The hunt for the intermediate host for SARS-CoV-2 continues. It’s pretty universally accepted (the odd conspiracy theorist aside) that this virus, like SARS and MERS, originated in bats. How it made it to people is an important question we’d like to answer to help understand this virus and future risks.
Figuring that out is challenging because we’re trying to answer a question about what happened in the past. If SARS-CoV-2 jumped to an intermediate host species, spread widely and is still hanging out in that species, then testing various kinds of animals might help answer the question. However, it might not be that easy to figure out:
- The presence of the virus in the intermediate host could have been a short-term event. If a virus jumped from a bat to another animal, then from that animal to a person, there’s no guarantee the virus is still circulating in the animal population. It’s possible that the animal wasn’t a great long-term host or that it didn’t effectively spread it. That could be particularly true for animals in markets whose lifespan is short. If a wet market was the source, the closer to the market time that the animal was infected, the shorter the time the animal had to pass it on to any other animals.
- Infection in the intermediate host might persist, but be uncommon. If an animal is a reservoir, it doesn’t mean most of that species is infected. Look at rabies – several animal species are reservoirs of different rabies virus strains. However, only a small percentage of individuals are infected. If we tested 100 skunks randomly caught in Ontario, we would probably find none infected with rabies and could wrongly conclude that skunks aren’t a rabies virus reservoir. Figuring out the status of an animal species requires thorough investigation, and can also depend a lot on geographical location.
That’s a long winded intro to a short post about another new paper in the journal Transboundary Emerging Diseases (Deng et al. 2020), for which the authors tested 1914 serum samples from 35 different animal species for antibodies against SARS-CoV-2. They didn’t find any.
So, that rules out the tested species, right? Wrong.
There are some major gaps in the information in this paper, consistent with many of the seemingly rushed papers and superficial peer reviews that we’ve been seeing related to SARS-CoV-2 research.
- The methods section doesn’t mention anything about the animal populations. That’s critical, since we need to know about the animals and sample collection process to put the results into context. Amazingly, the methods section doesn’t even specifically state the animals were from China. Some information can be teased out of the results but it’s still poor.
- Some of the animals were SPF (specific pathogen free). These are research animals raised under tight biosecurity to ensure they are free from selected pathogens. Those are not your typical animals with typical exposure to humans or other animals, so they’re not really relevant in this case if they’re negative.
- Some others were experimental animals, so they’d likewise be housed indoors under controlled conditions, and therefore exceedingly unlikely to have encountered a bat. So, finding no positive results there is not surprising.
More interesting was that some of the canine samples were from the city of Wuhan where the COVID-19 pandemic began (no idea where they got the rest of the canine samples). Negative results from those 15 pet and 99 street dogs are noteworthy (assuming their test is good).
- They also tested one dog that was owned by an infected person. That makes no sense in terms of looking for intermediate hosts. It was negative, but if it was positive, it wouldn’t indicate the dog was a potential intermediate host, since it would have most likely gotten infected from the owner.
Similarly, they tested 21 street cats. Maybe most of those were from Wuhan too but it doesn’t say. If so, that’s interesting because of the earlier report of positive antibody tests in 14.7% of cats from the area.
Overall, I don’t know what to say about this paper. They tested a lot of animals from a lot of species, but based on how the paper is written, it’s hard to draw any relevant conclusions (and it furthers my concerns about the state of scientific publishing and peer review). It certainly does not support the authors’ conclusion that this “excludes the above animal species as intermediate hosts of SARS-CoV-2.” It just means the animals they tested were negative.
The hunt for the intermediate host continues.