Headline writers are probably going to screw up my day tomorrow by sensationalizing this, but a new paper in the journal Animals (Fiorito et al. 2022) describes SARS-CoV-2 infection in cattle in Italy. (It’s an MDPI journal, and I take anything published there with a big grain of salt given their low standards, but this one seems ok). It’s also similar to a pre-print from Germany abobut SARS-CoV-2 in cattle that we’ve known about for a while.
Why do I say “take a deep breath and relax”?
I’ve talked about the need to consider spillback of SARS-CoV-2 into animals since 2020. However, there are different concerns in different situations. Livestock raise some big concerns because there are a lot of them, they live in large groups in close proximity to people, and we have lots of contact with food products from them (f not the animals themselves).
We always need to consider what the results of surveillance testing really mean. Data are great, but they need to be appropriately incorporated into our understanding of a given disease and what risks might be present (or not). So, let’s look at what this study tells us.
It’s a nice but small study of cattle from a farm where 13 of 20 workers had diagnosed or suspected COVID-19 – a great situations in which to do animal surveillance since there’s a clear risk of exposure of the animals.
Researchers collected nasal and rectal swabs, as well as milk, from 24 lactating cattle, and tested them by PCR for evidence of active infection (viral DNA).
- All the cattle tested PCR-negative for SARS-CoV-2.
We know that PCR testing is a challenge in animal surveillance, since infected animals may only shed the virus for a short period of time. As we saw in our dog and cat SARS-CoV-2 surveillance study, it can be a challenge to sample animals at the appropriate time to catch them when they’re shedding virus. Animals can be infected but eliminate the virus by the time we hear about the exposure, arrange to get samples and get there to collect them.
Researchers also collected blood samples from the cattle on this farm, which allowed them to look for evidence of previous exposure by detecting antibodies against SARS-CoV-2. The specificity of the test (how likely a positive result is really due to SARS-CoV-2 infection) in cattle isn’t clear, but they reported positive results in 11 (46%), 14 (58%) or 13 (54%) of cattle, depending on the test used. The 54% was based on a serum neutralization test – not the standard virus neutralization test that’s been used elsewhere, but probably fairly reliable nonetheless.
So, what does this study tell us?
It suggests that human-to-cattle transmission of SARS-CoV-2 was relatively common in lactating cattle on this farm, where there was an outbreak in people. It’s too bad they only tested a small number of cattle and all of them were lactating, because lactating cattle are presumably higher risk for exposure as they have a lot of human contact a couple times a day for milking. It would have been nice to see if there was evidence of infection in other cattle and calves; seeing low rates of transmission to low risk cattle would also have provided more weight to the positive results reported in the higher risk group. Regardless, this is pretty strong evidence that people infected cattle on this farm.
Did the cattle get sick from SARS-CoV-2?
There’s no evidence the cattle had any clinical illness. Lactating dairy cattle are pretty closely monitored, so we can be pretty confident they didn’t have any overt signs of disease.
Could the cattle have infected other cattle or people?
That’s hard to say. All the cows were PCR-negative. Whether that was because of timing of sampling or very low grade infection is impossible to say based on this study. Given the low predicted susceptibility of cattle, lack of evidence of disease and zero prevalence by PCR despite high seroprevalence, it’s more likely that these cattle had very minor infections that resulted in them developing antibodies but not shedding enough virus to pose a risk for transmission. We need more studies to properly evaluate this, but nothing here suggests a major concern.
Can cattle be a reservoir for SARS-CoV-2?
For cattle to be a reservoir, they’d have to be able to infect other cattle and there would have to be ongoing transmission within the cattle population (vs a short term situation where cattle get infected and the virus burns through the group quickly). How susceptible they are, how well they can transmit the virus (if at all) and the number of cattle in contact (to be able to sustain transmission) would impact this. Most likely, if cattle were able to transmit the virus at all, it would burn through the farm relatively quickly. Dairy cattle usually number in the 10s or 100s on a given farm, not thousands (like mink or wildlife, though there are exceptions), so sustained transmission seems unlikely on most farms.
What about cow-to-human transmission of SARS-CoV-2?
We don’t know what the risk is for sure. Most likely, cattle are “dead end hosts” for this virus, whereby they get infected but can’t pass it on. If they are able to spread it, the nature of dairy farms and the number of cattle per farm mean the risk would probably be short term and only to farm personnel (who are likely at greater risk of exposure from the other infected people, not the secondarily infected cattle). However, we need to look at this more. There’s been a lot of reluctance to test food animals, so our knowledge of how the virus behaves in this species is still pretty limited.
The take home message:
Cattle are one of many species that SARS-CoV-2 seems to be able to infect. The odds of it being relevant for cattle (or for infection of cattle to be relevant to public health) are low, but it’s worth further study.