Answering reader questions on H5N1

Tackling surveillance and detection, and pet risk factors

I've received a lot of questions about H5N1 lately, from concerns about cats to questions about how we know what's going on with this virus. So I'm going to do a series for paid subscribers answering the most common questions I've received. I've grouped them into a few categories, including prevention and control, understanding risk, and disease characteristics. Today I'll tackle two of these categories - surveillance and detection (how do we know what we know?) and animal risk factors (what to know about pet cats, birds, and other animals?). This post contains a paywall.

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Surveillance and Detection

What systems are in place to detect when H5N1 begins spreading between humans, and how reliable are they?

There are multiple different surveillance systems for influenza, and so I'm not worried that significant human-to-human transmission would go undetected. During peak flu season, for example, around 500-1,000 influenza A specimens are tested each week by the public health labs to identify the specific types and subtypes, including H5N1.

Also, specimens from hospitalized patients with influenza A undergo typing and subtyping. This is how the Missouri case was found back in September. There is also wastewater surveillance for H5N1 (more on that below), and some states have programs encouraging farm workers to get tested. Moreover, when someone is infected with H5N1, their close contacts are monitored for signs that the virus has spread.

To put some numbers behind it, one study by CDC scientists found that with 100 novel influenza infections (like H5N1) circulating in a population, the probability of detecting at least one case per month was about 72% in urgent care settings and 77% in random community testing, with urgent care being much more efficient since it required fewer tests. So although it's possible for some human-to-human transmission to go undetected, I don't think it would continue very long unnoticed.

How confident can we be that we're not missing cases, particularly mild or atypical presentations?

Individual mild cases are a different story. I wouldn't be surprised if some cases are going undetected. Not severe cases—I'm pretty confident in our ability to identify H5N1 in hospitalized patients. But in an outpatient setting, it's likely we are missing some; one-off, mild cases are like finding a needle in a haystack. But a large number of cases would likely show up in our surveillance systems.

These answers are specific to the United States. When thinking globally, surveillance and detection is more patchy. Some countries have very sophisticated systems for detection of novel influenza, and others are less equipped. Although the U.S. is in the spotlight right now because we are dealing with outbreaks in dairy cattle, spillovers could happen anywhere. For instance, Ghana reported a human infection with H9N2 to the WHO in September, and Cambodia reported a reassorted H5N1 in the fall as well. (Curiously, I have never seen a WHO Disease Outbreak News Alert come through for the recent human cases in the U.S.)