Nineteen percent of pro-ducers used anthelminticsexclusively in parasite man-agement. Eighty percent usesome form of pasture restand/or rotation, 31 percentgraze fields, and 7 percentare attempting to selectsheep and goats that areresistant to parasites.

Characterizationof Haemonchuscontortus

Although numerous gas-trointestinal nematodes arepresent, Haemonchus contor-tus presents the greatestconcern in Texas sheep andgoats. Common names forHaemonchus contortus

include stomach worm,Barber’s pole worm and wireworm. The adult worm,found in the abomasum, is10 to 30 mm in length.Females are very prolific,each capable of producing asmany as 5,000 eggs daily.The life cycle (egg to matureadult) is 17 to 21 days(Figure 1). Warm, moist soilsurface conditions favorpropagation while hot, dry orextremely cold conditionsare detrimental to larvae sur-vival.

An important adaptationof Haemonchus contortus isthe ability to overwinter inthe abomasum of its host ina dormant state known ashypobiosis. Ingested larvae

begin undergoing hypobiosisas they are acquired duringthe autumn, September andOctober. These larvae donot feed or lay eggs and dono damage to their hostswhile in this state. However,when the ewe or doe repro-duces, the worms receive sig-nals that it will soon bespring and they resumedevelopment. The lactatingewe or doe is unable to ridherself of the now adultworms and large numbers ofeggs are shed into the pas-tures. When the weatherwarms up, larvae will bewaiting for the newborn aswell as the ewe or doe toingest them.

While most of the wormsin hypobiosis survive, thelarvae in the pastures dieoff, especially if it is a drywinter. Therefore, most ofthe worm population is inthe host, not on the pastureas is the case during most ofthe year. The fact that lactat-ing ewes or does are unableto control adult parasite pop-ulations as well as dry ewesor does enables the worms toproduce more eggs over alonger time span further con-taminating the pasture. Thelarvae develop more slowlyin the late winter or earlyspring but survive until thereally hot dry summerweather conditions desiccatethem.

Signs of infection includeacute anemia, edema (bottlejaw), weak and listlessbehavior, and ultimatelydeath. Diarrhea frequentlyoccurs with heavy infesta-tions. Young animals andfemales 30 days either sideof parturition are most vul-nerable.

Figure 1. Life cycle of Haemonchus Contortus

Sources ofResistance

Anthelmintic productsavailable to Texas sheep andgoat producers are limited innumber. New product devel-opment, relative to the size ofits market, is cost prohibi-tive. Frequency of document-ed cases of resistant popula-tions is increasing. There-fore, resistance managementwarrants serious considera-tion.

Several scenarios canresult in resistance develop-ment. These include:

1. Insufficient dose. Themargin of safety for allapproved products is wide,at least twice the recom-mended dose. Therefore,the dosage selected for allanimals should be appro-priate for the heaviest ani-mal in that group(grouped by weight). Oncean appropriate dose isselected, equipmentshould be checked periodi-cally to calibrate theamount of product beingdelivered. Delivery shouldalways be checked whenchanging to a product ofdifferent consistency.Underdosing might save afew cents in the short termbut can be quite costlyshould resistance develop.

2. Inappropriate route ofadministration. Anthel-mintics available to live-stock producers may bedelivered in one or more ofthe following forms: oraldose, subcutaneous injec-tion, pour-on, or feed addi-tives. The appropriatemethod for sheep andgoats is oral administra-tion of products designedfor oral delivery. Followlabel directions.

3. Ineffective compound.Anthelmintics available toU.S. producers can bedivided into three groupsaccording to active ingre-dient (Table 1). Since twoof the groups contain onlyone compound, the con-cern of using an ineffec-tive compound is potential-ly more important in thebenzimidazole group. Useof an ineffective product isa waste of producer’smoney and could lead tothe development of resis-tance.

Rotate products annuallyor until resistance develops.When rotating products, theappropriate rotation isacross classes of compounds(not within a class of com-pound). For example, rotatefrom a benzimidazole toavermectin to imidothiazole,not from thiabendazole tofenbendazole to oxfendazole.

4. Massive re-exposure.Deworming animals andreturning them to a heavi-ly infested pasture is anexercise in futility.Animals will immediatelybegin the reinfectionprocess. Grazing manage-ment (pasture rotation) isan integral part of aninternal parasite manage-ment plan. Animals withsignificant worm burdenscan continue to shedviable eggs for severalhours or days after anthel-mintic administration. Ifpossible, hold treated ani-mals in the pen for 48hours post-treatment andthen release them to anuncontaminated pasture.

Table 1. Anthelmintics available to U.S. sheep and goat producers. Only three compounds are approved for use in small ruminants.

Class of Compound Trade Name Efficacy againstActive Ingredient Haemonchus contortus

Avermectin

ivermectin* Ivomec® +++

Benzimidazoles

albendazole Valbazen® +++

fenbendazole Safe-Guard®, ++Panacur®

mebendazole Telmin® ++

oxfendazole Synanthic®, ++Benzelmin®

oxibendazole Anthelcide® ++

thiabendazole* TBZ® +

Imidothiazole

levamisole* Tramisol®, +++Levasol®

*These products are approved for use in sheep and/or goats.

ResistanceManagementStrategies

Prevention, rather thancure, is the philosophy usedin developing managementprograms for Haemonchuscontortus. It must beassumed that this parasitecannot be eradicated but canbe limited to the extent thatit does not cause economicloss to the producer. The fol-lowing strategies attempt totake advantage of livestockmanagement and parasitepopulation dynamics whenimplementing a managementplan.

Strategic – when ananthelmintic is administeredwhile the parasite populationis concentrated in the host,transmission rate is veryslow and pasture contamina-tion is at an annual low. InTexas, the most appropriatetime of year for implement-ing a strategic treatment isafter the first hard killingfrost and before spring

greenup. The ideal treat-ment window for most of thestate is December 15 - Feb-ruary 15. The primary tar-gets of this mid-winter treat-ment are the hypobiotic lar-vae in the host. Therefore, aproduct labeled for inhibitedlarvae (L4 stage) must beused.

Another appropriate timefor strategic treatment iswhen animals are beingmoved from a contaminatedpasture to an almost para-site-free pasture. A pasturecan be considered relativelyfree of Haemonchus contor-tus larvae if: 1) it has beentilled since sheep and/orgoats last occupied the pas-ture or, 2) the pasture hasbeen completely vacant ofsheep and/or goats for atleast three months duringthe spring/summer or earlyfall or for at least six monthsduring the fall and winter.For many sheep and goatoperations with native rangeas a production base, defer-ring pastures for this lengthof time may not be feasible.

Tactical – when weatherconditions have been favor-able for development of para-sites. Moisture (heavy dewor rain) must be present onthe soil surface to facilitatelarvae movement from thefecal pellet to the forage.The purpose of the tacticaltreatment is to eliminate theactive worms in the gutbefore they have the opportu-nity to further contaminatethe environment. Timing ofthe tactical treatment maythen be based on 1) recentprecipitation and warmweather or, 2) increasingfecal egg counts.

Fecal Egg CountsFrequently during the

spring, summer, and earlyfall, based on subjectiveobservations, internal para-sites are cited as the causeof poor livestock perfor-mance. While parasites arefrequently the culprit, otherperformance inhibitors doexist. Fecal egg counting isa practical, cost-effectivediagnostic tool for determin-

Coccidiaoocyst Haemonchus

contortus

Nematodirusspathiger

Figure 2. Common parasite eggs and oocysts found in sheep and goat feces.

Table 2. Treatment Thresholds for Internal Parasites in Sheep and Goats

Time of Year Mature Yearlings andAnimals Younger

Spring Greenup - July 4 1000 epg* 500 epg

July 4 - First Frost 2000 epg 1000 epg

*epg = eggs per gram of feces

Egg counts equal to or above these levels warrant anthelminticadministration.

ing parasite burden. Thisprocedure involves collectinga fresh fecal sample, mixing2 grams of the fecal materialin a flotation solution andexamining a known quantityof that solution under amicroscope. For details onthe egg counting procedure,see TAEX leaflet L-5094,Monitoring Internal ParasiteInfection in Small Rumi-nants. For reference, sketch-es showing the relative sizesof eggs from commonlyobserved parasites areshown in Figure 2. Severalpoints concerning fecal sam-ple collection warrant fur-ther consideration.

1. Fecal pellets may be takenfrom the rectum or pickedup off the ground. Anexcellent opportunity forsample collection is earlyin the morning as the ani-mals are leaving their bed-grounds. Fresh samplesare easily distinguishedfrom older, weathereddroppings.

2. Collect 8 to 10 warm,moist, soft pellets per sam-ple and place them in asealable plastic bag (toprotect against dehydra-tion). Samples should bekept cool (<50o F ) untilanalysis. If immediate orsame day analysis is notpossible, samples may berefrigerated (not frozen)for up to 72 hours.

3. Collect at least 6 individ-ual samples per parasitemanagement unit prior toanthelmintic administra-tion. Management unitsmight be pastures, flocks,separate ranches, etc.Collection of a compositesample, while better thanno fecal egg count at all,

ance of parasites is lessthan later in the yearwhen nutrient require-ments are lower. Inaddition, resistance ofewes or does toHaemonchus contortusis weakened at the timeof kidding and duringearly lactation.

■ Many offspring areweaned and sold by mid-summer. Those remain-ing in the flock haveincreased in size andage such that they cantolerate a larger para-site burden.

■ As a compliment to ani-mal management, anoth-er objective of theseguidelines is to mini-mize pasture contamina-tion (tactical treatment)during the early part ofthe parasite season. Thelower thresholds of 500and 1,000 help to reducesuch pasture contamina-tion.

■ Relative to parasitemanagement, there isnothing significantabout July 4 other thanit is a mid-summer datethat is easy to remem-ber.

yields a less accurateassessment of parasiteburdens.

4. Early detection of resis-tance development andevaluation of anthelminticefficacy involves fecal col-lections and analysis 7 to10 days post-treatment.Again, a minimum of 6samples is suggested.Post-treatment samplesverify product efficacy.

Results of the analysisshould be reported in eggsper gram of feces. Need fortreatment can be based onthe “rules of thumb” listedin Table 2.

These rules of thumb arenot etched in stone for everyTexas sheep and goat pro-ducer. They are merelybenchmarks for producers touse in the development oftheir specific IPM program.These rules are most applic-able to spring lambing/kid-ding operations. The rea-soning behind these guide-lines includes:

■ During spring and earlysummer, nutrientrequirements for ewesand does are high dueto lactation, thus toler-

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