Cow and farmland at sunrise

A herd of cows in farmland at sunrise.

Whether you raise sheep, goats, cattle, swine or a combination of livestock, you will likely face the challenge of managing internal parasites.

Parasites cost the industry millions, if not billions, in lost revenue from poor animal performance, health care costs and deaths. And those challenges are compounded as parasites develop resistance to treatment options.

However, researchers are finding more preventative options for producers.

The first step in getting ahead is to figure out the specific internal parasite that may be vexing your operation, and then learn the life cycle of that parasite. This is extremely important because if there is a step in the life cycle of that particular parasite that can be eliminated by good management practices, it can greatly reduce any losses a producer may face from heavy infestation. Some of these practices include good sanitation, pasture rotation, or removing intermediate hosts.

The most common and effective way to identify the parasites you are dealing with is to work with a diagnostic lab or your veterinarian on a fecal egg count.

Internal parasites have one of two life cycles, direct or indirect. A direct cycle does not require the use of an intermediate host or vector to complete the parasite’s life or to pass along the parasite to a different host. An example of the stages of a direct life cycle would start with the initial host that is infected with adult parasites. These parasites will mate and release eggs that are excreted in the feces of that host. These eggs later hatch in the environment and may undergo one to three different growth, or larval, stages before they become infectious. It is at this larval stage that the parasites may be consumed by another host and develop into mature parasites, beginning the life cycle all over again.

For these direct infectious parasites, most larvae are found in the first two to three inches of forage growth in the pasture. That is why it is recommended that pastures not be grazed any shorter than four inches, to reduce infection to new hosts and reinfection to the initial host.

Producers can also break direct life cycles by rotating pastures before the larvae in the environment become infective. After the initial host releases the eggs in their feces, there is a time frame that the larvae will need to grow and develop into the infective stage. This time frame will vary depending on the parasite, but a good rule of thumb is to move animals every four to six days so that livestock are not ingesting larvae when they become infective.

Another technique is to let pastures rest long enough for the infective larvae to die off altogether. The challenge with this is that temperature and moisture can influence that time frame. Most parasites will live longer in cool, moist environments. Some can over-winter and be infective in the spring. Many of these larvae do not survive more than a few days in temperatures over 90 degrees with dry conditions.

Eliminating Intermediate Hosts

Parasites with indirect life cycles require the use of an intermediate host or vector of a different species in order to complete their life cycle or infect new hosts. This can start with the initial host being infected with adult parasites. These parasites mate and lay eggs that are then excreted into the feces of the host. The eggs then hatch into larvae in the environment where they are eaten by the intermediate host. While in the intermediate host, they continue to grow and develop and are then released back into the environment in an infectious form. This form, which can be a larva — or in the case of a liver fluke, a cyst — is then eaten by a new host. In the new host, or initially infected host, these parasites may move through the animal’s body to a specific organ or part of the digestive tract where they complete their growth phase and become mature adults.

Very similarly to the control of direct life cycle parasites, producers can break the indirect life cycle of parasites by controlling the length of grazed forage in pastures and rotating livestock through pastures following a time frame depending on the parasite’s infective stages (to both the intermediate and initial host). However, unlike the direct life cycle parasites, producers managing an indirect life cycle parasite have the advantage that they can take steps to manage or eliminate the presence of the intermediate host. For example, liver flukes require the use of the freshwater snail, Lymnaea tomentosa, as the intermediate host in their life cycle. This snail’s natural habitat is cool wet areas that frequently flood in the spring or fall. If this parasite is a problem in your herd, you could restrict livestock access to these areas or reduce the amount of moisture in those areas to reduce or eliminate the snail population in pastures.

Dealing with internal parasites is an unavoidable part of raising livestock. However, managing your flock or herd’s parasitic load doesn’t have to become a financial drain. By running fecal egg count tests and working with a veterinarian to diagnose which parasites are infecting your livestock, you can make sound management decisions to break the life cycles of the parasites before they become problematic.

Chelsea Hill is a Penn State Extension adult livestock and 4-H animal science educator in Wayne County.


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