Outcome of low pathogenicity avian influenza (H7N9) experimental infection of urban and rural house sparrows (Passer domesticus) is impacted by natural Avipoxvirus infection

Birds are constantly moving between the wild and the places where humans raise livestock, acting as invisible couriers for the microscopic viruses that cause disease. Among these travelers, the house sparrow is perhaps the most common, nesting in city parks and farmyards alike. Because they live so close to poultry, scientists have long wondered if these small birds could act as a bridge, carrying flu viruses from wild waterfowl into chicken farms. While wild ducks are the natural home for many flu viruses, sparrows are often infected with low-pathogenicity strains, which usually cause no obvious sickness. However, the real world is rarely simple. Wild birds often carry multiple infections at once, and their health is shaped by where they live. A sparrow in a city faces different stresses and eats different food than one in the countryside, factors that might change how their immune system fights off a virus. Understanding these subtle differences is crucial for predicting how flu might spread through the environment.

To explore this, researchers set up a controlled experiment to see how house sparrows from urban and rural areas reacted to a specific low-pathogenicity flu virus, known as H7N9. They captured adult sparrows from a city center park and from a rural area near a farm in central Spain. Before the experiment began, the birds were checked to ensure they did not already have the flu virus. They were then housed in separate, secure rooms to keep the groups distinct. Half of the birds from each location were given a small dose of the H7N9 virus through their nose, while the other half received a harmless saltwater solution to serve as a control. The scientists then watched closely for two weeks, checking the birds' weight, taking swabs from their mouths and cloacas to look for the virus, and drawing blood to see if they developed antibodies. They also monitored the birds for any signs of illness or death.

The results showed that the sparrows were indeed susceptible to the virus. The infected birds shed the virus in their droppings and respiratory secretions, primarily through their mouths, for a few days after infection. Most of the birds did not show severe symptoms, but a small number did become listless, and three birds in total died during the experiment. This mortality rate was higher than what is typically seen with this type of flu in other small birds, suggesting that something else might have been at play. The researchers found that the birds from the city were in slightly poorer physical condition than those from the country, and they took longer to recover their weight after infection. However, when it came to the virus itself, the city and country birds behaved almost exactly the same. They shed the virus for similar amounts of time and in similar amounts, and their immune systems responded in comparable ways. The location where the bird lived did not change how the flu virus moved through its body or how long it stayed there.

A surprising discovery emerged when the scientists looked deeper into the birds' health. A notable portion of the birds in the study, whether infected with the flu or not, were already carrying a different virus called Avipoxvirus, which causes skin lesions in birds. The researchers found that this poxvirus was more common and lasted longer in the birds that were also infected with the flu. The timing of the poxvirus detection overlapped with the peak of the flu virus shedding, suggesting that the two infections might be interacting with each other. The study identified three different types of this poxvirus circulating in the birds, with some types found only in the city and others only in the country. While the researchers could not prove exactly how the flu virus made the poxvirus more active, the data strongly suggests that having both viruses at the same time is a significant factor in how the disease progresses.

Ultimately, the study confirms that house sparrows can catch and spread the H7N9 flu virus, acting as potential carriers between wild birds and poultry. While the environment where a bird lives affects its general health and how quickly it recovers from the stress of infection, it does not seem to change how the flu virus itself behaves. The most important finding is that these birds are rarely infected by just one thing at a time. The presence of a second, common virus like avian pox appears to complicate the picture, potentially making the flu infection more severe or persistent. This interaction between different pathogens is a critical piece of the puzzle that scientists must consider when trying to understand how flu viruses move through the wild and into our farms. The study highlights that to truly understand disease spread, we must look at the whole picture of a bird's life, including the other invisible guests it carries, rather than just focusing on a single virus in isolation.