Role of H7 hemagglutinin in murine infectivity of influenza viruses following ocular inoculation

Mammalian pathogenicity and transmissibility of low pathogenic avian influenza H7N1 and H7N3 viruses isolated from North America in 2018

Low pathogenic avian influenza (LPAI) H7 subtype viruses are infrequently, but persistently, associated with outbreaks in poultry in North America. These LPAI outbreaks provide opportunities for the virus to develop enhanced virulence and transmissibility in mammals and have previously resulted in both occasional acquisition of a highly pathogenic avian influenza (HPAI) phenotype in birds and sporadic cases of human infection. Two notable LPAI H7 subtype viruses caused outbreaks in 2018 in North America: LPAI H7N1 virus in chickens and turkeys, representing the first confirmed H7N1 infection in poultry farms in the United States, and LPAI H7N3 virus in turkeys, a virus subtype often associated with LPAI-to-HPAI phenotypes. Here, we investigated the replication capacity of representative viruses from these outbreaks in human respiratory tract cells and mammalian pathogenicity and transmissibility in the mouse and ferret models. We found that the LPAI H7 viruses replicated to high titre in human cells, reaching mean peak titres generally comparable to HPAI H7 viruses. Replication was efficient in both mammalian species, causing mild infection, with virus primarily limited to respiratory tract tissues. The H7 viruses demonstrated a capacity to transmit to naïve ferrets in a direct contact setting. These data support the need to perform routine risk assessments of LPAI H7 subtype viruses, even in the absence of confirmed human infection.

Infection and Replication of Influenza Virus at the Ocular Surface

Although influenza viruses typically cause respiratory tract disease, some viruses, particularly those with an H7 hemagglutinin, have been isolated from the eyes of conjunctivitis cases. Previous work has shown that isolates of multiple subtypes from both ocular and respiratory infections are capable of replication in human ex vivo ocular tissues and corneal or conjunctival cell monolayers, leaving the determinants of ocular tropism unclear. Here, we evaluated the effect of several variables on tropism for ocular cells cultured in vitro and examined the potential effect of the tear film on viral infectivity. All viruses tested were able to replicate in primary human corneal epithelial cell monolayers subjected to aerosol inoculation. The temperature at which cells were cultured postinoculation minimally affected infectivity. Replication efficiency, in contrast, was reduced at 33°C relative to that at 37°C, and this effect was slightly greater for the conjunctivitis isolates than for the respiratory ones. With the exception of a seasonal H3N2 virus, the subset of viruses studied in multilayer corneal tissue constructs also replicated productively after either aerosol or liquid inoculation. Human tears significantly inhibited the hemagglutination of both ocular and nonocular isolates, but the effect on viral infectivity was more variable, with tears reducing the infectivity of nonocular isolates more than ocular isolates. These data suggest that most influenza viruses may be capable of establishing infection if they reach the surface of ocular cells but that this is more likely for ocular-tropic viruses, as they are better able to maintain their infectivity during passage through the tear film.IMPORTANCE The potential spread of zoonotic influenza viruses to humans represents an important threat to public health. Unfortunately, despite the importance of cellular and tissue tropism to pathogenesis, determinants of influenza virus tropism have yet to be fully elucidated. Here, we sought to identify factors that limit the ability of most influenza viruses to cause ocular infection. Although ocular symptoms in humans caused by avian influenza viruses tend to be relatively mild, these infections are concerning due to the potential of the ocular surface to serve as a portal of entry for viruses that go on to establish respiratory infections. Furthermore, a better understanding of the factors that influence infection and replication in this noncanonical site may point toward novel determinants of tropism in the respiratory tract.

The eyes have it: influenza virus infection beyond the respiratory tract

Avian and human influenza A viruses alike have shown a capacity to use the eye as a portal of entry and cause ocular disease in human beings. However, whereas influenza viruses generally represent a respiratory pathogen and only occasionally cause ocular complications, the H7 virus subtype stands alone in possessing an ocular tropism. Clarifying what confers such non-respiratory tropism to a respiratory virus will permit a greater ability to identify, treat, and prevent zoonotic human infection following ocular exposure to influenza viruses; especially those within the H7 subtype, which continue to cause avian epidemics on many continents.

A Novel A(H7N2) Influenza Virus Isolated from a Veterinarian Caring for Cats in a New York City Animal Shelter Causes Mild Disease and Transmits Poorly in the Ferret Model

In December 2016, a low-pathogenic avian influenza (LPAI) A(H7N2) virus was identified to be the causative source of an outbreak in a cat shelter in New York City, which subsequently spread to multiple shelters in the states of New York and Pennsylvania. One person with occupational exposure to infected cats became infected with the virus, representing the first LPAI H7N2 virus infection in a human in North America since 2003. Considering the close contact that frequently occurs between companion animals and humans, it was critical to assess the relative risk of this novel virus to public health. The virus isolated from the human case, A/New York/108/2016 (NY/108), caused mild and transient illness in ferrets and mice but did not transmit to naive cohoused ferrets following traditional or aerosol-based inoculation methods. The environmental persistence of NY/108 virus was generally comparable to that of other LPAI H7N2 viruses. However, NY/108 virus replicated in human bronchial epithelial cells with an increased efficiency compared with that of previously isolated H7N2 viruses. Furthermore, the novel H7N2 virus was found to utilize a relatively lower pH for hemagglutinin activation, similar to human influenza viruses. Our data suggest that the LPAI H7N2 virus requires further adaptation before representing a substantial threat to public health. However, the reemergence of an LPAI H7N2 virus in the northeastern United States underscores the need for continuous surveillance of emerging zoonotic influenza viruses inclusive of mammalian species, such as domestic felines, that are not commonly considered intermediate hosts for avian influenza viruses.IMPORTANCE Avian influenza viruses are capable of crossing the species barrier to infect mammals, an event of public health concern due to the potential acquisition of a pandemic phenotype. In December 2016, an H7N2 virus caused an outbreak in cats in multiple animal shelters in New York State. This was the first detection of this virus in the northeastern United States in over a decade and the first documented infection of a felid with an H7N2 virus. A veterinarian became infected following occupational exposure to H7N2 virus-infected cats, necessitating the evaluation of this virus for its capacity to cause disease in mammals. While the H7N2 virus was associated with mild illness in mice and ferrets and did not spread well between ferrets, it nonetheless possessed several markers of virulence for mammals. These data highlight the promiscuity of influenza viruses and the need for diligent surveillance across multiple species to quickly identify an emerging strain with pandemic potential.