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Practical Advice for Dealing With Parasites

A note from the Author: I’m not much of a scientist. While I was trained in science, I am, for the most part, just an observer of nature. I see things in the world around me and make up working hypotheses that appear to fit. The solutions I present here are not proven; they are not supported by research or data. Just by common sense.

I live in a horrible place.

The Willamette Valley on one of its better days.


How often do you hear someone say that? Not often, at least not in the ranching world. But seriously now, the place where I live is a haven for every creepy, slimy creature known to man. Animal Science Professors at our local Land Grant College commonly use the honorary title of “Crawliologist”. The name of our big valley here in Western Oregon is Willamette, (sounds like damn it) which the anthropologists tell us means Valley of Death in the native tongue. This country is laced with ditches, sloughs and swamps, all of them teeming with leeches, worms and flukes. Not only that, we rarely have a frost that extends below 1” of soil depth, so parasites emerge from winter refreshed and ready to find a new host. Cattle in this country – some of them, anyway—look pretty rough for at least part of the year. Skinny racks of bones with projectile diarrhea and sunken eyes. Just horrible.

With that in mind, perhaps I should be thankful for all of the articles in the conventional Ag magazines that focus on controlling parasites. Experts and professors from around the world describe aggressive treatment plans and constant vigilance, strategies aimed at saving our livestock from the wiggling scourge of creatures that await. The general theory seems to be that through the constant application of chemicals, we can keep our stock safe, healthy and profitable. Let me add this here:

I remain un-convinced by all of these arguments, and here’s why:

In my admittedly limited experience, I have observed that constantly battling against Mother Nature with ever-increasing chemical inputs has been a losing game for agriculture. In addition to the economic and technical issues, complete dependence on drugs appears to make producers evermore resistant to making strategic operational or management changes, adjustments that might help them become more profitable and more sustainable. People just become conditioned to reaching into the cupboard for a dose of the latest silver bullet.

Here are some things to think about.

Life Cycles of Stomach Worms and Liver Flukes

Parasites that inhabit the digestive tract of our livestock are generally called “worms”. Of these, Ostertsagia (the brown stomach worm) appears to be the most widespread problem. Ostertagia eggs are deposited on our pastures in the fecal droppings of infected individuals. These eggs hatch, producing larvae that take up residence in the soil and on the plants of the pasture. Each morning, these baby worms wiggle their way up the stems and leaves of the sward, swimming up through the dew, trying to reach a place where the next cow that happens by will take a bite, and bingo: the larvae now enter the gut where they mature and begin laying eggs of their own. These new eggs are deposited on the pasture and the cycle is renewed.

There are drugs that will interrupt this cycle, but if you place the now-treated cow back on a worm-infested pasture she will soon enough have a belly full of worms again.

Liver flukes, Fasciola hepatica, have a similar but much more fascinating life cycle that involves using a mud snail as a temporary host. Immature liver flukes soon leave the snail behind, migrating to your pasture grasses in the hope of infecting your livestock, much the same as worms. Generally speaking, flukes are most populous around wet, poorly drained areas, places where the mud snails live.

Parasites Learn Tricks: Just Say No to Drugs!

Aside from the problem of your sheep or cattle constantly re-infecting themselves, we have the problem of ever increasing drug resistance among the worms of the world: those drugs just don’t work as well as they used to. So, what do we do now?

Changing Grazing Management to Reduce Worms

Here are some common sense ideas for reducing worm problems on your ranch.

Leave Taller Residual
Because the worm larvae have a limited internal energy supply, they have a limited number of times that they can make the trek up and down the stem and leaves of your grass, and a limited number of days waiting for your stock to swallow them. The taller your residual, the farther your cow’s nose is from the ground, and the harder it will be for larvae to wind up in your cow’s belly. Taller residual probably means lower re-infection rates.

Tall may not be as tall as you imagine. This graph shows the distribution of larvae on grasses. At 15 centimeters, or about 6 inches, the number of larvae drops to almost 0. Below 5 cm or about 2 inches, their numbers increase dramatically.

See the liver flukes in this dewdrop? They’re not going to do so well with some time to dry out.

Work Towards a Longer Rest Period
Worm larvae have a limited life span. The longer they are exposed to natural ultra-violet sunlight, temperature variations, dry conditions, etc., the less likely individual larvae are to survive. It only makes sense that a longer rotation will result in fewer live larvae.

Seek a Shorter Grazing Period in Each Paddock
This is closely related to the two points above. Here’s the thing: if you have overly-long grazing residence periods, your animals will return to the most desirable individual plants and re-graze them, their mouths eventually moving closer to the ground, precisely where the larvae population is highest.

Getting the Cows Out of the Creek
One obvious way to reduce Fluke infection rates is to keep your livestock from grazing in the moist areas around surface water. Ditches, streams and ponds are the living room of mud snails, and if your cattle graze the vegetation that grows there, they will likely find flukes. A partial solution, then, is to move livestock away from surface water. I know, I know: allowing your stock to drink from surface water is easy and cheap. Switching to pumped water systems requires some thought and effort. But I will make a couple of not-so-bold predictions. Aside from reducing the fluke infection rate as we move animals away from surface water, I believe that

• Regulatory/social/political pressure will increasingly demand that we remove our animals from natural water sources. Maybe we should get out in front of this while we have a chance.

• Your life on the ranch will get better, more operationally easy, as you evolve toward using piped water sources. I say this based on 20+ years of total exclusion from riparian watering.

Happy Grazing!

Have you got some experiences of your own? Tell us what you’re doing to reduce your parasite problems.

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John Marble
John Marble
John Marble grew up on a terribly conventional ranch with a large family where each kid had their own tractor. Surviving that, he now owns a small grazing and marketing operation that focuses on producing value through managed grazing. He oversees a diverse ranching operation, renting and owning cattle and grasslands while managing timber, wildlife habitat and human relationships. His multi-species approach includes meat goats, pointing dogs and barn cats. He has a life-long interest in ecology, trying to understand how plants, animals, soils and humans fit together. John spends his late-night hours working on fiction, writing about worlds much less strange than this one.


  1. I enjoyed reading this article very much. It is a practical, common sense approach to manage animals to minimize effects of parasites.

  2. Great article! Recommended practically the same practices in Iowa. Amazing what happens when we work with creation instead of against it.

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