmitted mainly by midges of the genus Culicoides. In adult cattle, they can cause an acute but generally transient illness characterised by fever, lethargy and reduced milk yield. The greatest impact could be on cows infected during pregnancy, which may abort or give birth to malformed calves.

The rapid emergence of this virus appears to be linked to climatic conditions favourable to the insect vectors.

What solutions are available to veterinarians and farmers?

• The IHAP unit (INRAE-ENVT) first developed a PCR test specifically designed to detect this new virus. This enabled urgent testing of the initial suspected cases: several hundred samples from across France were processed in the laboratory in collaboration with the network of public laboratories (ADILVA).
  At the same time, with support from researchers, the main French veterinary diagnostic companies rapidly developed commercial PCR kits. These were made available to public laboratories just two weeks after the virus was discovered.
• The emergence of this virus was detected very rapidly thanks to close collaboration between practising veterinarians, departmental laboratories, research teams, diagnostic platforms, and government services and agencies. This collective effort will help to better characterise the current episode, monitor its spread, assess its health impacts and rapidly develop solutions to control the new virus.