7/20- H5N1 Aerosol Spread With Respiratory Infection Continues to be Underrated

Reality keeps disrupting the preferred talking points and research slowly provides more clues

(Illustration courtesy Med Page Today via Google Images)

Highly contagious influenza infections in all species has spread via respiratory shedding and transmission. Initial spillovers may include oral or oronasal routes (e.g. infected poultry remains fed to mink), but to really rock and roll across herds or flocks, the virus needs to become airborne. That basic fact contradicts the ongoing preoccupation/consensus that the current dairy H5N1 influenza epidemic is due to intramammary infection and transferred between herds and into neighboring poultry flocks predominantly by mechanical means via human biosecurity breaches. These just aren’t plausible routes that would allow explosive outbreaks as the U.S. has documented in Texas, Michigan, Idaho, Colorado, and to some extent Iowa and Minnesota. Mechanical transmission really gets tenuous in trying to explain repeated transmission into highly bio-secure large egg layer and turkey breeder facilities.

The swine industry has extensive experience with “area spread” of PRRS and influenza in pig-dense areas. Next week, I hope to devote more time to their experiences in dealing with this issue. In most cases the industry has moved past the “is it real” debate and on to “how do we deal with it”? But it still remains contentious because proving such transmission is nearly impossible.

Zoonotic H5N1 news this past week has been dominated by Colorado Deployment Response-H5 Human Outbreaks. Workers have recovered, treatment with antivirals has begun, and monitoring is ongoing. Improved compliance with PPE standards is being reviewed in the face of extreme heat making conditions difficult.

CDC deposited full genome sequences from a human isolate into GISAID Tuesday. USDA deposited H, N, and PB2 sequences from milk and chicken samples into GISAID (link) in the last 24 hours. The newly deposited samples closely align with the human sequence data from the Colorado human case deposited earlier in the week by CDC. Although the USDA data lacks date and location metadata, GISAID elected inclusion to allow phylogenetic comparisons. These timely new deposits this week indicate that USDA has provided the recent CO milk and chicken sequence data to allow comparisons with the human sample. In a Tuesday press conference CDC reportedly stated that the poultry, dairy, and human viruses were genomically closely related, as these GISAID trees indicate. Thus, we have phylogenetic evidence that the H5N1 dairy virus in Weld County likely jumped from cattle to a local bio-secure layer flock and then on to humans working to depopulate that flock.

Yesterday we learned of a second 1.3 million bird layer operation in Weld County infected with H5N1. According to Google maps this flock is located within 1 mile of a labeled dairy herd (of unknown H5N1 infection status) but is approximately 20 miles (as the crow flies) from the first poultry farm outbreak. Unless plausible contacts are found between the 2 poultry operations, it is likely the second flock was also infected via a jump from an area cattle infection. In less than 2 weeks these 2 events will require depopulation of over 3.1 million chickens in the heat of summer in Colorado. Nationally nearly 18.4 million birds will be depopulated (through July 18th) due to H5N1 2.3.4.4b b3.13 infections since the March 2024 discovery of the spillover of this virus into dairy herds in Texas.

The issue of how this virus transmits is critical and was covered this week in an article in Nature.com, which featured analyses on ARS H5N1 dairy cattle infection research reported out recently:

Can H5N1 spread through cow sneezes? Experiment offers clues (nature.com)

This Nature article delves extensively into the question of respiratory transmission of the B3.13 sub-clade of H5N1 2.3.4.4b in dairy cattle. I’ve pulled a section from Nature for your review:

Virus-laden mist

Amy Baker and her colleagues at the US Department of Agriculture (USDA) in Ames, Iowa, infected cows and calves with the specific H5N1 strain isolated in Texas cattle early in the outbreak. They exposed four female one-year-old calves to virus-laden mist through a mask that covered the animals’ noses and mouths. All of the infected animals produced neutralizing antibodies against the virus, confirming that they were infected.

The infected calves exhibited mild symptoms, and the researchers isolated infectious virus in the upper airways of two of the four calves. The study findings suggest that in an environment where hundreds of animals are held in close quarters, the virus could be spreading through the respiratory route.

But given that the animals did not shed virus at high levels in their airways, it is probably not a major source of spread, says Thomas Peacock, a virologist at Imperial College London. It’s possible, however, that cows could be getting indirectly infected through the respiratory route through aerosolized virus shed from the udder, he adds.

Wendy Barclay, a virologist at Imperial College London, agrees that because low levels of infectious virus were detected in the animals despite their exposure to high doses of H5N1, airborne transmission is probably not efficient, nor can it “explain what is going on at present”.

Instead, the study adds to previous work that suggests the virus is mainly spreading through infected milk, say researchers. “Udder-to-udder transmission still seems most likely to be the major route at present,” says Peacock. “That doesn’t mean the virus can’t change though, if this outbreak continues at the pace it currently is,” he says. “What we must now do is to keep a careful watch on the virus.”

The study mainly looked at the susceptibility to infection of cows, not transmission, which makes it difficult to draw conclusions about viral spread, says Jürgen Richt, a veterinary virologist at Kansas State University in Manhattan. Richt has done his own cow experiments together with colleagues in Germany, and the results should appear in a preprint within weeks.

I want to further comment on the above statements, which I don’t completely agree with, but first we should quickly review the proposed critical transmission work referenced by Dr. Richt and soon to be reported. I found an article back on May 7th in Science which described plans for an international collaborative transmission study between Kansas State University, The Loeffler Institute in Germany, and Vaccine and Infectious Disease Organization in Saskatchewan in Canada:

…One goal of the lab work is to investigate how the virus is spreading from cow to cow. The working theory is that lactating cows with infected udders are spreading the virus to other dairy cows during the milking process, but that’s based on circumstantial evidence such as the presence of H5N1 in environmental samples from milk parlors. “The big question right now is whether the virus is mechanically transmitted or can be transmitted from cow to cow via aerosol as well,” says Jürgen Richt, a virologist at Kansas State University.

The best way to test this is in well-designed animal experiments in highly secure labs, he says. “There are only a few facilities that can do this kind of work, so we have been coordinating with each other who is doing what.”

Richt, who runs one such facility, plans to inoculate the nose and mouth of male and female cattle with the same strain Diel sent Beer. “Then we will look at what’s happening within the animals,” he says. “We will kill some of them after 4 or 5 days and look where the virus is in the body.” Others will be watched for weeks to see whether they develop antibodies to H5N1 and how long they shed the virus in various ways. And 2 days after some of these initial infections, new cows, called sentinels, will be added to the mix to see whether they can catch the virus.

While Richt is doing these experiments with nonlactating cows, Beer will work with lactating cows, depositing the virus straight inside all four of the animals’ teats. And Volker Gerdts, director of the Vaccine and Infectious Disease Organization in Saskatchewan in Canada, is planning similar experiments with calves in the organization’s BSL-3 facility. …

No preprints have yet appeared from this work; however, Dr. Richt orally reported that his small study failed to show respiratory transmission between adult non-lactating cattle, and Dr. Beer reportedly showed localized infection with fevers in mammary tissues with intramammary infusions of both the American B3.13 clade, as well as a European clade of H5N1 2.3.4.4b H5N1 virus. Nothing has yet been shared publicly from the Canadian group.

I anxiously await the printed report from these groups, as well as transmission studies from ARS, which may well have already been completed but not yet reported. In the meantime, USDA allows that there may be a role for respiratory transmission, but continues to officially minimize its likely role both in public statements and in language in the preprint:

No to mild respiratory signs have been reported in affected dairy herds within the U.S. and very few bovine respiratory diagnostic samples have been confirmed to be positive for H5N1 HPAI. The clinical signs, RT-qPCR results, macroscopic lesions, histologic findings, and antigen distribution of this study align with these field observations. However, the mild acute macroscopic and histologic lung lesions, detection of replicating IAV by IHC in the lower respiratory tract, and RT-qPCR with virus isolation in two upper respiratory swabs confirmed a respiratory phase of infection following aerosol inoculation. Although respiratory infection was limited in the 4 heifers, the detection of viable virus in 2 out of 4 represents a mode of infection and transmission that, when applied to an animal facility that commonly holds hundreds of animals, implies there is a role for the respiratory route.

I would argue that the diagnostic sample argument is a red herring! Respiratory samples are actively discouraged; most diagnoses are based on milk samples. In addition, respiratory symptoms are subtle to non-existent and occur early in the course of infection prior to any signs of mastitis. With low mortality in clinical cases, very few diagnostic cases will undergo complete post-mortem examinations including respiratory tract histopathology.

Mastitis with extremely high viral loads in milk is conversely an overwhelming finding later in the course of infection. I find it interesting that intramammary infected cows in the study did not suffer infection beyond infected quarters and associated lymph nodes, according to pathology findings. Other quarters and organ systems were not infected. It almost seems that these cows suffered more of a systemic toxic reaction to a localized infection, rather than systemic H5N1 infection. More details from the researchers regarding pathological findings would be welcome.

It would also be useful to determine if intranasal infection of lactating cows leads to later mastitis as a sequela. It appears that intramammary infusion did not lead to lung lesions; does the opposite infection route produce a more systemic infection? It’s conceivable that intramammary infusion may truly be an unnatural form of viral transmission for influenza virus.

Returning to the intranasally infected heifers, here is a chart buried in the supplemental tables of the preprint that I found quite illuminating:

Note that 7 nasal swabs in the study were PCR positive with PCR cycle times as low as 24.8 and 27.4. A total of 6 sample were virus isolation positive over a 7-day sampling period. None of these results prove transmissibility; however, these are somewhat robust viral load levels for up to 5 days after inoculation. As the paper states, when thousands of incubating animals are housed together, all potentially intermittently shedding virus at these levels, the potential exists for ongoing respiratory spread of infection within the herd and conceivably between herds (and to poultry flocks) to some distance. Real world events since March in Colorado, Idaho, Texas and other locations would at least suggest that these possibilities should be further explored.

That brings me to my final article for review in a very busy week - please bear with me and read as much as you can stand to digest. This important paper compiles HPAI surveillance work from northern Vietnam’s live bird markets, painting a concerning picture of H5N1’s (and other AI’s) opportunities to find new ways to mutate. What I want to point out, however, is innovative use of new technology for surveillance, i.e. bioaerosols. This group used air collectors in live bird markets as a noninvasive sampling technology for AI viruses:

High Prevalence of Highly Pathogenic Avian Influenza: A Virus in Vietnam's Live Bird Markets

Here is a summary of the main findings of the studies at live bird markets. Note that air sampling provided somewhat comparable positive findings to more traditional labor-intensive sampling methods:

The papers conclusion states:

Furthermore, bioaerosol sampling appears to be a sufficient influenza virus surveillance tool that could potentially replace more invasive bird swab sampling. Such environmental sampling is relatively easy to use, highly portable, nonintrusive, and generally well accepted by market workers, making this surveillance tool a great alternative to the traditional methods of individual sampling of animals.

I bring this up because I feel some derivation of air sampling may be an alternative for the poultry industry to better document likely air transmission (versus biosecurity lapses) as sources for H5N1 outbreaks near cattle-dense areas. This knowledge won’t eliminate or necessarily even reduce the risk but may at least point the way towards where mitigation efforts need to be directed.

The entire agricultural sector needs to solve the H5N1 transmission question, including whether both dairy and non-dairy cattle are being sub-clinically infected by and intensifying transmission of this virus. Serology needs to be utilized strategically to answer these questions; we have enough confidence in its cut-offs to at least use it for some rudimentary research, if not for full regulatory purposes. Retail milk, air and wastewater sampling are additional non-invasive tools that could quickly aid in answering prevalence and transmission questions and also add to the sequencing pool for ongoing genomic analysis.

Good News (With a Caveat) to Close This Week

CDC H5N1 Bird Flu Response Update, July 19, 2024-Michigan Serology Study Preliminary Results

The Michigan human serology study returned all negative findings! None of the dairy workers in Michigan volunteering for serological sampling were positive for H5N1 titers. The virus as found in Michigan at that time apparently does not easily infect people sufficiently to generate a serological response.

My only caveat is that flu viruses are notorious adapters. With 6 confirmed natural infections in poultry response workers in Colorado, researchers will be busy looking for any genomic changes in all 8 segments between viruses found in Michigan cattle and human, and those found in Colorado cattle, poultry, and humans. Influenza is the story that never ends…

John