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Veterinary Drugs Residues: Control of Helminths
VETERINARY DRUGS RESIDUES
Control of Helminths T de Waal, University College Dublin, Dublin, Ireland M Danaher, Teagasc Food Research Centre, Dublin, Ireland r 2014 Elsevier Inc. All rights reserved.
Glossary Anthelmintics Drugs that acts against helminthic infections. Anthelmintic resistance A heritable change in the susceptibility to the attack of an anthelmintic in a population of parasitic nematodes. Cestodes Parasitic flatworms of the class Cestoda having a long flat body equipped with a specialized organ of attachment at one end. Also called tapeworms.
Introduction
activity against immature (larval) and mature stages of helminths, and mostly a broad spectrum of activity (Table 1).
Domestic animals are affected by a large variety of helminth parasites which includes nematodes (roundworms), trematodes (flukes), and cestodes (tapeworms). Since the first discovery of the potent broad-spectrum anthelmintics, it has been widely used and has led to major changes in clinical parasitism, and millions of dollars are being spent annually in efforts to reduce the effects of parasitism in food-producing animals. Both cattle and sheep producers list internal parasites as a common cause of significant economic impact on animal production. Producers are inflicted by losses at the global level that run to many billions of dollars (liver fluke infestation alone causes losses of an estimated $3 billion worldwide). Antiparasitic compounds are the dominant segment of the veterinary pharmaceuticals market with global sales of approximately $3.5 billion annually. Resistance to all major antiparasitic (anthelmintic) drug classes is now widespread in intestinal nematodes of sheep and goats. Modern anthelmintics generally have a wide margin of safety, considerable Table 1
Endectocide A drug effective against both endo- and ectoparasites. Nematodes Parasitic worms of the phylum Nematoda, having unsegmented, cylindrical bodies, often narrowing at each end, and including parasitic forms, such as the hookworm and pinworm. Also called roundworm. Trematodes Parasitic flatworms of the class Trematoda that have a thick outer cuticle and one or more suckers or hooks for attaching to host tissue. Also called fluke.
Summary of the Individual Drug Classes Anthelmintics are separated into classes on the basis of similar chemical structure and mode of action. The major drug classes, their introduction onto the market and mechanism of actions are summarized in Table 2. After the introduction of the first benzimidazole molecule on the market, approximately 20 new benzimidazole compounds have been synthesized with potent anthelmintic activity, and are in use today to control helminths. Levamisole is an anthelminthic and immunomodulator belonging to a class of synthetic imidazothiazole derivatives. Macrocyclic lactones (ML) (including avermectin/milbemycin) are endectocides with broad activity against both external and internal parasites. Since the first introduction of ivermectin on the market (a semisynthetic derivative of avermectin), many
Anthelmintic drug classes commonly used to treat helminth infections in food-producing animals
Chemical group
Activity spectrum (Helminth group)
Food-producing animals
Benzimidazole Imidazothiazole Tetrahydropyrimidine Avermectin/milbemycin Amino-acetonitrile derivatives Praziquantel Nitrophenolic compounds Salicylanilides Benzenesulfonamides
Nematodes and some trematodes Nematodes Nematodes Endectocide Nematodes Cestodes and trematodes Some nematodes and trematodes Some nematodes and trematodes Trematodes
Cattle, sheep, goats, pigs, and poultry Cattle, sheep, goats, pigs, and poultry Cattle Cattle, sheep, goats, and pigs Sheep Sheep and goats Cattle and sheep Cattle and sheep Cattle and sheep
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Table 2
Spectrum of activity, mode of action, and toxicity of the different anthelmintic chemical classes, with comments on the appearance of resistance Discovery and first release onto market
Mode of action
Anthelmintic spectrum
Pharmacokinetics
Toxicity
Route of administration
Resistance situation
Benzimidazole (BZ)
Thiabendazole was discovered in 1961 and subsequently a number of second generation BZ products had been commercially developed for use in domestic animals
Binds with parasite b-tubulin for subsequent disruption of tubulinmicrotuble dynamic equilibrium
Poor GI absorption
Low toxicity
Oral administartion
Imidazothiazole
The first veterinary product was tetramisole in 1967. Levamisole, the lisomer of tetramisole is currently the only compound in this group available
Nicotinic acetylcholine receptor agonist
Broad-spectrum against lung and GI nematodes. Some activity against cestodes and trematodes. One compound (triclabendazole) only effects against Fasciola Broad-spectrum against lung and GI nematodes, mainly adult stages
Rapid absorption following parental administration. Limited absorption following oral or topical application. Rapid elimination
Narrow safety margin
Oral administration, pour-on or by injection
Tetrahydropyrimidine
Pyrantel, introduced in 1966
Nicotinic acetylcholine receptor agonist
Oral administration
A series of macrocyclic lactone derivatives was introduced as an anthelmintic in 1981
Activity is mediated through g-aminobutyric acid and/or glutamate gated chloride channels
Poor GI absorption in ruminants, good GI absorption in pigs Highly lipophilic and rapidly absorbed, but reduced absorption from GI (in ruminants)
Good safety margin
Avermectin/ milbemycin
GI nematodes, narrow spectrum, mainly adult stages Very broad-spectrum against ecto- and endoparasites (endectocide) – both GI and lung nematodes
First reported in 1964. High resistance worldwide in sheep and goat trichostrongylid nematodes. Emerging resistance in cattle trichostrongylid nematodes reported First reported in 1979. High resistance worldwide in sheep and goat trichostrongylid nematodes. Emerging resistance in cattle trichostrongylid nematodes reported. No reports in food producing animals
Good safety margin in majority of target species
Oral administration, pour-on or by injection
First ivermectin resistance reported in 1988 and moxidectin in 1991. Resistance reported in sheep, goat, and cattle trichostrongylid nematodes. Ivermectin still effective in Europe and Canada. Moxidectin resistance so far only reported in goat trichostrongylid nematodes
Veterinary Drugs Residues: Control of Helminths
Chemical group
Amino-acetonitrile derivatives
2010
Spiroindoles
2012
Praziquantel
1978
Nitrophenolic compounds
Developed in the late 1960s as injectable fasciolicide for sheep and cattle
Salicylanilides
Rafoxanide was developed in 1969 and introduced for sheep and cattle in 1970
Abbreviations: BZ, benzimidazole; GI, gastrointestinal.
Complex. Uncouples oxidative phosphorylation in flukes but including other biochemical and physiological processes within the parasite
Effect on gut and integument of flukes
Broad-spectrum, recently introduced for use in sheep Narrow spectrum GI nematodes Narrow spectrum – adults cestodes in ruminants (at high dose rates) and schistosomes Narrow-spectrum, Highly effective against adult Fasciola and some abomasal nematodes Narrow-spectrum. Effective against mature Fasciola with some compounds showing activity against immature flukes. Also activity against some adult nematodes and some compounds against cestodes and Paramphistomum Narrow-spectrum. Effective against adult Fasciola
Oral administration
–
Oral administration
–
Good GI absorption
Wide safety margin
Oral administration
–
Rapid absorption following parental administration
Well tolerated in ruminants at recommended dose rates
Oral administration, but more effective when administered parentally
–
Well absorbed after both external and parenteral administration
Well tolerated in ruminants at recommended dose rates
Oral administration
–
Slow absorption following oral administration
Safe drug with high therapeutic index
Oral administration and injection
–
Veterinary Drugs Residues: Control of Helminths
Benzenesulfonamides
Nematode-specific acetylcholine receptors Nicotinic acetylcholine receptors Modulates cell membrane permeability and causes damage to parasite tegument Intrinsic mode of action not established. Disrupts tegument of Fasciola
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Veterinary Drugs Residues: Control of Helminths
ivermectin analogs were introduced for the control of parasites, including: moxidectin, milbemycin oxime, doramectin, selamectin, abamectin, and eprinomectin. The MLs exhibit no activity against cestodes, trematodes, or protozoa. Fecal excretion is the main route of elimination of most of the ML; however, in lactating animals, up to 5% of the dose may be excreted in the milk.
Anthelmintic Resistance to Drugs and Impact on Public Health The standards of anthelmintic efficacy usually demand that Z95% of parasitic nematodes be removed with a single drug treatment, and below this level, the efficacy indicates the evidence of anthelmintic resistance. Drug resistance in parasites results from the selection of a subpopulation, which can tolerate the lethal effects of the drug that are normally effective against them. This resistance has a genetic basis due to target gene mutation/alteration of the drug receptor, but is often more complex involving other nonreceptor based mechanisms (Table 2). In livestock, anthelmintic resistance to the broadspectrum drug, thiabendazole, was first reported by 1964. Today, significant levels of anthelmintic resistance have been recorded throughout the world to all three of the major anthelmintic classes used (benzimidazoles, imidothiazoles-tetrahydropyrimidines, and ML; see Table 2). Among sheep in particular, multidrug resistant nematode populations have become an important limitation for successful sheep production in many parts of the world. Anthelmintic resistance has also been reported in cattle and pig nematodes though not as widespread. Limited data is available on the resistance in cestodes against the drugs, but in the case of liver fluke, Fasciola hepatica, an important trematode infection of livestock in many temperate areas of the world, resistance (or decreased efficacy) to triclabendazole (a benzimidasole) has been reported. The mode of action of the benzimidazole drugs can be directly linked to the interruption of microtubular function in the target parasite. Benzimidazole resistance is thus mainly caused by transversion mutations leading to specific amino acid substitutions in the b-tubulin protein at codon 200 (TTC to TAC; phenylalanine to tyrosine), codon 167 (TTC to TAC), or codon 198 (GAA to GCA; glutamate to alanine). To date, there is no convincing evidence of a role for tubulin mutations in F. hepatica resistance to triclabendazole; however, there is evidence to indicate that altered uptake and metabolism of the drug maybe involved. A nicotinic acetylcholine receptor (ACh – the primary excitatory transmitter in nematodes) on the muscle cells of nematodes is associated with the sensitivity to levamisole. Levamisole, monepantel, and derquantel are cholinergic agonists that selectively produce depolarization and spastic contraction, followed by the paralysis of body muscle cells of nematodes. Monepantel, however, acts on a different class of nicotinic receptor than levamisole and derquantel. Resistance to levamisole is associated with mutation of the nicotinic acetylcholine receptor subunits, involved in forming the levamisole receptor. Both classes of ML mediate their nematocidal effect by interacting with a range of ligand-gated ion channels of the
muscles of the pharynx and somatic musculature. As a result, nematodes become paralyzed and starve to death. Resistance to ML is more complex and the mechanism of resistance to these compounds in parasitic nematodes seems to involve changes in the activity of multidrug ATP binding cassette (ABC) transporters such as P-glycoprotein. Treatment regimens against nematodes in humans often do not achieve the same high level of efficacy and therefore it may be much more difficult to notice an anthelmintic failure. Current mass treatment campaigns against onchocercosis, lymphatic filariosis, schistosomosis, and soil-transmitted helminths in humans are based on the use of drugs being employed for decades in the veterinary field. Data suggestive of praziquantel resistance in Schistosoma mansoni, and ML resistance in both Onchocerca volvulus and soil transmitted helminths has been reported. The cost of developing a new class of broad-spectrum anthelmintic for use in food-producing animals is becoming prohibitive and very few new classes of anthelmintic have been developed in the past 40 years. New anthelmintic drug classes, such as monepantel, derquantel, and emodepside that have been developed do not seem to have the breadth of spectrum in terms of target hosts and parasite species as the MLs and benzimidazoles, and have not so far been developed for use in humans.
Control of Anthelmintic Drug Residues in Food of Animal Origin Anthelmintic drug residues can potentially occur in food following the administration of veterinary medicinal products to farm animals or from cross contamination at feed mills. Maximum residue limits (MRL) have been set for the anthelmintic drug marker residues in edible tissues (namely, liver, muscle, kidney, and fat), eggs, and milk as markers of food safety. MRLs also play an important role in the harmonization of trade between regions worldwide including the EU, North America, Asia, Australia, and New Zealand. In addition, withdrawal periods have been set for veterinary medicinal products to ensure that residues have depleted below the MRL before tissues entering the food chain. A range of regulatory control measures to ensure food safety include the licensing of veterinary medicinal products, on-going national surveillance programs, networks of reference laboratories, and performance criteria for analytical methods. To monitor food safety, analytical methods have been developed for analysis of anthelmintic drug residues in food. Most of the early methods for the analysis of anthelmintic residues were based on high performance liquid chromatography coupled to ultraviolet, fluorescence, or electrochemical detection. In recent years, there has been a move to liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) technology, which allows the analysis of a wider range of anthelmintic drug residues at parts-per-billion. LC-MS/MS is advantageous because simpler sample preparation approaches can be employed and the sample throughput can be improved vastly. Several analytical methods have been developed that can analyze as many as 40 anthelmintic residues in both milk and animal tissues. Kinsella et al. (2011) have recently reported the application of this technology to the analysis of the new anthelmintic drug,
Veterinary Drugs Residues: Control of Helminths
Further Reading
N
S H N
O CF3
CF3
O
N
Monepantel N O
O S
H N
O CF3
85
CF3
O N Monepantel-sulphone H N
O OH
Salicylanilide (internal standard) Figure 1 Structure of monepantel, monepantel sulphone, and the internal standard salicylanilide.
monepantel and its sulphone metabolite in milk and animal tissue. Figure 1 shows the chemical structures of monepantel, monepantel sulphone, and salicylanilide, which was used an internal standard in the method.
Residue Findings and Implications Anthelmintic residues are occasionally detected in food samples and are typically found in o1% of tissue samples. The results of early residue surveillance indicate oxfendazole to be one of the most frequently detected anthelmintic residues, found mainly in ovine tissues. Many countries have commenced testing for triclabendazole residues in animal tissue, which has led to some reports of noncompliant residues in ovine tissue samples. Recently, anthelmintic analysis was extended to include the flukicide residues, namely, closantel, clorsulon, niclosamide, nitroxynil, oxyclozanide, and rafoxanide. From the inception of this testing, there have been some reports of MRL violations for closantel and rafoxanide residues in ovine tissue samples. However, in some countries, the most significant development has been the reported detection of low levels of closantel, nitroxynil, rafoxanide, and triclabendazole residues in milk samples. As a result of these positive milk samples, new MRLs have been established for four flukicide residues in milk.
See also: Foodborne Diseases: Overview of Chemical, Physical, and Other Significant Hazards. Veterinary Drugs Residues: Veterinary Drugs – General
Ballweber LR (2004) Parasites: Internal. In: Werner Klinth J (ed.) Encyclopedia of Meat Sciences, pp. 983–989. Oxford: Elsevier. Charlier J, Claerebout E, and Vercruysse J (2011) Diseases of dairy animals: Parasites, internal: Gastrointestinal nematodes. In: John WF (ed.) Encyclopedia of Dairy Sciences, 2nd edn., pp. 258–263. San Diego: Academic Press. Crawford LM and Franco DA (1994) Animal Drugs and Human Health. Boca Raton, FL: CRC Press INC. Danaher M, de Ruyck H, Crooks SRH, Dowling G, and O’Keeffe M (2007) Review of methodology for the determination of benzimidazole residues in biological matrices. Journal of Chromatography B-Analytical Technologies in the Biomedical and Life Sciences 845: 1–37. Fairweather I (2011) Raising the bar on reporting ‘triclabendazole resistance’. Veterinary Record 168: 514–515. Haber CL, Heckaman CL, Li GP, Thompson DP, Whaley HA, and Wiley VH (1991) Development of a mechanism of action-based screen for anthelmintic microbial metabolites with avermectinlike activity and isolation of milbemycin-producing Streptomyces strains. Antimicrobial Agents and Chemotherapy 35: 1811–1817. Kaplan RM (2004) Drug resistance in nematodes of veterinary importance: a status report. Trends Parasitol. 20: 477–481. Kinsella B, Byrne P, Cantwell H, McCormack M, Furey A, and Danaher M (2011) Determination of the new anthelmintic monepantel and its sulfone metabolite in milk and muscle using a UHPLC-MS/MS and QuEChERS method. Journal of Chromatography B-Analytical Technologies in the Biomedical and Life Sciences 879: 3707–3713. Kinsella B, Whelan M, Cantwell H, et al. (2010) A dual validation approach to detect anthelmintic residues in bovine liver over an extended concentration range. Talanta 83: 14–24. Lespine A, Me´nez C, Bourguinat C, and Prichard RK (2012) P-glycoproteins and other multidrug resistance transporters in the pharmacology of anthelmintics: Prospects for reversing transport-dependent anthelmintic resistance. International Journal for Parasitology: Drugs and Drug Resistance 2: 58–75. Mehlhorn H (2008) Encyclopedia of Parasitology. Heidelberg: A-M. Springer-Verlag GmbH. vol. 1. xix þ 860 pp. Prichard R, Me´nez C, and Lespine A (2012) Moxidectin and the avermectins: Consanguinity but not identity. International Journal for Parasitology: Drugs and Drug Resistance 2: 134–153. Riviere JE and Papich MG (2009) Veterinary Pharmacology and Therapeutics, 9th edn. Ames, IA: John Wiley & Sons. Takeba K, Itoh T, Matsumoto M, Nakazawa H, and Tanabe S (1996) Simultaneous determination of five fasciolicides in milk by liquid chromatography with electrochemical detection. Journal of Aoac International 79: 848–852. Vercruysse J, Albonico M, Behnke JM, et al. (2011) Is anthelmintic resistance a concern for the control of human soil-transmitted helminths? International Journal for Parasitology: Drugs and Drug Resistance 1: 14–27. Vercruysse J and Rew RS (eds.) (2002) Macrocyclic Lactones in Antiparasitic Therapy, p. 432. Wallingford, UK: CABI Stylus Pub Llc. Whelan M, Kinsella B, Furey A, et al. (2010) Determination of anthelmintic drug residues in milk using ultra high performance liquid chromatography-tandem mass spectrometry with rapid polarity switching. Journal of Chromatography A 1217: 4612–4622. Wolstenholme AJ (2011) Ion channels and receptor as targets for the control of parasitic nematodes. International Journal for Parasitology: Drugs and Drug Resistance 1: 2–13. Wolstenholme AJ, Fairweather I, Prichard R, von Samson-Himmelstjerna G, and Sangster NC (2004) Drug resistance in veterinary helminths. Trends in Parasitology 20: 469–476.
Relevant Websites http://ec.europa.eu/food/food/chemicalsafety/residues/control_en.htm European Commission: Residues of Veterinary Medicinal Products - Control and Monitoring. http://www.efsa.europa.eu European Food Safety Authority.
Control of Helminths T de Waal, University College Dublin, Dublin, Ireland M Danaher, Teagasc Food Research Centre, Dublin, Ireland r 2014 Elsevier Inc. All rights reserved.
Glossary Anthelmintics Drugs that acts against helminthic infections. Anthelmintic resistance A heritable change in the susceptibility to the attack of an anthelmintic in a population of parasitic nematodes. Cestodes Parasitic flatworms of the class Cestoda having a long flat body equipped with a specialized organ of attachment at one end. Also called tapeworms.
Introduction
activity against immature (larval) and mature stages of helminths, and mostly a broad spectrum of activity (Table 1).
Domestic animals are affected by a large variety of helminth parasites which includes nematodes (roundworms), trematodes (flukes), and cestodes (tapeworms). Since the first discovery of the potent broad-spectrum anthelmintics, it has been widely used and has led to major changes in clinical parasitism, and millions of dollars are being spent annually in efforts to reduce the effects of parasitism in food-producing animals. Both cattle and sheep producers list internal parasites as a common cause of significant economic impact on animal production. Producers are inflicted by losses at the global level that run to many billions of dollars (liver fluke infestation alone causes losses of an estimated $3 billion worldwide). Antiparasitic compounds are the dominant segment of the veterinary pharmaceuticals market with global sales of approximately $3.5 billion annually. Resistance to all major antiparasitic (anthelmintic) drug classes is now widespread in intestinal nematodes of sheep and goats. Modern anthelmintics generally have a wide margin of safety, considerable Table 1
Endectocide A drug effective against both endo- and ectoparasites. Nematodes Parasitic worms of the phylum Nematoda, having unsegmented, cylindrical bodies, often narrowing at each end, and including parasitic forms, such as the hookworm and pinworm. Also called roundworm. Trematodes Parasitic flatworms of the class Trematoda that have a thick outer cuticle and one or more suckers or hooks for attaching to host tissue. Also called fluke.
Summary of the Individual Drug Classes Anthelmintics are separated into classes on the basis of similar chemical structure and mode of action. The major drug classes, their introduction onto the market and mechanism of actions are summarized in Table 2. After the introduction of the first benzimidazole molecule on the market, approximately 20 new benzimidazole compounds have been synthesized with potent anthelmintic activity, and are in use today to control helminths. Levamisole is an anthelminthic and immunomodulator belonging to a class of synthetic imidazothiazole derivatives. Macrocyclic lactones (ML) (including avermectin/milbemycin) are endectocides with broad activity against both external and internal parasites. Since the first introduction of ivermectin on the market (a semisynthetic derivative of avermectin), many
Anthelmintic drug classes commonly used to treat helminth infections in food-producing animals
Chemical group
Activity spectrum (Helminth group)
Food-producing animals
Benzimidazole Imidazothiazole Tetrahydropyrimidine Avermectin/milbemycin Amino-acetonitrile derivatives Praziquantel Nitrophenolic compounds Salicylanilides Benzenesulfonamides
Nematodes and some trematodes Nematodes Nematodes Endectocide Nematodes Cestodes and trematodes Some nematodes and trematodes Some nematodes and trematodes Trematodes
Cattle, sheep, goats, pigs, and poultry Cattle, sheep, goats, pigs, and poultry Cattle Cattle, sheep, goats, and pigs Sheep Sheep and goats Cattle and sheep Cattle and sheep Cattle and sheep
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Table 2
Spectrum of activity, mode of action, and toxicity of the different anthelmintic chemical classes, with comments on the appearance of resistance Discovery and first release onto market
Mode of action
Anthelmintic spectrum
Pharmacokinetics
Toxicity
Route of administration
Resistance situation
Benzimidazole (BZ)
Thiabendazole was discovered in 1961 and subsequently a number of second generation BZ products had been commercially developed for use in domestic animals
Binds with parasite b-tubulin for subsequent disruption of tubulinmicrotuble dynamic equilibrium
Poor GI absorption
Low toxicity
Oral administartion
Imidazothiazole
The first veterinary product was tetramisole in 1967. Levamisole, the lisomer of tetramisole is currently the only compound in this group available
Nicotinic acetylcholine receptor agonist
Broad-spectrum against lung and GI nematodes. Some activity against cestodes and trematodes. One compound (triclabendazole) only effects against Fasciola Broad-spectrum against lung and GI nematodes, mainly adult stages
Rapid absorption following parental administration. Limited absorption following oral or topical application. Rapid elimination
Narrow safety margin
Oral administration, pour-on or by injection
Tetrahydropyrimidine
Pyrantel, introduced in 1966
Nicotinic acetylcholine receptor agonist
Oral administration
A series of macrocyclic lactone derivatives was introduced as an anthelmintic in 1981
Activity is mediated through g-aminobutyric acid and/or glutamate gated chloride channels
Poor GI absorption in ruminants, good GI absorption in pigs Highly lipophilic and rapidly absorbed, but reduced absorption from GI (in ruminants)
Good safety margin
Avermectin/ milbemycin
GI nematodes, narrow spectrum, mainly adult stages Very broad-spectrum against ecto- and endoparasites (endectocide) – both GI and lung nematodes
First reported in 1964. High resistance worldwide in sheep and goat trichostrongylid nematodes. Emerging resistance in cattle trichostrongylid nematodes reported First reported in 1979. High resistance worldwide in sheep and goat trichostrongylid nematodes. Emerging resistance in cattle trichostrongylid nematodes reported. No reports in food producing animals
Good safety margin in majority of target species
Oral administration, pour-on or by injection
First ivermectin resistance reported in 1988 and moxidectin in 1991. Resistance reported in sheep, goat, and cattle trichostrongylid nematodes. Ivermectin still effective in Europe and Canada. Moxidectin resistance so far only reported in goat trichostrongylid nematodes
Veterinary Drugs Residues: Control of Helminths
Chemical group
Amino-acetonitrile derivatives
2010
Spiroindoles
2012
Praziquantel
1978
Nitrophenolic compounds
Developed in the late 1960s as injectable fasciolicide for sheep and cattle
Salicylanilides
Rafoxanide was developed in 1969 and introduced for sheep and cattle in 1970
Abbreviations: BZ, benzimidazole; GI, gastrointestinal.
Complex. Uncouples oxidative phosphorylation in flukes but including other biochemical and physiological processes within the parasite
Effect on gut and integument of flukes
Broad-spectrum, recently introduced for use in sheep Narrow spectrum GI nematodes Narrow spectrum – adults cestodes in ruminants (at high dose rates) and schistosomes Narrow-spectrum, Highly effective against adult Fasciola and some abomasal nematodes Narrow-spectrum. Effective against mature Fasciola with some compounds showing activity against immature flukes. Also activity against some adult nematodes and some compounds against cestodes and Paramphistomum Narrow-spectrum. Effective against adult Fasciola
Oral administration
–
Oral administration
–
Good GI absorption
Wide safety margin
Oral administration
–
Rapid absorption following parental administration
Well tolerated in ruminants at recommended dose rates
Oral administration, but more effective when administered parentally
–
Well absorbed after both external and parenteral administration
Well tolerated in ruminants at recommended dose rates
Oral administration
–
Slow absorption following oral administration
Safe drug with high therapeutic index
Oral administration and injection
–
Veterinary Drugs Residues: Control of Helminths
Benzenesulfonamides
Nematode-specific acetylcholine receptors Nicotinic acetylcholine receptors Modulates cell membrane permeability and causes damage to parasite tegument Intrinsic mode of action not established. Disrupts tegument of Fasciola
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Veterinary Drugs Residues: Control of Helminths
ivermectin analogs were introduced for the control of parasites, including: moxidectin, milbemycin oxime, doramectin, selamectin, abamectin, and eprinomectin. The MLs exhibit no activity against cestodes, trematodes, or protozoa. Fecal excretion is the main route of elimination of most of the ML; however, in lactating animals, up to 5% of the dose may be excreted in the milk.
Anthelmintic Resistance to Drugs and Impact on Public Health The standards of anthelmintic efficacy usually demand that Z95% of parasitic nematodes be removed with a single drug treatment, and below this level, the efficacy indicates the evidence of anthelmintic resistance. Drug resistance in parasites results from the selection of a subpopulation, which can tolerate the lethal effects of the drug that are normally effective against them. This resistance has a genetic basis due to target gene mutation/alteration of the drug receptor, but is often more complex involving other nonreceptor based mechanisms (Table 2). In livestock, anthelmintic resistance to the broadspectrum drug, thiabendazole, was first reported by 1964. Today, significant levels of anthelmintic resistance have been recorded throughout the world to all three of the major anthelmintic classes used (benzimidazoles, imidothiazoles-tetrahydropyrimidines, and ML; see Table 2). Among sheep in particular, multidrug resistant nematode populations have become an important limitation for successful sheep production in many parts of the world. Anthelmintic resistance has also been reported in cattle and pig nematodes though not as widespread. Limited data is available on the resistance in cestodes against the drugs, but in the case of liver fluke, Fasciola hepatica, an important trematode infection of livestock in many temperate areas of the world, resistance (or decreased efficacy) to triclabendazole (a benzimidasole) has been reported. The mode of action of the benzimidazole drugs can be directly linked to the interruption of microtubular function in the target parasite. Benzimidazole resistance is thus mainly caused by transversion mutations leading to specific amino acid substitutions in the b-tubulin protein at codon 200 (TTC to TAC; phenylalanine to tyrosine), codon 167 (TTC to TAC), or codon 198 (GAA to GCA; glutamate to alanine). To date, there is no convincing evidence of a role for tubulin mutations in F. hepatica resistance to triclabendazole; however, there is evidence to indicate that altered uptake and metabolism of the drug maybe involved. A nicotinic acetylcholine receptor (ACh – the primary excitatory transmitter in nematodes) on the muscle cells of nematodes is associated with the sensitivity to levamisole. Levamisole, monepantel, and derquantel are cholinergic agonists that selectively produce depolarization and spastic contraction, followed by the paralysis of body muscle cells of nematodes. Monepantel, however, acts on a different class of nicotinic receptor than levamisole and derquantel. Resistance to levamisole is associated with mutation of the nicotinic acetylcholine receptor subunits, involved in forming the levamisole receptor. Both classes of ML mediate their nematocidal effect by interacting with a range of ligand-gated ion channels of the
muscles of the pharynx and somatic musculature. As a result, nematodes become paralyzed and starve to death. Resistance to ML is more complex and the mechanism of resistance to these compounds in parasitic nematodes seems to involve changes in the activity of multidrug ATP binding cassette (ABC) transporters such as P-glycoprotein. Treatment regimens against nematodes in humans often do not achieve the same high level of efficacy and therefore it may be much more difficult to notice an anthelmintic failure. Current mass treatment campaigns against onchocercosis, lymphatic filariosis, schistosomosis, and soil-transmitted helminths in humans are based on the use of drugs being employed for decades in the veterinary field. Data suggestive of praziquantel resistance in Schistosoma mansoni, and ML resistance in both Onchocerca volvulus and soil transmitted helminths has been reported. The cost of developing a new class of broad-spectrum anthelmintic for use in food-producing animals is becoming prohibitive and very few new classes of anthelmintic have been developed in the past 40 years. New anthelmintic drug classes, such as monepantel, derquantel, and emodepside that have been developed do not seem to have the breadth of spectrum in terms of target hosts and parasite species as the MLs and benzimidazoles, and have not so far been developed for use in humans.
Control of Anthelmintic Drug Residues in Food of Animal Origin Anthelmintic drug residues can potentially occur in food following the administration of veterinary medicinal products to farm animals or from cross contamination at feed mills. Maximum residue limits (MRL) have been set for the anthelmintic drug marker residues in edible tissues (namely, liver, muscle, kidney, and fat), eggs, and milk as markers of food safety. MRLs also play an important role in the harmonization of trade between regions worldwide including the EU, North America, Asia, Australia, and New Zealand. In addition, withdrawal periods have been set for veterinary medicinal products to ensure that residues have depleted below the MRL before tissues entering the food chain. A range of regulatory control measures to ensure food safety include the licensing of veterinary medicinal products, on-going national surveillance programs, networks of reference laboratories, and performance criteria for analytical methods. To monitor food safety, analytical methods have been developed for analysis of anthelmintic drug residues in food. Most of the early methods for the analysis of anthelmintic residues were based on high performance liquid chromatography coupled to ultraviolet, fluorescence, or electrochemical detection. In recent years, there has been a move to liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) technology, which allows the analysis of a wider range of anthelmintic drug residues at parts-per-billion. LC-MS/MS is advantageous because simpler sample preparation approaches can be employed and the sample throughput can be improved vastly. Several analytical methods have been developed that can analyze as many as 40 anthelmintic residues in both milk and animal tissues. Kinsella et al. (2011) have recently reported the application of this technology to the analysis of the new anthelmintic drug,
Veterinary Drugs Residues: Control of Helminths
Further Reading
N
S H N
O CF3
CF3
O
N
Monepantel N O
O S
H N
O CF3
85
CF3
O N Monepantel-sulphone H N
O OH
Salicylanilide (internal standard) Figure 1 Structure of monepantel, monepantel sulphone, and the internal standard salicylanilide.
monepantel and its sulphone metabolite in milk and animal tissue. Figure 1 shows the chemical structures of monepantel, monepantel sulphone, and salicylanilide, which was used an internal standard in the method.
Residue Findings and Implications Anthelmintic residues are occasionally detected in food samples and are typically found in o1% of tissue samples. The results of early residue surveillance indicate oxfendazole to be one of the most frequently detected anthelmintic residues, found mainly in ovine tissues. Many countries have commenced testing for triclabendazole residues in animal tissue, which has led to some reports of noncompliant residues in ovine tissue samples. Recently, anthelmintic analysis was extended to include the flukicide residues, namely, closantel, clorsulon, niclosamide, nitroxynil, oxyclozanide, and rafoxanide. From the inception of this testing, there have been some reports of MRL violations for closantel and rafoxanide residues in ovine tissue samples. However, in some countries, the most significant development has been the reported detection of low levels of closantel, nitroxynil, rafoxanide, and triclabendazole residues in milk samples. As a result of these positive milk samples, new MRLs have been established for four flukicide residues in milk.
See also: Foodborne Diseases: Overview of Chemical, Physical, and Other Significant Hazards. Veterinary Drugs Residues: Veterinary Drugs – General
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Relevant Websites http://ec.europa.eu/food/food/chemicalsafety/residues/control_en.htm European Commission: Residues of Veterinary Medicinal Products - Control and Monitoring. http://www.efsa.europa.eu European Food Safety Authority.