By Alejandro I. Lopez, El País
Gary García Espinosa, a specialist in viral diseases of birds at UNAM, talks about what to expect from the outbreak that has put poultry farms worldwide in check and threatens to trigger infections in humans.
The H5N1 virus, a highly pathogenic subtype of influenza virus that causes avian flu, has been responsible for an unprecedented number of outbreaks worldwide, triggering alarms in both the poultry industry, which has had to cull hundreds of millions of birds to curb infections, and the World Health Organization (WHO), which has been warning of its pandemic potential for two decades. The virus, which has recently demonstrated its ability to infect other mammals and humans, is being closely monitored for the emergence of new strains with a greater capacity for human-to-human transmission.
What is avian flu?
This is an infectious disease caused by various influenza A viruses that primarily affects chickens, turkeys, and other poultry. In nature, most viruses responsible for avian influenza reside in waterfowl, such as wild ducks, geese, and other coastal species, which typically experience mild or asymptomatic illness. However, the highly pathogenic H5N1 subtype has proven relentless when it infects poultry, with a fatality rate reaching between 90% and 100% of infected birds.
H5N1: Bird flu jumps to humans
Influenza viruses are classified according to the two types of proteins on their surface, abbreviated H and N according to their scientific names (hemagglutinins and neuraminidase, respectively). Among dozens of combinations, two have caught the attention of the scientific community: H5 and H7. “These are the ones that really concern us, because they have the highest mortality rate and spread. At some point, it is expected that they may affect some people, which is why they have been under constant surveillance for several decades,” Gary García Espinosa, PhD in Veterinary Sciences from the National Autonomous University of Mexico (UNAM) and an expert in viral diseases of birds, explained to EL PAÍS.
Although avian influenza has been known for over a century, it wasn't until 1997 that an H5N1 outbreak affecting more than 70,000 poultry in Hong Kong took an unexpected turn: for the first time, the world witnessed the transmission of avian flu from birds to humans . The government recorded 18 positive cases, six of which resulted in death from complications of the virus. It then made a radical decision and ordered the culling of all 1.5 million chickens raised on the island's farms. Although the outbreak did not reach epidemic levels, it demonstrated the virus's ability to overcome the barriers that, in theory, prevented it from infecting humans.
When did H5N1 arrive in Mexico?
Highly pathogenic avian influenza (HPAI H5N1) was detected in Mexico in October 2022. The first documented case was a trained falcon in Metepec, State of Mexico, which died from complications of the disease on October 9. Two days later, laboratory tests confirmed the infection with this subtype. “The H5N1 virus we have in Mexico is not new. Although it differs slightly genetically from the one detected in China in 1997, it is still the same virus that causes high mortality in susceptible birds,” says García Espinosa. Since then, the WHO has maintained close surveillance of avian influenza outbreaks worldwide.
How is avian flu transmitted?
Among birds
The viruses that cause avian influenza are concentrated in the saliva, mucus, and fecal matter of infected birds. When a healthy bird comes into contact with these respiratory droplets, infection is almost guaranteed. In poultry farms, which house tens of thousands of birds in overcrowded conditions, the lethality rate of H5N1 can reach 100%. “After confirming the diagnosis, and within days, all the infected birds will become ill and die,” explains García Espinosa.
Bird-to-human transmission
Although transmission of H5N1 from birds to humans is uncommon, when the virus infects a person, the likelihood of severe illness and death is high. In the last twenty years, the WHO has reported 868 confirmed cases of H5N1 and 457 deaths, with a case fatality rate exceeding 50%. Unlike other influenza viruses that affect humans, there is no scientific evidence that H5N1 is transmitted from person to person. Therefore, most cases occur on poultry farms, resulting from direct contact with infected birds. A smaller proportion is due to indirect contact, through contaminated surfaces and enclosed spaces on farms, where respiratory droplets expelled by sick birds enter through the eyes, nose, or mouth of people. “It is a highly contagious virus; it can be easily transmitted if care is not taken with hands and all the equipment used in a production unit,” says García Espinosa.
Impact on the poultry industry
As of mid-February 2023, health authorities in Mexico reported no active outbreaks following the culling of 5.6 million poultry birds in late 2022, which led to an unprecedented increase in the price of eggs . According to García Espinosa, the biosecurity measures implemented by the National Service for Agrifood Health, Safety and Quality (Senasica) have been effective in mitigating the impact of H5N1 on the country's poultry industry, the fourth largest in the world. In the United States, the disease has forced the culling of a record 58 million poultry birds and driven a 109% increase in egg prices . Elsewhere in the Americas, Canada, Chile, Colombia, Ecuador, Honduras, Panama, Peru, and Venezuela are also experiencing active outbreaks of avian influenza.
A virus with pandemic potential
Aside from the damage to the poultry industry, the main concern shared by García Espinosa and other experts in zoonotic diseases lies in the possibility that the widespread circulation of the virus could produce an enhanced version, capable of easily transmitting from birds to humans and overcoming the barriers that prevent human-to-human transmission. “The risk exists because we know from the virus's history and biological characteristics that it can recombine with other influenza subtypes. What could happen is that H5N1 adapts to a person's biological characteristics. The virus will seek to adapt to another organism because it has the characteristics to do so, and this has happened in the past,” he explains. In early January 2023, the Pan American Health Organization (PAHO) issued an epidemiological alert due to the “unusual persistence” of H5N1 and called on countries in the region to strengthen their surveillance systems, especially biosecurity measures on farms. Given the characteristics of influenza viruses, each new infection presents an opportunity to produce random mutations, from which a version better adapted to our species could emerge.