Officially, there is only one documented case of bird flu spilling over from cows into humans during the current U.S. outbreak.
But epidemiologist Gregory Gray suspects the true number is higher, based on what he heard from veterinarians, farm owners and the workers themselves as the virus hit their herds in his state.
"We know that some of the workers sought medical care for influenza-like illness and conjunctivitis at the same time the H5N1 was ravaging the dairy farms," says Gray, an infectious disease epidemiologist at the University of Texas Medical Branch in Galveston.
"I don't have a way to measure that, but it seems biologically quite plausible that they too, are suffering from the virus," he says.
Gray has spent decades studying respiratory infections in people who work with animals, including dairy cattle. He points out that "clustering of flu-like illness and conjunctivitis" has been documented with previous outbreaks involving bird flu strains that are lethal for poultry like this current one.
Luckily, genetic sequencing of the virus doesn't indicate it has evolved to easily spread among humans.
Still, epidemiologists say it's critical to track any possible cases. They're concerened some human infections could be flying under the radar, especially if they are mild and transient as was seen in the Texas dairy worker who caught the virus.
"I think based on how many documented cases in cows there are, probably some decent human exposure is occurring," says Dr. Andrew Bowman, associate professor of veterinary preventive medicine at The Ohio State University. "We just don't really know."
Limited testing raises concerns
There have been 36 herds affected in nine states. Local and state health departments have tested about 25 people for the virus and monitored over 100 for symptoms, federal health officials said at a briefing on Wednesday.
These people are in "the footprints of where the bovine detections are," says Dr. Demetre Daskalakis, who's with the Centers for Disease Control and Prevention, although he didn't provide details on the actual locations.
"There's a very low threshold for individuals to get tested," he adds.
The lack of testing early in the outbreak isn't necessarily surprising. In places like Texas and Kansas, veterinarians weren't thinking about bird flu when illnesses first cropped up in early March and it took time to identify the virus as the culprit.
But the total number of tests done on humans at this point seems low to Jessica Leibler, an environmental epidemiologist at Boston University School of Public Health.
"If the idea was to try to identify where there was spillover from these facilities to human populations, you'd want to try to test as many workers as possible," says Leibler, who has studied the risk of novel zoonotic influenza and animal agriculture.
Also, notes Gray, the virus is probably much more geographically widespread in cattle than the reported cases show, "possibly spilling over much more to humans than we knew, or then we know."
The federal government has been quick to assess the safety of the dairy supply. On Wednesday, the Food and Drug Administration released findings, showing that infectious virus wasn't present in about 200 samples collected from dairy products around the country. Initial results on ground meat are also reassuring.
However, there still remain "serious gaps" in public health officials' ability to detect bird flu among those who work with cows, a task made all the more difficult by the fact that some cases may not be symptomatic, says Leibler. "There's really widespread opportunity for worker exposure to this virus."
Only complicating matters — the true scale of the outbreak in cattle remains murky, although new federal testing requirements for moving cattle between states may help fill out the picture.
"Some of the dairy herds seem to have clinically normal animals, but potentially infected and [that] makes it really hard to know where to do surveillance," says Bowman.
Calls for proactive steps to track down possible human cases
The health care system would likely catch any alarming rise in human cases of bird flu, according to modeling done by the CDC.
Federal health officials monitor influenza activity in emergency departments and hospitals. Hundreds of clinical laboratories that run tests are tasked with reporting findings. And in early April, a CDC health alert was sent to clinicians advising them to be on the lookout for anyone with flu-like symptoms or conjunctivitis who'd worked with livestock.
But even these safeguards may not be sufficient to get ahead of an outbreak.
"I worry a bit that if we wait until we see a spike in those systems that perhaps we would already be seeing much more widespread community transmission," says Dr. Mary-Margaret Fill, deputy state epidemiologist for the Tennessee Department of Health. Instead she says there should be proactive testing.
Fill notes there are anecdotes about farmworkers with mild illness while working with cattle in some of the areas where the virus has spread and "not enough visibility on the testing that's happening or not happening in those populations to understand what might be going on."
To get ahead of the virus, Leibler says not only do workers need to be screened but also their family members and others in the community, in the event that the virus does evolve to spread easily among humans.
Dr. Rodney Young says doctors in the Texas panhandle have been vigilant about any cases of influenza, particularly among those who are around livestock, but so far there are no indications of anything out of the ordinary.
"We just haven't seen people who fit that description in order to suddenly be testing a lot more," says Young,regional chair of the Department of Family and Community Medicine at the Texas Tech Health Sciences Center School of Medicine in Amarillo.
Getting buy-in from dairy farms
Gray says it can be hard to detect and measure the illness in these rural workers for many reasons — their remote location, a reluctance to seek out health care, a lack of health insurance, concerns about immigration status, and a reticence among farmers "to wave the flag" that there are infections.
The farms he works with consider protecting workers and curbing the spread of this virus "a huge priority," but right now they bear all the risks of going public, he says.
Dr. Fred Gingrich says this is a major barrier to closer cooperation between federal health officials and the industry during the current crisis.
Dairy cattle farmers currently don't get compensated for reporting infections in their herds — unlike poultry farmers who receive indemnity payments for losses related to culling birds when they find cases, says Gingrich, executive director of the American Association of Bovine Practitioners.
"So what is their incentive to report?" he says, "It's the same virus. It just doesn't kill our cows."
Gray has managed to start collecting samples from humans and cattle at several dairy farms that recently dealt with the virus. It's part of a study that he launched before the H5N1 outbreak in response to concerns about SARS-CoV-2 spillover on farms.
They'll look for evidence of exposure to novel influenza, including bird flu --something he's able to pull off because of his background in this area and his guarantee that the farms will be kept anonymous in the published work.
What concerns him most is the possibility the outbreak could wind up at another kind of farm.
"We know when it hits the poultry farms because the birds die, but the pigs may or may not manifest severe illness," he says, "The virus can just churn, make many copies of itself and the probability of spilling over to those workers is much greater."
