The parasite Ostertagia ostertagi, or brown stomach worm, is a pathogen affecting cattle globally, particularly in temperate climates. Its presence causes economic losses from production setbacks like reduced weight gain, lower milk yields, and decreased feed efficiency. Understanding the parasite’s life cycle, which begins with an egg passed in the feces, is necessary for diagnosis and implementing effective control strategies.
The Parasite Life Cycle
The life cycle of Ostertagia ostertagi is direct, with a free-living stage on pasture and a parasitic stage inside the host. It begins when an adult female worm in a cow’s abomasum, or “true stomach,” lays thousands of eggs. These eggs are then passed in the dung onto the pasture.
In the dung pat, a first-stage larva (L1) hatches from an egg. This larva feeds on bacteria, grows, and molts into a second-stage larva (L2). The L2 larva continues to feed and develop until it molts into the third-stage larva (L3), which is the infective stage for cattle.
The L3 larva retains a protective outer sheath from its L2 phase, which prevents it from feeding but makes it more resilient to environmental conditions. These larvae migrate from the dung pat onto surrounding moist grass. When a cow grazes, it ingests these infective L3 larvae, which then travel to the abomasum and burrow into the gastric glands of the stomach lining.
Within the gastric glands, the larva undergoes two more molts, developing into an L4 and then an L5 stage. It then emerges onto the surface of the abomasal mucosa as an immature adult worm. The worms mature, mate, and females begin producing eggs that are shed in the feces, continuing the cycle. This process from ingestion of L3 to the appearance of new eggs in the dung takes about three weeks.
Egg Identification and Diagnosis
The method for detecting a parasite burden in live cattle is a Fecal Egg Count (FEC). This laboratory test involves analyzing a fecal sample to determine the number of parasite eggs present per gram of feces. The count helps estimate the worm burden within an animal and the level of pasture contamination.
An Ostertagia egg is a “strongyle-type” egg, which is oval-shaped with a thin, smooth shell and contains a developing embryo. These characteristics are not unique. The eggs of Ostertagia are visually indistinguishable from those of other common gastrointestinal nematodes in cattle, such as Cooperia or Haemonchus.
Because of this limitation, a laboratory report will identify the eggs only as “strongyle-type” rather than specifying Ostertagia. A definitive diagnosis of ostertagiasis, the disease caused by this parasite, requires combining FEC results with other information. Veterinarians consider the age of the animals, clinical signs like diarrhea and poor growth, the time of year, and the herd’s grazing history.
Environmental Development and Survival
The development of Ostertagia eggs and larvae outside the host is heavily dependent on environmental conditions. Temperature and moisture govern the speed at which eggs hatch and larvae develop to the infective L3 stage. Under warm, moist conditions, this development can occur in as little as two weeks, while it takes much longer during colder periods.
The infective L3 larvae are the most resilient of the free-living stages and can survive on pasture for extended periods. They are capable of surviving over winter in colder climates, creating a source of infection for cattle turned out to pasture in the spring. Larvae require a film of moisture to move from dung pats onto blades of grass and are susceptible to drying out in hot, dry weather.
A survival strategy of this parasite is hypobiosis, or arrested development. Larvae ingested from pasture at certain times of the year can pause their development as early L4 larvae within the host’s gastric glands. This arrested state can last for several months before development resumes, and the synchronized emergence of a large number of worms can cause severe disease, known as Type II ostertagiasis.
Control and Pasture Management
Control of Ostertagia ostertagi focuses on breaking its life cycle by minimizing the ingestion of infective larvae. This involves managing the parasite population within the host and reducing the contamination of pastures. Strategic deworming with anthelmintic treatments reduces the number of adult, egg-laying worms, thereby decreasing the number of eggs shed onto the pasture.
Pasture management is a component of sustainable control. Rotational grazing systems, where cattle are moved through a series of paddocks, help to break the parasite life cycle. By not allowing animals to graze a pasture for several weeks or months, many of the L3 larvae present will die off before cattle are reintroduced, reducing the infection risk.
Maintaining appropriate forage height can also lower the intake of larvae, as most are found in the bottom few inches of the grass. In some climates, harrowing pastures during hot, dry weather is effective. This practice breaks up dung pats, exposing the eggs and larvae to lethal heat and desiccation, which disrupts their development and survival.