The main transmission route of avian influenza in the outbreak affecting cattle in the US appears to be through milk and milking techniques. This is indicated by the results of new research published in the latest issue of the journal Nature, which shows that respiratory transmission does not play a key role in the spread of this outbreak that has experts concerned.
In March 2024, an infection of cattle on a farm in Texas (US) was detected, caused by a highly pathogenic strain of the H5N1 avian influenza virus (specifically clade 2.3.4.4b). This was the first case of an outbreak that has already affected 231 farms in 14 states across the country, which is extraordinary as this type of pathogen had never before affected a ruminant species in this way. During these months, 14 people in the US have been infected with avian influenza, four of them through direct contact with cattle, 10 through contact with birds, and one of them with an unknown source of exposure.
Currently, the virus remains inefficient in infecting humans, but the characteristics and known potential of the pathogen demand very close monitoring.
Jürgen Richt's team from the Department of Pathobiology and Diagnostic Medicine at the College of Veterinary Medicine in Kansas (US) is one of the groups closely monitoring the outbreak. After the initial cases of contagion, they wanted to analyze the possible transmission routes of the clade 2.3.4.4b virus, for which they conducted two in vivo experiments, one in calves and one in dairy cows.
Their objective was to compare the susceptibility to the pathogen and its transmission. Therefore, they initially orally and nasally infected nine calves, which were then housed with healthy animals in facilities with high biological security.
Similarly, they inoculated the circulating virus in US cattle into the mammary glands of three milk-producing cows. In another three dairy cows, they carried out the same procedure but with a strain of the virus circulating normally in birds in Europe.
The infection of the calves caused mild disease symptoms, such as coughing or nasal secretions, but the virus did not spread to the healthy animals they lived with.
In contrast, the infected dairy cows developed severe conditions, including fever, mastitis, and a reduction in milk production of up to 90% compared to before the infection, as well as a loss of appetite. The milk also showed a significant loss in quality. However, no signs of systemic infection or respiratory tract involvement were observed. These effects occurred in both cows infected with viruses circulating among US cattle and those that had received strains of European origin.
"Cow-to-cow transmission likely occurs through the milking process, appears to be related to procedures, and therefore represents a mechanical and anthropogenic event," the scientists emphasize in their conclusions. "Milking and milking processes, in this sense, seem to be the central mediator of pathogen spread on farms," they add.
According to Elisa Pérez Ramírez, a researcher at the Center for Animal Health Research (CISA-INIA-CSIC), the data provided by this study is "important and necessary."
"We needed an experimental infection study that would allow us to answer the questions we have about what is happening in the US, and to answer them, an experimental infection study like this was necessary, which, as the authors indicate, may have some limitations, but it is the best way we have to try to understand in controlled conditions how the virus's pathogenesis is, how it is excreted, what clinical symptoms it presents in cows, etc."
The work "confirms that the virus has a very strong affinity for the mammary glands of cows and, unlike what happens in other species or with other subtypes of influenza, this virus concentrates a lot in the mammary gland and does not cause systemic infection; that is, it does not massively affect other organs," adds the specialist, who emphasizes that the research shows that the virus is mainly excreted through milk, where there are very high concentrations of the pathogen.
Therefore, the milking process would allow the virus to be transmitted due to the contact of infected milk and udders with the same machinery used on different cows.
Susceptible to European-Origin Viruses
Pérez also highlights another fundamental aspect of the research, the fact that it has demonstrated the susceptibility of dairy cows to two H5N1 viruses from different genotypes and two different continents.
"The researchers have not only inoculated the genotype that is being detected in cows in the US but have also inoculated another H5N1 virus of European origin. And this virus causes exactly the same clinical picture as the American genotype," emphasizes the researcher.
Both pathogens cause severe mastitis, cows develop very severe clinical symptoms, apathy, lack of appetite, dehydration, and a decrease in milk production. "Both the American and European viruses are capable of reproducing at least under experimental conditions the clinical picture we are seeing on US farms. And this would imply that the fact that there has been no outbreak outside the US is not due to the virus itself, it is not an intrinsic reason related to the virus, but there must be some other factor that explains this. This means that, at least theoretically, the H5N1 virus circulating in Europe could also cause this in cows," raising new questions.
"Now we need to answer the question of why this is only happening in the US. We will have to wait for further epidemiological studies to explain what is unique or special about these US farms where the outbreaks have occurred, whether there is closer contact with wildlife or what the key factor is," she suggests.
Additionally, the work also provides important information on the clinical conditions caused by the disease in dairy cows, Pérez Ramírez points out.
Inoculating the virus into the mammary glands of the animals involved in the experiment produced severe manifestations of the disease, including fever, lethargy, and general discomfort accompanied by a drastic reduction in food intake, milk production, and the onset of clinical mastitis. Four animals had to be euthanized earlier than planned during the study due to the severity of their symptoms, as noted by the scientists in their work.
"Initially, it was said that the disease was quite mild in cows, but this work shows that perhaps this clinical process is more severe than previously thought and can have serious repercussions because cows do not recover and, of course, do not regain the milk production they had before the infection, which has many economic implications on farms," Pérez Ramírez points out.
In their work, the scientists acknowledge that the methods used to inoculate the virus into the cows "may not be a perfect representation of how milking equipment can infect udders," although they emphasize that "the similarity of our virological and clinical results with field studies seems to validate the conditions used in our experiment."
Although the mechanisms involved are not yet known, the scientists suggest that this ability of the virus to replicate in the mammary glands may stem from mutations in the PB2 sequence of its genome.
Fortunately, human-to-human transmission of this virus has not yet occurred, but the frequent and direct contact between people and this type of animals provides the opportunity for the virus to adapt to a new host, the scientists remind, calling for measures to control the pathogen's spread, as well as to better understand the transmission mechanisms and its behavior in the dairy industry.