Bird flu continues to appear to pose a "low risk to the general public" for now, the Centers for Disease Control and Prevention says.
Epidemiologists from the agency were ultimately unable to access a Texas dairy farm where a human was infected with the virus in March, they disclosed in attachments to the report published Friday by the New England Journal of Medicine. That prevented investigators from being able to investigate how workers might have been exposed to the virus on the farm.
That is because the dairy worker who came to a Texas field office for testing "did not disclose the name of their workplace," said Lara Anton, a spokesperson for the Texas Department of State Health Services.
They also were unable to collect follow-up samples from the dairy farm worker or their contacts, which could have revealed missed cases as well as tracking the virus and antibodies against it in the body after an infection.
H5N1 was likely spreading through dairy farms via the high concentrations of the virus found in the raw milk of infected cows, authorities said previously.
The virus had been circulating in cows for an estimated four months before it was confirmed by labs on March 25, according to a draft report from U.S. Department of Agriculture scientists released Thursday.
A mutation to the virus in wild birds, a specific "clade" of the virus that scientists call 2.3.4.4b, appears to have enabled bird flu to jump into cows. Multiple herds were likely infected during that initial spillover before the birds migrated north, officials have said.
Since then, at least nine states have detected cow infections from the virus. Cows largely recover from H5N1, unlike the mass die-offs seen in other species. Some herds with infected cows have also remained asymptomatic and are continuing to produce milk.
Experiments run by the Food and Drug Administration show that pasteurized milk remains safe to drink, despite traces of the virus found in samples from grocery stores. The outbreak has also prompted a renewed warning not to drink raw milk, which has been linked to deaths of other animals like cats.
The ongoing outbreak is also in stark contrast to how the virus has spread in other mammals infected by the virus, which have generally resulted in what USDA scientists called "dead end hosts."
A handful of variants with potentially worrying mutations have also since been spotted in cows, the USDA analysis found. If those variants become dominant, it could change the disease caused by H5N1 or make spread to other animals or humans more likely.
The virus from cows has also been spotted spreading out of dairy farms into nearby wild birds and poultry, likely ferried by contaminated milk droplets and surfaces.
Questions also remain about the exact origins of the virus that infected the Texas dairy worker. While the H5N1 sequence from the human case is closely related to those found in dairy herds, the agency's analysis found it also differs in some key ways.
Those genetic differences suggest the human was infected by "an early, slightly different virus" that was circulating in cows before the current cases, or that multiple spillovers may have actually occurred.
While sequences collected from sick cows on the worker's dairy farm could have helped CDC scientists answer those questions, samples were "not available for analysis."
Recovery of Human Worker from Bird Flu
Following a bird flu infection, a human worker has successfully recovered. The individual experienced conjunctivitis, commonly known as pink eye, without any fever or other typical flu symptoms. Treatment with oseltamivir, an antiviral medication for bird flu, was administered to the worker and their close contacts as a preventive measure against further infections.
Possibility of Missed Infections
Recent reports from a local veterinarian suggest that other workers on Texas dairy farms exhibited symptoms of flu or conjunctivitis, as confirmed by Anton on April 30.
However, health officials in Texas and neighboring New Mexico have disclosed that some of these symptomatic workers tested negative for H5N1. Anton mentioned, "It's possible that there were additional individuals with symptoms who chose not to undergo testing, making it uncertain whether there were other cases of H5N1. Nonetheless, it is confirmed that individuals were ill with other respiratory viruses while working on dairy farms."
In light of the reliance on dairy workers and their associates to voluntarily report symptoms and undergo testing, health authorities and experts are exploring alternative data sources to detect any potential undetected spread of the virus.
Recent analysis of wastewater samples in a small town in northwest Texas revealed a concerning increase in traces of H5N1 virus in the sewers. Interestingly, during the same period, there was a noticeable decline in emergency room visits in the area. Researchers have put forth a hypothesis suggesting that the spike in H5N1 levels could be attributed to the disposal of waste from dairy farms housing infected cows, rather than human transmission.
To alleviate concerns about potential undetected cases of H5N1, the Centers for Disease Control and Prevention (CDC) have highlighted the significance of monitoring emergency room data.
