Avian Flu Diary: PrePrint: HPAI H5N1 Infections in Wild Red Foxes Show Neurotropism and Adaptive Virus Mutations

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In May of 2021, in Netherlands: Two Foxes Test Positive For (non-Zoonotic) Avian H5N1, we looked at an unusual report from Wageningen Bioveterinary Research (WBVR) where they reported finding two foxes in the province of Groningen that had been infected with avian H5N1.

Five months later, in EID Journal: HPAI A(H5N1) Virus in Wild Red Foxes, the Netherlands, 2021, we saw a research letter that shed additional light on these HPAI H5 infections in foxes, including unusual neurological presentations.

Since then we've seen additional reports of terrestrial mammals infected with HPAI H5N1 and presenting with neurological symptoms, including Ontario: CWHC Reports HPAI H5 Infection With Severe Neurological Signs In Wild Foxes (Vulpes vulpes) and CDC EID Journal: Encephalitis and Death in Wild Mammals at An Animal Rehab Center From HPAI H5N8 - UK.

These findings are concerning as they are reminiscent of past reports of infections with the far more pathogenic Asian H5N1 virus, which over the past 2 decades has infected more than 860 people, killing more than half.

In 2015, after the death of the first imported H5N1 case in Canada, we saw a study (see CJ ID & MM: Case Study Of A Neurotropic H5N1 Infection - Canada), where the authors wrote:

These reports suggest the H5N1 virus is becoming more neurologically virulent and adapting to mammals. Despite the trend in virulence, the mode of influenza virus transmission remains elusive to date. It is unclear how our patient acquired the H5N1 influenza virus because she did not have any known contact with animals or poultry.

Similarly, in a Scientific Reports open access study on the genetics of the H5N1 clade 2.3.2.1c virus - Highly Pathogenic Avian Influenza A(H5N1) Virus Struck Migratory Birds in China in 2015 – the authors warned:

This suggests that the novel Sanmenxia Clade 2.3.2.1c-like H5N1 viruses possesses tropism for the nervous system in several mammal species, and could pose a significant threat to humans if these viruses develop the ability to bind human-type receptors more effectively.

While the Eurasian H5N1 virus has thus far only infected 2 people (that we know of), and both of those cases were mild, the fact that it can produce severe (even fatal) illness in mammals is of concern.

The researchers from Wageningen Bioveterinary Research (WBVR) are back with a new preprint, published in BioRxiv, that finds evidence that the H5N1 virus was actively adapting to its new-found mammalian hosts by generating PB2-627K mutations which promote increased viral replication at lower temperatures.

You may recall that last week, in EID Journal: Pathogenesis & Transmissibility of North American Highly Pathogenic Avian Influenza A(H5N1) Virus in Ferrets, one of the limiting factors cited in that study was a lack of the PB2-627K mutation.

Today's study suggests that acquiring the PB2-627K through host adaptation is well within the reach of HPAI H5N1. While that, alone, isn't enough to make this virus ready for prime time, it represents an important escalation in its threat potential.

Highly pathogenic avian influenza H5N1 virus infections in wild red foxes (Vulpes vulpes) show neurotropism and adaptive virus mutations

Luca Bordes, Sandra Vreman, Rene Heutink, Marit Roose, Sandra Venema, Sylvia B E Pritz-Verschuren, Jolianne M Rijks, José L Gonzales, Evelien A Germeraad, Marc Engelsma, Nancy Beerens
doi: https://doi.org/10.1101/2022.07.21.501071

ABSTRACT

During the 2020-2022 epizootic of highly pathogenic avian influenza virus (HPAI) several infections of mammalian species were reported in Europe. In the Netherlands, HPAI H5N1 virus infections were detected in three wild red foxes (Vulpes vulpes) that were submitted with neurological symptoms between December 2021 and February 2022.

Histopathological analysis demonstrated the virus was mainly present in the brain, with limited or no detection in the respiratory tract and other organs. Phylogenetic analysis showed the three fox viruses were not closely related, but were related to HPAI H5N1 clade 2.3.4.4b viruses found in wild birds. In addition, limited virus shedding was detected suggesting the virus was not transmitted between the foxes.

Genetic analysis demonstrated the presence of mammalian adaptation E627K in the polymerase basic two (PB2) protein of the two fox viruses. In both foxes the avian (PB2-627E) and the mammalian (PB2-627K) variant were present as a mixture in the virus population, which suggests the mutation emerged in these specific animals. The two variant viruses were isolated and virus replication and passaging experiments were performed.

These experiments showed mutation PB2-627K increases replication of the virus in mammalian cell lines compared to the chicken cell line, and at the lower temperatures of the mammalian upper respiratory tract. This study showed the HPAI H5N1 virus is capable of adaptation to mammals, however more adaptive mutations are required to allow efficient transmission between mammals. Therefore, surveillance in mammals should be expanded to closely monitor the emergence of zoonotic mutations for pandemic preparedness.

IMPORTANCE

Highly pathogenic avian influenza (HPAI) viruses caused high mortality amongst wild birds in 2021-2022 in the Netherlands. Recently three wild foxes were found to be infected with HPAI H5N1 viruses, likely by feeding on infected birds. Although HPAI is a respiratory virus, in these foxes the viruses were mostly detected in the brain. Two viruses isolated from the foxes contained a mutation that is associated with adaptation to mammals. We show the mutant virus replicates better in mammalian cells than in avian cells, and at the lower body temperature of mammals.

More mutations are required before viruses can transmit between mammals, or can be transmitted to humans. However, the infections in mammalian species should be closely monitored to swiftly detect mutations that may increase the zoonotic potential of the HPAI H5N1 viruses as these may threaten public health.

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Although HPAI H5N1 has come close in the past, it has never acquired the ability to transmit effectively among humans, and perhaps it never will. There may be some - as yet unidentified - species barrier that protects us.