The severity of epidemic and pandemic influenza outbreaks is dictated in part by the efficiency with which the causative strain transmits between human hosts. The mechanisms underlying influenza virus spread are poorly understood, in part because of the lack of a convenient animal model to study this phenomenon. Indeed, despite extremely efficient transmission among humans and virulence in the mouse model, we have shown that even the 1918 pandemic influenza virus does not transmit between mice. We therefore evaluated the guinea pig as a model mammalian host for influenza virus. Using the recent human isolate A/Panama/2007/99 (Pan/99) (H3N2) virus, we found that guinea pigs were highly susceptible to infection with the unadapted virus (ID50 = 5 plaque-forming units). Pan/99 virus grew to high titers in the upper respiratory tract and was shed in nasal washings of infected animals. Moreover, influenza virus was transmitted from infected guinea pigs to noninfected guinea pigs housed in the same cage, an adjacent cage, and a cage placed 91 cm away. Our results demonstrate that influenza virus can pass between guinea pigs by means of droplet spread and thereby establish the suitability of the guinea pig as a model host for influenza virus transmission studies.

https://www.pnas.org/content/pnas/103/26/9988.full.pdf

The guinea pig as a transmission model for human influenza viruses

Animal models for influenza virus pathogenesis, transmission, and immunology

A human isolate of type A Hong Kong influenza virus (H3N2) was adapted to mice by serial passage. Lung homogenates from mice who received low passage levels contained about the same quantity of virus (10(6.2-6.95) 50% tissue culture infective doses/ml) as those from mice who received high passage levels (10(5.95-6.45) 50% tissue culture infective doses/ml); however, death occurred only in animals given high-passage virus. Passage 3 (P3) and passage 9 (P9) viruses were selected as representative of low-passage and high-passage viruses, respectively. Although minimal differences were detected in infectivity for rhesus monkey kidney tissue cultures and mice, P9 virus was at least 10,000 times more lethal for mice (mean lethal dose = 10(4.2)). Infection with P3 virus was accompanied by minimal bronchitis and bronchiolitis only, whereas P9-infected animals exhibited marked bronchitis, bronchiolitis, and pneumonia. Striking thymic cortical atrophy was also demonstrable in the P9-infected animals and, although virus was more commonly recovered from thymuses from these animals, immunofluorescent studies revealed only a few cells containing influenza virus antigens. To further explore the participation of thymus-derived lymphocytes in influenza, athymic nude mice and furred immunocompetent littermates were given 500 50% mouse infectious doses of P9 virus. Nude mice exhibited an increased survival time and, in contrast to the extensive lung pathology seen in furred littermates, manifested minimal cellular infiltration and no tissue destruction in lungs. Brains from nude mice exhibited encephalomalacia with lymphocytic perivascular cuffing, which was not seen in furred animals. Virus was recovered from brains of 6 of 13 nude mice and 1 of 10 furred animals. The contrasting models suggest that thymus-dependent cells play a significant role in the inflammatory response to influenza virus infection and should prove useful for probing host-virus interactions which characterize influenza virus virulence.

Effects of Low- and High-Passage Influenza Virus Infection in Normal and Nude Mice

Transfer of primary or secondary influenza-immune spleen cells to mice infected intranasally with influenza virus resulted in a significant clearance of virus from the lungs and the protection of the recipients from death. The antiviral activity was associated only with intact, viable cells and was not due to carryover of virus. The effector cell population responsible for the antiviral effect was shown to be T cells. Thus, the removal of adherent, phagocytic and Ig+ cells did not affect the antiviral activity, whereas it was destroyed with antitheta serum and complement. Antiviral activity was specific and was best expressed if the virus used to infect the recipients and to generate immune cells was the same strain. Further work will be necessary to define rigorously the role of different viral antigens in cell-mediated immune response to influenza virus infection.

The Recovery of Mice from Influenza Virus Infection: Adoptive Transfer of Immunity with Immune T Lymphocytes

At present, new influenza A (H1N1)2009 viruses of swine origin are responsible for the first influenza pandemic of the 21st Century. In addition, highly pathogenic avian influenza A/H5N1 viruses continue to cause outbreaks in poultry and, after zoonotic transmission, cause an ever-increasing number of human cases, of which 59% have a fatal clinical outcome. It is also feared that these viruses adapt to replication in humans and become transmissible from human to human. The development of effective vaccines against epidemic and (potentially) pandemic viruses is therefore considered a priority. In this review, we discuss animal models that are used for the preclinical evaluation of novel candidate influenza vaccines. In most cases, a tier of multiple animal models is used before the evaluation of vaccine candidates in clinical trials is considered. Commonly, vaccines are tested for safety and efficacy in mice, ferrets and/or macaques. The use of each of these species has its advantages and limitations, which are addressed here.

https://www.tandfonline.com/doi/pdf/10.1586/erv.09.148

Animal models for the preclinical evaluation of candidate influenza vaccines.

Development of systemic delayed hypersensitivity was demonstrated in inbred strain 2 and strain 13 guinea pigs experimentally infected with influenza A/Hong Kong/1968(H3N2) virus or parainfluenza 3 virus. Groups of seronegative guinea pigs were inoculated intranasally with live influenza A virus or parainfluenza 3 virus, or ether-inactivated parainfluenza 3 virus, and nasal washings, sera, and suspensions of peritoneal exudate cells were collected at varying intervals. Animals receiving the live virus suspensions shed virus for 6 to 8 days, developed significant rises in serum neutralizing antibody titers, and demonstrated systemic delayed hypersensitivity 6 or 7 days after inoculation, which persisted 25 days after inoculation. The group inoculated with ether-inactivated parainfluenza 3 virus failed to shed virus or manifest a systemic delayed hypersensitivity response although one animal had a rise in serum antibody titer. The development of the delayed hypersensitivity correlates in time with the cessation of virus shedding and the appearance of circulating antibody. However, the role of delayed hypersensitivity in resolution of respiratory viral infections remains to be defined.

Roger E. Wetherbee

Papers

Induction of Systemic Delayed Hypersensitivity During Experimental Viral Infection of the Respiratory Tract with a Myxovirus or Paramyxovirus