On November 13, 2024, the Public Health Agency of Canada (PHAC) confirmed the first domestically acquired human case of avian influenza (also known as bird flu) caused by the influenza A(H5N1) virus in Canada.
On November 9, the Office of the Provincial Health Officer for British Columbia reported that a teenager was hospitalized and tested presumptive positive for H5 avian influenza. On November 13, the Public Health Agency of Canada’s National Microbiology Laboratory in Winnipeg confirmed that the individual had avian influenza H5N1. The genomic sequencing result indicates that the virus is related to the avian influenza H5N1 viruses from the ongoing outbreak in poultry in British Columbia (Influenza A (H5N1), clade 2.3.4.4b, genotype D1.1).
The PHAC notes that no cases of avian influenza have been reported in dairy cattle in Canada and there is no evidence of avian influenza in milk samples from Canada. They also emphasized that the genotype of H5N1 avian influenza in dairy cattle in the United States differs from the genotype confirmed in the human case in British Columbia. The subsequent analysis has shown that the sequence from an infected teenager has some mammalian adaptation signature.
The avian influenza A (H5N1) virus was first identified in southern China in 1996, leading to substantial outbreaks among poultry in Hong Kong in 1997. These outbreaks resulted in 18 human infections. Even though the 1997 avian epidemic was controlled, the virus persisted in birds, and two decades later, it has spread extensively among birds throughout Asia, reaching Africa, Europe, the Middle East, and for the first time South America and Antarctica. Millions of wild bird and poultry deaths across multiple continents were caused by highly pathogenic avian influenza A (HPAIV) subtype (H5N1) viruses, which continued to diversify genetically.
Human infection with avian influenza A(H5N1) usually occurs after close contact with infected birds, other infected animals, or highly contaminated environments. After an outbreak of avian influenza A(H5N1) clade 2.3.4.4b in Texas dairy cattle herds in March 2024, the first human infection was identified in Texas in early April. From February 2022, there have been 52 confirmed human cases of avian influenza A(H5N1) in the United States (26 in California, 10 in Colorado, 2 in Michigan, 1 in Missouri, 1 in Oregon, 1 in Texas, and 11 in Washington) (CDC data). In a recent study, the authors from the United States examined two dairy farms in Texas in April 2024, which were recovering from incursions of their cattle with HPAIV influenza A H5N1 virus, and found elevated titers of neutralizing antibodies against the recombinant H5N1 virus of clade 2.3.4.4b in the sera of 14% of farm workers (2/14). https://discovermednews.com/texas-dairy-farmworkers-exposed-to-hpaiv-h5n1/
In recent human infections with the H5N1 2.3.4.4b virus in the United States, farmers have exhibited significantly lower case fatality rates than previous H5N1 outbreaks in Asia, where half of individuals with reported infections died. Since human infections in the United States have been characterized as conjunctivitis and mild respiratory symptoms and have not required hospitalizations, these milder symptoms have been attributed to the absence of viral pneumonia.
The differences in the disease presentation of the current H5N1 human cases could be due to many factors, including changes in the viral genome or the impact of prior immunity to H1N1 strains that circulated widely after 2010. As a result of genomic reassortment, the currently circulating panzootic H5N1 viruses are genetically distinct from previous strains. Some authors suggest that older people may have partial immunity to H5N1 because of their childhood exposure (“imprinting”) to seasonal H1N1 and H2N2 viruses, whereas younger people born since the 1968 H3N2 pandemic may be more susceptible to severe disease in a H5N1 pandemic. A certain degree of cross-reactivity between H5N1 and the avian-origin N1 neuraminidase that has circulated in humans since the 2009 pandemic may also provide partial protection. https://doi.org/10.1038/s41586-024-08054-z
However, experts are warning that human cases of the H5N1 avian flu could be going undetected due to poor surveillance and a lack of diagnostic testing in at-risk groups. Continuous monitoring and reporting of results from farmworkers is crucial in understanding outbreak trends. Genome analyses of future H5N1 strains are also extremely important, not only for determining circulating viral strains but also for evaluating genetic markers associated with increased virulence and resistance to antivirals. For example, the GenoFLU tool was developed to classify HPAI H5N1 goose/Guangdong clade 2.3.4.4b viruses detected in North American flyways. https://github.com/USDA-VS/GenoFLU/blob/main/README.md
Reference
Statement from the Public Health Agency of Canada. November 13, 2024, Ottawa, ON, Public Health Agency of Canada https://www.canada.ca/en/public-health/news/2024/11/update-on-avian-influenza-and-risk-to-canadians.html
