Influenza is a zoonotic disease that can infect a variety of host species. Strains can be transmitted between species, and new strains can emerge through co-infection and genetic recombination in intermediate hosts. Wild ducks and wading birds are considered to be a reservoir for influenza because they can carry all subtypes, and the virus is avirulent to its avian hosts. Avian viruses are also found in other birds such as domestic ducks and poultry. New strains of avian influenza have recently emerged in South East Asia and have infected humans. These strains are not transmissible from human to human; however, they are highly virulent in humans and have killed approximately 70% of infected individuals [37
]. Besides humans, avian influenza viruses infect a variety of other mammals including seals, whales, and pigs [38
]. Considerable attention has been focused on avian influenza as it has been expected that pandemic strains would arise from transmission from birds to humans. However, surprisingly, influenza A (H1N1) emerged through cross-species transmission from pigs to humans and has been shown to have arisen due to recombination between swine, avian, and human strains.
The first modeling paper on influenza A (H1N1) has recently been published [39
]. By fitting an SIR model to initial outbreak data from La Gloria in Mexico Fraser et al
. estimated the R0
for this novel strain to be between 1.4 to 1.6 [39
]. This value is on the lower end of previous values for the 1918–1919 strain (R0
mean ~2: range 1.4 to 2.8 [18
]) and is comparable to R0
values estimated for seasonal strains of influenza (R0
mean 1.3: range 0.9 to 2.1 [11
]). (It is important to note that there is considerable overlap in the estimates of R0
for seasonal and pandemic strains.) The public health measures that were widely applied in Mexico appear to have been successful in mitigating the outbreak of H1N1; this observation appears to corroborate results from earlier modeling studies [23
] that show behavioral interventions can be very effective if R0
is below two. The R0
results of Fraser et al
. from La Gloria (R0
for H1N1 lies between 1.4 and 1.6) indicate that it is theoretically possible to control this pandemic. However, as we have discussed previously, an effective control strategy that has been identified by modeling may not be a feasible control strategy. If a vaccine is available by the autumn there is likely to be high uptake, due to the publicity surrounding the initial outbreak of this strain. If the initial estimates of the R0
for H1N1 are correct then this high vaccination coverage could have a significant effective on mitigating the pandemic, at least in resource-rich countries. However, H1N1 has now been disseminated worldwide through air travel. Consequently, it will be necessary for resource-rich countries to share vaccines and antivirals in order to mitigate a pandemic. Such a global cooperative strategy will be essential to prevent resource-constrained and resource-poor countries suffering from a significantly disproportionate burden of morbidity and mortality.
Fraser et al
. modeled the transmission dynamics of influenza A (H1N1) in the human population, but did not include cross-species transmission [39
]. The emergence of H1N1 has shown the necessity for developing more biologically complex models that can provide a comprehensive understanding of strains that arise due to cross-species transmission. Coburn has recently developed one such complex model that tracks influenza transmission dynamics within three species (birds, pigs, and humans), as well as between these species [40
]. His model includes several species-specific strains that infect birds, pigs, and humans. He models pigs as 'mixing vessels' which can be co-infected with avian, swine, and human strains of influenza. Species-specific strains can then undergo recombination in infected pigs and generate 'super-strains' that can be transmitted from pigs to humans. Analysis of his model generates significant insights into understanding the emergence of novel recombinant strains of influenza (such as H1N1), as well as in predicting their epidemic and pandemic potential. Surprisingly, his results show that an epidemic with an intermediate value of R0
could result in significantly more infected individuals than an epidemic with a high value of R0
; see Figure (the value of the transmissibility of the 'super-strain' in humans corresponds to the value of the R0
). Specifically, the contour map in Figure illustrates that the greatest outbreak occurs when the transmissibility/infectivity of the "super-strain" is greater than 0.024 and less than 0.04; this implies 2.3 < R0
< 3.8. In addition, Coburn's modeling shows that at low values of R0
the number of individuals that become infected will be very dependent on the degree of interaction between humans and pigs (Figure ). Coburn's results illustrate how, by modeling cross-species transmission and determining the degree of interaction between pigs and humans, it may be possible to predict the emergence of pandemic strains of influenza.
Figure 5 Results from a cross-species multi-strain transmission model where pigs act as 'mixing vessels'. This figure shows the severity of the epidemic that occurs when a 'super-strain' emerges into the human population from pigs, as a function of the cross-species (more ...)
To the best of our knowledge there are only two published studies that have modeled interventions for influenza strains that arise due to cross-species transmission. Iwami et al.
modeled epidemics that result as a consequence of cross-species (that is, avian-human) transmission [41
]. Their results show the potential effectiveness of quarantine as a control strategy, and also the importance of simultaneously controlling influenza in the avian population [41
]. Saenz et al.
modeled the potential effect of pigs (or poultry) on amplifying the number of infections that would arise as the result of a new strain of influenza [42
]. They modeled the transmission dynamics in a confined feeding operation (CAFO) as a result of interactions between three groups: CAFO species (either swine or poultry), CAFO workers, and the rest of the local population. Their results show that amplification would be prevented if at least 50% of the CAFO workers could be successfully vaccinated [42
]. They suggest that a vaccination strategy targeted at CAFO workers could be an effective strategy for containing a pandemic. Notably, the interventions suggested by Iwami et al.
] and Saenz et al.
] are interventions that cannot be identified unless cross-transmission is included in the model.