The complex situation of Avian Influenza in Mexico, Part I

Published: June 5, 2024

By: Karina Gaviña / Tryadd

Avian influenza was an exotic disease in Mexico until 1993 when the first H5N2 low-pathogenic avian influenza (LPAI) virus was identified. After many months of accumulating mutations, this low-pathogenic predecessor evolved into a highly pathogenic strain. This inevitably resulted in the authorities reporting the first H5N2 highly pathogenic avian influenza (HPAI) virus in 1994.

To contain the HPAI virus, the government implemented a massive vaccination program, active surveillance, and reinforcement of biosecurity measures. Although the country was declared free from HPAI in December 1995, the sanitary measures could not hold back the H5N2 LPAI virus, so the virus became endemic. Little did anyone suspect at the time that the country would hold a second and even worse sanitary emergency.

Despite the little research conducted on the circulation of H7N3 AIV in wild birds in this country, in 2006, an outstanding investigation published by Cuevas-Domínguez confirmed the presence of a new subtype characterized by the official laboratories as an LPAI H7N3 virus (A/cinnamon teal/Mexico/2817/2006). Six years after this finding, on 13 June 2012, a new H7N3 virus caused three HPAI outbreaks in Acatic and Tepatitlán (in the western state of Jalisco), affecting commercial egg-laying hens. It was not until 21 June 2012 that the Mexican government reported the presence of the virus to the OIE. This new subtype caused substantial damage in the states of Jalisco, Puebla, and Guanajuato. The measures taken to control the H7N3 strain had to be different this time since the most affected regions are key egg suppliers for the entire country.

After reporting the presence of a second highly pathogenic AIV in our territory, the official laboratories had no choice but to produce an emergency vaccine using the previous LPAI H7N3 virus (A/cinnamon teal/Mexico/2817/2006) available from 12 July 2012. The vaccine was able to offer protection though it did not stop the spread of the virus to other states. There have been many hypotheses about how this subtype disseminated to other regions. The most accepted is that the transport of living infected birds, poultry products, and by-products caused the problem. In the end, the virus killed over 22 million hens, leading to a temporary shortage in the supply of eggs which in turn caused a dramatic increase in their price. Such was the magnitude of the second outbreak that international scientists became particularly interested in studying this HPAI H7N3 virus.

Compared with HPAI viruses from other countries, the viral genome of the Mexican isolates has changed more rapidly, making it possible for the virus to diversify into three distinct clades. Generally speaking, the hypothesis claiming that the Mexican H7N3 HPAI virus had emerged from wild birds is now widely accepted. It is the evolution of this virus that intrigued the international veterinary community. On this topic, Maurer-Stroh and Kapczynski made a remarkable contribution by reporting that the cleavage site insert in the Mexican H7N3 virus originates from the host ribosomal RNA (rRNA) and not from the viral genome. What is important is that up to that point this phenomenon had only been observed in a laboratory strain. Therefore, this finding represented the very first report of natural recombination of this kind.

Avian influenza still poses the biggest threat to poultry farming in the country. In fact, the latest Mexican H7N3 HPAI outbreak was reported to the OIE on 27 April 2022. This unfortunate event caught the attention of the press because it affected breeder hens in the northern state of Coahuila, once considered free of H7N7 HPAI. Sadly, the Mexican H7N3 HPAI outbreak is now the longest-running H7N3 sustained outbreak in modern history. It seems fair to say that if we look at the big picture of the H7N3 HPAI virus in Mexican territory, there is still a lot of work to be done.

As can be expected, some researchers have shown concern about the sanitary situation in Mexico. According to Trovão: “surveillance and genetic sequencing of H7N3 subtype viruses in Mexico have been low commensurate to the severity of the outbreak in poultry”. Moreover, the H5N2 LPAI viruses persist in Mexican poultry, and they have even managed to diversify into five clades. On this topic, Lupiani considers that the fact that “the control strategies in Mexico relied solely on vaccination and not on accompanying measures like monitoring and quarantine may have resulted in the evolution of H5N2 variants for which current vaccines are not efficacious”. Further collaboration between the authorities and poultry farmers is needed to overcome this sanitary issue.

References

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