Bluetongue (Ovine Catarrhal Fever): What You Need to Know

Since early August, bluetongue (BT) has been spreading across 10 departments in northeastern France. BT, also known as “bluetongue,” primarily affects sheep and ewes. The Ministry of Agriculture and Food Sovereignty reported 41 outbreaks on August 14, then 190 a week later, and 342 on the 29th. To understand the risks posed by this disease, we spoke with Stéphan Zientara, director of the ANSES Animal Health Laboratory, which houses the national reference laboratory for bluetongue.    

The Conversation: What are the symptoms of bluetongue?

Stéphan Zientara : Bluetongue (bluetongue) is a non-contagious disease that infects a wide variety of domestic and wild ruminants and produces a broad range of clinical signs. Fever, lethargy, respiratory distress, and anorexia are symptoms typically observed in affected animals. Cyanosis of the tongue is sometimes reported: an abnormal bluish or grayish discoloration of the skin caused by insufficient oxygenation of the blood, hence the name “blue tongue.” In the most severe cases, bluetongue causes hemorrhagic fever and death in animals, particularly in sheep, which are the most susceptible species. Goats and cattle generally show few or no signs of the disease. However, prolonged viremia (the presence of viral particles in the blood) is observed in cattle, which are traditionally considered a reservoir for the infection. Due to its economic and public health impact on affected herds, bluetongue has been included on the list of notifiable diseases with the WOAH (World Organisation for Animal Health) since the 1960s.  

T.C.: What causes this disease? How is it transmitted?

S.Z.: The causative agent of bluetongue is the bluetongue virus (BTV), which belongs to the genus Orbivirus within the family Sedoreoviridae . Like other members of this family, BTV is a non-enveloped virus. Its genome consists of 10 double-stranded (ds) RNA segments that can undergo reassortment, similar to influenza and other segmented viruses. It encodes seven structural proteins (VP1 through VP7) and five non-structural proteins (NS1 through NS5). VP2 forms the virus’s outer capsid and is considered the primary protein defining the virus’s serotype. A serotype is defined as the set of characteristics of certain viruses that allows strains belonging to the same species to be distinguished from one another. As of 2008, there were 24 known serotypes of this virus, but new ones have been discovered in recent years. In total, 36 BTV serotypes have been described, and others have recently been identified, particularly in countries bordering the Mediterranean basin. BTV is an arbovirus transmitted by the bite of female midges of the genus Culicoides during a blood meal on an infected animal. Serotypes 1 through 24 are primarily transmitted by midges, while the others are transmitted through direct contact.  

T.C.: Are there any risks to human health, either from the virus itself or from contaminated meat consumption?

S.Z.: The virus can infect many animal species, but it causes clinical signs primarily in sheep (hence the name bluetongue) as well as in cattle. This virus is not transmitted to humans. Furthermore, consuming products derived from infected animals (meat, milk, etc.) poses no danger to us.  

T.C.: What is the current health situation in France? How is the epizootic progressing?

S.Z.: Before 1998, Europe was free of bluetongue. From 1998 to 2006, a few serotypes emerged in the Mediterranean basin, but it was after 2006 that bluetongue became a real concern in the field of animal health. In August 2006, the first epizootic outbreaks associated with an unknown strain of BTV-8 were identified in the Netherlands, at a latitude never before reached by this virus. The virus then spread rapidly to Belgium, Germany, Luxembourg, and northern France by the end of 2006. Contrary to some initial predictions, the virus survived the winter and reappeared in 2007, coinciding with the resumption of vector activity, in all countries affected in 2006, from which it rapidly spread to the United Kingdom, Denmark, Switzerland, and the Czech Republic, affecting nearly 60,000 farms. At the same time, a strain of BTV-1, present in North Africa in 2006, spread to Sardinia on the one hand and, on the other, crossed the Iberian Peninsula to reach southwestern France during the summer of 2007. In 2008, BTV-8 and BTV-1 continued to spread, with France being the hardest-hit country, reporting more than 24,000 cases of BTV-8 and more than 6,000 farms infected with BTV-1. In 2008, a large-scale vaccination campaign using inactivated vaccines was launched across Europe, which significantly reduced the number of outbreaks and thus contained the spread. However, it was not until the following year and a second mass vaccination campaign that there was a drastic reduction in BTV-8 cases in France (83 outbreaks by the end of 2009). Most European countries regained their BTV-8-free status between 2010 and 2012. In France, the use of inactivated vaccines targeting BTV-1 and -8 led to the eradication of BTV in 2010, and the country was deemed free of the disease in December 2012. Three years after the eradication of BTV-8 in France, this serotype was detected again in September 2015 in the Allier department in a ram exhibiting clinical signs suggestive of bluetongue (fever, respiratory problems, and facial edema). Despite the control measures implemented by health authorities, BTV-8 spread rapidly across much of France in the following years (more than 3,000 cases). In April 2019, the virus spread to Germany, Switzerland, and Belgium. From a pathophysiological standpoint, very few clinical signs were observed compared to the BTV-8 outbreaks in France between 2006 and 2008. In November 2017, a serotype 4 virus was introduced from Corsica. Since then, France has been enzooticly infected with both serotypes 4 and 8. In 2023, a serotype 3 virus (BTV-3) was identified in the Netherlands. This virus caused the deaths of more than 55,000 sheep. It spread to Germany, Belgium, and England. In 2024, this virus crossed the French-Belgian border. As of August 28, France has recorded more than 190 outbreaks in the northern departments of the country.  

T.C.: How can bluetongue be controlled?

S.Z.: There is no antiviral treatment (medication). Vaccination helps prevent infection and reduce viral circulation. Inactivated vaccines against serotypes 3, 4, and 8 are available on the market. In 2008, a large-scale mandatory vaccination campaign for cattle and sheep using inactivated vaccines was launched across Europe, which significantly reduced the number of outbreaks and thus contained the spread. However, it was not until the following year—and a second mass vaccination campaign—that a drastic reduction in BTV-8 cases was achieved in France. In France, the use of inactivated vaccines targeting BTV-1 and -8 led to the eradication of BTV in 2010, and the country was declared disease-free in December 2012. However, the cost of mass vaccination is high, which limits its implementation. Movement restrictions and vaccination measures aim to reduce the clinical consequences of infection and limit viral spread. During the 2006–2010 epizootic, losses amounted to several hundred million euros. Today, when an outbreak is detected, a 15-km restricted area is established. Animals may leave this area only if they test negative via a PCR test. The vaccination program will help reduce the severity of the disease or mitigate clinical signs. When animals are exported, PCR testing and individual insecticide treatments are also carried out. The objectives of these measures are to reduce the impact of the disease on infected animals, limit the spread of the virus within the country, and prevent its transmission to disease-free regions or countries.

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Interview by Benoît Tonson. Stéphan Zientara , Veterinarian and Virologist, French Agency for Food, Environmental and Occupational Health & Safety (ANSES) This article is republished from The Conversation under a Creative Commons license. Read the original article.