#18,613
Despite hundreds of poultry outbreaks, eleven months ago the United States had only detected 1 (probable) human infection with H5N1 (Colorado, 2022). Since then (at least) 68 human infections have been reported, with 41 of those linked to exposure to cows.
Cattle infections - at least until a couple of weeks ago - were all attributed to the B3.13 genotype, leading researchers to believe that all of those herds were infected due to a single spillover of the virus from birds in late 2023 or early 2024.
News of a new spillover - this time of the D1.1 genotype from birds to cattle in Nevada - raise new questions about how the virus is spreading. What may have been true in 2024 may no longer hold true in 2025.
While we await more specific information on the spread of D1.1, we've an excellent overview of what we know - along with important gaps in our knowledge - about the transmission of the B3.13 genotype in cattle published yesterday in mBio.
Due to the delays in publication, this review has a cut-off date of Dec 12th, 2024, and a lot has happened in the two months since then.
Due to its length, I've just posted the abstract and a brief excerpt. Follow the link to read it in its entirety.
mGem: Transmission and exposure risks of dairy cow H5N1 influenza virus
Authors: A. J. Campbell , Kayla Brizuela, Seema S. Lakdawala
https://doi.org/10.1128/mbio.02944-24
ABSTRACT
In March 2024, highly pathogenic H5N1 was detected in dairy cows; as of 12 December 2024, it had spread to over 800 herds in 16 states. The ongoing outbreak is a public health crisis affecting both humans and animals, as interspecies transmission has emerged as a common characteristic of this virus. As of 12 December 2024, >30 humans have been infected in the United States related to dairy cow exposure. In this mGem, we discuss transmission modalities between cows within herds, the spread of the virus between dairy farms, and exposure risks for humans. We also highlight major gaps in knowledge constituting barriers to our ability to effectively control the spread of H5N1 in dairy cows and reduce the risks to humans.
PERSPECTIVE
Highly pathogenic avian influenza (HPAI) H5N1 virus of the 2.3.4.4b clade has been circulating in North America since 2022 (1–3). On 25 March 2024, a strain in this clade, genotype B3.13, was confirmed to be infecting dairy cows in Texas and Kansas (Fig. 1A). Since then, dairy herds in several other states tested positive for this strain and spillover into dairy workers has followed (4–7). Infections of other domestic and peridomestic animals, such as cats and raccoons, have also occurred on the same premises containing infected cows (7, 8). Notably, the spread of cow H5N1 to poultry flocks has resulted in the culling of millions of birds and the infection of workers involved in culling operations on multiple occasions (9).
As of 12 December 2024, over 800 dairy herds in 16 states have had confirmed infections, including three of the five biggest dairy-producing states (Fig. 1B) (10, 11). At least 58 known human cases of H5N1 including those exposed to dairy cows, poultry, or unknown exposure source have been reported by the CDC as of 12 December 2024 (12). Disturbingly, in September, cow H5N1 infections were confirmed in two housemates in Missouri without livestock contact (13, 14). Understanding how cow H5N1 transmits within and between farms is critical for curtailing the outbreak. Below, we discuss knowledge regarding the spread of cow H5N1 between farms, between cows, and the risk to humans.
(SNIP)
URGENT NEEDS
To contain the current outbreak, the following steps are needed:
i. Longitudinal sampling of individual cows to assess the length of the infectious period and whether cows can become reinfected.
ii. Tracing of infected cows and animals using the same milking equipment to provide important data regarding transmission.
iii. Broader testing of dairy worker populations, as well as the individuals they interact with, which is critical to accurately assess the frequency of virus spillover and initiate timely responses.
iv. Robust biosecurity measures such as personal protective equipment (PPE; eye, respiratory protection, and hand washing), education of key populations about the virus and symptoms of infection, and disinfection of equipment moving between farms.
v. Treatment of contaminated milk waste from dairy farms to inactivate the virus prior to disposal to ensure no livestock, pets, or peridomestic wildlife is further infected by it.
vi. Deployment of stockpiled H5 vaccines that can confer protection against the cattle 2.3.4.4b clade.
The widespread outbreak in cattle and frequent spillover into humans highlights the urgent needs for increased surveillance and mitigation efforts to combat the spread of cow H5N1.
The recent discovery of the spillover of the D1.1 genotype from birds to cattle is a serious complicating factor, as no longer can other countries point to their lack of the B3.13 genotype (only detected so far in North America) as shielding them from similar outbreaks.
And if that can happen with D1.1, it can presumably happen with other current (and future) genotypes, which could make devising a broadly effective vaccine challenging.
Given the number of HPAI H5N1 milestones we've over the past 12 months (see recent reviews here, here, and here), I won't even begin to speculate as to what the bird flu landscape might look like a year from now.
Other than to say we should expect the unexpected.
