Aujeszky's disease: a swine virus that is fatal to dogs

Early this year, cases of Aujeszky’s disease were detected in the Hautes-Alpes. An epidemiological investigation is underway, in particular to determine the source of the infection with the herpesvirus in question. Here is an overview of this viral disease in pigs, which is not transmissible to humans but is fatal to dogs and other mammalian species. Fortunately, it remains relatively rare in France.   Aujeszky’s disease, also known as pseudorabies in dogs, is a viral infectious disease affecting wild and domestic swine. Although highly contagious, it is not a zoonosis; however, it is most often fatal to infected carnivores and ruminants. It is therefore classified as a Category 1 hazard for all species. Although rare, new cases are cause for concern and must be closely monitored.  

A Swine Herpesvirus Is to Blame

Aujeszky’s disease is caused by a herpesvirus. It primarily affects pigs and wild boars. Clinical signs vary in severity, ranging from respiratory problems to reproductive disorders, including neurological symptoms. The mortality rate remains low among adults (2%), but once a pig is infected, it remains a lifelong carrier of the virus and transmits it to other pigs via saliva, nasal secretions, and genital secretions. Although not transmissible to humans, this virus can, however, accidentally infect other species, including dogs, cats, horses, and cattle. Infection occurs through close contact with infected animals or contaminated objects, or by ingesting infected pork or wild boar offal. In these species, the disease progresses so rapidly that they do not serve as a source of infection and are therefore considered epidemiological dead ends. Clinical manifestations vary, including behavioral changes, intense itching, respiratory failure, coughing, fever, and convulsions, rapidly progressing to death within 48 to 72 hours. The clinical picture resembles that of an infectious encephalomyelitis, very similar to that of rabies, hence its nickname “pseudo-rabies.”  

A Difficult-to-Control Infection

Unfortunately, to date, there is no effective treatment against the virus, nor is there an effective vaccination protocol for dogs. Control measures therefore rely primarily on surveillance and prevention. In the 1980s, an effective swine vaccination campaign and strict hygiene measures had eradicated the disease across most of France. Since 2006, however, vaccination has been banned in regions of the world officially free of the disease, in accordance with recommendations from the World Organisation for Animal Health (WOAH), so that screening tests can detect any new suspected cases. To date, these tests cannot distinguish between vaccinated and infected animals. The protection of livestock herds therefore relies on an active prevention policy through the implementation of strict biosecurity measures, facilities designed to prevent any physical contact with outbreaks in wild boar populations, and rigorous health standards for exports and imports. However, limiting the spread of Aujeszky’s disease in livestock operations relies above all on vigilance and early, rapid reporting by farmers and veterinarians. As such, clinical and serological surveillance is mandatory every quarter on breeding farms and once a year on free-range farms. In the event of clinical suspicion, the farm is placed under a prefectoral order for placement under surveillance (APMS). This entails, in particular, a ban on the movement of animals to and from the farm pending clarification of its status as part of an epidemiological investigation. If the case is confirmed, the farm is placed under a prefectoral order of declaration of infection (APDI), which mandates the culling of all pigs, as well as the destruction of derived products, the cleaning and disinfection of the premises and equipment, and an epidemiological investigation of neighboring farms or those likely to have had contact with the infected farm. The emergence of new outbreaks is therefore taken very seriously. Every effort is made to prevent the infection from spreading too widely. This is why the latest cases in France are cause for concern.  

A Rising Number of Cases Causes Concern

Recently, a free-range domestic pig farm (11 breeding sows and 15 piglets) tested positive as part of the mandatory annual prophylaxis (one positive animal out of 17 tested) in Vars in the Hautes-Alpes. These cases are in addition to those confirmed in 2018 at a free-range pig farm in the Pyrénées-Atlantiques, two outbreaks in April 2019 at free-range fattening farms in the Alpes-de-Haute-Provence and Vaucluse, one in December 2019 in Haute-Garonne, followed by three other outbreaks in 2020 at wild boar farms in Allier. Although the number of outbreaks remains low, this marks the first resurgence of the disease on French territory since September 2010. Metropolitan France had been recognized as free of Aujeszky’s disease in pig farms by Decision 2008/185/EC¹ and vaccination was therefore no longer practiced there, as in many other European Union countries. This increase in cases is therefore cause for concern. Farmers face significant risks, since as soon as one animal is infected, the entire herd must be culled. Barn fallowing must then be observed before operations can resume, which is a major blow to the industry. Hunters are also affected, as their dogs are particularly vulnerable to the virus. Indeed, the disease continues to circulate among wild suids. A study conducted between 2000 and 2004 shows that, on average, 6% of wild boars over one year of age in France carry the virus, which would explain the sporadic outbreaks in French farms. The emergence of these new cases on farms could indicate increasing viral circulation within wild populations. To date, an estimated 30% of wild boars are believed to be seropositive. Vigilance is therefore essential to prevent a loss of control over the epidemic in the future.