Understanding Parasites on Pasture

Brady Campbell, Program Coordinator, OSU Sheep Team

Image of an adult Haemonchus parasite recovered from the abomasum of a lamb

Recently, I had a sheep producer ask me, “when do I need to start thinking about parasites on my pastures?” This is a great question and certainly a valid concern as livestock are making their way to pastures this spring.

Now I know what some of you are thinking, “I don’t have issues with parasites. If I did, my sheep would be showing clinical signs of disease such as decreased appetite, decreased  activity, or even death.” However, this is a common mistake that we as producers make too often. Typically, clinical signs of parasitic infection are only noticed when the cases become severe. According to Dr. Thomas Craig, DVM, PhD, DACVM, most losses associated with parasitic infection are economic rather than clinical. Parasitized livestock are extremely inefficient as demonstrated by a decrease in overall animal performance, such as decreases in average daily gain and reproductive performance. In order to understand the effects of parasitism, we must first be familiar with how and why our livestock become infected.

Parasite Life Cycle
To begin, lets us first start with the life cycle of a parasite. For the purpose of this article, we will use Haemonchus contortus, more commonly known as the Barber’s Pole Worm, as an example. With that we will start where it all begins, the egg. Parasite eggs are shed from an infected animal (sheep or goat) in the manure and start their infective life cycle on pasture. This is important to note as most parasites must hatch outside of the host and be consumed in order to the life cycle to continue.

Once the eggs hatch, the L1 and L2 stages of larvae develop and feed off of the bacteria present in the manure. Parasitic larvae are considered infective once they reach the L3 stage. The development of an egg to L3 infective larvae can occur in a matter of 3-10 days and is dependent upon environmental conditions (temperature, moisture, humidity). L3 stage larvae will move out of the manure and use moisture, in the form of rain or dew, to move onto and up and down the forage leaflet. Once the larvae are present amongst the forage canopy, it will only be a matter of time until they are consumed by grazing livestock. Once consumed, L3 larvae will de-sheath and attach to the wall of the abomasum (fourth compartment of the ruminant digestive system). Upon attachment, larvae will feed off blood and begin to develop into a mature adult. Once reaching adulthood (approximately 18 days after attachment), the parasites will mate, females will produce eggs, and the vicious cycle continues. For those that didn’t catch the math, these parasites consumed on pasture can reinfect the flock in a matter of 21 days. In addition, a mature H. contortus female has the capability of producing 5,000 – 10,000 eggs per day, talk about productivity!

Now having a better understanding of the parasite life cycle, the next question becomes “what factors influence the productivity and longevity of parasites on pasture?” Parasites thrive in an environment in which is hot, humid, and among forages that provide a protective canopy from ultraviolet sunlight. Environmental temperature and moisture play a critical role in the development of parasites. In a lab setting, Besier and others (2016) found that the normal development of infective L3 stage larvae is anywhere in the range of 50°F – 104°F. To further support this claim, Dr. Craig (2018) states that a more ideal temperature range for optimal development is 72°F – 79°F with 100% humidity. Beyond these ranges, parasite hatchability and survivability decreases.

However, this does not mean that grazing livestock are safe to graze without fear of parasites outside of these temperature ranges. Parasite development tends to be rapid in summer like conditions, but life expectancy is short due to heat and sunlight. Remember the de-sheathing process that I mentioned once the L3 stage larvae were ingested? The sheath of the L3 larvae is important as it prevents the parasitic larvae from drying out in the intense heat. However, with limited energy reserves, the larvae are not able to survive in these conditions for an extended period of time. On the flip side, parasite development is slow during spring, fall, and winter conditions, but survival is lengthy. As drying out becomes less of an issue during these periods, the L3 larvae are able to cope in these conditions and survive for a longer period of time. In areas with moderate to low temperatures, parasitic larvae are able to survive on pasture for up to a year, but in general most survive for 2-6 months (Craig, 2018).

Remember, parasites are not only found on forages within the pasture. As survival tools, parasites are also able to protect themselves by remaining in the manure pack or moving into the soil when environmental conditions are not suited to their development. Therefore, short periods of dry weather and intense heat or cold temperatures may not be enough to destroy an entire parasitic population. Furthermore, your flock and herd can be a reservoir of parasites as well. During unfavorable environmental conditions, parasites can undergo a period of hypobiosis, an arrested or stalled stage of development. This allows for the parasites to resume development and infection once conditions are favorable (i.e. warm weather in the spring and summer).

Parasite management
So now that we have an understanding of how the parasite life cycle works and what factors influence their development, the next question always becomes “what can I do to get rid of the parasites?” Unfortunately, this is currently not an option. There is no silver bullet to parasite management and producers must adapt and learn how to raise their livestock in an environment in which parasites exist. Fortunately, due to continued research efforts at The Ohio State University and around the world, producers today have a list of management practices and strategies in their management tool boxes to help combat parasites aside from the use of anthelmintic or deworming products.

In order to keep this weeks post short, I’ll save the discussion of on-farm parasite management for next week. In the meantime, be thinking about what management strategies that you currently use on your operation. You never know, sharing what you do and comparing it to our discussion next week may help both you and fellow shepherds down the road while combating issues associated with parasitic infection. Until next week, happy shepherding!


  • Besier, R.B., Kahn, L.P., Sargison, N.D., van Wyk, J.A., 2016. The pathophysiology, ecology  and epidemiology of Haemonchus contortus infection in small ruminants. Adv. Parasitol. 93, 96-133.
  • Craig, T.M., 2018. Gastrointestinal nematodes, diagnosis and control. Vet. Clin. Food Anim. 34, 185-199.