Ectoparasiticides

 

Ectoparasiticides

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Ectoparasiticides are the agents that are used to kill or eradicate animal parasites infesting the external body surface. Ectoparasites of veterinary importance mainly belong to the classes of insecta and arachnida. The class insecta includes flies, fleas and lice whereas class arachnida contains ticks and mites. Ectoparasites are responsible for the transmission of diseases (by acting as vectors), loss of condition, production deficit and damage to fleece or hide.

 

Techniques for the application of ectoparasiticides

 

Several methods have been employed for the application of ectoparasiticides on animals. Some important methods are mentioned below.

1. Dips: In this method, the entire body of animal is immersed in medicated water for a sufficient period of time.  This technique is more useful for sheep and goats. In most cases, a permanent dipping vat is constructed in the farmhouse for this purpose.

2. Sprays: Spray machines (which are generally portable, may be manually-operated or motor driven) can be used for the application of ectoparasiticides on all animal species.

3. Pour-on: This method involves the pouring of ectoparasiticide solution along the animal’s dorsal midline behind the shoulders to the hipbones. Pour-on treatments are useful for the control of horn-flies, lice and some other arthropods of animals.

4. Spot-on: In this method, a specified amount of ectoparasiticide (in the form of ointment or cream) is applied over the head or back region of the animal.

5. Ear-tags: In this method, a fragment of polyethylene is impregnated with ectoparasiticide and tagged in one or both ears of the animal. The insecticide impregnated tags can also be attached to the tail (called tail tags) or halters (in horses). The drug from the tag is slowly released by volatile action or by the body movements of animal and spreads over the body surface. Most tags act for about 4 to 5 months.

6. Back rubber: A cloth material (called back rubber) is charged with ectoparasiticide diluted with oil. This back rubber is wrapped around a wire or chain which is then suspended between the two poles of a pasture. When cattle rub their bodies against the cloth, insecticide is transferred to their bodies. This technique is quite useful to control flies and other insects that visit cattle for short duration.

7. Dusting: Dusting is mostly done manually in which the insecticide powder is applied over the parasite-infested area of the animal body.

8. Collars: Collars mostly consist of a solid thermoplastic resin with a liquid ectoparasiticide on relatively high vapour pressure distributed throughout the collar. The drug is slowly released in the form of vapours and covers the animal body. 

 

Classification of ectoparasiticides

 

Ectoparasiticides are classified on the basis of their chemical structure into following categories.

(a) Pyrethrins and Pyrethroids: Pyrethrins are natural insecticides produced from the extract of Pyrethrum flowers while Pyrethroids are the synthetic analogues of Pyrethrins. These agents act on sodium channels in the axonal membrane and enhance inward sodium flow and suppress outward potassium flow. This action causes repetitive discharge of membrane depolarization and subsequent death of target ectoparasites. Pyrethrins are lipophilic and readily absorb orally, dermally or via lungs. Their metabolites are excreted in the urine after conjugation with glycine, glutamic acid or sulphate. Although Pyrethrins are still included in many ectoparasiticide formulations, their use has declined mainly due to the development of more efficacious and stable synthetic

 

Pyrethroids.  Pyrethrin I, Pyrethrin II, Cinerin I, Cinerin II, Jasmolin I and Jasmolin II are the important examples of Pyrethrins. While important Pyrethroids include Permethrin, Cypermethrin and Deltamethrin. Pyrethroids are amongst the safest ectoparasiticides with minimal adverse effects when used according to labeled instructions. Overdosage or accidental oral exposure results in acute toxicity characterized by hypersalivation, inco-ordination, vomiting, diarrhea and seizures. Dermal exposure may lead to dermatitis, blister formation and itching or burning sensation in the skin. The toxicity of Pyrethroids is inversely proportional to change in ambient temperature. There is no specific antidote of Pyrethroids toxicity however seizures can be controlled by the administration of Diazepam.

 

(b) Juvenile hormone analogues (Juvenoids): These compounds mimic (enhance) the action of endogenous insect growth hormones (called Juvenile hormones) which are normally responsible to maintain the larval stage of insect by preventing its metamorphosis and maturation to pupa and then adult stage. Juvenoids are synthetic analogues of Juvenile hormones that are given from exogenous sources to impair the metamorphosis of target insects. Juvenoids maintain the larval stage of target insects and inhibit their further development (to pupa and subsequently adult stage) which ultimately leads to the death of these insects.  Juvenoids also possess ovicidal, larvicidal and embryocidal effects. Although Juvenoids are primarily developed for the control of fleas but they are also effective against certain other insects. Methoprene and Fenoxycarb are common examples of Juvenoids.

 

(c) Chitin synthesis inhibitors: Chitin is an integral component of the exoskeleton (cuticle) of insects. The survival of insects largely depends upon the acquisition of cuticle as it serves as the first line of defense against predators, pathogens and it also helps in locomotion by providing attachment for muscles. Lufenuron and Diflubenzuron are commonly used compounds for the inhibition of chitin synthesis of insects.

Lufenuron: On oral administration it accumulates in adipose tissues of the treated animal and is then slowly released for few weeks into the systemic circulation. Fleas take up the drug from the host’s circulation and transfer it to their eggs where it inhibits chitin synthesis by interacting with the action of chitin synthase enzyme.

Diflubenzuron: After oral administration almost 80% of this drug remains unabsorbed and is passed into the faeces of host animal where it acts upon the eggs and larvae of manure-breeding insects and ultimately inhibits their development.

 

(d) Formamidines: Amitraz is the only member of this group that is used as an ectoparasiticide. It indirectly enhances the level of Epinephrine and Norepinephrine (and thus enhances the occurrence of adrenergic neurotransmission) by inhibiting the action of MAO (Monoamine oxidase) enzyme that is normally responsible for the metabolism/degradation of amine neurotransmitters (including Epinephrine and Norepinephrine etc.) in the nervous system of susceptible insects. The toxicity of Amitraz occurs as a result of inhibition of mammalian MAO enzyme and is manifested by sedation, decreased pulse rate, hypothermia, vomiting and diarrhea. α₂ receptor antagonists such as Yohimbine or Tolazoline should be used to treat the adverse effects associated with Amitraz toxicity.

 

(e) Organophosphates: They indirectly increase the level of acetylcholine (by irreversibly inhibiting the action of actylcholinesterase enzyme that is responsible for the metabolism/cleavage of acetylcholine) and thereby mimic cholinergic neurotransmission in susceptible insects which leads to spastic paralysis and subsequent death of parasites. Trichlorfon, Dichlorvos, Coumaphos, Parathion, Malathion and Ethion are frequently used Organophosphates. The detailed pharmacological considerations of Organophosphates have already been discussed in the chapter dealing with anthelmintic drugs.

 

(f) Carbamates: Their pharmacological and toxicological aspects are quite similar to those of Organophosphates except that Carbamates reversibly inhibit the actylcholinesterase enzyme. Carbaryl is the most commonly used member of this category.

 

(g) Macrocyclic lactones: They are known as endectocides as they are active against endoparasites (including nematodes, lung worms and immature heart worms) as well as ectoparasites (like cattle grubs, mange mites, lice and horn flies). The detailed pharmacological considerations of Macrocyclic lactones have already been discussed in the chapter dealing with anthelmintic drugs.