Why has bird flu affecting US cows and what’s the human risk?


The bird flu virus, initially detected in US poultry farms across 14 states in early 2022, has now extended to cows and even two humans, marking the first cases of bird flu in humans in the United States. This strain of bird flu, classified as a “global pandemic for animals” by experts, is also spreading in other nations.

Why has bird flu affecting US cows and what's the human risk
Why has bird flu affecting US cows and what’s the human risk

US health authorities are closely monitoring dairy cow herds, as well as beef and milk products, since the first cow outbreak was reported in March. While the US is the sole country reporting bird flu in cattle, there are concerns about potential risks to humans.

In April, a Texas farm worker contracted the virus, believed to be the first known case of transmission from cattle to a human. Fortunately, the individual experienced mild conjunctivitis and fully recovered. The first US human case of bird flu occurred in 2022 when a poultry farm worker in Colorado was exposed to infected chickens, with fatigue being the main symptom reported.

Despite these relatively mild cases, bird flu can pose significant dangers to humans. Since its initial detection in China nearly 30 years ago, the virus has infected approximately 860 individuals across 23 countries, including China, Egypt, Vietnam, and Turkey, according to the World Health Organization (WHO). Among these cases, 463 resulted in death, translating to a substantial 52% mortality rate.

The Centers for Disease Control and Prevention (CDC) have identified the virus in over 200 cows, 9,000 wild birds, and around 90 million chickens throughout the US.

Recently, Canada and Colombia have tightened import checks on US cattle due to the spread of bird flu.

The key questions remain: How is bird flu spreading, and could it pose a more severe threat to humans in the future?

What is bird flu?

Bird flu is a viral infection primarily affecting birds, but certain strains can also infect other animals, including humans. It falls under Influenza A, one of four types of influenza known to cause flu pandemics.

There are various subtypes of avian influenza, with some like H5N1, H5N6, and H7N9 known to cause illness in humans. The letters and numbers in their names denote specific protein combinations and subtypes.

H5N1, the most widely recognized subtype, was first identified in domestic geese in China’s Guangdong province in 1996. This strain is currently affecting birds and cattle in the US.

In birds, the virus can cause severe illness and death. Infected cows, especially older ones, exhibit symptoms such as reduced appetite and lactation.

Meghan Davis, an associate professor at the Johns Hopkins Bloomberg School of Public Health and trained dairy veterinarian, described H5N1 as a strain causing a global animal pandemic (or epizootic disease) impacting domestic poultry production significantly.

What is the extent of bird flu prevalence in US cattle?

Since late March, H5N1 has been detected in approximately 200 animals across 36 dairy cattle herds spanning several US states including Colorado, South Dakota, Kansas, Michigan, North Carolina, Idaho, Texas, Ohio, and New Mexico.

Certain experts speculate that the outbreak might have started spreading to cows prior to March, possibly as early as late 2023, although it went unreported.

Meghan Davis emphasized the uncertainty surrounding the extent of this spread, stating, “What we don’t know is just how widespread this might be. This calls for extensive surveillance, multiple perspectives, and varied approaches to address this challenge.”

Could bird flu spread to cattle in other countries?

While there have been no confirmed instances of bird flu affecting cows outside the United States to date, the World Health Organization (WHO) has cautioned that there is a potential for international spread through migratory bird movements.

Wenqing Zhang, the head of WHO’s Global Influenza Programme, highlighted this risk during a late April news briefing in Geneva, explaining, “Given the virus’s transmission by migratory birds across the globe, there is indeed a possibility for cows in other countries to become infected.”

How dangerous is bird flu for humans?

H5N1 is classified by the CDC as “highly pathogenic,” indicating its strong disease-causing potential in birds. It has also proven deadly to humans in previous cases. Since its discovery in 1996, only 860 individuals worldwide have contracted the virus, but over half of them have succumbed to it, according to the WHO.

Individuals in direct contact with infected animals, such as farm workers, are at the highest risk of infection. Human-to-human transmission is rare, with only one suspected case reported in Indonesia in 2006.

In humans, symptoms of bird flu can manifest within two to eight days after exposure, ranging from fever, cough, sore throat, and muscle aches to more severe complications like pneumonia and organ failure.

How do we treat bird flu in humans?

At present, there is no vaccine accessible for avian flu in humans. When individuals are infected, they are usually treated with antiviral medications like oseltamivir or zanamivir to alleviate symptoms. Health organizations like the CDC advise promptly administering these medications once symptoms appear to effectively manage the illness.

How do farmers manage virus outbreaks?

In many regions worldwide, farmers typically cull animals exposed to a deadly virus outbreak like bird flu. For instance, in early April, a poultry farm in Texas euthanized 1.5 million chickens to contain the spread of bird flu.

As of now, there have been no reports of cattle being culled. Andrew Stevens, assistant professor of agricultural and applied economics at the University of Wisconsin-Madison, suggests that bird flu has been less lethal in cows compared to chickens and turkeys.

Stevens noted, “In this context, culling would be an extreme and costly measure to prevent further disease spread. This is especially true given our limited understanding of how contagious the bird flu virus is among cattle.”

How is bird flu transmitted to farm animals and humans?

Wild birds and waterfowl such as ducks and geese can transmit bird flu through their droppings or saliva. When poultry or other animals scavenge in contaminated environments or come into contact with infected birds, they can also contract the virus. Researchers are still investigating potential modes of transmission involving infected wild birds on farms that could affect cows through water or feed consumption, according to Davis.

The exact method of viral transmission among cows remains unclear, but evidence suggests that mammary glands can harbor a significant viral load. This suggests that milking processes may facilitate virus spread, as equipment could become contaminated or the virus might become aerosolized during cleaning procedures.

Humans who have close or prolonged interactions with farm animals or wild birds can become infected when the virus enters their system through the eyes, mouth, or inhalation of droplets or aerosol particles through the nose.

Is it possible to get bird flu from consuming milk and beef products?

Since April, the US Food and Drug Administration (FDA) has conducted tests on 297 dairy products, including milk, cottage cheese, and sour cream. Initially, traces of the virus were detected in 20% of the first 96 samples. However, subsequent testing confirmed that the pasteurization process, used by farmers to eliminate harmful pathogens and bacteria from dairy products, effectively destroys the virus, ensuring consumer safety.

Health experts, including the CDC, strongly recommend consuming only pasteurized dairy products not only to mitigate the risk of avian flu but also to prevent bacterial infections such as salmonella and brucella.

Although beef samples tested by the US Department of Agriculture (USDA) have shown no signs of bird flu, the FDA continues to advise against consuming raw beef for safety reasons.

Aren’t all dairy products pasteurised?

There is a growing movement against food regulation in Western countries like the US, viewed by some as a symbol of freedom. For instance, in 2015, Wyoming enacted the Food Freedom Act, allowing farmers to directly sell raw milk and other farm products to consumers.

Australia, which along with Canada has a blanket ban on raw milk sales, is witnessing a rise in the “Raw Milk Movement,” advocating for regulated distribution of unpasteurized products. Proposed regulations would ensure hygienic production, proper packaging, and testing for harmful pathogens.

While raw milk sales are permitted in 30 US states, federal law prohibits interstate distribution of unpasteurized milk. Additionally, milk from sick cows must be entirely discarded under federal regulations. However, if raw milk is packaged before cow illness is detected, products with virus traces could potentially reach store shelves.

Although human studies on this matter have not been conducted due to ethical considerations, research on cats in the US demonstrated that those consuming raw milk infected with H5N1 became ill or died as a result.

What measures are being taken by authorities in the US to address the bird flu outbreak?

Experts and public health authorities, such as the CDC, have been studying bird flu for decades, noting that the COVID-19 pandemic has prompted positive changes in outbreak preparedness.

Regulatory agencies are increasing virus surveillance efforts. In late April, the USDA mandated testing for bird flu in all dairy cattle moving between states.

However, Davis noted that testing within state boundaries remains voluntary, which might not be sufficient to contain the virus spread. She emphasized the need for a shift from passive to active surveillance, where the focus is on comprehensively understanding the scale of the issue.

Davis suggested employing antibody serology testing, which detects recent virus exposure by analyzing blood or saliva samples. Another method is polymerase chain reaction (PCR) testing, which uses nasal swabs to identify current infections but may not be ideal for asymptomatic individuals or those with minimal exposure.

Rural communities in the US may face challenges during outbreaks due to limited access to healthcare services and protective equipment. Medical facilities could be distant and unaffordable, posing obstacles to testing for both farmers and the workforce.

Davis highlighted the uncertainty regarding testing opportunities in such areas, particularly if farmers are unwilling to have their cows tested.

How are other countries reacting to the US bird flu crisis?

In late April, Colombia became the first country to restrict trade with the US due to bird flu in cows, imposing limits on beef and beef product imports from states where infected cows were identified.

Mexico, a significant market for US beef and dairy, has also intensified surveillance of incoming cattle for signs of respiratory issues, according to the agriculture ministry.

Recently, the Canadian Food Inspection Agency implemented stricter import controls on US cattle, requiring exporters to furnish negative bird flu test results for lactating dairy cattle and mandating retail milk testing for virus traces.

Stevens emphasized the importance of such measures for ensuring the long-term sustainability of supply chains. He noted that while increased import checks may slightly raise US export costs, these costs are minimal compared to the potential impacts of an import ban or moratorium.



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