ANTHELMINTICS - RATIONAL USE
Dr.Pramod Jog*, Dr. Krunal Shah**
Consultant Pediatrics, Associate Professor Dr. D.Y.Patil Medical College, Pimpri, Pune Director Medi Point Hospital, Pune.*, Resident in Pediatrics, Dr. D.Y.Patil Medical College, Pimpri, Pune.**
Anthelmintics are drugs that either kill (vermicide) or expel (vermifuge) infesting helminthes. Helminthiasis is prevalent globally (1/3rd of world population harbors them and is more common in developing countries with poorer personal and environmental hygiene).

The common worm infestations are shown in Fig.1. Multiple infestations in same individual are not infrequent. In human body, GIT is a common site of many helminthes, but some also live in tissue, or their larvae migrate into the tissue. They harm the host by depriving him of food, causing blood loss, injury to organs, intestinal or lymphatic obstruction and by secreting toxins. Helminthiasis is rarely fatal but it is a major cause of ill health.

Fig. 1. Percentage of different worm infestations


Properties
In developing anthelmintic drug, the pharmaceutical companies measure newly discovered and promising compounds against an ideal standard. While no anthelmintic discovered yet has met all criteria of an 'Ideal Anthelmintic', some of the more recently developed drugs have come quite closer.

The properties of an ideal anthelmintics are:

  1. Broad spectrum of activity
  2. Non toxic at therapeutic dose
  3. Should possess a wide margin of safety
  4. Metabolism and excretion should be rapid
  5. Administration should be easy and
  6. The drug should be cost effective

Development of resistance has not been a problem in the clinical use of anthelmintics. Benzimidazole group of drugs are:
  1. Thiabendazole
  2. Mebendazole and
  3. Albendazole

In animals, drug is embryotoxic, therefore it is safe to avoid in pregnancy.
Mechanism of Action

Drug attacks

B tubulin of parasite (microprotein)

Microtubules of cells of parasite lost

Glucose of host can not now enter inside parasite cell

No ATP Synthesis

Death of cell



Thiabendazole: 25 mg/kg/day in 2 divided doses after meals x 2 days; tablet must be chewed.

Mebendazole: Round, hook and whip worms:
<2 year- 100 mg BD x 3 days
> 2 year- 200 mg BD X 3 days

Thread worm: 100 mg single dose, repeat after 3 weeks.

Albendazole: Round, hook and whipworms - 400 mg single dose; Tape worm - 400 mg daily x 3 days; Hydatid disease - 400 mg bd x 4 weeks, repeat 2nd dose after 2 weeks, Neurocysticercosis - 15 mg/kg/day x 28 days with steroids.

Piperazine: Highly active against A lumbricoides and Enterobius vermicularis, 100% cure rate. Now considered as a 2nd choice drug even for these worms. Causes hyperpolarization of Ascaris muscle by GABA agonistic action, opening chloride channel that causes relaxation and depresses responsiveness to contractile action of acetylcholine leading on to flaccid paralysis and worms expelled alive. Parents can literally see the worms being expelled in stools.
Dose : Round Worm : 75 mg/kg/day x 2 days
Whip Worm : 75 mg/kg/day/ 7 days

Nitazoxanide (NTZ): Nitroimidazole, broad spectrum anthelmintic with antiparasitic action.
Mechanism of action : Pyruvate ferredoxin oxidoreductase inhibitor.
Pharmacokinetics : NTZ is partially absorbed from GIT with approximately 1/3rd of oral dose is excreted in urine and 2/3rd in feces. In blood, NTZ is immediately metabolized to tizoxanide.
Dose : <4 years 100 mg BD x 3 days, and> 4 years 200 mg BD x 3 days

Pyrantel pamoate (Tetrahydropyrimidine)

Mechanism of action:

Activation of nicotinic cholinergic receptors in worm

Persistent depolarization

Slowly developing contracture and spastic paralysis leading to worm expulsion



Dose: Ascaris, Ancylostoma, Enterobius - 10 mg/kg single dose
Necator Strongyloides - 10 mg/kg/day x 3 days

Praziquantel: Isoquinoline pyrazine compound

Mechanism of action:

Praziquantel taken-up by body of worm

Increases permeability of intracellular Ca ++ in cell membrane of worm

Increases Ca ++ inside cells of worm

Contracture and paralysis and worms expelled


Dose: Tape worm - 10 mg/kg single dose.
Neurocysticercosis: 50 mgm/kg, two divided doses for 15 days.

Table 1. Current choice of drugs for worm infestations in Indian subcontinent
Worm 1 st Choice Alternative
Round Worm
Ascaris lumbricoides

Mebendazole
Albendazole
Pyrantel
Piperazine
Hook WormA. duodenale
N. americanus
Pyrantel
Mebendazole
Albendazole
Mebendazole
Albendazole
Levamisole
Pyrantel
Thread Worm
E. Vermicularis
Pyrantel
Mebendazole
Albendazole
Piperazine
Whip Worm
Trichuris trichiura
Mebendazole
Albendazole
Guinea Worm
D. medinensis
Metronidazole
Thiabendazole
Tape Worm T. saginata
T. solium
H. nana
Neurocysticercosis
Praziquantel
Praziquantel
Praziquantel
Albendazole
Albendazole
Niclosamide
Niclosamide
Praziquantel


Table 2. Activity of anthelmintics against different worms

- Round worm Pinworm A. duodenale N. americanus Whip worm Strongyloides Tape worm
Albendazole

+ + + + + + +

Mebendazole

+

+

+

+/-

+

+

+/-

Pyrantel

+

+

+

+/-

-

-

-

Piperazine

+

+

-

-

-

-

-

Levamisole

+

+

+

+

+/-

-

-

Niclosamide

-

-

-

-

-

-

+

Praziquantel

-

-

-

-

-

-

+



+ active
- not active
+/- moderately active

Conclusion:
As far as antibiotics, we mostly use narrow-spectrum antibiotics, however for worm infestations broad-spectrum anthelmintics is a rational choice.
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