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Outbreaks, Alerts and Hot Topics: Bird Flu in Bovines

Column Author: Kathleen J. Berg, MD, FAAP | Medical Director, Office of Evidence-Based Practice

Column Editor: Chris Day, MD | Director, Transplant Infectious Disease Services; Medical Director, Travel Medicine Program

 

Readers of this column have likely heard about the outbreak of bird flu in dairy cattle that began earlier this year and the two associated human cases of HPAI (highly pathogenic avian influenza) A(H5N1). The affected people were both dairy workers that had conjunctivitis and apparently no other symptoms. Conjunctivitis has been known with avian influenza infections previously.1

The “highly pathogenic” in HPAI refers only to effects of the virus in poultry, where these viruses cause severe disease and high mortality, and does not refer to their effects on mammals, where illness (if it occurs at all) will be variable.2 Since the original emergence in 1996 of HPAI A(H5 N1), many millions of wild birds and poultry have died from either direct infection or, in poultry, from culling to prevent ongoing transmission of infection. HPAI A(H5N1) was initially detected infecting domestic waterfowl in China. In 1997, the first human infections were noted in Hong Kong. It would go on to cause more than 900 human infections (most occurred prior to 2016) with a mortality rate around 50%. For a time, it was replaced in birds by H5N6 and H5N8 viruses, but in 2021 a new strain of HPAI A(H5N1) emerged in the Eastern Hemisphere. That strain started the current HPAI A(H5N1) wild bird outbreak in North America in March 2022. The first and, until recent events, only human case in the United States was noted shortly thereafter.3

Genomic analysis from specimens obtained in 2024 suggests the outbreak in U.S. dairy cattle began around December 2023 or January 2024, when there was likely a single crossover event from wild birds to cattle. The crossover could have occurred as a result of contamination of cattle feed by bird feces. The outbreak was recognized with the first reports of clinical cases in cows in March 2024 in southwestern Kansas and northeastern New Mexico. HPAI A(H5N1) infections in cows have now been detected in multiple states, including Michigan, Idaho and Ohio. In these three states, infections appeared following transport of infected cows from sites that are known to have been affected earlier, providing further evidence of some form of cow-to-cow transmission.4 The exact mechanism of transmission does not appear to be clear, though there is some speculation about a possible association with milking machines or other equipment.2 Signs of infection in cows include reduced feed intake and rumination as well as an abrupt drop in milk production. Nevertheless, affected cows shed large amounts of viral particles in their milk. Deaths in cattle have been infrequent. Some post-mortem cows (three euthanized, three died naturally) that were examined had evidence of mastitis. While prior to this occurrence there has been serologic evidence of influenza A infections in cattle, clinical infections have been reported rarely, and this rarity likely played a part in the delay in recognizing the current outbreak. In those rare prior outbreaks, a drop in milk production was also often seen.

On at least one dairy farm in the Texas panhandle, deaths due to HPAI A(H5N1) occurred in domestic cats that were fed milk products from sick cows. Cats are well known to be susceptible to HPAI A(H5N1) and frequently have severe disease. In the past, infections in cats have typically occurred in association with consuming wild birds or contaminated poultry products.4

While the current HPAI A(H5N1) outbreak in dairy cattle is an unusual manifestation of the virus and the implications for human health are not entirely clear, the risk to the general public seems to be low.1 Reassuringly, only two human cases have been reported, and no human-to-human transmission has been documented. While influenza viruses do have pandemic potential and avian influenza viruses (including HPAI A(H5N1)) acquired directly from birds have been deadly in individual cases and in smaller outbreaks, efficient human-to-human transmission is required for a pandemic to start.3 Apparently, the current strain may require genetic viral reassortment with a human strain of influenza A or need to undergo significant changes to adapt to humans before human-to-human transmission could reliably occur.5 The serious threat to human health posed by influenza viruses has typically been through respiratory disease, and this form of illness has not occurred in humans with the outbreak in cattle, likely at least in part because this particular virus does not currently have adaptations that are compatible with respiratory infections in humans.2 Prevention of further transmission to humans (as well as halting the outbreak in cattle) is an important public health effort. The CDC (Centers for Disease Control and Prevention) advises individuals to “avoid close, long, or unprotected exposures to sick or dead animals, including wild birds, poultry, other domesticated birds, and other wild or domesticated animals (including cows)” and “also avoid unprotected exposures to animal poop, bedding (litter), unpasteurized (“raw”) milk, or materials that have been touched by, or close to, birds or other animals with suspected or confirmed A(H5N1) virus.”3 Pasteurized milk remains safe both because it kills the virus and because only milk from healthy cows is processed for consumption. Additional efforts by CDC, state health departments, and the U.S. Department of Agriculture are ongoing, including advice for worker protection, efforts to supply personal protective equipment to farmworkers, and assistance for milk producers affected by the outbreak.

Figure 1. Epidemic Curve of Human Cases of A(H5N1) by Illness Onset Date, 1997-2024 by Country (N=912). (Source: CDC, reference 3).

References:

  1. CDC reports second human case of H5 bird flu tied to dairy cow outbreak. News release. Centers for Disease Control and Prevention; May 22, 2024. https://www.cdc.gov/media/releases/2024/s0522-human-case-h5.html
  2. Racaniello V. (Host). TWiV 1113: influenza virus H5N1 in cows’ milk with Richard Webby [Audio podcast episode]. In This Week in Virology. MicrobeTV. May 12, 2024. https://www.microbe.tv/twiv/twiv-1113/
  3. Technical report: highly pathogenic avian influenza A(H5N1) viruses. Centers for Disease Control and Prevention, National Center for Immunization and Respiratory Diseases (NCIRD). June 9, 2024. CDC.gov. https://www.cdc.gov/flu/avianflu/spotlights/2023-2024/h5n1-technical-report-06052024.htm
  4. Burrough ER, Magstadt DR, Petersen B, et al. Highly pathogenic avian influenza A(H5N1) clade 2.3.4.4b virus infection in domestic dairy cattle and cats, United States, 2024. Emerg Infect Dis. Published online April 29, 2024. doi:10.3201/eid3007.240508
  5. The Lancet Infectious Diseases. What is the pandemic potential of avian influenza A(H5N1)? Lancet Infect Dis. 2024;24(5):437. https://www.thelancet.com/journals/laninf/article/PIIS1473-3099(24)00238-X/fulltext
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