Part 1: A parasite primer
Anyone who raises sheep or goats is well aware of the parasite issues that come with those species. Dr. Andrew Weaver, assistant professor and Extension small ruminant specialist at North Carolina State University, provided up to date information on how to best manage small ruminant parasites.
Lots of different worms can affect animals, said Weaver. Many fall into a family of parasites called strongylid nematodes. That includes Haemonchus contortus, Osterstagia, Trichostrongylus species, Cooperia and Nematodirus. We also have to worry about protozoan parasites like Eimeria (coccidia).
The parasites that cause the most challenging issues for sheep producers are H. contortus and Eimeria. Sheep producers in the Northeast report that Haemonchus is the most prevalent and is responsible for the most parasite-related economic losses.
Haemonchus is a blood feeder, and during one stage of its life uses the tooth-like structure (lancet) on its mouth to slash the walls of the abomasum – the fourth compartment of the ruminant stomach. This results in blood release, which the worm feeds on. Haemonchus gets its name “barber pole worm” from the appearance of the worm’s blood-filled intestines wrapped around the reproductive structures of the female worm.
The result of Haemonchus infection is severe anemia in the host, which leads to decreased performance and, in severely infected animals, death. “This is a highly prolific parasite,” said Weaver. “A single female can release thousands of eggs in a day. That female can mate with multiple males, so not only is she releasing thousands of eggs, those eggs can be fertilized by multiple males.” The ability to release so many eggs plays a role in resistance to dewormers, and anything producers can do to manage Haemonchus and minimize infection level in animals will improve performance and the bottom line.
Understanding the lifecycle of parasites is critical to successful parasite management. Haemonchus eggs are deposited in feces and hatch into L1 larvae within a day or two. Larvae reside in the environment and consume bacteria, and as they grow, they molt and enter the L2 stage. L2 larvae continue to eat and grow, then molt into L3 larvae.
“The L3 stage is the only infective stage to the host,” said Weaver. “The sheep can consume L1 and L2 larvae and will never get infected – only the L3 infect the host. The lifecycle of this parasite does not continue after the L3 stage until it’s inside the host.”
A unique characteristic of L3 larvae is its secondary covering, a sheath, that protects it from harsh environmental conditions. The covering also prevents the worm from feeding. “At the L1 and L2 stage, the worm is storing energy reserves,” said Weaver. “Whatever they have when they get to the L3 stage is all they’re going to get because they can no longer feed. Anything we can do to extend the time L3 larvae are on pasture before they are consumed increases the chance of the L3 using up energy reserves and eventually dying.” Producers can take advantage of this trait and incorporate it as part of a management strategy.
L3 larvae have a fairly limited lifespan, and can be found in dewdrops on grass blades. Weaver said dew is usually heaviest in the evening, overnight and early morning, which corresponds with ruminant livestock grazing periods.
When L3 larvae are consumed, they find the abomasum where they molt into the L4 stage. “The L4 larvae feed on blood just like the adults,” said Weaver, “but they are not sexually mature and do not produce eggs. We can have L4s in animals, consuming blood and causing anemia, but a fecal egg count wouldn’t reveal any eggs.” L4 larvae continue to develop and mature into adult worms that will become sexually mature and egg-producing.
Between the time L3 larvae are consumed by the host until eggs are visible in a fecal sample is about 21 days. “There’s essentially a three-week lag time between when the parasite is consumed from the environment to when we can actually observe eggs in a fecal sample,” Weaver said. “What we’re seeing in a fecal today is what they consumed three weeks ago or maybe more – not what they consumed yesterday.”
Outside the host, the L3 survivability timeframe is highly variable (usually between seven and 14 days). But Weaver said during hot, humid weather, larvae can mature faster, sometimes in three to four days. During cooler, drier weather, maturity can take longer. Adult Haemonchus does not bind to the epithelial surface of the abomasum – they must swim and remain in continuous motion to maintain their position within the gut. Weaver said this is important when considering the way in which dewormers and the sheep immune system works to mitigate parasite infection.
“Anything that prevents parasites from moving will result in their expulsion from the GI tract and removal from the host,” said Weaver. “It’s a common misperception that these parasites attach – they do not attach. They have to maintain a swimming motion to maintain position in the gut.”
Another characteristic of Haemonchus is its ability to enter hypobiosis, a state of arrested development. “The parasite senses environmental conditions are not favorable for continued development,” said Weaver. “As we enter winter months when larval survival in the environment is quite low, or times of extreme drought when the likelihood of completing the lifecycle is low, the parasite can enter hypobiosis where they hang out in the L3 to L4 stage. They set up camp and hang out until the environment becomes more favorable for their continued development.”
Weaver said hypobiosis allows parasites to survive harsh winter conditions but isn’t well understood. There’s data suggesting reproductive-related hormones are involved to signal favorable conditions for survival. During the periparturient rise, there’s a temporary loss of natural immunity to internal parasites associated with an increase in fecal egg count and parasite burden in adult female sheep. This rise is associated with the stress at lambing time.
The first key to managing parasites is to correctly identify the problem. Weaver suggested understanding traits associated with parasitism and what can be measured. Start with deworming records: track treated animals, note which animals require multiple treatments and which animals clear up immediately. Track trends regarding the time of year deworming is taking place and determine whether products used are effective.
Part 2 will include assessing treatment options and using FAMACHA.
by Sally Colby