WORM WARFARE

An Update on Small Ruminant Gastrointestinal Parasite Related Research



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+) Background Information

Gastrointestinal nematode parasites ("worms") are the leading threat to the health and productivity of goats and sheep in the southern United States. The warm humid climate and abundant rainfall provide ideal conditions for survival and transmission of two important worms: Trichostrongylus colubriformis and Haemonchus contortus. Also known as the wireworm and the barber pole worm, Haemonchus contortus is the most pathogenic, prevalent, and economically devastating worm of pastured sheep and goats. This worm is amazingly prolific: one healthy female wireworm can lay as many as 5,000 eggs per day! Once infective larvae hatch, they ride up grass blades with the dew, and await ingestion by a suitable host. After being eaten by a sheep or goat, the infective larvae attach to the lining of the abomasum, where they cause massive numbers of tiny bleeding ulcers. A small ruminant harboring as few as 1,000 Haemonchus contortus worms can lose 1/4 to 1/2 a cup of blood every day. If unchecked, the blood loss can be fatal. Periparturient ewes and does, and recently weaned kids and lambs are at greatest risk of succumbing to worm-related blood loss. Clinical signs of severe haemonchosis include weight loss, weakness, pale mucous membranes, mandibular edema, and unformed, pasty feces. Producers are faced with the increasingly daunting task of managing worms in their herds and flocks in order to maintain profitability.

For the past 40 years, frequent use of anthelmintics ("dewormers") has been the mainstay of worm control in sheep and goats. Producers and veterinarians have become complacent about controlling worms through management, and instead, have relied on the availability of a steady stream of inexpensive and effective dewormers. In the southern United States, it has been common practice to "deworm" (drench) all the sheep and goats at regular intervals of time to suppress worm populations. This "suppressive strategy" initially appeared successful, but unfortunately has proven to be profoundly unsustainable. We are now reaping the devastating long term consequences of this approach to worm control. Anthelmintic resistance to all classes of anthelmintics is becoming rampant in small ruminant worms, particularly in Haemonchus contortus populations.

+) Well Educated Worms

In 1964, only three years after thiabendazole became available in the United States, thiabendazole resistant Haemonchus contortus were being reported in sheep. Newer, more effective benzimidazoles such as fenbendazole soon became commercially available. By 1988, resistance to fenbendazole, as well as to thiabendazole and mebendazole, was reported in a Pennsylvania dairy goat herd. In 1990, ivermectin resistance was reported in a herd of Angora goats in Texas. This report marked the first documented proof of ivermectin resistance in the United States. In 1994, ivermectin resistance was reported again, this time in H. contortus populations in a flock of sheep in Louisiana. In 2000, worms with resistance factors to multiple classes of dewormers were found in a goat herd in Virginia. This finding was particularly alarming because it documented that a single population of worms could carry resistance factors against all three of the major classes of anthelmintics that are commonly used in sheep and goats: macrocyclic lactones (ivermectin, moxidectin), benzimidazoles (fenbendazole, albendazole), and imidazothiazoles (levamisole, morantel).

In 2000, multiple anthelmintic resistance patterns were found in the worms that parasitized goat herds at the University of Georgia and Fort Valley State University. The observation of multiple drug resistant worms in these studies provoked the obvious question: were these isolated observations, or did they represent a widespread resistance problem emerging in small ruminant worms across the southeastern United States? In order to address this question, researchers at the University of Georgia College of Veterinary Medicine and Fort Valley State University combined their efforts to conduct a large prospective study on 18 privately owned goat farms in Georgia in 2001. Criteria for inclusion included having 50 or more adult goats available for use in the study. Meat goats (primarily Boers) comprised over 90% percent of the study population. A common observation on many of these farms was a high stocking rate, and frequent use of dewormers. The study results were nothing short of alarming. Resistance to albendazole was documented on 14/15 farms, resistance to ivermectin on 17/18 farms, and resistance to levamisole on 6 of 18 farms. Multiple resistance to albendazole and ivermectin was detected on 14/15 farms, and to albendazole, ivermectin and levamisole on 5/15 farms. These findings supported the conclusion that multiple-anthelmintic resistant worms are the rule rather than the exception in many herds!

+) The Rise (and Downhill Slide) of Moxidectin

The findings of the 2001 study had at least one bright spot: moxidectin (CydectinR pour on used orally at 400 ug/kg) was found to be a highly effective dewormer on all 18 farms studied. CydectinR (extra label use) was steadily gaining popularity among producers at that time. Moxidectin was the anthelmintic of choice to use in "salvage" situations where animals were dying from worms because it was both safe and effective. Even though it is chemically similar to ivermectin, moxidectin was able to kill most ivermectin resistant worms because of it's superior potency. Parasitologists warned that moxidectin efficacy would be short lived by virtue of this chemical kinship, and recommended that moxidectin be used selectively and sparingly. The warnings went unheeded. Many producers started using moxidectin in a "suppressive fashion" in lieu of altering management practices that promoted worm build up in pastures and in the animals.

Although parasitologists speculated that it would not take long for ivermectin resistant worms to become moxidectin resistant, no one was sure of the time frame. A follow up study in 2003 provided some sobering insights. Seven of the original 18 farms studied in 2001 were evaluated to determine moxidectin sensitivity in the resident worm populations. Two moxidectin doses were evaluated: a low (100 ug/kg) dose, and a "high" dose (400 ug/kg). On all 7 farms, ivermectin resistance had been documented in 2001, and moxidectin had since been used as the primary anthelmintic to control worms. Two additional farms were added to the study as controls. Moxidectin had not been used on the control farms. One of the control farms had worms known to be ivermectin sensitive, and the other control farm had worms known to be ivermectin resistant. The data revealed low level moxidectin resistance on all seven farms where moxidectin had been used extensively (in a suppressive fashion) for the past few years. Three of these 7 farms also had resistance to the "high" dose of moxidectin. On the control farms, worms were still sensitive to both doses of moxidectin. This study proved the existence of moxidectin resistance in caprine worms in the United States. One lesson that can be taken from this study is that moxidectin resistance can emerge in populations of vermectin resistant worms within several years of using the dewormer in a nonselective, frequent fashion.


Important Aspects of "Smart Drenching"


+) Resistance Testing

Since anthelmintic resistance is becoming the rule rather than the exception in the southeastern United States, it is difficult to know which class of drugs will perform well in a particular herd without appropriate testing. A farmer can test the resident GIN for anthelmintic sensitivity by using the fecal egg count reduction test (FECRT), or by sending a composite fecal sample to a laboratory capable of performing a larval developmental assay (LDA).

The FECRT involves dividing the goats in the herd into groups (i.e., albendazole, ivermectin, levamisole, moxidectin and control group) that contain at least 6 animals. The goats are drenched according to the group designation, and individual fecal egg counts are performed 10-14 days later. The group average is then compared to the control group average for percent reduction in the egg count. The LDA (Drenchrite test) is an in vitro test that involves hatching out larvae from a composite fecal sample, and testing them for vulnerability to levamisole, a benzimidazole, and avermectin. The test is expensive ($125) but well worth the money, considering how costly it is to lose animals to parasitism and to buy drenches (anthelmintics) that are ineffective. Dr. Ray Kaplan at the University of Georgia College of Veterinary Medicine is offering the Drenchrite test through the Diagnostic Laboratory.

+) Use Selective deworming rather than whole-herd treatment

Selective (A.K.A. "targeted") deworming involves treating individual animals that show signs of parasitism, rather than treating the whole herd or flock at regular intervals. The producer must routinely evaluate each animal in the herd or flock for body condition, and degree of conjunctival pallor. Only the pale and/or thin animals with pasty feces are dewormed. Worm burdens are not evenly distributed among host populations. As a general rule, about 80% of the parasites are harbored by a small percentage (20%) of animals in any given herd or flock. Typically, the animals with the highest worm burdens also show the most clinical signs of parasitism. So, treatment of the most clinically affected animals will significantly decrease worm burdens on pasture.

In Haemonchus infected small ruminants, the degree of conjunctival pallor correlates well with the degree of anemia and with the magnitude of the worm burden in the animals. Recently, a system dubbed "FAMACHATM" was developed in South Africa to allow identification of anemic sheep and goats. Producers compare the color of the animal's lower conjunctiva to pictures on a color coded FAMACHATM card. A FAMACHATM score of 1 indicates the conjunctiva is red (not anemic) whereas a FAMACHATM score of 5 indicates the conjunctiva is extremely pale-to-white (very anemic). Small ruminants that score as 3s, s and 5s are generally treated, and those in categories 1 and 2 are left untreated. Weekly checks are needed during periods of high worm transmission (late summer and fall after a period of heavy rainfall) and less frequently (every 2-3 weeks) during other times of the year. Use of the FAMACHA System in South Africa has resulted in 50% less animals being drenched. This reduction in drench usage benefits the producer in 2 ways. The most obvious benefit is it saves money on anthelmintics. Of even greater long term importance is that it leaves a portion of the parasite population unselected by the dewormer. This unselected parasite population is called refugia; it provides a pool of sensitive genes that will dilute out the resistant genes in the worm population. Although the system seems elementary, training is necessary to understand the principles and use the FAMACHATM System effectively. People using the System need to understand that incorrect application of the principles could result in animal mortalities. FAMACHATM training work shops are being organized and conducted throughout the southeastern United States. A work shop is being planned for August, 2005 in Athens, Georgia. For more information on FAMACHA, contact famacha@vet.uga.edu

+) Dose accurately.

Weigh animals and dose them according to weight to insure a therapeutic dose of anthelmintic is given. Goats metabolize anthelmintics more rapidly than cattle and sheep. Therefore, all anthelmintics should be administered to goats at 2X the sheep/cattle dosage with the exception of levamisole. Because the toxicity of this drug is dose dependent, use only 1.5X the cattle/sheep dose when using levamisole in goats.

Dose orally.

All anthelmintics should be administered by the oral route in order to avoid creating long post treatment periods where the anthelmintic is present in the body in sub-therapeutic concentrations.

+) Use slow (annual) rotatio

Rotation amongst effective classes of anthelmintics rather than rotation with each treatment. It is important to realize that many drenches are being used in an extra label fashion (particularly in goats), and therefore should be under the direct advisement of a veterinarian. FDA guidelines for extra label drug use should be closely followed in these situations.

+) Use pharmacokinetics to maximize efficacy of the benzimidazoles.

The efficacy of the benzimidazoles is improved by administering a second dose 12 hours after the first dose. The second dose keeps the drug level in the "killing zone" within the gut for a longer period of time, thereby increasing the efficacy of the drug. A recent study showed that that fenbendazole efficacy increased from 50% (single dose) to 92% when 2 doses where administered 12 hours apart.

+) Use 2 classes of anthelmintics when necessary.

There is substantial evidence that treating with anthelmintic combinations can increase the efficacy of treatment as compared to that achieved with individual drugs. This increase in efficacy is probably achieved by 2 separate mechanisms: (1) a synergistic effect whereby each drug weakens the parasite sufficiently to kill it in combination; and (2) parasites that carry alleles for resistance to 1 drug, but not to the other unrelated drug.

+) Adopt Common Sense Management strategies

Strategies that decrease parasite transmission Avoid overgrazing. Goats should be allowed access to browse as an alternative to grazing. Stocking rates should be kept low (5-7 small ruminants per acre). Producers that fail to follow this simple guideline are fighting a "no win" situation Co-grazing. Species such as adult cattle and horses do not share small ruminant worms. The horses and cattle will ingest infective Haemonchus larvae (but will not establish an infection), thereby making them less accessible to the small ruminants.. Pasture rotation. Free-living larvae cannot survive for more than 3 months (generally less) in hot dry conditions, or for more than 6 months in cooler moist conditions. If inadequate pasture is available for rotation, then small ruminants can be removed from pastures and placed in dry lots or barns during times of peak parasite transmission (warm, wet times of the year) to break the parasite's life cycle.

Do NOT treat all the goats and then move to a new "clean" pasture. This recommendation is a completely opposite of what was being preached a few years ago. The main problem with the "treat and move" plan is that the only worms repopulating the new pasture are the resistant ones! Browse rather than graze. Worms can not climb tees, shrubs, and vines, so these food sources are virtually parasite free. Investigate the best ones iin your geographic area to plant for goats.

Dry lot, or indoor housing during periods of high transmission. Prevent accumulation of worms by keeping animals off contaminated fields. Dick Henry (Bellwether Solutions, Tallahassee, Florida) runs a large sheep operation. He built shelters with slatted floors to house his lambs in over the summer and fall. Even though he had to feed hay, this housing plan was more profitable than maintaining them on pasture because Haemonchus related losses were extremely high in pastured animals. The manure collected from the stabled sheep provided an additional source of income.

Good health care. Disease free small ruminants receiving excellent nutrition are less vulnerable to parasitism.

+) Prevent introduction of resistant parasites through new acquisitions.

"Diseases are not free, they are bought and sold…" Mary Smith. This quote is especially applicable to worm resistance. A producer can create resistance within a herd through overuse of dewormer, but it is quicker to introduce it through new purchases. All new goats should be isolated in a stall or on dry lot, and dewormed with oral moxidectin and levamisole. Fourteen days after treatment, perform a fecal egg exam. If the feces are free of ova, allow the animal into the herd. If eggs are detected, do not integrate the goat into the herd.


Novel Worm Control Strategies

In response to the growing threat posed by worms to the future of the small ruminant industry in the southern United States, a international group of parasitologists, research scientists, veterinary clinicians, and producers formed a research group called the Southern Consortium for Small Ruminant Parasite Control (SCSRPC). Funded in 2002 by SARE, the Consortium began researching new non-anthelmintic worm control methods for sheep and goats. More information about the Consortium and their projects can be viewed at their web site http://www.scsrpc.org/. There are no "silver bullets" on the horizon for control of parasites, but there are a few promising products under investigation that might be useful for producers using an integrated approach to worm control.

+) Nematode-Trapping Fungi

Studies conducted in Denmark and the United States are investigating the ability of nematode trapping fungi such as Duddingtonia flagrans to reduce the availability of infective worm larvae. The beneficial fungi had previously been shown to reduce the infective larvae in fecal pads from cattle and horses. Fort Valley State University recently reported that feeding D. flagrans spores to does resulted in a significant reduction (93.6% at highest dose) in the development of infective larvae in fecal cultures during the feeding trial (Terrill TH, 2004). Daily feeding was necessary to achieve optimal results. Three to 6 days after ceasing the spore treatment, the number of infective larva rebounded. They concluded that D. flagrans was effective in reducing the development of L3 larvae under controlled conditions, and that it holds promise as a biocontrol agent for parasitic nematodes in goats.

+) Dewormers

At Pearson Pond, we use the following medications, but consult your veterinarian.

  • Ivomec injectable or Dectomax injectable
  • Safeguard or Panacure, oral (paste or liquid)
  • Valbazen, oral, not to breeding llamas
  • Corid or Albon, for Coccidia
  • Quest paste for horses is excellent
  • Cydectin
  • Marquise paste for emac - not a dewormer, emac is not a parasite

+) Dietary Condensed Tannins (CT)

Condensed tannins have been shown to have an antiparasitic effect. Plants such as clover, vetches, chicory, and sericea lespedeza contain condensed tannins. Research is currently underway to test the effect of 'AU Grazer' sericea lespedeza hay on sheep and goats infected with Haemonchus contortus. Preliminary data shows that goats consuming sericea lespedeza hay while being experimentally infected with H. contortus larvae maintained higher packed cell volumes and lower fecal egg counts (reduced by 80%) compared to controls. The tannins appear to exert their antiparasitic effect by reducing the number of abomasal worms, and by reducing fecundity. The main problem with implementing the condensed tannins as parasite control substance is that it is not very palatable. A good way of getting the condensed tannin into the goat or sheep needs to be developed.

+) Support Care with Dewormers if necessary

  1. FIBReviveTM ( High-fiber formula for use in camelids, macropods, goats, sheep, and livestock when illness, injury, or surgery prevents normal feeding behavior.)
  2. Red Cell Solution, if anemic
  3. MasterPlan Llama Minerals Chelated with Probiotics
  4. Probiotic - Master Blaster MEGA Probiotics Symbiotics Paste ( 1 billion micro organisms)
  5. Fortified Multiple Vitamin B Complex



We welcome visitors by appointment , call or write
Jack and Tracy Pearson:
Pearson Pond Ranch & Llama Co.
242 Llama Lane (Charles Lane), #6017
Ellijay, GA 30540
Phone: (706) 276-3658
Fax: (706) 276-3680