Two studies recently appeared on July 2nd in Science. In both of them, ferrets (Mustela putorius) were experimentally infected with the new H1N1 (swine flu) to determine how efficiently the virus is transmitted from one individual to another and to get an idea of how dangerous the virus is.
One of the first questions people ask about these studies is: why ferrets ? Ferrets are among the animals that respond to influenza in ways most similar to humans. This idea has been confirmed many times with various subtypes and strains of flu.
Another question people may ask is: why do animal experiments? After all, there are plenty of humans infected with H1N1 that we can study. The answer is control. If one examines reports of how transmissible the new H1N1 is, one gets widely varying estimates. The same is true in terms of how serious the average case is and what percent of the people infected die. This is because we do not know all the people who have been infected. If there were a very large number of people with mild symptoms, we would have to conclude that serious symptoms and deaths are extremely rare events. On the other hand, if most the mild cases have been identified, then the new H1N1 virus is as bad as, or worse, than the 1918 “Spanish” influenza. Ideally, seroprevalence studies should help address these important questions. However, until they are done, animal studies are the best way to determine how efficiently the virus is transmitted and how dangerous it is.
The two studies in ferrets were conducted by two different groups: one lead by Terence Tumpey at the CDC and other one lead by Albert Osterhaus and Ron Fouchier at the National Influenza Center at Erasmus Medical Center in the Netherlands.
In both studies, ferrets were inoculated intranasally with live flu viruses. Both studies had two groups, one which received seasonal flu and one which received the new H1N1 pandemic flu. In light of some of the differences in results obtained below, it is worth mentioning that the ferrets in the two studies did not receive the same viruses. The CDC investigators used A/California/04/2009 (obtained from a child with mild disease), A/Mexico/4482/2009 (obtained from 29 year old woman with severe respiratory disease) and Texas/15/2009 (obtained from a child that died of severe respiratory disease). The Erasmus Medical Center investigators used A/Netherlands/602/2009 (obtained from 3 year old child who had an uncomplicated recovery after treatment with Tamiflu). The ability of the infected ferrets to transmit the virus to other ferrets was examined. Animals were also sacrificed after varying amounts of time and tissues examined for pathology.
Both studies found that the new H1N1 caused more severe disease than seasonal influenza. Further, both noted involvement of the lower respiratory system with the new H1N1, but not with seasonal flu. However, there were some differences in the results of the two studies.
In general, CDC investigators noted more severe symptoms in the ferrets inoculated with the new H1N1 than did investigators with the Erasmus Medical Center. Even within the CDC study, there was variation in the severity of symptoms observed. The ferrets inoculate with A/California/04/2009 exhibited less severe disease than the ferrets inoculate with A/Mexico/4482/2009 or Texas/15/2009. Further, one of the ferrets inoculated with Texas/15/2009 needed to be euthanised after 10 days due to excessive weight loss.
Another important difference between the two studies involved transmission of the virus. The Erasmus Medical Center investigators found relatively easy transmission with the new H1N1, presumably by respiratory droplets, from inoculated ferrets to non-inoculated ferrets. In contrast, the CDC researchers found that although there was some transmission from ferrets inoculated with the new H1N1 to non-inoculated ferrets, this was significantly less than that observed when seasonal flu viruses were used.
Both studies are in agreement that the involvement of the lungs is much more likely to occur with the new H1N1 virus than with seasonal flu. This finding alone suggests that the current pandemic will result in greater morbidity, and likely mortality, than seasonal flu. Interpretation of the differences in severity observed within and between studies is more complex. However, one is lead by the available results to suspect that different strains of the virus have differing ability to cause severe disease. Although too few strains were examined to propose a meta-analysis of the data with any confidence, the available data is consistent with the suggestion that the strains that caused mild disease in humans caused mild disease in ferrets while strains that caused severe disease in humans caused severe disease in ferrets. Since we are early in the pandemic, the new H1N1 virus may still be unstable. That is to say, there may be many strains with different properties, all competing for dominance with each other. The final case fatality rate that we observe may depend on which strain wins this battle and becomes the predominant strain that infects most people.
Explanation for the different results between the two studies with respect to transmission is more difficult. It may be that the strains of virus used by the two groups had intrinsic differences in their ability to spread from individual to individual. Alternatively, subtle aspects of experimental design (temperature, humidity, etc.) could have affected the results.
To determine whether the differences observed between the two groups are due to intrinsic properties of the different strains of virus, it would be useful if the two groups were to trade strains and repeat the experiments under the same conditions as they had previously done.
Both studies suggest that the new H1N1 is likely to cause more severe disease than seasonal flu. The possible instability of the new H1N1, which may be hinted at when both studies are considered together, suggests that we do not know what the upper boundary of severity is for this virus.
Maines TR, Jayaraman A, Belser JA, Wadford DA, Pappas C, Zeng H, Gustin KM, Pearce MB, Viswanathan K, Shriver ZH, Raman R, Cox NJ, Sasisekharan R, Katz JM, Tumpey TM. (2009) Transmission and Pathogenesis of Swine-Origin 2009 A(H1N1) Influenza Viruses in Ferrets and Mice. Science, Jul 2.
Munster VJ, de Wit E, van den Brand JM, Herfst S, Schrauwen EJ, Bestebroer TM, van de Vijver D, Boucher CA, Koopmans M, Rimmelzwaan GF, Kuiken T, Osterhaus AD, Fouchier RA. (2009) Pathogenesis and Transmission of Swine-Origin 2009 A(H1N1) Influenza Virus in Ferrets. Science, Jul 2.