One of my favorite old-time gospel songs tells the story of a man pondering the eternal nature of family while grieving the loss of a loved one. It walks the listener through poignant moments of grief and mourning as the protagonist describes flashbacks to the day they laid his beloved mother to rest. Perhaps it is a manifestation of my sentimental nature, but when I hum the words of the chorus softly to myself, I can’t help but think about internal parasite control.

The circle that is the helminthic life cycle is one that seems to be unbroken far too often. Much like my exposure to gospel music, my experience with parasite life cycles increased substantially a few years ago when I moved to Mississippi to work on a master’s degree in epidemiology. 

Over the course of my time in the South, I realized that much of what I had assumed to be true about parasite control was actually pretty crummy advice. I recall being taught in veterinary school, several years earlier, that anthelminthics should be applied often, in large doses, and to as many animals in the herd as possible. Strangely, this advice seemed to line up pretty closely with what I was told at dinner meetings paid for by the manufacturers of these drugs.

This approach to parasite control is based on the assumption that the mere existence of parasites within a herd of cattle is an abomination that cannot be tolerated. It also assumes that all animals share an equal risk of disease, or at least production losses, if they are not dewormed regularly. 

Breaking the cycle

However, investigation into the dynamics of parasitism reveals several chinks in this logic. It turns out that many mature cattle can carry a low level of internal parasites and live otherwise normal, productive lives. Furthermore, mass treatment of a herd ensures any parasite eggs that are shed into the climes cattle inhabit are much more likely to carry genetic traits that will ensure future generations of parasites are more resistant to anthelminthics than their predecessors. This is especially true when we deworm an entire group of animals and place them immediately onto a “clean” pasture. This practice ensures the only parasite larvae in the pasture will be those with a demonstrated ability to survive whatever chemicals have been thrown at them.

Maybe a better way to break the circle of parasitism would be to think strategically. This requires a recognition that many animals can strike an immune-mediated balance with internal parasites, and therefore do not need to be dewormed regularly. Limiting anthelminthic treatment to those animals most at risk (e.g., young calves) can help to maintain a population of worms that are under less selection pressure for resistance. An understanding of parasite life cycles can also be useful if we are trying to break them. 

Most internal parasite eggs shed onto pastures require several days to develop into a larval stage that is capable of infesting a new host. Intensively rotating cattle through pastures, with long rest periods in between, is not only good for forages, but can effectively break the life cycle and limit the parasite burden. 

Lastly, avoiding practices that create ecosystems where the vast majority of surviving parasites are resistant to anthelminthic drugs can help slow the development of resistance. These include turning cattle that have been dewormed back out onto pastures where susceptible parasite larva are present; or better yet, only deworming prior to feeding in confinement so the larvae that are shed are unable to access green grass and complete their life cycle.

If we take a more thoughtful, strategic approach to parasite management, we may not expect to hear the sound of tiny funeral dirges emanating from the cow pasture, but it is possible we may break the circles of parasite transmission and resistance, and maintain the effectiveness of our anthelminthic tools long enough to pass on an antiparasitic world that is better, or at least no worse than the one we inherited. 

Editor’s note: Todd Gunderson is a clinical assistant professor, beef production medicine, at Kansas State University in Manhattan, Kan.