What is it? What does it mean for you?
By Sean Ray
Summary Points:
- Bird flu has been circulating amongst cattle in the US
- Meat and dairy products (pasteurized) are still safe for consumption
- Risk to humans remains very low
- Scientists are concerned about further spread of bird flu among mammals
Bird flu, as the name suggests, is typically found in birds. Though very uncommon, mammals (including humans) can become infected, and the result is often severe disease and even death. Fortunately, this recent outbreak in cattle has resulted in mild disease, and pasteurized meat and dairy products are still safe for consumption. However, this outbreak is concerning to scientists because it is part of a larger trend of bird flu becoming more prevalent in mammals. Bird flu emerging in mammals increases the chances of a pandemic influenza virus emerging, so scientists are working to understand why and how this uptick of bird flu in mammals is occurring. This article summarizes the latest work on understanding bird flu in cattle, and what the greater implications of this outbreak are.
What is the flu?
Influenza (aka, “the flu”) is a respiratory disease. There are many different types of flu and humans, other mammals, and birds can all get the flu.
In humans, the seasonal influenza A subtypes in humans are H1N1 and H3N2; these circulate year-round in human populations and are the “flu” you are used to hearing about (in addition to influenza B). There is no clinical difference between H1N1 and H3N2. They are both capable of causing severe disease, though they most commonly cause the mild “flu-like” symptoms that people are most familiar with.
What is “bird flu”?
Avian influenza, colloquially known as “bird flu”, is a term used to describe subtypes of influenza A virus that naturally circulate in birds, specifically waterfowl. Waterfowl are the natural reservoir for influenza A so they can be infected with “bird flu” and be just fine. The bird flu viruses that cause little to no disease in birds are referred to as low-pathogenic avian influenza (LPAI).
Some avian influenza viruses can cause severe disease in birds, poultry, and humans. These viruses are referred to as highly-pathogenic avian influenza (HPAI) viruses and are most commonly of the H5N1 and H7N9 subtypes. For the remainder of this article, we will be discussing the HPAI H5N1 “bird flu” that has recently found its way into cows.
How did bird flu get into cows?
The short answer is: scientists are not completely sure, but they have a reasonable hypothesis. Sequencing data suggests that the spillover, or introduction, of bird flu into the cows, was a single transmission event. This means that a single cow got infected by a single bird and bird flu has spread from cow to cow ever since. Scientists are not completely sure how the first cow became infected, though it is likely that it stumbled across a dead/infected bird and interacted with it. On the other hand, the transmission mechanism from cow-to-cow is becoming clearer. It has been suggested that the main method of transmission is through the milk and milking equipment of cattle. This is supported by data indicating large amounts of virus (i.e. high titers) in the milk of infected cows as well as mammary glands. Moreover, the spread of HPAI H5N1 to other herds can be associated with dairy cattle movement and the sharing of milking equipment. Studies are underway to explore other routes of transmission (e.g. respiratory droplets), though preliminary data has not indicated any other major route of transmission.
What is the current threat to humans?
At time of writing this article, there have been thirty-nine confirmed human infections with HPAI H5N1 bird flu following exposure to cows. There have also been twenty-three reported infections of HPAI H5N1 virus closely related to the one found in cows following exposure to poultry. Fortunately, these human cases have been relatively mild with no deaths reported and no human-to-human transmission. This is important because human infections with HPAI H5N1 typically have a staggering 50% mortality rate; and the current situation with mild symptoms, no mortality, and no human-to-human transmission is encouraging. Therefore, experts say the risk of human infection remains very low. Testing of commercial, pasteurized milk and beef and chicken has not detected any live, replicating virus. However, there have been high levels of live, replicating virus detected in raw milk, so raw milk consumption is not recommended. Overall, if a person is avoiding exposure to infected poultry and infected cows (check here for states with confirmed HPAI cases in livestock) and not consuming raw milk, their risk is incredibly low! If they are exposed, they should monitor for symptoms for 10 days after last exposure. Symptoms include eye redness (like pink eye) and cough.
What are the long-term health implications?
Though the risk to humans is currently very low, the introduction and sustained transmission of HPAI H5N1 in cattle is an alarming occurrence and is part of a continuing trend of bird flu becoming more prevalent in mammals. Flu mutates rapidly, and the more animals (including humans) that it can continuously infect, the more mutations it can accumulate. This can lead to more serious flu seasons, where the vaccines have low efficacy against the circulating flu strains, and, more seriously, the emergence of a pandemic flu strain that the population is immunologically naive to (e.g. 2009 H1N1 swine flu pandemic). For these reasons, experts are watching the situation closely and scientists are conducting experiments to assess the risk of current HPAI H5N1 virus strains infecting more mammals.
hpai-h5n1-dairy-cattle-mi-epi-invest.pdf
About Bird Flu | Bird Flu | CDC
H5 Bird Flu: Current Situation | Bird Flu | CDC
CDC Confirms New Human Cases of H5 Bird Flu in California | CDC Newsroom
Signs and Symptoms of Bird Flu in People | Bird Flu | CDC
H5N1 and Safety of U.S. Meat Supply | Animal and Plant Health Inspection Service
HPAI Confirmed Cases in Livestock | Animal and Plant Health Inspection Service