Agent causal
Influenza viruses
Influenza (flu) viruses are RNA viruses of the family of Orthomyxoviridae. Four (4) types of influenza viruses are known: A, B, C and D. The subtypes of influenza A viruses are classified according to two proteins found on the surface of the virus:
- Hemagglutinin (H), of which there are 18 subtypes AND
- Neuraminidase (N), of which there are 11 subtypes.
The combination of these different subtypes makes it possible to classify influenza A viruses (eg: influenza A H1N1 or H5N1). Each subtype (e.g.: H1N1) can subsequently be subdivided into different clades and subclades.
Seasonal influenza: Influenza viruses types A and B circulate in humans every year. These viruses are transmissible from one human to another and regularly evolve through progressive mutations.
Zoonotic influenza : Influenza A viruses circulate in animals (birds, pigs, etc.). Although these can occasionally be transmitted to humans following close contact with an infected animal, they are difficult to transmit from human to human. Zoonotic influenza viruses are monitored because they can, through recombination with other zoonotic or seasonal influenza viruses, generate pandemic influenza viruses.
Pandemic influenza: Pandemic influenza viruses occur when zoonotic influenza viruses recombine with each other or with seasonal influenza viruses. These reassortments can occur in animals or humans, and give rise to a new subtype of virus, transmissible from human to human. As the population is not immune to this new virus, it can spread quickly. This was the case, for example, during the H1N1 subtype pandemic in 2009.
The hosts
Zoonotic influenza viruses are usually named in relation to the species of animal they primarily infect: avian influenza, swine influenza, equine influenza, etc.
Wild aquatic birds are the natural reservoir for most avian influenza viruses. For example, some species of ducks can shed virus in their intestines and respiratory tract without being very symptomatic. Since these are often migratory birds, they can spread the virus quite quickly from one place to another. These viruses are contagious and can therefore sicken and kill other species of birds, particularly poultry. The pathogenicity of avian influenza clades (highly pathogenic (HPAI), low pathogenic (LPI)) is classified according to disease severity and lethality in poultry. This pathogenicity is not correlated with pathogenicity in humans.
Avian influenza viruses can also infect other animals (e.g. mammals) that feed on wild birds or whose environment is contaminated by wild bird droppings.
The remainder of this chapter will be devoted solely to avian-type zoonotic influenza viruses.
Transmission
Bird-to-human transmission
Transmission of the avian influenza virus from birds to humans is rare, since the virus is difficult to transmit to humans.
Transmission is usually done by:
- Contact direct : contact with a living or dead animal, such as during slaughter, plucking, handling of carcasses, preparation for consumption of poultry;
- Contact indirect : contact with the environment contaminated by birds. For example, viral particles released into the air during sweeping of soil contaminated with bird droppings;
- Food contamination: A few anecdotal cases have been associated with the consumption of raw blood from contaminated poultry. Cooking food would eliminate the risks.
Transmission between animals
During the HPAI H5N1 avian influenza epizootic of clade 2.3.4.4b which has been raging since 2020, many mammals have contracted the disease. Both terrestrial (raccoons, bears, wolves, lynx, coyotes, etc.) and marine (otters, seals, dolphins, etc.) mammals have been contaminated. In 2024, several cattle were contaminated on dairy farms in the United States.
Mammal-human transmission
Although rare, transmission of the avian influenza virus to humans is also possible via infected mammals. A few human cases have been reported on American cattle farms in 2024.
Furthermore, the avian influenza A H5N1 virus was detected by nucleic acid amplification test (NAAT) in several cow’s milk products in the United States in 2024. However, the virus detected was considered non-viable by the Food and Drug Administration (FDA). Pasteurization would therefore be effective in inactivating the virus. The virus has not been found in cattle meat. As of September 2024, the virus has not been found on cattle farms or in milk in Canada.
Transmission interhumaine
Human-to-human transmission of avian influenza is exceedingly rare. A few clusters of human-to-human transmission have been reported among household members living under the same roof, but no sustained human-to-human transmission has been documented. Furthermore, no human-to-human transmission of HPAI H5N1 clade 2.3.4.4b has been documented to date. However, as previously mentioned, avian influenza viruses can undergo genetic modification and acquire the capacity for sustained transmission between humans, which poses a pandemic risk.
Clinical picture
The clinical presentation of avian influenza in humans is variable. In some people, the infection is asymptomatic. Other people will experience conjunctivitis or flu-like illness, which results in fever, cough, sore throat, muscle or joint pain and/or gastrointestinal symptoms. Complications can occur, such as pneumonia which can lead to hospitalization, and sometimes even death. Severe and unusual presentations have been reported, including myocarditis and encephalitis.
Treatment of the infection includes supportive care and antiviral treatments. These are used according to clinical severity.
