Cheese Lovers Newsletter (6.6.2021): Jessica Vines this Saturday

Question of the Week: Were the cows themselves - their immune systems - susceptible to COVID-19?

Answer from Stephanie Langel, Ph.D.; Alise and Lucas' college friend, cow lover (so we say) and Scientist and Medical Instructor at Duke University Medical Center:

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a novel respiratory virus that emerged in December 2019. It is a coronavirus - a group of related viruses that cause disease in birds and mammals, including cows. Coronaviruses can be divided into subgroups called ‘genera’. These groups are called alpha, beta, delta and gammacoronaviruses. There are seven known human coronaviruses, four that cause the common cold (their names are 229E, NL63, OC43, and HKU1) and three that can cause severe disease including SARS that emerged in 2003, MERS that emerged in 2012 and SARS-CoV-2 that emerged in 2019. These viruses are similar enough that they all cause disease in one main host; humans. However some are different enough from each other that they fall into different genera. For example, 229E and NL63 are closely related and belong to the alphacoronavirus genus while OC43, HKU1, SARS-CoV, MERS-CoV and SARS-CoV-2 all belong to the betacoronavirus genus.

Now a little bit about bovine coronaviruses (BCoVs). BCoVs cause respiratory and enteric infections in cattle and wild ruminants. BCoV is a pneumoenteric (think lung and gut) virus that infects the upper and lower respiratory tract and intestine. It is shed in feces and nasal secretions and also infects the lung. BCoV is the cause of 3 distinct clinical syndromes in cattle: (1) calf diarrhea, (2) winter dysentery with hemorrhagic diarrhea in adults, and (3) respiratory infections in cattle of various ages including the bovine respiratory disease complex or shipping fever of feedlot cattle. BCoVs belong to the betacoronavirus genus.

Why do scientists go thru the hassle of categorizing viruses in this way? It is a very helpful way to compare human and animal coronaviruses to each other. Coronaviruses love to jump from animals to humans and back again… it’s what they do! They are ‘zoonotic’ viruses. Categorizing coronaviruses in this way helps us understand where coronaviruses came from and potentially, where they are going. For example, the common cold coronavirus OC43 first emerged in rats and then ‘jumped’ to cows before emerging in humans. This process of ‘jumping’ from one host to another can take a long time, possibly hundreds or thousands of years. However, because of the way coronavirus ‘replicate’ (make more copies of themselves) they can ‘adapt’ themselves and become better suited for a new host (for instance, a cow instead of a rat). The question of ‘are cows susceptible to COVID-19’ is an important one because it helps us understand if there could be animal reservoirs for SARS-CoV-2. To determine this, the first thing scientists do is to take a concentrated amount of the virus and place it in the mouth and nose of cows to see if it can replicate productively (or in other words, infect a cow enough that they can transmit to other cows). This was attempted in Germany by a well known group of coronavirologists. In this experiment, they inoculated (another word for infected) 6 cattle with SARS-CoV-2 and kept them together with 3 uninoculated cattle. The scientists did observe viral replication and seroconversion (antibody production indicating infection) in two of the animals. However, the scientists had one more test. To truly determine if an animal is ‘productively’ infected, they have to transmit the virus to another animal. If they don’t, it is likely the virus doesn’t infect ‘well enough’ and may not be a suitable host. However the scientists did not observe intraspecies (between cattle) transmission from infected to uninfected cattle. Therefore, the scientists had no indication that cattle play any role in the human pandemic, and no reports of naturally infected bovines exist.

This question of ‘what animals DO replicate and transmit SARS-CoV-2’ is an important one. Natural or experimental infections have demonstrated the susceptibility of fruit bats (Rousettus aegyptiacus), ferrets, felids, dogs, and minks to the virus; however, cows, pigs, chicken, and ducks are not susceptible. In fact, there are multiple reports of outbreaks of SARS-CoV-2 on mink farms, likely from SARS-CoV-2 infected people transmitting the virus to minks. In countries with mink farming (including the US), entire groups of ferrets had to be culled to make sure SARS-CoV-2 didn’t spread further and infect more minks and more people.

Coronaviruses will be with us as long as humans and animals populate this earth. It is imperative that scientists continue to sample and surveil wildlife and domestic animal populations for indications of ‘spillover’. For decades, coronavirologists have been communicating both to the government and general public about the pandemic potential of coronaviruses. As we start to go back to ‘normal’ in a post SARS-CoV-2 vaccine world, it is important to remember that SARS-CoV-2 has not gone anywhere. It is just now unable to infect more people and vaccination rates increase. However, it is still replicating in the respiratory tracts (noses and lungs) of unvaccinated and otherwise susceptible individuals. Even in a person who never has symptoms (asymptomatic) or has mild symptoms, they can replicate SARS-CoV-2 on the order of billions of times in their nose and spread it to others. Only by continuing to vaccinate and test for SARS-CoV-2 can we keep this virus under control and know where it is going and if it’s going be ‘hiding out’ in another animal population. But for now, we can confidently say that SARS-CoV-2 does not infect cows and they are not an animal reservoir.

Image from "Origin and evolution of pathogenic coronaviruses"

The German study where they infected cows with SARS-CoV-2: https://wwwnc.cdc.gov/eid/article/26/12/20-3799_article

Thank you for such a great question!!
Lucas, Alise, Linda and Jerry