What to know about bird flu – and the potential for another pandemic

ATLANTA – For influenza viruses, imperfection is a strength. They constantly mutate, producing new strains that challenge immune systems primed to fight earlier varieties. That is what makes flu a lifelong threat to humans and the animal species – mainly birds – that are vulnerable to it. 

Since 2020, a strain of highly pathogenic avian influenza called H5N1 has been decimating both wild and domestic birds.

Now the same strain is spreading among US dairy cows, and has also infected several US farm workers exposed to sick cattle.

A man in Mexico recently died with a strain of bird flu, H5N2, that has not been seen in humans before.

The US Centres for Disease Control and Prevention (CDC) and the World Heath Organisation (WHO) say the overall risk to the general public remains low, but public health officials remain on alert for any indication of the most feared outcome: human-to-human transmission that could trigger a pandemic.

Former CDC director Robert Redfield said in June that it is not a matter of if a bird flu pandemic will arise, but when. 

1. When did H5N1 begin spreading?

The H5N1 strain was first discovered in 1996 in geese bred in southern China. It is proven to be alarmingly adept at jumping continents and species and has rampaged through captive and commercial birds across the globe. Governments have ordered millions of birds slaughtered to limit the contagion. 

A variant that emerged in 2020, clade 2.3.4.4b, paved the way for spread among mammals such as farmed mink in Spain and Peruvian sea lions. Wild migratory birds are thought to be the initial source of the infections in US dairy cows, possibly via contamination of their feed or water in late 2023. 

2. What do we know about H5N2?

Not much yet. The strain is related to H5N1. Both are in the influenza A virus family. But there has been little research on H5N2. While the deceased patient in Mexico marks the first laboratory confirmed case of H5N2 in humans, researchers have identified people with antibodies that could signal prior infections.

The death in Mexico was surprising because the 59-year-old patient, who had underlying health conditions, had no history of exposure to poultry or other animals, which are the most likely source of disease.

“It’s so crazy that we don’t know where this is coming from,” said Dr Katrine Wallace, an epidemiologist with the University of Illinois Chicago School of Public Health. The Mexican government is now monitoring wild birds for H5N2 as the strain has previously been reported in poultry in the Mexican state where the person lived.

3. How bad is bird flu for humans? 

Bird flu can be fatal. But the health implications for H5N1 in the US so far have been limited. Since the start of 2020 through May 3, 2024, 28 confirmed human cases of H5N1 bird flu have been reported to the WHO, resulting in eight deaths. People in close contact with sick animals have a higher risk of becoming infected. 

4. What are the symptoms?

The three people who have tested positive in the US in 2024 – all farm workers in close contact with dairy cows, the first in Texas in March and two in Michigan in May – reported irritated eyes.

The health authorities are looking out for conjunctivitis, a type of eye infection, as an indication that bird flu may be involved.

The latest case in Michigan also experienced an upper respiratory infection, raising concerns about transmission. The CDC is conducting genetic analyses to see if the virus has mutated in a way that could make it more dangerous.

The victim in Mexico – who also had chronic kidney disease, Type 2 diabetes and longstanding hypertension – died one week after developing a fever, shortness of breath and diarrhoea. 

5. How are people infected with bird flu treated? 

Anyone with a suspected or confirmed infection should be treated immediately with one of the anti-flu drugs known as neuraminidase inhibitors, such as Roche Holding’s Tamiflu, according to the CDC.

Patients should be treated regardless of the severity of the disease or the length of time since symptoms began, the agency said, and the start of treatment should not await confirmation of an infection.

6. Is there a bird flu vaccine for humans?

Not yet, but the US has two vaccine candidates that are well-matched to H5N1, according to Ms Dawn O’Connell, who leads the Administration for Strategic Preparedness and Response. One of those candidates is now being manufactured. The US selected vaccine maker CSL Seqirus to fill and finish 4.8 million doses this summer. 

The US is also in talks with Moderna and Pfizer about the development of mRNA vaccines.

The government is working out support for a late-stage trial of Moderna’s bird flu shot, the Financial Times reported. Whether any of these vaccines also protect against H5N2 remains unknown.

Regular flu vaccines do not protect against bird flu.

7. How many US dairy cows have been impacted?

As at June 18, the CDC has reported 101 herds in 12 states have tested positive for bird flu. The agency has previously said it believes the cows are transmitting it to each other via contact with infected milk.

US officials have not recommended the destruction of the cows, which seem to recover from the infection, unlike the millions of birds that have been culled after the virus was detected among them. 

While milk from cows that are thought to be infected is being diverted or destroyed, these limited losses should have no major impact on the supply and no impact on the price of milk or other dairy products, according to the US Department of Agriculture. 

8. Are milk, cheese, ice cream and other dairy products safe? 

Yes, so long as the products have been pasteurised. Nearly all commercial milk produced on US dairy farms complies with standards that include pasteurisation, a process that kills viruses and most bacteria, though some inactivated particles may remain.

On May 1, the Food and Drug Administration (FDA) said that it had tested about 300 samples of dairy products – including pasteurised milk, sour cream, cottage cheese and baby formula – all of which proved to be safe. Traces of the virus were found in about 20 per cent of commercial milk samples tested by the FDA.

9. What about raw milk products? 

Scientists fear that raw milk could possibly transmit the virus, though no human cases have been identified that relate to consumption of the unpasteurised product.

US health agencies suggest Americans should avoid unpasteurised products, such as raw milk cheese, and advise consumers to ask grocers when in doubt as to whether a product is pasteurised.

Consumers also should not buy milk products from farmers’ markets or roadside stands unless certain they have been pasteurised.

While federal law prohibits the sale of raw milk and raw milk products across state lines, 14 states including California allowed the sale of raw milk in retail stores as at 2019. In states where raw milk is banned, it still remains easy to get, posing a threat to consumers. 

10. Are there implications for the meat supply?

Bird flu virus particles have been detected in the beef tissue of only one dairy cow, which has already been condemned so its meat will not enter the food supply. So far, there have been no reported cases of bird flu in beef cattle, and officials have said they are very confident in the safety of the meat supply based on their tests. BLOOMBERG