Animal health matters: Grass fires and herd immunity

Russ Daly Special to the Farm Forum
Farm Forum

As a veterinarian, I smile a little inside when I hear human health experts talk about “herd immunity.” I like the idea that high level policymakers might be envisioning cattle or pigs in the back of their mind while discussing disease dynamics in the human population.

Disease immunity in herds is the goal, of course, for livestock farmers and their veterinarians as they work to prevent the illnesses that depress productivity and well-being of their animals. In a similar manner, public health officials are seeking the same thing for all of us amid the COVID-19 pandemic.

I suspect that when most people consider herd immunity – whether in the public or a barn full of hogs – vaccines come immediately to mind. Agreed: vaccines are critically important for achieving this group immunity. Thankfully, effective COVID-19 vaccines are going into people right now.

But achieving herd immunity has to be more than just vaccinating everyone. In the case of COVID-19, the vaccines are very good but not perfect. A small percentage of vaccinated people are still vulnerable to disease and could still spread the virus. And we know that not everyone will be able to – or choose to – be vaccinated.

Herd immunity is defined as, “the resistance of a group to attack by a disease to which a large proportion of the group is immune.” When herd immunity is achieved, it means that even if an individual is not immune themselves, they’re protected against getting the disease. This is because the chance of that person contacting an infectious person is very small when most individuals in their community are resistant themselves.

One of my colleagues and mentors, Dr. Dave Smith at Mississippi State University, illustrates this tricky concept very well. He uses the analogy of a lit match (representing an infectious person or animal) and grass (representing people or animals in the population). If you drop a lit match in your pasture in April when the grass is lush and green, what happens? Nothing. The grass is resistant (“immune”) to the fire. How about in early summer, when some of the grass is turning brown (“susceptible”)? Dropping a match now means a few of those brown blades will burn where the match was dropped but because most of the grass is still green, a grass fire doesn’t break out. This is the value of herd immunity: even if some of the population is susceptible, disease doesn’t run unchecked.

Imagine what will happen if that lit match is dropped in August after a dry summer has turned the grass brown. In this case, the entire population is susceptible. The ensuing inferno might even be enough to burn the few remaining green blades. The fire (disease epidemic) eventually ends when everything that’s susceptible is burned (no longer vulnerable to the effects of disease or…dead!).

In these scenarios, Mother Nature dictated the underlying conditions and much of what happened. One might think things were out of our control. Consider, however, the pastures across the fence from the grass fire.

On one side, the landowner is starting to break up the pasture. Most of the land is disked up black dirt, but patches of that dry grass still exist. Embers from our fire land on this pasture, but the whole pasture doesn’t burn because fire can’t easily pass from one patch of grass to the next. He’s maintained separation among his susceptible population. On the other side, the pasture is irrigated with green (immune) grass, so our embers don’t start this pasture on fire. These human interventions stopped the fire (disease) – improving the resistance of the population (vaccines) or spacing the susceptible subjects out (wearing masks and avoiding groups).

Herd immunity won’t work for every disease, but it will for COVID-19. It’s directly transmitted from person to person and doesn’t have a source in the environment or other species. We’ll never have herd immunity to a disease such as tetanus – which isn’t spread between people and has its reservoir in the environment.

While the immunity gained through vaccines will be the greatest contributor to herd immunity against COVID-19, the other tools will remain necessary. Whatever we can do to make sure the virus doesn’t find its way from an infected person to a susceptible person will also help close the book on COVID-19.