Avian Influenza has killed millions of poultry birds in the U.S., giving rise to repeated outbreaks every year that wax and wane, depending on the season. The risk to humans has always been touted as “low” despite some infections in poultry workers over the years. Then in March of this year, the H5N1 strain of the virus jumped to cattle, and the number of people confirmed with the virus in the U.S. has proceeded to climbed to the highest levels ever. Now, H5N1 has been detected in at least two pigs in Oregon, which is raising big alarms among scientists.
For those that missed the news, the USDA at the end of October reported that a pig on a backyard farm in Oregon had tested positive for H5N1, the first known case in swine ever. A miniature potbellied pig on the farm also tested positive. All the pigs at the farm were euthanized and so far, no other cases in swine have been reported elsewhere. Still, the case has further raised concerns that bird flu could become a human pandemic.
In 39 human cases that have been confirmed in the U.S. this year, almost all been in people who are in close contact with livestock. The one exception was a case in Missouri, where transmission remains a mystery. Of the remaining 38 cases, 18 involved workers on farms that culled infected chickens. The other 20 cases were in dairy farm workers – at least 395 cow herds have tested positive for H5N1.
Researchers say H5N1 is causing infections in lactating cows via the mammary glands rather than the respiratory tract, which is the main site of infection for other influenza viruses in mammals. Since the infection in cows is largely restricted to the mammary glands, researchers believe that H5N1 is being transmitted by contaminated milking equipment.
The humans that have been infected with H5N1 this year have experienced mild symptoms that so far seem to be isolated to eyes, with symptoms including eye redness or discharge (conjunctivitis). None were reportedly hospitalized.
Birds and humans have different types of receptors in their respiratory tract that flu viruses attach to. This is a key reason why Avian flu viruses do not circulate unchecked in the human population.
Pigs have receptors in their respiratory tracts which both avian and human flu viruses can bind to. When multiple viruses infect the same cell and replicate, they can swap genetic material in a process known as “reassortment.” This reassortment can create a virus that contains features of both parent viruses, making it more transmissible and virulent to humans.
This type of genetic swap is believed to have produced the 2009 H1N1 swine flu from a mix of U.S. and European strains of pig flu virus, which did launch a pandemic but it was thankfully very mild. However, pigs are also the suspected source of the strain behind the 1918 “Spanish flu” pandemic. The pandemic broke out toward the end of World War I, so it’s difficult to form an accurate count, but estimates of deaths range from 17 million to 50 million.
Technically, reassortment could occur in cattle as well, but so far infections are limited to mammary glands and transmission to humans seems to require direct contact with infected animals. The pig cases, however, have left more questions than answers. The biggest mystery is how the pig was infected, which we may never know for sure. Hopefully it remains isolated to that one backyard farm but if it makes its way to a larger commercial operation, scientists say the risk of a human pandemic would become much more concerning. (Sources: Scientific American, Stat, CDHP)
