A number of animal models have been used to study how influenza viruses may work in humans: mice, hamsters, guinea pigs, and both cotton rats (Sigmodon) and ordinary rats (Rattus). However one of the most used is the ferret model. This species is very susceptible to infection with human influenza viruses and unlike some other animal models, such as mice, there is no need for prior host adaptation of the human viruses. Hence ferrets are considered by many to be the most suitable small animal model for influenza research relating to humans. For decades the degree of antigenic match of drifted influenza viruses to existing vaccines has been judged by seeing how reactive they are with ferret anti-sera raised against the current vaccine strains. Many studies on ferrets have modelled the pathogenesis mechanisms of influenza viruses and the potential for these to cause disease in humans through similar pathogenesis patterns including not only human influenza A viruses but also highly pathogenic avian influenza A(H5N1) and other animal influenza viruses .
Ferrets also show clinical signs following infection with influenza viruses that are similar to those that humans may experience. The infected ferret may sneeze, be feverish and have a nasal discharge. The ferret respiratory tract is considered to resemble that of humans as both species have a predominance of alpha 2,6-linked sialic acids on the upper airway epithelia to which some influenza viruses may bind, more so than for example the mouse [3–5]. Also avian A(H5N1) and human A(H3N2) influenza viruses exhibit similar patterns of virus attachment to tissues from both humans and ferrets [6,7].
Replication of low-virulence human influenza viruses is generally restricted to the respiratory tract in ferrets, though entry through other portals can also be demonstrated. For example, entry through the ocular epithelial  and, with A(H5N1), through the gastrointestinal route . Most of the low-virulence strains cannot be detected in other systemic tissues of the ferret though some will appear in the intestinal tract post-inoculation. However, the dose of virus given, and how the introduction is undertaken is important .
The ferret model showed its particular value following the emergence of the 2009 pandemic. Groups in Europe and North America rapidly used it to estimate how transmissible and pathogenic the new A(H1N1)pdm09 viruses were likely to be in humans. This resulted in publications as early as 2 July 2009 [11,12]. The results demonstrated that these viruses had some pathogenicity in ferrets and transmitted naturally from ferret to ferret. However, the results also show the limits of the model. The similarities and differences observed in ferrets between the behaviour of A(H1N1)pdm09 and the preceding seasonal A(H1N1) were not entirely borne out in the ensuing pandemic in Europe [11–13]. One reason being that the ferret model did not take into account the acquired immunity in humans [14,15]. In summary, work using the ferret model is complementary to, but does not substitute for studies of the virology, immunity, epidemiology, transmission and pathogenicity of influenzas in humans. Work with ferrets can also contribute to how to potentially induce protection in humans while at the same time producing findings that can be difficult to interpret or to turn into interventions. Consider, for example, the observation that ferrets when infected with human seasonal A(H3N2) influenza acquired some heterosubtypic immunity, that is immunity against potentially novel pandemic subtypes like A(H5N1). However, this effect is lost if the ferret is first immunised against human A(H3N2). A finding that leaves the public health community unsure as to what protective strategy to adopt .
Some highly pathogenic avian influenza viruses such as A(H5N1) will multiply in the respiratory tract of ferrets and spread to extra-pulmonary organs. The effect can be lethal, causing lethargy, weight loss, lymphopenia and neurological signs in animals en route to death. Another unusual finding seen in ferrets is direct spread of A(H5N1) from the upper respiratory tract of the ferret to the central nervous system . However, not all highly pathogenic avian A(H5N1) viruses are virulent in ferrets and different results are experienced in different laboratories . Though these viruses can be pathogenic they often require direct introduction into the trachea in high doses to do so , which is not at all a natural form of transmission.
Wild A(H5N1) viruses have not transmitted from ferret to ferret through the air . In order to determine whether A(H5N1) viruses have any capacity to become a pandemic strain (i.e. to transmit efficiently from human to human through the air) a number of groups have been undertaking genetic manipulations and using other techniques in carefully controlled conditions to see if air-borne ferret-to-ferret transmission characteristics can be acquired by these viruses .
ECDC Comment: 5 March 2012
Recent work undertaken in highly secure laboratories in two institutions (in Madison, USA and Rotterdam, the Netherlands) have found that certain combinations of genetic changes and other manipulations can result in an A(H5N1) influenza virus capable of air-borne ferret-to-ferret transmission [19,20]. Publication of the manuscripts from the Madison and Rotterdam work has been delayed by concerns over biosecurity (whether the findings could be used for ill purpose) . ECDC published a risk assessment concerning this development, because it raises a number of important issues. Two of the key unknowns highlighted in the risk assessment were just how pathogenic the laboratory-modified viruses would be in ferrets and how transmissible they would be between animals . Both research groups have now clarified that though, like human influenza, the laboratory-modified A(H5N1) viruses can be lethal when introduced directly into the lungs in high doses, natural transmission through the air only results in mild symptoms and a self-limiting illness in the ferrets and infection is more often without any symptoms at all [20,23]. The original peer-review papers produced by the two groups have been held up by an American Board (the funding is from the US National Institutes of Health - NIH). At the same meeting where the Dutch group presented the information about the pathogenicity and transmissibility of the Rotterdam viruses an official from NIH the US National Institutes of Health announced that the US Board was now going to reconsider its decision .
Scientific Advance prepared by Vicente Lopez Chavarrias and Angus Nicoll with thanks for comments and suggestions from Ron Fouchier
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