Ferrets and flu transmission studies

Professor Wendy Barclay was awarded funding to reduce the number of ferrets used to study influenza transmission.

Research details

Principal Investigator: Professor Wendy Barclay
Organisation: Imperial College London
Award type: Project grant
Start date: 2013
Duration: 3 years
Amount: £400k

Case study

Influenza viruses cause flu-like symptoms in a range of species, including birds, pigs, horses, dogs and humans. Each year, seasonal flu affects 10% to 15% of the UK population and is associated with 12,000 deaths. Children, the elderly and those with pre-existing medical conditions are especially vulnerable. Influenza viruses evolve rapidly giving rise to novel antigenic variants that can evade natural or vaccine-induced immune responses. Occasionally, influenza strains, which would ordinarily cause disease in one species, acquire the ability to transfer to a new host. This can enable rapid viral spread in the absence of host immunity, leading to influenza pandemics.

Ferrets are considered the ‘gold standard’ for modelling influenza airborne transmission because they exhibit similar clinical signs and receptor distribution in the airways as humans. Studies in ferrets show good correlation with human transmission patterns. Animals are lightly anaesthetised and infected intranasally with the virus and daily viral titres are measured in nasal flushes or swabs. Post-infection, the ferrets (referred to as donors) are housed next to naïve sentinel animals for up to 14 days and subsequent nasal flushes of sentinels at multiple time points are used to establish whether transmission has occurred. Exposure can be by close contact where donor and sentinel animals are housed in the same cage, akin to a household setting, or to model respiratory droplet and airborne routes, animals are placed in adjacent cages separated by perforated sides.

A typical experiment designed to test when during the infection transmission occurs uses four donor ferrets and depending on the number of time points up to 64 sentinel animals. The studies do not completely recapitulate what happens in humans where contact is much shorter in duration and may be over a greater distance. Better understanding of how much infectious virus a ferret exhales and its stability in airborne droplets could improve the relevance of the studies to humans and help explain why some viral strains are reported to be more transmissible than others.

3Rs benefits (actual and potential)

Wendy and her team have designed, manufactured and tested a novel piece of equipment called the influenza virus transmission tunnel (IVTT) which avoids the need to use sentinel ferrets. The IVTT consists of a 100 cm long tube along which sentinel cell culture plates containing MDCK cells (an immortalised canine cell line that is highly susceptible to influenza viruses) are positioned. Infected donor ferrets are placed for a maximum of ten minutes in a chamber attached to the IVTT and viral plaque counts on the plates are used to determine the titre of infectious virus in the animal’s exhaled breath. The IVTT can be attached to a nebuliser allowing fully in vitro experiments. It is also amenable to use in human infection studies.

Scientific and technological benefits

Using the IVTT, the Barclay laboratory have demonstrated that the amount of virus in the ferret’s nose is not a reliable indicator of the amount of viable virus in exhaled air. They showed that factors affecting virus survival in airborne droplets, such as stability of the virus particle, can be more readily studied in the IVTT than in the animal-to-animal transmission studies. Virus recovered from the MDCK cells infected by the airborne route can be easily interrogated using molecular techniques, for example, to define the genetic determinants that enhance transmission – studies which are challenging to do in the ferret as the site from which virus is transmitted is unknown or inaccessible without culling the animals.

Added value

In 2017, Wendy presented the IVTT at the ‘Transmission of Respiratory Viruses: From basic knowledge to evidence-based options for control’ conference in Hong Kong. Subsequently groups in the USA and Japan have adopted a similar device, further reducing the use of ferrets. Based on the work supported by the NC3Rs funding, Wendy has secured a £1.2M Wellcome Collaborative Award to study the evolution of the influenza virus, which will exploit the use of the IVTT. There have been three papers published from the NC3Rs-funded research. Wendy has collaborated with a small company to use the IVTT to test materials suitable for use in face masks that aim to protect against respiratory virus transmission. She is currently working with a major pharmaceutical company using the IVTT to test the effects of antiviral drugs on influenza transmission.

This case study was published in our 2019 Research Review.