Tuesday, 23 July 2013

Ferrets are not small furry humans.

Zeng Guang, Chief epidemiologist at the Chinese Center for Disease Control and Prevention (CDC), commented on the recent H7N9 study from his own country (covered here)...
The findings are mainly based on animal tests in the lab that have not been witnessed or substantiated among the H7N9 human cases reported. So it shouldn't affect current intervention efforts or strategy at all

So far, no substantial evidence of H7N9 spreading among humans has been detected

And Zeng, back in April, made other comments about H7N9 transmissibility...

Humans' susceptibility to the H7N9 virus is only a little bit higher than to that of H5N1

Sure. I think we all understand that a tad over 130 cases in an area with 10s of millions people living in it, does not indicate a great deal of human-to-human transmission. As I noted yesterday, I wonder what active PCR screening of well human populations would yield?But ferrets are not supposed to be used for direct comparisons: 1 infected ferret in 3 exposures, under experimental conditions, probably does not equal 1 infected human in 3 exposures. Ferrets have tails, are much shorter, have a lot more body hair and a tendency to bite - not the usual description of a human. So there are other differences.

Ferrets are however, meant to be a great alternative to placing two humans in a cage separated by 7cm with one downwind of the other, inoculating one then harvesting tissues from them both to see where the virus ended up. Such human infections (reviewed here), acceptable for some viruses (e.g. rhinoviruses), would most likely yield the most relevant data to wild human infections. But we can't do that sort of thing for a virus we know to be detected in people who die. The risks to the participants are too high.

Also, we can and do inoculate ferrets and other animals with "unnatural" amounts and unnaturally pure preparations of virus to get the results we are after...in this case infection to let us observe transmission. Sure ferrets are not tiny humans, but we still have work to do to prove they are not a very useful indicator of what might be happening in the transmission of H7N9.

Let's turn Zeng's question around then: What evidence is there that 33% of ferrets being infected in various H7N9 studies, is not what happens to humans in the context of previous exposures, underlying diseases, the human immune system, chance encounters with animals and people, exposure to lower amounts of virus etc?