Avian influenza is a disease caused by influenza virus type A that primarily affects birds but can affect mammals, including humans, after prolonged exposure. Since its discovery in 1876, the outbreaks in poultry have become more frequent. Between 2014 and 2015, over 51 million birds in the US were culled due to outbreaks.
Milk
The main hosts are wild aquatic birds, which can be transmitted to domestic birds like chickens. The virus spreads usually by contact with infected birds via secretions from the mouth, nostrils, eyes, and feces. It can also spread through fomites like contaminated farm equipment.
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Many strains of avian influenza virus exist. They were initially classified into highly pathogenic (HPAIV) and low pathogenic (LPAIV) strains based on their ability to cause mass deaths in a poultry population. This classification now takes into consideration the various structural differences of viral haemagglutinin.
The severity of HPAIV strains varies amongst different birds, from less severe in wild birds to deadly in poultry birds. The H5N1 subtype is an emerging strain that threatens to cause a pandemic. It has been particularly responsible for mass poultry deaths in various countries.
Highly pathogenic Avian flu in cows
Recently, avian flu, particularly the H5N1 strain, has been detected in dairy cattle in nine US states. Proper diagnosis were made following symptoms and signs that were noticed; reduced milk production, abnormal appetite, and abnormal appearance of milk. However, the disease has not been so severe in cattle compared to its manifestation in poultry.
The affected dairy cattle usually recover with little or no fatality. This also shows that possible cow-to-cow transfer is possible, through contaminated farm equipment or direct contact. Cattle owners, veterinarians, and dairy producers are currently encouraged by the United States Department of Agriculture to practice intensive bio-surveillance; testing and isolation of any sick cow, monitoring of new cattle for 30 days, and adequate testing of lactating cows, adequate testing and pasteurization of milk.
Risk of AIV in Retail Dairy Products
So far, dairy products have not been contaminated in a way that causes illness in humans, however, a study carried out by Erica Spackman et al. detected strands of RNA in 20.2% of dairy samples that they collected and tested.
Usually, milk from cattle goes through a purification process called pasteurization. This involves applying heat to dairy to eliminate pathogens. Different countries have their pasteurization protocols, however, a temperature of 72°C for at least 15 seconds is applicable in the US (according to the Pasteurized Milk Ordinance, PMO). This system has several rules that ensure food safety.
Erica’s team wanted to find out whether the virus had entered the food market. They also wanted to determine if pasteurized dairy’s viral particles can be amplified with quantitative real-time RT-PCR (qrRT-PCR). They collected 197 pasteurized dairy samples from processing locations in 38 states.
Amongst all the viral RNA strands that were replicated, none showed any infectious tendencies. They also tried to replicate the virus in embryonated chicken eggs, however, no infectious particles were detected.
Clinical significance
The study vividly presents evidence that the avian influenza virus entered the circulation through dairy products, however, we are not sure at what stage or in what quantity since these samples were obtained from different states. The production location, pasteurization, and even marketing might differ for different dairy products.
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So far human cases recorded have been very few, and we intend to keep it that way. We are confident that the avian influenza virus can be safely eliminated if specific guidelines concerning dairy products and biosecurity are followed. Most importantly milk from sick cows should be destroyed.
References
Spackman, E., Jones, D. R., McCoig, A. M., Colonius, T. J., Goraichuk, I. V., & Suarez, D. L. (2024, May 22). Characterization of highly pathogenic avian influenza virus in retail dairy products in the US. medRxiv (Cold Spring Harbor Laboratory). https://doi.org/10.1101/2024.05.21.24307706
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