First, let me clarify that I am not a veterinarian. I am not a nutritionist nor a scientist nor a formal researcher. I am just a girl that loves goats. I want to take care of them to the best of my ability, and so I strive to LEARN. I like to share what I learn, but please do check out all the resources provided. This piece was originally published in the Winter 2018 KGBA Newsletter. I revised it as of 02/2020.
Most goat owners know and dread the name “Barber Pole Worm.” Though small ruminants can be infected by a wide variety of internal parasites, the Barber Pole worm, scientific name Haemonchus contortus, is one of the most prevalent and economically important gastrointestinal nematode worms.
Dr. Steve Hart, a research specialist and extension goat specialist of Langston University, recently wrote a six-part article on the goat’s common internal parasites. Dr. Hart states, “One cannot eradicate worms on your farm; you have to learn to live with them and use management to control them to levels which do not harm the animal population.” A low level of internal parasites triggers the goat’s immune system to respond. An immune system response can keep the parasite infection limited, or a particularly strong response can eliminate internal parasites altogether. Excessive parasite infestations, however, can be problematic. Understanding the life cycle and biology of worms, especially the too-common Barber Pole worm, is critical to keeping their population in check.
A Barber Pole worm is classified in the taxonomist group trichostrongyles, or round worms, and is also called a twisted wire worm, or simply the wire worm. The adult Barber Pole worm is visible to the naked eye, and is about the diameter of a paper clip. Adult females are 20 to 30mm long and males are around 15mm long. The female displays the barber pole design – the white stripe contains her sexual organs, and the red stripe contains her intestinal tract, usually red from feeding on the host’s blood. The male Barber Pole is solid-colored, usually pink or red. Both lose coloration when exposed to air. The average life span for an adult Barber Pole is 4-6 months, but there is evidence the parasites can live over a year if conditions are right.
The Barber Pole has a voracious appetite, sucking half to one drop of blood per day, even though its mouth is only 5 micrometers in diameter – about the same diameter as an average capillary. A drop (.05cc) may not seem like much, but consider that only 1000 worms can suck two ounces of blood per day, or two quarts in a month.
The female is an egg-laying machine, producing anywhere from 2000 to 10,000 eggs in a single day. Most sources report the average eggs laid per day at around 5000. A goat with as few as 500 adult female Barber Pole worms can shed 2,500,000 eggs in a single day.
Haemonchus contortus is especially concerning to producers because the life cycle can complete in as little as four to five weeks in optimum conditions. The Barber Pole worm is a tropic worm, thriving in heat and humidity. These worms are less of an issue, or are a concern for a shorter duration, in areas that receive less than 25 inches of rain annually.
Eggs can hatch in as little as four to ten days of the fecal pellets being deposited into the environment. Eggs cannot hatch into first stage larva – L1 – without adequate moisture and warm temperatures. Ideal hatching occurs at about 86°F and with around a half inch of moisture or at least 60% relative humidity. If hatching conditions are not met, eggs can die in an inhospitable environment – very hot, dry, or cold. Fewer eggs hatch at temperatures lower than 50°F or higher than 96°F. If the pellet is heated over 100°F, nearly all of the eggs and larvae will die. Eggs can survive 5-21 days.
If the egg does hatch, the L1 larva travels through the fecal pellet, eating bacteria and growing. It molts and develops into a second-stage larva – L2 – that also lives inside the pellet, eating bacteria and growing. These two stages of larva are more susceptible to drying out and dying than either the eggs or the infective-stage larvae.
If the L2 larva survives and molts, the larva enters the L3 stage – this is the larva the goat picks up and ingests. The Barber Pole can grow from egg to infective L3 larva in as little as four to six days. However, the molt is incomplete. L3 larvae have only partially shed their protective cuticle. Partial protection makes them less susceptible to drying out than previous stages, but according to the article “Managing the Barber Pole Worm” by Dr. Steve Hart “…the partially shed skin covers his (L3 larva) mouth which prevents him from eating. This means that he must live off his body stores until he gets into your goat. If his body stores run out before he gets into your goat, he dies.”
L3 rely on moisture to soften the fecal pellet enough to emerge, and to migrate from the pellet to migrate from the pellet to forage using random movement, especially utilizing dew drops. Infective larvae usually do no move more than a few inches vertically and horizontally from the fecal pellet. In warmer conditions, the cold-blooded larvae wiggle more, using more energy reserves which thereby shortens their life spans. In cooler conditions, L3 move little and can live much longer, sometimes months. This table from WormBoss illustrates this - Source: Modeled from death rate of the L3 population in ‘Simulation of pasture larval populations of Haemonchus contortus’ by IA Barger, PR Benyon & WH Southcott. Proceedings of the Australian Society of Animal Production (1972) 9: 38
If an L3 larva is ingested by a goat, it travels to the abomasum. Along the way, it completely molts its protective, partially-shed skin, becoming an L4 stage larva. The exsheathment process is important because it initiates parasitic infections and is host-specific. Environmental conditions in the rumen trigger signals to the L3’s nerve ring, which start the process. If the conditions are not favorable or the animal is not a ruminant, the larva does not shed the cuticle and eventually perishes in its uninhabitable host.
If the host environment is favorable, the nerves stimulate hormones. According to “Parasites
and Parasitic Infections of Domestic Animals” –
"These hormones act directly on the excretory cell to stimulate the uptake of water which, in turn, activates enzymes…Activated enzymes, including leucine aminopeptides, are released into the excretory duct and pass into the space between the two cuticles. Enzyme action weakens the cuticle (A). The cuticle breaks at this point and the anterior detaches as a cap (B). The exsheathed L3 (C) then wiggles out leaving the sheath behind (D)." This is why single-stomach animals like horse are a dead-end host for Barber Pole worms - without the proper host environment, the ex-sheathement process cannot take place.
Able to feed now, the L4 attaches to abomasum, and blood forms a clot around the larva. It develops sexual organs while feeding inside the blood clot. The larva emerges from the clot in about three days, and molts into a fully-mature adult. The adult Barber Pole worm now attaches to the mucosa lining of the abomasum and begins sating its gluttonous appetite. It mates, and females lay eggs in as little as two to three weeks.
Sometimes, the development of the L4 larva becomes arrested. It can enter a dormant stage, residing in the abomasum but not maturing; this is called a hypobiosis. L4 dormancy is ultimately a survival mechanism, enabling the Barber Pole worms to survive the winter when the eggs are not hatching and adult worms are dying from old age. Dr. Steve Hart writes that, “The arrested larvae are in reserve, safely nestled down in the glands in the stomach where they do not trigger an immune response…The main trigger for the arrested form to develop into an adult is kidding or lambing. The exact mechanism for triggering the development of the arrested worms is not known, although, longer day length, kidding, lactation and green grass are all supported by scientific evidence.” The rise in gastrointestinal parasites around kidding is referred to as the periparturient rise and can occur up to two weeks before kidding and as long as eight weeks after.
Understanding the biology and life cycle of Haemonchus contortus is essential, and especially important in terms of pasture management strategies and utilizing the most effective deworming strategies.
References –
“Parasites and Parasitic Diseases of Domestic Animals” - Colin Johnstone PhD, BVMS, MRCVS
"Understanding the bioactivity of plant tannins: developments in analysis methods and structure-activity studies" - Marcia Engström (Source for the LifeCycle graphic).
“Biology of Parasites” Anne M. Zajac, DVM PhD, Dipl. ACVM-Parasitology
“Parasite Information Session” by Dr. David Fernandez – used with permission from Goat Vet Corner