Following the declaration in recent weeks of an outbreak of equine viral arteritis (EVA) involving several stallions at a breeding centre in the Calvados department (14), with possible secondary outbreaks, the RESPE crisis team* met on Wednesday, November 19, to assess the situation. The meeting also determined the precautionary measures to be implemented and disseminated in the coming days and weeks.
Equine viral arteritis (EVA): A reminder about the disease
Equine viral arteritis is a viral disease that is widespread throughout the world.
It is one of the most contagious diseases in equids. It is transmitted directly, primarily through the respiratory route. Abortion is one of the clinical consequences of this mode of transmission and remains uncommon. The second mode of transmission is the venereal route via stallions that have contracted the disease and remain carriers of the virus in their semen (approximately 40% of stallions). During the acute phase of the disease, the animal will excrete the virus for one to two weeks, via nasal secretions or, exceptionally, in cases of high viral load in the blood, any other biological fluid including semen, urine, etc. Although the virus is not very resistant in the external environment, indirect contamination is possible through contaminated equipment (care equipment, mannequin, feeder, waterer, truck, etc.) or through people in contact with horses (clothing, hands and any instrument used in livestock management, etc.).
The classically described signs are fever, nasal discharge, watery eyes (clinical signs quite similar to those of influenza and rhinopneumonitis), swelling of the limbs, oedema of the sheath and scrotum, facial oedema, abortion. The disease can also go unnoticed with a simple spike in temperature or even be asymptomatic.
Some stallions become chronic viral carriers for several years: they no longer show any signs, are no longer contagious through the respiratory route, but excrete the virus in their semen, which compromises their breeding career. This carrier status depends on testosterone and therefore disappears after castration or hormonal suppression.
In females, once the viral infection is complete, they do not remain carriers of the virus even if they remain seropositive (antibodies in the blood). The diagnosis is based on a PCR analysis, which can be performed on a nasal swab, blood or semen, or on serology, which requires 10 to 15 days for antibodies to appear. Viral arteritis is among the diseases that must be reported, even though the State leaves the health management to the industry.
Epidemiological situation to date
On October 17, 2025, an outbreak of viral arteritis was detected at a breeding centre in Normandy during a routine check. Five serological tests revealed a positive result in nine stallions, whereas all the horses had tested negative upon entering the centre and in previous tests. Further tests (on blood, swabs and semen) were carried out as soon as the positive results were known and revealed that most of the horses had already eliminated the virus at the respiratory level. Of the 9, however, 7 to 8 stallions continued to test positive in their semen.
After analysis of all the stallions present, 19 stallions are positive, affecting the sport (breeding and competition) and purebred Arabian flat racing sectors. The likely source of infection would be initial respiratory transmission but also possibly indirect contamination (equipment, personnel, collection mannequin). Analysis of entries into the centre and the results of previous tests as well as the traceability of batches of straws produced would indicate an initial contamination at the end of September. The ongoing epidemiological investigation seeks to identify the "index" horse, which probably arrived at the centre around September 23, but several possible origins exist, and no establishment of origin has yet been formally identified.
Between October 7, the date of the samples that came back positive, and October 17, some stallions left the centre to join different sites: several competition stables now have horses that are positive, often only in serology, with negative nasal and blood PCR tests, indicating viral passage but no longer any apparent active viral circulation. Some sites still have analyses in progress. Stallions carrying the virus in their semen could be found there. At this stage two sites are affected and a third is awaiting analysis results. Some sites have taken strict measures on their own initiative to block entry and exit (28 days), and others have carried out screenings to ensure safe participation in competitions.
Since the index horse has not yet been identified, it is likely that at least its original structure, before its arrival at the breeding center, was subject to a viral transmission.
Strict vigilance is needed to limit the spread of the disease
Viral arteritis caused an epizootic in Normandy during the summer of 2007 (more than 200 horses affected). In 2011, an outbreak was identified in southeastern France with one fatality in a foal. In 2018, two abortions were recorded in the Orne department. Since 2018, the few strains of the virus circulating in the country had not caused any clinical signs.
The centre's management has notified all horse owners and keepers whose horses were housed on the premises during the period in question. All owners and keepers concerned are strongly encouraged to implement temperature monitoring to detect sick horses as early as possible, and also to conduct nasopharyngeal swab tests to screen for infected but asymptomatic horses, with the aim of further limiting the circulation of the virus and the spread of the disease. In the event of a positive result, the unit reminds owners and/or their veterinarian of the importance of declaring these results to RESPE, in addition to their DDPP. After October 18, all horses left the facility after obtaining a negative PCR result on a respiratory and blood sample.
As a reminder, in order to limit the spread of the virus and the increase in the number of outbreaks, any horse owner in a confirmed outbreak is urged not to move their horse and, above all, not to participate in any events. Failure to comply may result in criminal penalties under Article L228-3 of the Rural Code: "The act of intentionally causing or contributing to the spread of an epizootic disease in domestic vertebrates […] is punishable by five years' imprisonment and a fine of €75,000. Attempted offenses are punishable as completed offenses."
Organizers, owners and keepers, as well as any other person involved with equines, are therefore asked to strictly comply with the regulations in force or, in their absence, to apply a strict health protocol.
Enhanced biosecurity
The members of the crisis cell urge vigilance and the implementation of precautionary measures for livestock farms and livestock and sport gatherings in the coming weeks, particularly for stallions, with testing for the EVA virus by PCR on a nasopharyngeal swab (NPS) carried out within 72 hours before the arrival of the equine on site remains recommended. For entire horses, they must not show hyperthermia in these same 72 hours and a search for the EVA virus by PCR must also be carried out on a blood sample (in addition to an NPS).
In this particular context of detecting non-sick horses, this test must be carried out in an approved analysis laboratory and not on a rapid test. Temperature monitoring before the gathering (at least one week), during, and after the gathering (at least one week) must also be implemented and recorded on a document. A strengthening of all biosecurity measures is also strongly recommended.
For example, a PCR test was carried out on nasopharyngeal swabs for all the horses present at the male qualifier of the Selle Français Studbook in Villers Vicomte on 19 November 2025 (100% negative results) and the owners of the horses present at the qualifiers in Lamballe and Saint Lo (during the first half of November) were encouraged to have tests carried out afterwards on their male animals.
Note: A vaccine exists for this disease. Currently, it is used only in stallions, produced and distributed in limited quantities by a single laboratory in Europe, and at a high price. The protocol consists of three injections for the initial vaccination, followed by six monthly boosters. Given the small number of vaccine doses and the vaccination protocol, their use must be judicious and sparing. It is not recommended to vaccinate stallions that are seropositive or whose status with regard to the virus is unknown.
* The RESPE crisis unit
It brought together on this day the Association of Private Stallion Owners (ASEP), the French Equine Veterinary Association, the French Equestrian Federation, the National Horse Federation, France Galop, the French Institute of the Horse and Riding, LABEO Frank Duncombe, the French Society of Working Equines (SFET), the French Equestrian Society (SHF), the Selle Français Stud Book (ANSF), the General Directorate of Food, RESPE and Professor Jean Luc Cadoré, present as a scientific expert.