A Detailed Study on Anthelmintic Resistance and its Diagnosis
Dr. Yash Bhargava, MVSc (Teaching Associate IPVS, DUVASU) Dr. Pradeep Kumar MVSc, PHd (Assistant Professor Department of veterinary Parasitology, COVSc & AH, DUVASU, Mathura) Dr. Shardendu Narayan Giri MVSc (Teaching Associate IPVS, DUVASU) Dr. Shubham Kumar, MVSc (Teaching Associate IPVS, DUVASU)
Dr. Sudaksha Saraswat BVSc & AH (Teaching Associate IPVS, DUVASU)
*Corresponding author- Dr. Shardendu Narayan Giri, dr.shardendu007@gmail.com
Anthelmintics are a group of antiparasitic drugs that helps to expel the parasitic worms (helminthes) and other internal parasites from the body by either stunning or killing them, without causing significant damage to the host organism. Anthelmintics are also called as antihelmintics. Anthelmintic compounds share a common mode of action. In the free living nematode Caenorhabditis elegans, AMs act on a group of glutamate-gated Cl−channels which appear to be expressed in defined muscle cells, and possibly neurons, in the pharynx of worms. Glutamate is an inhibitory neurotransmitter in the pharynx and Glu, Ivermectin and milbemycin D act by increasing Cl− currents when added to expressed receptors. It is likely that AMs paralyze worm muscle either by mimicking and/or influencing the binding of Glu to its receptor and by these types of mechanism of action the helminthes are killed, paralyses or inhibit there growth which ultimately leads to reduction in their number by their removal from the body or by partially digestion in the GIT. But now days many of these compounds are ineffective to the helminths due to genetic changes, adaption, selection and biological changes in the body. Anthelmintic resistance in the parasite of the cattle is an emerging problem.
Resistance is the ability of worms in a population to survive treatments that are generally effective against the same species and stage of infection. However, one problem in recognizing resistance is that some species and stages of parasites differ in their inherent tolerance to chemicals. As an example, drugs are less effective against mucosal stages than luminal stages of the cyathostomes. Shifts in anthelmintic efficacy are often difficult to detect in mixed populations of worms. Thus now resistance to anthelmintic drugs is became a big problem which results for the discovery of new drugs. There are some natural methods to cure it by using diatomous earth and certain plants are also used to cure the infestation helminthes in the body as they are not having any adverse effect on the body of the animal and the helminthes are also unable to develop resistance against it.
Anthelmentic are of two types-
- Vermicide: These are the agents which are destructive to worms which means by the killing of the worms as piperazine and hexylresorcinol used for round worms (ascarids), quinacrine for tapeworms. These drugs are toxic thus also given with care.
- Vermifuge: These are the agents which help in the expulsion of worms and other parasites from the intestinal tract.
Some of the anthelmentics are used for nematodes are- Diethylecarbamazine, Ivermectin, Mebendazole, Pyrental pamoate and Thiabendazole for trematodes- Praziquantal for cestodes- Albendazole, Niclosamide are used. Mebendazole- in roundworms and hookworms it produces 100% cure rates in egg counts. It blocks the glucose uptake and binds with the beta-tubulin and it also inhibits microtubules polymerization. Albendazole- it is the congener of mebendazole. It is a drug of choice for roundworms, hookworm, threadworm, whipworm, tapeworm, dog tapeworm but alternatively it can also be used for filariasis, taenia solium and taenia saginata. Thiabendazole- the machenism of action is just similar to mebendazole. Pyrantel palmoate- it activates the nicotinic cholinergic receptor in the worms due to which, there is persistent depolarization and which results in contracture and spastic paralysis.in worms. A single dose of 10mg/kg of the body weight is sufficient for round worms and hookworms and threadworms.
Piparazine- it acts on GABA receptor. And acts as a GABA agonist and open the Cl-channels which results in the relaxation of the ascaris muscle which causes flaccid paralysis and expels alive worms.
Levamisole and tetramisole- it stimulates the ganglia which causes tonic paralysis with the expulsion of live worms and it also interfere in the carbohydrate metabolism and it inhibit the fumarate reductase it is the second choice of drug for ascaris lumbricoides and ancylostoma duodenale. Diethyl carbamazine citrate (DEC)- it is having piperazine moiety which causes muscular relaxation and it also causes hyperpolarization and causes muscular relaxation and it also causes alteration of the Mf membrane (microfilaria membrane) which is phagocytosed by the tissue fixed monocytes. Ivermactin- it acts on the glutamate gated Cl- channels thus causes tonic paralysis. Praziquantel- it causes leakage of the IC calcium from the membrane and causes contraction of the mucles and paralysis which results in the loss of grip in the GIT it can be used in tapeworm, neurocysticercosis, flukes and schistosomes. Niclosamide-it acts on the mitochondria by inhibition of the oxidative phosphorylation due to which there is interference with the generation of ATP (in anaerobic condition) thus causes injury and partly digested in the intestine.
Anthalmentic resistance is defined as the lack of the drug efficacy in a population of helminth parasites that was previously sensitive to the drug at a defined dose and in a defined host. Resistance is present when there is a greater frequency of individuals within a population able to tolerate doses of compound than in a normal population and is heritable or Anthelmintic resistance is inherited. The development of resistance first requires that resistance genes are present. The rate of development of resistance is determined by selection pressure and the extent to which worms surviving treatment pass their genes on to the next generation. With continued selection and reproduction of resistant worms, the frequency of resistance genes in the population increases to the point where treatment fails. This point may not be immediately recognized. Drugs are marketed at dose levels determined by efficacy against the species which is hardest to kill (the dose limiting parasite). Some parasites which are easily killed by recommended dose rates continue to be controlled during the early stages of the development of resistance, although some survive. As resistance develops further, more worms survive treatment until treatment failure occurs for these species or stages too. Once resistance is present, the population does not appear to revert to susceptibility, so the aims of resistance control are to prevent the first steps in the development of resistance and then to delay the accumulation of resistance genes.
The alternative alleles are already present in the genome of the parasite for the resistance of the drugs due to very high genetic diversity of the parasites, thus because the response of the drug is not uniform to the parasite population and there is also the spontaneous multiple origin of resistance by spontaneous and recurrent mutations. Some of the mutant allele parasite also grow/ develop factor in the population by their life cycle and decrease in the in the generation time increase in the fecundity by it there is spread of the resistance in the population will increase. Selection for resistance can be done by estimate the numbers of free living stages and stages within the host, includes the effects of immunity and the efficacy of treatment, and considers the genetics of resistance. Under dosing, like using an ineffective drug, is likely to enhance selection for resistance. The efficacy above 99.99% will tend not to select for resistance because there are too few survivors to effectively reproduce. Refugia refers to the population of worms not exposed to treatment (and hence, selection) with the compounds used. The higher the proportion of worms in refugia, the more slowly resistance develops and remain susceptible, and will dilute resistant worms in the population.
Now a day’s Haemonchus is 100% resistance to Ivermactin and 92% resistance to Fenbendazol.
Anthelmintic resistance develops due to-
- Increase frequency of treatment
- Underdosing
- Improper administration of drug
- Introducing of animal resistant worms
- Dosing in winter or before turn out leads to rapid development of anthelmintic resistance in haemonchus.
The resistance in benzimidazole group- thiabendazole was produced in 3 years after introduction to market and 11 years for albendazole. The resistsnce to imidazoles group in levamisol takes about 9 years develop resistance and macrocylic lactones group for pyrantel it takes 22 years and ivermactin 7 years.
The possible mechanisms of resistance development in different groups of anthelmintics-Benzimidazoles Altered target structure (beta -tubulin isotype 1 mutations). In
Avermectins – beta- tubulin isotype 2 mutations, deletion, altered metabolism or uptake. Mutations inGluCl or GABA-R genes And Overexpression of P-glycoproteins altered target (structure of GluClchannel & subunits Levamisole- there is changes in nicotinic acetylcholine receptors
Test for diagnose anthelmintic resistance
- Drench response
- Fecal egg count reduction test (FECRT)
- Larval development assay (lab based) (FDA)
- FAMACHA
- SCOOP
- Larval paralysis
- Egg hatch assay(EHA)
- Tubulin binding
- Adult development
- Larval migration inhibition assay(LMIA)
- Micromotility meter test(MMT)
- Larval feeding assay (LFA)
- WAAVP(molecular detection technique)
Each test is used for diagnosis of resistance to a specific anthelmintic which works on the specific stage of the life cycle of the parasite.
