Managing Pasture Parasites

Keep your livestock healthy
By John Hibma

During the spring, summer and fall months, most folks who are involved in animal agriculture enjoy having their livestock outside, grazing pastures, staying healthy and productive as they take advantage of nature's bountiful offering of fresh forages. However, virtually all animals that spend any time on pasture will eventually be infected with internal parasites that are found in the pasture environment. Dairy cattle, beef cattle, goats, sheep, alpacas, llamas and horses are all susceptible to parasites that find their way to the lungs, stomach or lower digestive tract as these animals consume pasture as part or all of their diets. Depending upon the type of parasite and the level of infection and loading, all parasites will have some negative impact on both the health and productivity of animals.

Actual fecal sample with a tapeworm egg and a stomach worm egg in the same sample. Look closely at the characteristic three-sided tapeworm egg and you can see the teeth of the still-undeveloped worm. (The large, black dots are air bubbles)
Photos by John Hibma.

Parasite infestation, also known as parasitism, is a production disease that primarily affects grazing livestock. Animals will often only be affected subclinically, meaning that the infestation and infection is at a marginal or low level. Producers or owners may not even be aware that their herds or flocks have a parasite problem. Subclinical parasitism manifests itself somewhat subtly and insidiously with decreases in productivity such as...

If parasitism goes untreated for too long, more serious, clinical problems will develop such as...

During the life cycle of internal parasites, adult worms in the animal's stomach or intestines will shed eggs, which then exit by way of the animal's feces. If environmental conditions are favorable, the eggs will hatch into larvae, which then find their way onto grasses where they can live and survive for days or weeks. When livestock graze the grasses that harbor the larvae, they reinfect themselves and the larvae grow to adulthood, and then the cycle is repeated. The worms then find nourishment from the tissue and blood supply of their host, robbing them of essential nutrients they need for their own health and productivity.

Actual fecal sample of a whipworm egg.

Young cattle, along with goats and sheep, are the most visibly affected by parasitism. If young stock are already, in some way, experiencing health problems, such as pneumonia or vitamin and mineral deficiencies, their immune systems may not be strong enough to fight off a severe infestation of parasites and their health will quickly deteriorate. Mature livestock that have recently calved, kidded or lambed and are in early lactation are especially susceptible to parasitism problems that may exacerbate metabolic problems such as milk fever or ketosis, as well as decreasing milk production and reproductive efficiencies as their immune functions are compromised.

Parasitic infestation of pastures, unfortunately, is a continual process, and eradicating pastures of parasites is nearly impossible. Many of the parasites have developed the ability to overwinter and come back to life in the spring when the weather warms up. In other instances, livestock will remain infected over the winter with mature worms and as soon as they are turned out on pasture in the springtime they will begin redepositing eggs by way of their manure. The pasture grasses provide both the vehicle on which worms are carried to livestock and also the protection to survive unfavorable conditions, allowing them to reappear the following grazing season.

For the most part, larvae and eggs prefer and thrive in moist, temperate conditions. Hot, dry weather can help in desiccating and killing eggs and larvae. However, some manage to survive under protected vegetation, in a manure plop or in the soil. Virtually all permanent pastures are infested by parasitic larvae and eggs.

Since the elimination of parasitism is nearly impossible, the problem must be managed in such a way that infestation is kept to a minimum. The primary focus should be to prevent livestock from recontaminating pastures with worm eggs during the early portion of the grazing season. This is accomplished most effectively by the use of commercial deworming products known as anthelmintics. Dewormers essentially come in two forms for livestock, those that are added to feed and consumed orally, and those that are poured on the hide and absorbed through the skin.

To be most effective, the administration of dewormers must be timed to when livestock will be introduced to pastures. Animals should also be dewormed at the end of the grazing season to kill off mature worms, which will greatly reduce the recontamination of pastures in the spring. However, parasite larvae that have survived through the winter will reinfect livestock as soon as they are turned out on pasture. Therefore, another deworming should be conducted shortly after introduction to the pasture in the spring. The strategy of this two-time worming program is to kill mature parasites in the animal before they can lay eggs that are excreted through the manure and back onto the pasture.

On most farms, though, animals are not managed as single groups for very long. Rather, animals are brought into a herd during midseason that may not have been dewormed. These animals, if infested with worms, will reinfect the rest of the herd that has been dewormed. Or, a stray may get into another pasture that is heavily infested and upon returning will recontaminate the pasture that was thought to be clean. Therefore, herdsmen and owners must develop strategies to deal with parasitism over the course of the grazing season that will keep parasites to a minimum. In practice, the strategic use of anthelmintics along with rotation of pastures is an effective way of managing parasitism.

There are five main categories of parasitism in livestock: Stomach worms, intestinal worms, liver flukes, protozoa and lungworms. All of these worms cause different types of problems in the animal they are infecting.

Stomach parasites include the following:

Intestinal parasites include the following:

Liver flukes include the following:

Protozoa include the following:

In cattle, the most prevalent worm is the brown stomach worm. In the larval stage, the brown stomach worm invades and destroys gastric glands in the abomasum, diminishing acid production and digestion efficiency. Goats are most susceptible to the barber pole worm, which sucks copious amounts of blood from the stomach to cause anemia and eventually death. Young calves are more susceptible to the threadneck worm, which causes diarrhea. Tapeworms can grow to be 1 inch wide and 6 feet long, completely blocking the intestine.

Slide depicting relative size of different eggs. (Remember: an ?m is 1/1000 of a millimeter)

Monitoring of worm levels in herds and flocks through the collection of fecal samples should be done periodically to determine if and when deworming may need to be done. There are a host of different deworming products on the market, and they are classified in two groups. Class I dewormers kill only the adult worms while in the animal. (Examples include: Levasole and Rumatel.) Class II dewormers kill both mature and immature worms. (Examples include: SafeGuard and Ivomec.) Some manufacturers of these products offer free parasite clinics to which customers may bring fecal samples that can be quickly evaluated for worms.

Over the years, the deworming of livestock has become increasingly more sophisticated, allowing for the early detection and treatment of those animals and avoiding unnecessary and costly economic losses. As with many health challenges, preventing the problem before it occurs is often much less costly than having to treat animals once they are sick. All livestock owners who graze animals will eventually run amuck with parasitism and should work with their veterinarian or feed company professional to design parasite management programs that will keep their animals both healthy and productive.

Thanks to Intervet, Inc., Millsboro, Del., for information in this article.

The author is a dairy nutritional consultant and works for Central Connecticut Farmer's Cooperative in Manchester, Conn.