The National Health Commission of China confirmed the first known human case of H3N8 avian influenza virus infection in a four year old. And yes, it is bad news.
Influenza A viruses are important pathogens for humans and animals, including domestic animals such as pigs and poultry. A few lineages of avian influenza viruses have been established in mammals such as humans, pigs, horses and dogs, although at the moment only subtypes H1, H2 and H3 in combination with N1, N2 or N8 are of great relevance.
Current evidence supports that interspecies transmission of influenza viruses occurs frequently. However, they usually result in a dead end transmission, possibly due to the lack of viral adaptation to the new host species. This may be one of the reasons why, at the moment and fortunately, H3N8 does not seem to transmit well between humans.
Robot portrait of influenza A viruses
In reality, influenza viruses are a very diverse group. There are four types of seasonal flu viruses which are referred to as influenza A viruses, influenza B viruses, influenza C viruses, and influenza D viruses.
All known influenza pandemics have been caused by influenza A type influenza viruses. The ancestral reservoir of all type A influenza viruses is waterfowl, particularly those included in the order Anseriformes, to which geese and ducks belong, and in the order Charadriiformes, where gulls are found. Type A viruses are the only ones responsible for avian influenza, in addition to causing common influenza in humans and swine, equine or canine influenza, for example.
Influenza viruses have a high mutation rate and a phenomenon known as genetic reassortment that allows the virus to generate many different combinations. Influenza A viruses can be divided into different subtypes depending on the genes that make up their surface proteins, hemagglutinin (H) and neuraminidase (N). The hemagglutinin (H) and neuraminidase (N) proteins of the virus are the main sites of antigen recognition by the host immune system.
There are 18 hemagglutinin subtypes and 11 different neuraminidase subtypes, H1 to H18 and N1 to N11 respectively. This implies that there are potentially dozens of combinations of the influenza A subtype, being H3N8 one of them. It is noteworthy that within each subtype there is considerable genetic, antigenic and phenotypic variability, which affects the pathogenicity of the different strains.
Most of the influenza virus subtypes pathogenic for humans originate in birds and pigs. The latter are considered an ideal recombination vessel for variants of various origins. In waterfowl, influenza A viruses replicate and are cleared from the digestive tract, allowing waterborne transmission.
Viruses can survive in the environment for quite long periods. This creates conditions conducive to viral exchange with marine mammals such as pinnipeds (seals, sea lions, etc.), whose habitats and prey intersect with those of waterfowl.
A virus that infects horses and seals
Several cases of infection of marine mammals with avian influenza viruses of many subtypes (H1N1, H3N3, H3N8, H4N5…) with a spectrum of effects ranging from massive deaths to subclinical events in most of these cases. In 2012, H3N8 was blamed for the deaths of more than 160 seals off the northeast coast of the United States after it caused deadly pneumonia in the animals.
All known subtypes of influenza A viruses can cause infections in birds, except subtypes H17N10 and H18N11, which have only been found in bats. Only influenza A virus subtypes H1N1 and H3N2 are currently circulating among people, but other subtypes such as H5N1, H10N8, H10N7, H6N8, H7N9, etc. they can also infect and affect humans. Some subtypes cause diseases in other infected animal species. For example, the H7N7 virus affects horses, and the H3N8 virus that has now burst onto the scene is capable of to sicken dogs, horses and seals among other mammals.
H3N8 is found in birds, but it also circulates in horses and is one of two viruses that cause dog flu. A different subtype of equine influenza A H3N8 was first reported in horses in Florida during an outbreak of disease that began in January 1963 in animals recently imported from Argentina. This virus was associated with a major transcontinental pandemic that reached Europe in early 1965.
A virus that has already caused pandemics
The pandemic was distinguished in its early stages by its appearance in horses of all ages rather than being restricted to younger horses, as had been the case in previous epidemics of presumably H7N7 influenza.
In 1989 and 1990, two equine influenza virus outbreaks with high morbidity (up to 80%) and mortality (up to 20%) affected large populations of horses in northeastern China. The virus that caused these outbreaks was of the H3N8 subtype, although it was more phylogenetically related to avian than equine viruses. Some researchers have also suggested that H3N8 was the virus that caused the so-called “Russian flu” of 1889 that caused the death of more than a million people worldwide.
The prevalence of H3N8 viruses in birds and multiple mammalian species, including recent isolates from pigs, and the possibility that it may have been a human pandemic virus in the past make surveillance and risk analysis of these viruses vital. importance for public health.
Raul Rivas GonzalezProfessor of Microbiology, University of Salamanca
This article was originally published on The Conversation. read the original.