For the moment, the pandemic risk is “low”, according to specialists. But they remain on alert in the face of the possibility of a mutation of the virus which would facilitate its transmission between humans.
Epidemiologists and virologists around the world have their eyes on the United States this spring. The subject of these concerns? Bird flu. A third person contracted influenza A H5N1 there, linked to an epidemic of this virus in cows. This is an employee working on a farm in the state of Michigan, American health authorities said on Thursday, May 30. The tension is great because, among the threats identified by the world’s health authorities, that of a pandemic involving an avian virus is at the top of the list. Franceinfo reviews the situation and the possible risks.
Three human infections on three different farms
The first human case of avian flu linked to an epidemic in cattle dates back to April 1, in Texas, in the southern United States. The second case was detected in Michigan, in the northern United States, on May 22. The third was announced eight days later, in the same state, but on a different farm. The Centers for Disease Prevention and Control (CDC) has not discovered a direct link between these three cases at this stage.
All of these patients work on farms with cattle and none wore protective equipment. “They were in very close contact with them all day”underlines to franceinfo Guillaume Fournié, veterinarian and epidemiology researcher at the National Research Institute for Agriculture, Food and the Environment (Inrae).
Variable symptoms
The first two patients only experienced mild eye symptoms. For the second, the infection occurred “after a splash of milk in the eyes”, specified the Michigan health official. The third person showed symptoms “more typical” of a respiratory illness, including cough, but without fever, the CDC said. Treated with oseltamivir, an antiviral, she isolated herself at home and “his symptoms are improving“added the CDC. His relatives did not develop symptoms.
To prevent further cases, people exposed to sick or potentially infected animals should take some precautions. For example, avoiding contact “close together, prolonged and without protection” with animals as well as with their excrement, or even with raw milk, American health authorities warned.
The US Medicines Agency has announced that milk sold in stores in the United States is “on”the pasteurization process being “effective” to kill the bird flu virus. Heating raw milk to high temperatures, as pasteurization does, destroys virtually all traces of the virus after a few seconds and all of the pathogen after several minutes, researchers confirmed in a study published in the New England Journal of Medicine. According to a recent United States-wide investigation, all samples of pasteurized milk did not contain viable virus. However, the virus in an inactive state, that is to say unable to spread, was found in around 20% of the samples.
Cattle so far spared
The virus in question is a type A avian virus of the H5N1 subtype. We speak of avian influenza when animals are infected, and of avian flu when humans are affected, as explained by the National Agency for Health, Food, Environmental and Occupational Safety (ANSES).
This family of H5N1 viruses has been well known for around twenty years. But Guillaume Fournié reports that the scientific community was surprised by noting the pattern of transmission in the United States and the involvement of cattle. The H5N1 virus, which circulates mainly in wild birds, generally infects humans through domestic birds and then a new host species. Pigs have already played the role of intermediaries allowing the crossing of the species barrier in the past, but scientists do not know if they are involved in the current circulation of the virus in the United States.
“We did not expect that this virus could reach cattle. This is the first time that influenza A has been identified in them.”, remarks Guillaume Fournié. Yet, “the catalog of species affected was important”, including horses or dogs.
“What’s really worrying is when we see new species of mammals that are infected and can transmit the virus to each other.”
Guillaume Fournié, researcher in epidemiology at INRAEat franceinfo
This epidemic which affects cattle herds has so far been encountered in nine American states (Michigan, Ohio, Idaho, South Dakota, North Carolina, Kansas, Connecticut, New Mexico and Texas). Guillaume Fournié discusses the need to maintain a “high vigilance” “to continue to monitor traffic” And “better understand the level of contamination of farms in the United States”.
Transmission between humans not observed at this stage
“There is no indication of person-to-person transmission of the virus” of influenza A H5N1 “at this time”, emphasized the CDC. As early as mid-April, the World Health Organization (WHO) announced its “huge concern” faced with the increasing spread of avian flu to new species. In France, the Committee for Monitoring and Anticipation of Health Risks (Covars) is reassuring about the pandemic risk, while recommending reinforced surveillance.
“The scientific community, health authorities, health services and veterinary services are watching these recent events very closely.”
Guillaume Fournié, researcher in epidemiology at INRAEat franceinfo
“The H5N1 virus is closely monitored by the WHO because there is a fear of relatively high mortality as has been observed in the past when it was transmitted to humans”, added to franceinfo Thibaut Crepin, CNRS researcher at the Institute of Structural Biology. Since the Hong Kong episode in 1997 (18 cases, six deaths), during which an avian flu epidemic was noted with the first cases of direct contamination from birds to humans, cases of H5N1 flu ( therefore affecting humans) amount to just over 850, in 16 countries, with a fatality rate greater than 50%, explained Michel Bublot, veterinarian and virologist, during a conference at the Museum of Biological Sciences in 2022. .
If the pandemic risk is “weak” Currently, there is concern about the possibility of human-to-human transmission. The probability of a pandemic involving an avian virus is “close to 100%”, scientists explained to franceinfo in 2023. This threat is not weakening, according to the specialists interviewed in June 2024, which justifies the close monitoring in progress. In other words, experts expect a new pandemic, but do not know when and where it will emerge.
All continents potentially affected
The epidemic affecting the United States is relatively limited at this stage. However, “we must not minimize the situation”supports Thibaut Crepin. Without forgetting that “geographic distance is no longer a barrier” and that it would be misleading to believe we are safe on our side of the Atlantic, warns Guillaume Fournié.
Although the wild birds that have infected American cattle migrate mainly between North America and South America, there are “overlapping areas” between the different migration corridors, he summarizes. These birds may have found themselves in gathering areas with other birds, coming from elsewhere, on their way to the European continent. These crossing areas are conducive to exchanges, allowing viruses to circulate and conquer new territories.
“The involvement of wild birds in the epidemiology of this virus is a game-changer and we can now expect significant outbreaks around the world.”
Guillaume Fournié, researcher in epidemiology at INRAEat franceinfo
The scientific community may well have a certain “predictive power”it can be taken by surprise by “upgradable capabilities”es viruses, Guillaume Fournié readily recognizes. “It is important to remain humble and continue to monitor their progress, without being alarmist.”