Possible transmission of influenza D from cattle to pigs

The disease was detected on a farm in Brittany rearing both pigs and cattle. Following the onset of coughing and sneezing in some of the pigs, the farm's veterinarian sent samples to a laboratory for analysis. After these initial tests, ANSES's Ploufragan-Plouzané-Niort Laboratory confirmed that the influenza D virus was the cause of the clinical signs observed. "Influenza D mainly affects cattle," explains Gaëlle Simon, head of the Pig Immunology and Virology Unit at ANSES's Ploufragan-Plouzané-Niort Laboratory. "It was already known that the virus could affect pigs, but this is the first time that it has been isolated in this species in France." The virus has already been detected in pigs in several countries, including the United States, Italy, Luxembourg, Ireland and the Netherlands. It generally causes only a few clinical signs in pigs. The risk of transmission to humans is very low, and the possible cases of transmission identified so far remain uncertain.

Crossing from one species to the other facilitated by their physical proximity

Scientists from ANSES visited the farm to better understand how the pigs could have become contaminated. Passage of the virus from cattle to pigs within the farm could not be confirmed, but is highly probable given the proximity of the pigsty's ventilation inlet to the cowshed. Moreover, according to the farmer, some young bulls had developed a respiratory illness shortly after arriving on the farm. The virus could therefore have been transmitted by air, or via tools or clothing. Analysis of samples taken from pigs of different ages showed that the virus had circulated among the pigs before being eliminated.

Avoiding adaptation to new species

When studying the genome of the virus isolated from the pigs, the scientists discovered that it carried two new mutations. Given their location, these mutations could make it easier for the virus to adapt to new species. However, this theory remains to be confirmed. "This example is a reminder of the importance of keeping species apart on farms and ensuring that non-disinfected equipment is not transported from one area to another", reiterates the scientist. Adaptation of the influenza D virus to pigs represents a risk to animal health. Moreover, pigs are known to be intermediate hosts, facilitating the adaptation of influenza viruses to humans, as is the case particularly with type A influenza viruses, which can cause zoonotic influenza. Strict surveillance is therefore needed.