Avian influenza (bird flu)

Avian influenza (bird flu) is a highly contagious viral disease caused by various virus strains that primarily affects birds. Under certain circumstances, other animals and humans can also become infected with some of the known virus strains.

The outbreak in cows in the US that is currently dominating the headlines is caused by the influenza virus type A(H5N1). While previous reports of human infection with H5N1 have almost always been clearly traced back to close, direct contact with infected birds or their droppings, the current outbreak is the first to observe transmission to humans from other mammals - namely dairy cows [1]. At the end of May 2024, at least 69 cow herds in nine US states were proven to be infected. So far, three dairy workers have been infected with H5N1 and suffered from mild illness with conjunctivitis or mild respiratory symptoms [2]. The transmission of the pathogen to the milking machine is believed to be the starting point of the outbreak. Although all three people have recovered from the infection, the avian influenza virus can cause serious and sometimes fatal disease in humans.

Researchers believe it is realistic to expect that in the future, avian influenza viruses will mutate to become even more adaptable to mammal-to-mammal transmission. While the current outbreak in cows in the US suggests this, evidence already derived in 2023 in Argentine elephant seals using genome analysis [3]. One thing is certain: The closer the contact between humans and the affected mammalian species, or the more widespread use of their animal products, the greater the risk of a possible pandemic in humans. In order to be prepared for this scenario and to prevent further transmission among dairy cows, vaccine development approaches for cows are now being discussed. Yet, there is still a long way to go before such vaccines can be produced. First, fundamental aspects must be examined in more detail - for example, whether bird flu in cows is actually only transmitted via milk(ing) or also via droplets [4]. The high virus concentrations detected in raw milk of infected cows, for example, support the assumption that transmission occurs during the milking process [5]. As the large-scale use of avian influenza vaccines in cattle is still a long way off, current containment measures focus mainly on increased surveillance. Since the wastewater from dairy farms likely contains virus fragments, wastewater testing will soon be used on a larger scale [6].

Avian influenza infections in humans have been caused predominantly by the subtypes A(H5N1) and A(H7N9). The World Health Organization WHO therefore carefully monitors any new strains with the potential to infect humans. In April 2024, a person in Mexico was infected with A(H5N2) for the first time and died shortly afterwards. The exact route of infection in this case is still unclear [7]. Since no further human cases have occurred, and various outbreaks of H5N2 in poultry have been identified in Mexico since the beginning of 2024, transmission by mammals - as in the US - is unlikely.

Despite the current outbreak in cows, the health risk for the general population in the US is currently considered low [8]. However, the US Centers for Disease Control and Prevention (CDC) recommend that people should not consume raw milk and other insufficiently heated animal products [9]. Likewise, the risk of infection with bird flu for the European population is also considered to be low [10]. Here, the general population is only advised not to touch sick or dead wild birds and animals. People exposed occupationally should take appropriate precautions, such as wearing personal protective equipment and disinfecting their hands [10].

Nevertheless, the extent of the global H5N1 pandemic in birds - with more than 50 million birds killed since 2020 - and its massive spread to various mammalian species are reasons for increased vigilance. International experts are therefore calling for greater consideration to be given to the One Health approach in the future [11]. This approach is based on the recognition that human, animal, and environmental health are closely linked and that only interdisciplinary and global cooperation can prevent or contain future pandemics.

Sources:

1. https://www.who.int/emergencies/disease-outbreak-news/item/2024-DON512 (accessed on June 19, 2024)
2. Abbasi J (2024) As a Third Worker Tests Positive for Bird Flu in US Dairy Cattle Outbreak, Here’s What to Know. JAMA. Published online on June 5, 2024.
3. https://www.universityofcalifornia.edu/news/bird-flu-increasingly-adapting-mammals (accessed on June 19, 2024)
4. Cohen J (2024) A bird flu vaccine for cows? It's complicated. Science 384: 837-838.
5. Kozlov M. Huge amounts of bird-flu virus found in raw milk of infected cows. Nature. Published online on June 5, 2024.
6. https://www.statnews.com/2024/05/22/bird-flu-wastewater-surveillance-specific-h5-testing-by-wastewaterscan-network/ (accessed on June 19, 2024)
7. https://www.who.int/emergencies/disease-outbreak-news/item/2024-DON520 (accessed on June 19, 2024)
8. https://www.cdc.gov/bird-flu/situation-summary/index.html (accessed on June 19, 2024)
9. https://www.cdc.gov/media/releases/2024/p0401-avian-flu.html (accessed on June 19, 2024)
10. https://www.ecdc.europa.eu/en/zoonotic-influenza/facts/faq-avian-influenza (accessed on June 19, 2024)
11. Petersen E et al. (2024) Avian 'Bird' Flu - undue media panic or genuine concern for pandemic potential requiring global preparedness action? Int J Infect Dis 145: 107062.