By Lisa Williamson, DVM, MS, DACVIM
Associate Professor of Large Animal Medicine
University of Georgia College of Veterinary Medicine
Haemonchus contortus, also referred to as the ‘barberpole worm,” is achieving notoriety as a major health threat to alpacas. In temperate parts of the world, Haemonchus contortus is a major killer of sheep, goats, llamas, and alpacas. This worm does three things extremely well: it drinks the host’s (alpaca’s) blood, it makes lots of eggs (females lay up to 500 eggs per worm per day), and it is highly adaptable in the face of challenge by dewormer treatments. Barberpole worms reside in the glandular aspect of the third compartment, where they are visible to the naked eye on necropsy. The female worms are responsible for the name, “barberpole worm,” as the white and red intertwined structures are the ovary and digestive tracts, respectively. The males are smaller, and appear red.
A recent study conducted on privately owned alpacas in the southeastern United States showed that Haemonchus contortus was a common isolate on alpaca farms, but the amount of disease in the herd associated with its presence varied widely, from practically no recognizable disease among the herdmates, to observation of severe anemia, weight loss, and death among herd members. Those observations illustrate an important point: parasitism is a natural state, but disease from parasitism is not. Low-level burdens of most parasites are well tolerated by hosts such as alpacas. Intervention is necessary when the burden is significant enough to cause recognizable health issues. The clinical disease caused by heavy Haemonchus contortus burdens is termed “haemonchosis.” Since this worm feeds on blood, the clinical sign that distinguishes it from other nematode infections is profound anemia (blood loss). Clinically, inner eyelid pallor is noticed in clinically compromised alpacas. Blood work reveals a low hematocrit, and reduced blood proteins. Weight loss/poor body condition also reflects a chronic health problem.
Risk factors associated with clinical disease include weather factors and management factors. Wet, warm conditions favor transmission of Haemonchus contortus. During periods of drought, transmission is low, and producers are often lulled into a sense of false security. In some herds that we followed for inclusion in the study for several years during a period of sustained drought, haemonchosis was not apparent until several weeks after a period of heavy rainfall. That finding was not surprising. Water liberates infective larva from eggs in dry fecal pellets, and also disperses fecal material and larvae away from the communal dung piles, thereby contaminating large areas. In parts of the country where temperatures get into the freezing range for sustained periods, transmission declines significantly. The onset of cold weather triggers a decrease in egg production, and developing worms in the lining of the third compartment become dormant over the cold winter months. Development to adult blood-feeding worms resumes in the spring.
Management factors associated with greater risk of clinical disease from haemonchosis include high pasture stocking rates, lax sanitation practices (dung pile are not being removed at regular intervals, and spreading feces on pastures), having pregnant/lactating females and young animals, feeding suboptimal dietary protein sources, and lack of biosecurity. You might say, well heck, we need to make babies to be profitable, and you will not get an argument from me! However, you need to realize that production needs to be accompanied by management practices that reduce risk of parasitism. In particular, I want to emphasize that moving animals from farm to farm without utilizing quarantine and testing/treatment protocols prior to allowing an alpaca into the herd is very risky, as resistant worms can be spread through this practice. Our ability to kill these worms with dewormer agents is becoming increasingly more challenging. We term this phenomenon “anthelmintic (dewormer) resistance.” As more and more of the worms in a given population become refractory to treatment, the level of resistance increases. Laboratory tests can detect resistance in a worm population when it is in relatively early stages, but by the time 50% or more of the worms are resistant to the dewormer, the producer will recognize the treatment failure based on poor response in the animals.
Multi-anthelmintic resistance (i.e, resistant to more than one class of dewormers) in sheep and goat Haemonchus contortus isolates has reached critical levels in many parts of the world, including my backyard in north Georgia. Camelid H. contortus isolates are showing similar trends. Larval developmental assays were performed on Haemonchus contortus isolates harvested from 32 camelid farms in the southeastern United States by Dr Ray Kaplan’s laboratory at the University of Georgia College of Veterinary Medicine. This work was funded by the Morris Animal Foundation, and by the Alpaca Research Foundation. The results were nothing short of alarming: dewormer resistance to the benzimidazoles (white dewormers like Safe-Guard®, Panacur®, Valbazen®), ivermectin, levamisole, and moxidectin was found on 100%, 88%, 22%, and 22% of the farms, respectively. Total anthelmintic resistance (resistance to all these dewormers) was identified on one camelid farm! A “take home” message is that moxidectin would be a reasonable choice for treating an alpaca suffering with haemonchosis, whereas treatment with ivermectin or a white dewormer (benzimidazole) is less likely to be effective in situations where the dewormer sensitivity pattern is unknown. We currently recommend that moxidectin be given orally at a dose of 0.4 mg/kg, and that a product formulated for oral use be used, ie Cydectin Oral Sheep®, rather than a moxidectin product formulated for topical use. Nonselective use of moxidectin on a whole-herd basis is strongly discouraged, as resistance to this dewormer will be the end result! This is not a theoretical statement. The evolution of small ruminant Haemonchus contortus isolates from moxidectin-sensitivity to moxidectin-resistance has been documented and it only took a couple of years of nonselective use of moxidectin to render it useless.
The reasons why the problem of dewormer resistance continues to emerge in Haemonchus contortus isolates goes back to basic biology: the process of natural (or in our case, unnatural) selection! Every time you “deworm” an alpaca, you place the worms in that alpaca under selection pressure. The old practice of deworming all the alpacas on the farm at the same time for Haemonchus contortus contol has been shown to be a very poor practice, as it rapidly eliminates all the susceptible worms, leaving only resistant worms to repopulate. Insult is added to injury by the recommendation of moving the newly treated herd to a “clean” pasture (field with little parasite contamination), as only resistant worms will contribute to the next generation of worms in that location. Believe me, you will not kill them all!
The currently recommended strategy for control of Haemonchus contortus revolves around the practice of frequent hands-on herd checks, and dewormer treatment ONLY of animals with signs of disease (not the whole herd). Selective use of an effective anthelmintic successfully reduces pasture contamination, alleviates disease, and does one more crucial thing: it preserves some worms that are still susceptible to the dewormer. These worms will pass on this drug susceptibility to subsequent worm generations. These susceptible worms that were spared exposure to a dewormer are called “refugia”. The size of the refugia influences the rate at which dewormer resistance develops; the larger the refugia, the slower the evolution of dewormer resistance.
Selective deworming decisions are made on physical and/or laboratory observations. Since Haemonchus contortus is a voracious blood feeder, anemia is a key clinical sign. Additionally, body condition is a good indicator of overall health, including parasitic burdens. In a future issue, we will discuss how to physically assess the herd in more detail, and discuss laboratory parameters that are relevant to control of Haemonchus contortus in alpaca herds, including how to determine if a dewormer is still effective, or not.