Thread by @DiseaseEcology

What wildlife could be reservoirs for SARS-CoV-2?
New paper suggests North American big brown bats are not. Here's why this is important & why we need more studies like this.
Thread
onlinelibrary.wiley.com/doi/10.1111/tb…

We still don't know the natural reservoir for SARS-CoV-2. Some similar viruses were found in horseshoe bats (Rhinolophus spp.), but the difference between those viruses & SARS-CoV-2 is large enough that SARS-CoV-2 may have different reservoir.
ncbi.nlm.nih.gov/pmc/articles/P…

Regardless of where SARS-CoV-2 originally came from, many have worried that SARS-CoV-2 might be transmitted from humans into other animals that might be able to sustain the virus & transmit it back to humans.

Most worrisome, the virus might evolve in animals hosts to become more deadly, more transmissible, or able to infect a person vaccinated with the current set of vaccines being developed or immune from previous infection.

Which animals do we know can get infected & which can transmit virus to other animals?
Susceptible & Infectious*: felids, mustelids (mink, ferrets), Rousettus bats, primates, hamsters, shrews, mice
Susc., not Infect: dogs, cows
Not Susc.: Pigs, chickens, ducks
* see next tweet

*Note that experimental infection data CANNOT indicate whether a species can maintain the virus in a population; only whether they can become infected & shed live virus. These are necessary but NOT sufficient conditions for species being a reservoir for a pathogen.

Refs for susceptibility & infectiousness:
thelancet.com/journals/lanmi…
onlinelibrary.wiley.com/doi/10.1111/tb…
wwwnc.cdc.gov/eid/article/26…
science.sciencemag.org/content/368/64…
eurosurveillance.org/content/10.280…
onlinelibrary.wiley.com/doi/epdf/10.11…

Several papers have also tried to use models based on binding to ACE receptors to predict susceptibility but these models poorly match experimental studies (onlinelibrary.wiley.com/doi/10.1111/tb…).

However, experimental infections are hard to do & people worry there may be many other animal species that are susceptible & infectious & some *could* become reservoirs.

One obvious group people have worried about is bats. Of special concern is bats in the Americas, since they would be novel hosts for the virus. (Old world bats might also be a novel host or might be a/the natural reservoir; we don't know yet).

A large group of scientists who have worked on bats tried to assess the risk of spillover of SARS-CoV-2 from humans to bats & especially North American bats. But they had no actual data on SARS-CoV-2, so it was just speculation.
journals.plos.org/plospathogens/…

This new paper (w/ one shared author from paper in last tweet; Hon Ip) found that big brown bats were completely resistant to infection. No evidence of infection or transmission to cage mates.
onlinelibrary.wiley.com/doi/10.1111/tb…

What does this mean for N American bats serving as reservoirs for SARS-CoV-2? Strictly, it only implies that this single species is unlikely to be a host. Extrapolating to other groups (e.g. abundant Myotis & Tadarida) could be risky.

However, since we can't test all species, one might ask why we should suspect, a priori, that North American bats are likely to be competent hosts for SARS-CoV-2? Some papers that have done so (e.g. biorxiv.org/content/10.110…) were clearly wrong about big brown bats & SARS-CoV-2.

The failure of multiple papers using indirect assessments (ACE receptors, trait based models, etc.) to correctly identify hosts for SARS-CoV-2 clearly indicates these indirect approaches have poor accuracy at the species level.

Why does this matter? B/c quantifying the risk that a species or taxa could be a reservoir determines what measures we should take to avoid human-animal interactions.

There are variable levels of contact b/w humans & different animal taxa & the chance of virus spillover (& spillback) depends on contact rates & chance animal can sustain virus. For high risk species we clearly should avoid contact.

For farmed mink, risk was clearly high (spillover, spillback & viral evolution in animal host demonstrated), so millions were culled & many have suggested mink farming be outlawed.
nytimes.com/2020/11/04/hea…

What about other species? Few, if any, have been shown to be as high risk as mink. Should we try to stop all contact b/w humans & all animals known or suspected to be possible hosts? This would mean no contact w/ cats, weasles, mice, primates. Clearly this would be difficult &

it hasn't happened for any of these groups.

But research on N American bats, including work on the biggest current threat to hibernating bats, white-nose syndrome, has been stopped in many (but not all) places. Is this justified? It is clearly a risk-benefit tradeoff.

Hard evidence for risk is absent, so far, & new paper shows it is clearly lower than suggested by indirect modeling papers. Conservation benefit of halted research & monitoring is important, but hard to put in the same currencies (public health vs wildlife conservation).

New paper should be used to revise risk-benefit assessments, especially given total absence of data in previous assessment (journals.plos.org/plospathogens/…).

Summary
Identifying potential animal reservoirs for a novel pathogen like SARS-CoV-2 that can both infect & transmit among widely disparate mammalian taxa including primates, felids, weasels & hamsters is difficult. Indirect approaches do a poor job identifying competent hosts...

but experimental infections are difficult so we can't examine most species or taxa.

Restricting contact (closing/culling mink farms, avoiding contact w/ domestic pets, stopping research or animal rehabilitation) has costs. Rigorous cost-benefit assessments are difficult.

I'm grateful for work like new paper & hope for more studies on other taxa that could lead to better assessment of risks & benefits.
In meantime, people contacting animals should reduce transmission risk, like we do b/w humans, w/ PPE, isolating when sick, etc.