Perhaps this is one you didn’t see coming, but there have been lots of discussions about SARS-CoV-2 and marine mammals. You may think, “people don’t have much contact with marine mammals,” and of course you’d be correct, if you meant direct contact. However, human activity (and waste) can significantly influence marine mammal health.
What’s the risk of direct transmission of SARS-CoV-2 from humans to marine mammals?
This is a concern for a very small group of people, but human contact with marine mammals does occur, with both captive animals and sometimes during field studies. Any direct contact poses some risk of transmitting pathogens (of many kinds) in either direction. We saw human-to-marine mammal transmission a few years ago during an MRSA outbreak in dolphins and walruses that we investigated. If people can pass MRSA to marine mammals, we can presumably do the same with the SARS-CoV-2 virus. However, in the grand scheme of things, infection of captive marine mammals is of limited concern (at least beyond the individual animals in a collection).
How would wild marine mammals get exposed to the SARS-CoV-2 virus?
The main issue is exposure of marine mammals to the virus via sewage. The virus doesn’t like being outside of a warm body, but it will survive for a short period of time in sewage. Unfortunately we don’t know exactly how long it can survive in such waste water, or how different sewage treatment approaches influence survival of the virus.
We know that exposure to human sewage can result in transmission of pathogens to wild animals, so the concern isn’t unrealistic in areas where there is close proximity of marine mammals to human sewage effluent, especially if there are sewage infrastructure challenges that lead to release of poorly treated or untreated sewage.
Are any marine mammals actually susceptible to SARS-CoV-2?
We don’t know. As I’ve discussed before, we can look at their ace2 receptors (structures the virus uses to attach to and invade cells) for clues. We have to be cautious putting too much faith in predicted susceptibility based on ace2 receptors, and most of these studies have not included marine mammals anyway. However, a couple of studies ranked various marine mammals (including a variety of whales and porpoises) as having potentially “high” susceptibility (Damas et al., Luan et al.).
There is a pre-print available of a marine mammal-focused study modeling potential susceptibility to SARS-CoV-2. Th study predicted various whales, dolphins, seals and otters would be highly susceptible to the virus. Sea lions were lower risk, which would be a very good thing given how they congregate in large populations, often close to human populations. In contrast, the models predicted some species like beluga whales and bottlenose dolphins may be even more susceptible than people. High-risk species included a large number of species that are already vulnerable or endangered.
Overall, the risk to marine mammals is likely very low, especially in terms of creating a sustained problem. This virus isn’t as hardy as most pathogens that we know can be spread via sewage. If infected, infected marine mammals would likely only spread the virus over short distances and short periods of time. A small pod of whales poses much less risk overall than a large population of sea lions, where there are enough individuals for sustained transmission in the group.
However, since some marine mammal populations are highly threatened, an outbreak localized to an individual pod or population could still have significant consequences.
Is there a risk of SARS-CoV-2 transmission to people from marine mammals?
The risk posed by marine mammals to people is likely negligible. We’re pooping in their habitat more than they’re doing it in ours. Sure, an infected marine mammal would likely pose a risk from direct contact, but the odds of such contact are very low. Outside of some very densly populated areas, I suspect the potential impact of marine mammal infections (should they occur at all) on human health is very low.
What are the recommendations with regard to marine mammals?
- We need to learn more about sewage, as an indicator of infection of people in the community, but with that we should also aim to learn about the potential for viable virus escaping into nature and exposing wildlife (both terrestrial and marine). Detection of viral bits in sewage by PCR is one thing. Knowing how that correlates to infectious virus is key to assessing the risk of spillover into wildlife.
- The risk of human-to-animal transmission can’t be ignored. Facilities with captive marine mammals should limit contact with them (just like they should be limiting human-to-human contact), and obviously there should be no contact with people who are infected or quarantined because they are high-risk.
- Similar precautions apply to field research. This has been a tough topic for any field researcher, but the goal is to prevent problems, not react to them. So, we’re better off limiting direct contact with wild mammals (both terrestrial and marine) as much as possible. When contact is required, we need to maximize protective measures (e.g. masks), minimize contact time, minimize the number of people involved, and do everything possible to make sure infected or otherwise high-risk people don’t participate.