What’s in a drench?

Dairy News Australia - - ANIMAL HEALTH - GEMMA CHUCK

LAST MONTH we dis­cussed the com­mon gas­troin­testi­nal worms and how they can af­fect dairy heifers. This month we fo­cus on how gas­troin­testi­nal worms can be man­aged through chem­i­cal con­trol. The term ‘drench’ refers to the anti-par­a­sitic group of chem­i­cals called an­thelmintics. It is quite an in­ac­cu­rate term as it im­plies ei­ther thor­oughly wet­ting the an­i­mal or giv­ing some­thing orally. In the past this could have re­flected the oral and pour-on meth­ods of ap­ply­ing the an­thelmintic but now they are avail­able in in­jectable forms, the term seems a lit­tle out-dated. Flu­ki­cides are a sep­a­rate group of anti-par­a­sitic and are specif­i­cally de­signed for the con­trol of liver fluke The ma­jor­ity of an­thelmintics regis­tered for use in dairy cat­tle are con­sid­ered broad-spec­trum, mean­ing they are ef­fec­tive against a wide range of gas­troin­testi­nal worms and at dif­fer­ent stages of their life­cy­cles. There are three classes of an­thelmintic avail­able for use:

1. Ben­z­im­i­da­zoles (the ‘white’ drenches)

First dis­cov­ered in the 1960s, the white drenches were highly ef­fec­tive against adult and im­ma­ture gas­troin­testi­nal worms. Un­for­tu­nately, their overuse has led to a re­sis­tance of gas­troin­testi­nal worms on up to 50 per cent of prop­er­ties in some coun­tries. The species of worms rel­e­vant to this prob­lem in dairy cat­tle in­clude Osterta­gia spp., Coope­ria spp., Hae­monchus spp. and Tri­cho-strongy­lus spp. They are gen­er­ally ad­min­is­tered orally and have lit­tle to no resid­ual ef­fect. This means that a sin­gle treat­ment will kill the par­a­sites present in the an­i­mal at the time of treat­ment, but it will not pro­tect the an­i­mal against re-in­fes­ta­tions. More re­cently, the ben­z­im­i­da­zoles have been used in com­bi­na­tion drenches. An ex­am­ple of a white drench regis­tered for use in dairy cat­tle is fen­ben­da­zole (Panacur®, Coop­ers An­i­mal Health).

2. Le­vamisole (the ‘clear’ drenches)

Le­vamisole has been used for the con­trol of gas­troin­testi­nal worms in dairy cat­tle for ap­prox­i­mately five decades. It is ef­fec­tive against ma­ture worms and lar­val stages of some species, al­though is in­ef­fec­tive against the ar­rested stage of Oster­gia os­tertagii which lim­its its sole use in cer­tain re­gions of Aus­tralia dur­ing at-risk pe­ri­ods. Like the white drenches, re­sis­tance is com­mon par­tic­u­larly from Osterta­gia spp. but also Tri­cho-strongy­lus spp. Le­vamisole is gen­er­ally avail­able as an oral drench or a pour-on. Pour-on prod­ucts con­tain­ing le­vamisole should be used with cau­tion in hot weather as tox­i­c­ity can de­velop due to rapid ab­sorp­tion. Le­vamisole has min­i­mal resid­ual ef­fect but is fre­quently used in com­bi­na­tion drenches.

3. Macro­cyclic Lac­tones (the ‘mectin’ drenches)

In the 1980s the first macro­cyclic lac­tone, iver­mectin, was in­tro­duced and it rev­o­lu­tionised the con­trol of ve­teri­nary par­a­sites in live­stock, horses and pets. They are highly ef­fec­tive against a broad-spec­trum of gas­troin­testi­nal worms but are also classed as en­dec­to­cides, mean­ing they are ef­fec­tive against ec­topar­a­sites as well (lice, mites, ticks). How­ever, they are in­ef­fec­tive against flukes (trema­todes) or tape­worms (ces­todes). Un­for­tu­nately, re­sis­tance of gas­troin­testi­nal worms to the macro­cyclic lac­tones has been found in many species, in­clud­ing cat­tle. In Aus­tralia, the worm species that have shown re­sis­tance in­clude Coope­ria spp, Osterta­gia spp, Tri­cho-strongy­lus spp and Hae­monchus spp. The ‘mectin drenches’ can be ad­min­is­tered orally, by in­jec­tion or as a pour-on. They have a much longer resid­ual ac­tiv­ity than the other classes of an­thelmintic, mean­ing they of­fer a pe­riod of pro­tec­tion after a sin­gle treat­ment. This is due to them be­ing stored in the body fat after ad­min­is­tra­tion and then re­leased back into the blood over sev­eral weeks or months. Ex­am­ples of macro­cyclic lac­tones regis­tered for use in dairy cat­tle in­clude do­ramectin (Dec­tomax®, Zoetis), mox­idectin (Cy­dectin®, Vir­bac) and epri­nomectin (Ivomec® Eprinex®, Me­rial). Com­bi­na­tion drenches In the past few years, the macro­cyclic lac­tones have been used in com­bi­na­tion drenches with the ben­z­im­i­da­zoles and/or le­vamisole. Re­search has shown that us­ing ef­fec­tive com­bi­na­tions of two or more classes of an­thelmintic at the same time helps pre­vent and slow the de­vel­op­ment of re­sis­tance.

By the con­cur­rent ad­min­is­tra­tion of mul­ti­ple an­thelmintics, gas­troin­testi­nal par­a­sites that have shown re­sis­tance to one of the classes of an­thelmintic can still be tar­geted. The best time to use a com­bi­na­tion drench is be­fore re­sis­tance has de­vel­oped and while the ef­fec­tive­ness of the in­di­vid­ual ac­tive in­gre­di­ents is still high. Com­bi­na­tion drenches regis­tered for use in dairy cat­tle in­clude Eclipse® (Me­rial) and Tri­fecta® (Coop­ers An­i­mal Health). En­sure your choice of an­thelmintic is regis­tered for use in dairy cat­tle and ob­serve when and how the prod­uct should be ad­min­is­tered and what the with­hold­ing pe­riod is. Some prod­ucts are regis­tered for use in dairy heifers and dry cows, but not for lac­tat­ing an­i­mals. As dis­cussed, gas­troin­testi­nal par­a­sites have shown re­sis­tance to all classes of an­thelmintic on a global scale. Whilst the use of com­bi­na­tion drenches will help slow this process, we need to look at al­ter­na­tive meth­ods to pre­vent the un­nec­es­sary use of drenches. Next month we will dis­cuss non-chem­i­cal gas­troin­testi­nal par­a­site con­trol in dairy cat­tle.

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