As we approach spring working of cows and calves and the growth of new pasture, it is important to understand the life cycle of cattle's most common problem parasite, the brown stomach worm (Ostertagia ostertagi). It thrives in temperate climates such as Kentucky's, especially in the moist and cool weather we usually experience in the spring. However, it is very susceptible to hot and dry conditions- a fact we can use to our advantage when choosing when to deworm.

This roundworm ("nematode") goes through a developmental process of 4 stages of larva (immature worm) and 1 egg-laying adult stage. Adult worms live in the abomasum or "true stomach" of a cow or calf for approximately 30 days where they reproduce and lay eggs. These eggs are passed out on the ground in fecal material and then go through a two week developmental stage while feeding on the manure. Larvae can survive up to a year in fecal pats, even in drought conditions or underneath snow. When the immature worm reaches the L3 stage, it must get away from the fecal pat and out on the forage to be eaten by cattle. Since the larva have no legs, they are forced to move in a film of moisture and generally move less than 1' away from the fecal pat and no more than 4" up the forage. One drop of dew may contain hundreds to thousands of immature larvae.

Once swallowed, the disease process begins. Young cattle (7-15 months old) experience "Type I Ostertagiasis" in which the larvae enter the stomach glands, grow and become adults in 3 weeks. When the adult worm leaves the gland, it tears its way out and begins feeding on the lining of the stomach. This destruction of the gastric glands makes it very difficult for the animal to digest protein which in turn causes diarrhea. Older cattle (12-20 months old) experience a "Type II Ostertagiasis". In this case, the L3 larvae migrate to the stomach glands, develop into 4th stage larvae then enter a sort of hibernation known as "hypobiosis". The immature worms remain in the stomach glands while the weather is extremely hot and dry outside, waiting for better conditions for survival of their eggs. When the weather gets cooler and wetter in the fall, all of the dormant larvae that have accumulated in the stomach glands tear out all at once, causing severe, rapid and sometimes catastrophic damage.

Typical clinical signs of Type I (younger cattle) and Type II (older cattle) disease include:
* Off feed (anorexia)
* Diarrhea (and usually dehydration)
* Weight loss/Poor doer
* Rough hair coat
* Bottle Jaw due to poor protein digestion
* Anemia/Pale mucous membranes
* - Subclinical Disease - May be most economically important because there are no obvious signs of disease yet gain is 0.2-0.4 lbs /day less than it would be without parasites.

Control of this parasite involves treating to kill the adults, the larvae in the stomach glands as well as any new larvae ingested from the grass. Broad spectrum dewormers known as "macrolides" (Cydectin, Dectomax, Ivomec, Eprinex) are very effective and last several weeks so incoming larvae are killed before they reach the stomach glands. The "benzamidazoles" (Valbazen, Safeguard, Synanthic) are also very effective but short acting. In Kentucky, research has proven late June to early July to be the optimal time for deworming of cows and calves. All young stock under 2 years old should be dewormed again in the fall since they are most susceptible to parasite problems. Although adult cattle are often considered "immune" to worms in the stomach, these adult worms will reproduce and lay eggs during times of stress, especially at calving. Therefore deworming of all cattle is essential to decreasing pasture contamination.

Beyond chemical control with dewormers, several management practices will result in cleaner, safer pastures. Some examples include:
* Resting Pasture-May not be physically or economically possible.
* Alternating grazing cattle with sheep or goats- Worms are "host specific" and can't survive in a different species
* Hay or Silage Production- Large numbers of larvae are removed in the hay/silage and the reduction in ground cover exposes the remaining parasites to harsh conditions
* Growing a crop of cereal grains or renovating pastures- Parasite levels are reduced by the long duration without grazing and working of the soil

In summary, control of the brown stomach worm comes from an understanding of the host-parasite-environment interaction. Bear in mind that anything done to break up manure and expose the larvae to light and heat will reduce their numbers in pasture. Avoid overgrazing that forces cattle to eat too close to manure and the ground. Strategic deworming for all cattle on the farm in late June-early July and a second round in the fall for young stock will help to eliminate parasitic disease and economic loss as well as reduce pasture contamination.

Source: Dr. Michele Bilderback, Extension Veterinarian, University of Kentucky