Understanding the Internal Parasite Life Cycle
One of the key challenges to small ruminant producers across the country is managing internal parasites, particularly Haemonchus contortus, also known as the barberpole worm. These worms cost producers in the United States millions of dollars a year through reduced growth in young animals, reduced reproductive performance, health care treatments, and increased mortality. Having a better understanding of their life cycle can help producers target their management strategies to prevent high levels of infection.
The life cycle begins when a sheep, goat, llama, or alpaca passes eggs through its manure. The egg hatches into its first larval stage, called an L1, usually in under 24 hours. The larva feeds on bacteria in the manure and undergoes two molts as it grows. The L1 and L2 stages remain within the manure pellet. During these stages, the larvae are susceptible to heat and low humidity, which can kill them.
Once the larva reaches the L3 stage, it does not molt. At this stage in its life cycle, the larva is covered by two cuticle layers that protect it from the environment. The second cuticle layer also covers its mouth, so it cannot eat. However, the L3 larva can survive on stored energy reserves within its body for about 30 to 60 days in hot weather and as long as 120 to 240 days in cool weather. It can take as few as 6 to 14 days to reach the L3 stage.
While in this non-feeding stage, the larva moves out of the manure and onto nearby blades of grass in the pasture. The L3 larva wiggles and uses water films from rain or dew to migrate up a leaf blade. For the most part, these larvae only move up two inches from the ground. However, splashing rainwater can sometimes allow them to reach higher up a blade of grass. The larva may also travel under leaf debris, which can protect it from the weather.
As an L3, the larva's goal is to be consumed by a small ruminant to continue the worm life cycle. Once consumed, the larva will spend two to three weeks in the rumen before developing into the L4 stage and moving into the abomasum. At this time, the L4 can either continue its life cycle or enter a hypobiotic (dormant) stage by burrowing into the mucosal lining of the abomasum.
While in this hypobiotic stage, the larva can be very difficult to kill with deworming treatments. This stage often occurs during the winter, when temperatures are too cold for larvae to survive, or during the summer, when it's hot and dry. Basically, these weather conditions are not conducive to the larva completing its life cycle outside of the animal.
At the L5 stage, the larva develops ovaries and a uterus, becoming capable of laying up to 5,000 eggs per day. At the L5 stage, the larva now has a lancet-type tooth that allows it to penetrate the mucosal lining of the abomasum and suck blood. Each larva may consume 1 to 5 blood droplets per day. This doesn't sound like much, but if an animal has 1,000 worms in its abomasum, those worms can consume nearly a pint of blood in a week. This blood loss leads to anemia, resulting in poor performance and, if left untreated, could result in death.
The time it takes for an egg to progress through all larval stages to an adult and for the adult to begin laying eggs varies greatly with temperature and humidity. Under ideal weather conditions, the entire life cycle could be completed in as little as three weeks. And even though only about 2-10% of the eggs passed in the manure will reach the L3 stage, this still leaves thousands of infective larvae in a pasture with the potential to be ingested by a small ruminant.
To determine the extent of a parasite infection, producers often evaluate the level using FAMACHA scores or fecal egg counts. Keep in mind that FAMACHA scores only estimate the level of anemia within the animal caused by Haemonchus contortus. Other internal parasites do not cause anemia, so a small ruminant can have a high parasite load without showing signs of anemia. Fecal egg counts may not show the whole picture either. A small ruminant can have a high parasite load, but if those parasites have not reached the adult stage where they are producing eggs, the fecal egg count may be low even though parasite numbers present in the animal are high enough to warrant treatment.
Therefore, producers should follow an integrated approach, using diagnostic methods and observation to determine an animal's need for treatment. The 5-Point Check examines 5 areas: eye, jaw, back (body condition), tail (dag score), and nose. Using the FAMACHA scoring system, score the eye mucous membrane color for the level of anemia. Check the jaw for signs of fluid accumulation known as bottle jaw. Assess body condition for weight loss. Inspect the rear end for signs of diarrhea. And look at the nose for signs of nasal discharge.
Producers can also consider several parasite prevention methods. Divide pasture areas into smaller sections that will provide enough feed for the flock or herd to eat for 4 to 5 days before moving the animals to a new pasture. The goal is to stay ahead of the parasite life cycle by moving the animals before the larvae reach an infective stage. Animals could return to a pasture area previously grazed that year when the forage is at least six inches tall, or they have remained off the pasture for a minimum of 30 days.
Maintain at least 4 inches of growth in the areas where small ruminants graze to decrease the likelihood of ingesting internal parasite larvae. Graze with other species that don't share the same parasites as the small ruminants, such as cattle or horses, to break the parasite life cycle. And, save the "cleanest" pastures for young animals. These would be pastures that have had the longest rest period since sheep, goats, llamas, or alpacas grazed them, or fields where you made hay.
Understanding the Haemonchus contortus life cycle, recognizing signs of parasitism, knowing when to deworm, and following internal parasite prevention methods should help you better manage internal parasites in your flock or herd.
Resources:
- Sheep Production Handbook, Vol. 8, 2015.
- Naeem, M., et al. (2021). Ovine haemonchosis: A review, Tropical Animal Health and Production, 53, Article 19.
- Paddock, R. (2011). Haemonchus contortus in sheep and goats: An insidious killer, Indiana Animal Disease Diagnostic Laboratory Newsletter, Fall 2011
