Benefits and Risks of Pesticide Usage in Pets

Abstract

The purpose of this chapter was to highlight the advantages of applying pesticides for the optimum care of pet animals, while also outlining the adverse effects that may be associated with their use. Pesticides can be defined as substances that can be applied for the prevention, control or eradication of unwanted organisms in living systems or in the environment. Companion animals, fondly called “pets” include dogs, cats, ferrets, pet birds and some laboratory animals like albino rats, rabbits, guinea pigs, etc. Pesticides are usually applied on pets to control ectoparasites like ticks, fleas, mites, among others. However, pets may be poisoned by pesticides if their dosages and appropriate routes of administration are not strictly adhered to. Pesticides should be administered to pets by Veterinarians and other suitably qualified personnel. Subsequently, the pets should be monitored for signs of toxicity and be treated promptly if such develop.

1. Introduction

Pesticides are substances or mixtures of substances that possess unique chemical properties for the control of detrimental pests and insect vectors [1, 2]. Pests are living organisms that pose health risks such as biting and sucking, transmission of allergy-inducing constituents, diseases, as well as parasites, thereby causing harm to humans, animals and various components of the ecosystem [3]. Pesticides can be classified as algicides, insecticides, fungicides, herbicides, rodenticides, pyrethroids, fumigants, miticides, molluscicides, etc. with discrete chemical characteristics that decrease economic, health, and environmental risks elicited by pests [4, 5]. The inappropriate application of pesticides can evoke deleterious outcomes in several organisms and the environment. Notably, pesticides do not usually differentiate between pests and other living things, consequently they may cause injury to the organisms they encounter [1].

It has been observed that pesticides may gain access into biological systems through diverse routes. For instance, organophosphate and carbamate insecticides are quickly absorbed after dermal, oral, and inhalation exposures [6]. Damalas and Koutroubas [7] reported that pesticide applicators are commonly exposed to pesticides through the dermal route. Besides, pesticides may be absorbed dermally through a splash, spill, or spray device, when being mixed, loaded or disposed of [8]. Liquid preparations of pesticides are more readily absorbed through the dermal route and other body tissues compared to powders, dusts and granular types [7]. According to [8], oral exposure to a pesticide may occur by accident or intentionally. Moreover, marked damages to the nasal, throat and pulmonary tissues have been observed after inhalation of appreciable quantities of pesticides [7].

Furthermore, exposure of populations to pesticides have been associated with negative health conditions including cancers, congenital disorders, immunological aberrations, respiratory, neurobehavioral and reproductive deficits [9]. These undesirable effects of pesticides may be evoked in several tissues and organs through genetic impairments, epigenetic alterations, mitochondrial dysfunction, oxidative damage, endoplasmic reticulum stress, endocrine disruption, among others [10]. Some of the clinical manifestations of pesticide toxicosis are confusion, agitation, lacrimation, salivation, emesis, bronchospasm, respiratory failure, micturition, diarrhoea, muscle weakness, paralysis, fasciculations, etc. [11].

Pets are animals that are domesticated and catered for by human beings for companionship, pleasure, provision of services and assistance, among others. They include dogs, cats, ferrets, pet birds, rodents, rabbits, guinea pigs, as well as exotic species like cubs, reptiles, etc. Pets are an essential part of human lives and they have been existing with human beings for thousands of years [12]. They are continually exposed to fleas and ticks. These ectoparasites may cause distress, itching, anaemia and systemic infections in the pets [13]. It is crucial to control ectoparasites in companion animals to prevent vector-borne diseases that may eventually result in high morbidity and mortality [14]. Moreover, the presence of fleas and ticks on pets may make their owners vulnerable to parasitism and zoonosis [15, 16].

The purpose of this chapter was to highlight the advantages of using pesticides for the optimum care of pet animals, while also outlining the adverse effects that may be associated with their applications.

2. Benefits of pesticide usage in pets

Insecticides such as organophosphates (e.g., malathion, diazinon, phosmet, fenthion, chlorfenvinphos, and cythioate) and carbamates (e.g., carbaryl and propoxur) are used to control insect and nematode infestations in animals [17]. They are formulated as sprays, pour-ons, baits, collars, etc. [17]. Carbamates are used more frequently because they are considered safer than organophosphates. However, some signs of intoxication linked to the application of carbamates are abdominal cramping, emesis, diarrhoea, dyspnoea, seizures, among others [18]. Organophosphate and carbamate insecticides competitively impede acetylcholinesterase by binding to its esteric site [19]. The excessive acetylcholine that ensues brings about unwarranted stimulation of smooth muscles and glandular secretions [17]. However, the inhibition of acetylcholinesterase by organophosphates is irreversible, while the inhibition by carbamates is reversible [20]. The classification, examples, routes of administration and mechanisms of toxicity of some insecticides applied to pets are shown in Table 1.

Classification of insecticides	Examples	Mode of administration	Mechanisms of toxicity
Organophosphates	Diazinon, phosmet, cythioate	Sprays, pour-ons, collars	Inhibition of acetylcholinesterase [19]
Pyrethroids	Cypermethrin, permethrin	Shampoos, dips, spot-ons and sprays	Interruption of sodium channels in neurons [21, 22]
Carbamates	Carbaryl and propoxur	Sprays, pour-ons, collars	Inhibition of acetylcholinesterase [19]
Neonicotinoids	Imidacloprid, dinotefuran, nitenpyram	Imidacloprid and dinotefuran are applied as spot-on topical products. Nitenpyram is administered per os.	Act as agonists on the postsynaptic acetylcholine receptors in insects [23, 24]
Isoxazolines	Fluralaner, afoxolaner, sarolaner	Oral administration	Blockage of arthropod ligand-gated chloride channels [17]
Benzoylphenylurea derivative	Lufenuron	Oral suspension and injectable preparation for cats. Oral tablets for dogs.	Chitin (exoskeleton) synthesis inhibitor [25]
Insect growth regulators	Methoprene, fenoxycarb, pyriproxyfen	Oral suspensions, sprays and spot-ons	Mimic insect hormones, thereby interfering with the growth and development of insects [26]
Oxadiazine insecticide	Indoxacarb	Administered topically in a spot-on formulation	Bioactivation to an active metabolite that blocks the voltage-gated sodium ion conduits in insects [18]
Phenylpyrazole insecticide	Fipronil	Topical administration	Binds to gamma-aminobutyric acid receptors and the glutamate-gated chloride channels in the central nervous systems of invertebrates [13, 27, 28, 29]
Macrocyclic lactones	Selamectin, aprinomectin, milbemycin	Topical administration	Bind to glutamate-gated chloride channels in the nervous systems of parasites [18]
Formamidines	Amitraz	Available as a dip. Also formulated as impregnated collars for dogs	Binds to octopamine receptors for its insecticidal effects [18]
Spinosyns	Spinosad	Formulated as edible tablets for dogs and cats	Targets the binding sites on nicotinic acetylcholine receptors [18]

Table 1.

Classification, examples, route of administration and mechanisms of toxicity of some insecticides applied to pets.

Pyrethroids are synthetic derivatives of natural pyrethrins derived from the plant, Chrysanthemum cinerariaefolium, and they contain esters of chrysanthemum acid [21]. They are 2250 times more poisonous to insects compared to higher organisms [30]. This is because insects possess additional sensitive sodium channels, a reduced conformation and lower body temperature [30]. Permethrin, a type I pyrethroid, exists in the form of a liquid, yellow-brown and brown crystals, and it is soluble in organic solvents [31]. It may enter the body through the dermal, oral and inhalational routes [32, 33]. It is found in shampoos, dips, spot-ons, and sprays for the control of ectoparasites in companion animals [33]. Also, it is used for the treatment of scabies and lice [31, 34, 35]. Permethrin evokes injury to insect neurons by elevating the impulse conduction, thereby causing paralysis and death of insects [21]. It is broken down in the body by hydrolysis, esterification, oxidation and conjugation [30, 36].

Its metabolites include cis-3-(2,2 dichlorovinyl) 2,2 dimethylcyclopropane-1-carboxylic acid, trans-3-(2,2-dichlorovinyl)-2,2 dimethylcyclopropane-1-carboxylic acid) and (3 phenoxybenzoic acid) [31]. The metabolites of permethrin are principally excreted in the urine and faeces [21].

Furthermore, cypermethrin, a type II pyrethroid insecticide, is used for the control of pests in agricultural, public and animal health programmes [37]. It evokes toxicity through the interruption of sodium channels in neurons, thereby disrupting neuronal transmission [22]. Also, it produces oxidative stress in living organisms [38, 39, 40]. Type II pyrethroids are more neurotoxic relative to type I pyrethroids because of their α-cyano constituents [41].

Another class of insecticides administered for pest control in pets are neonicotinoid insecticides such as imidacloprid, nitenpyram and dinotefuran (stated in Table 1). Imidacloprid is structurally similar to nicotine, and is endorsed as a topical spot-on for dogs, as well as for agricultural purposes [14, 23]. It exerts its insecticidal activities by binding to the acetylcholine receptor on the postsynaptic region of insect neurons, thereby averting acetylcholine binding [23, 24]. Besides, imidacloprid has been reported to elicit oxidative stress and cause injury to crucial biological molecules such as deoxyribonucleic acid, proteins and lipids [42]. Moreover, nitenpyram is administered per os to eliminate fleas in dogs and cats [18]. It undergoes fast absorption with utmost blood concentrations attained within one and a half hours, and thirty-six minutes in dogs and cats respectively [18]. Dinotefuran is applied as a topical spot-on with different formulations for dogs and cats against external parasites like fleas, flies, lice, etc. [43].

Fluralaner (an isoxazoline) is a systemically administered insecticidal and acaricidal formulation that elicits long-acting efficacy after oral administration to dogs [44]. Another isooxazoline, afoxolaner, has been reported to be efficacious in dogs and cats against fleas [45, 46, 47], ticks [46], and mites [47, 48, 49, 50]. It is detected in plasma 20–30 minutes following administration through the oral route and it attains its uppermost level in 2–4 hours [51]. Sarolaner is a broad spectrum isooxazoline with efficacy against fleas, ticks and mites in dogs [52, 53]. Isoxazolines bind to the ligand-gated chloride channels in insects and acarines [17]. Consequently, the presynaptic and postsynaptic transmission of chloride ions across the cell membranes ensue, thereby causing hyperexcitation and uninhibited activity of the central nervous system, ultimately resulting in the death of ectoparasites [17].

Lufenuron, a benzoylphenylurea derivative, is a chitin synthesis inhibitor [25]. It is available as an oral suspension and injectable formulation for cats, and an oral tablet for dogs [17]. It eliminates emerging larvae within the egg or after hatching, and female fleas feeding on treated animals are hindered from producing viable eggs or larvae [25].

Methoprene is an insect growth regulator that mimics insect hormones, thereby interfering with the growth and development of insects [26]. It is formulated as suspensions, emulsifiable and soluble concentrates, sprays and spot-ons, etc. [17]. It is used for flea control in dogs and cats, marine mosquito control, as well as agricultural and domestic pest control [54].

Fipronil is a phenylpyrazole insecticide that is approved for agricultural usage, pest control, as well as topical flea and tick treatment for companion animals [55]. It dissolves in sebum because of its high lipid solubility and it is disseminated throughout the body for the manifestation of its insecticidal effect [13]. It has been shown that fipronil binds non-competitively to γ-aminobutyric acid (GABA) receptors and the glutamate-gated chloride channels in the central nervous systems of invertebrates (e.g., fleas and ticks), thereby eliciting excessive excitation [13]. Additionally, fipronil also binds to mammalian GABA receptors, [27], and engenders oxidative stress through the production of reactive oxygen species [28, 29]. Some investigators have asserted that the foremost metabolite of fipronil, fipronil sulfone, exerts a more robust inhibitory effect on GABA_A receptors and brings about cell impairment at lesser concentrations compared to fipronil [27, 28, 29].

Selamectin, aprinomectin and milbemycin are macrocyclic lactones that are used for the control of endoparasites and ectoparasites in dogs and cats [18]. They are widely administered for the prevention of heartworm disease in dogs [56]. Selamectin and aprinomectin are semisynthetic avermectins, while moxidectin is semisynthetic. These substances bind to glutamate-gated chloride channels in the nervous systems of parasites, and this culminates in a speedy and sustained entry of chloride ions into neurons [18]. As a result of this, the activity of the neurons is impeded and paralysis of the parasites occurs. The macrocyclic lactones are administered topically, and are swiftly absorbed through the dermal route. Selamectin exhibits effective control against the flea, Ctenocephalides felis [57, 58], biting lice (Felicola subrostratus) and ear mites (Otodectes cynotis), among others in cats [59].

Indoxacarb is an oxadiazine insecticide that is administered topically in a spot-on formulation for the control of fleas on companion animals [18]. It is found in insect baits for home use and granules, as well as liquids for agricultural applications [60]. Moreover, it is bioactivated to an active metabolite that blocks the voltage-gated sodium ion conduits in insects [18].

Furthermore, formamidines are acaricidal compounds that exert their effects through binding to octopamine receptors [18]. Amitraz is the only approved formamidine for use in veterinary medical practice, and it is applied primarily as an acaricide to control ticks and mites [18]. It is available as a dip for the control of demodicosis in dogs, as well as the control of scabies. An amitraz-impregnated collar is also marketed for the control of ticks on dogs.

Spinosyns are a family of insecticides obtained from the fermentation of an actinomycete, Saccharopolyspora spinosa [18]. Spinosyns A and D are the main products of the fermentation procedure, as well as the principal components of Spinosad [61]. Spinosyns mostly target the binding sites on nicotinic acetylcholine receptors, and they also influence GABA receptor function [18]. This ensues in spontaneous muscle contractions, prostration, tremors, and paralysis of insects. Spinosad is used to control numerous insects and it is formulated as edible tablets for dogs and cats [61].

3. Risks of pesticide usage in pets

There is a predominant exposure of human and animal populations to pesticides and this may be associated with detrimental effects on their health status [4, 62]. According to [17] , clinical signs of pesticide intoxication can occur within a short or long duration of exposure, depending on the dose, route, and noxiousness of the pesticide administered. It has been documented that those pesticides have severe effects on non-target organisms, including various components of the ecosystem [63].

Various pesticides, especially, insecticides applied to pets for the prevention and control of ectoparasites may be associated with some adverse effects. For instance, permethrin poisoning may produce symptoms including epidermal lesions, pharyngitis, salivation, nausea, emesis, abdominal pain, gastrointestinal mucosal irritation and dyspnoea in animals [32, 63, 64]. Cats are more likely than dogs to develop pyrethroid toxicosis because the feline liver cannot conjugate glucuronide efficiently, and conjugation with glucuronide is essential for permethrin metabolism [33]. Permethrins are regarded as the commonest aetiology of poisoning in cats in the United States of America [65]. Cats may be exposed to permethrin from dermal application of topical formulations, oral intake, and direct contact with dogs administered with it topically [66]. The commonest clinical signs of permethrin intoxication in cats are muscle tremors and seizures, but hypersalivation, depression, emesis, anorexia and even death may ensue [33].

Moreover, alpha-cypermethrin (a synthetic pyrethroid like permethrin) intoxication can cause lacrimation, salivation, nausea, emesis, diarrhoea, mucosal irritations, motor coordination dysfunction, chorea, inactivity, tremors and clonic seizures [30, 36]. It has been observed that dogs usually exhibit signs of intoxication such as shaking of their limbs, slight muscle fasciculation, rubbing of the application site, distress and uneasiness after dermal administration of pyrethrins/pyrethroids [67, 68, 69].

Cats are more susceptible to insecticides that inhibit acetylcholinesterase such as organophosphates and carbamates compared to dogs [70]. Also, neonate, geriatric and incapacitated animals are more vulnerable to these groups of pesticides. Organophosphates and carbamates elicit muscarinic, nicotinic, and central nervous system signs of toxicity in biological systems. The muscarinic signs are salivation, lacrimation, urination, defecation, respiratory distress, vomiting, pupillary constriction and reduced heart rate [70]. The nicotinic symptoms include muscle tremors, fasciculations, feebleness, incoordination, and paresis that may culminate in paralysis [71], while the central nervous system signs of toxicity comprise hyperactivity, incoordination, convulsion and unconsciousness [71].

The predominant clinical signs linked to isoxazoline toxicity are emesis, anorexia, diarrhoea and exhaustion in dogs and cats [17]. The administration of lufenuron to cats causes pain at the site of injection and oedema [17]. Additionally, dogs treated with the parenteral formulation of lufenuron developed a marked local reaction [25].

Some investigators asserted that young animals are more likely to exhibit exhaustion and incoordination after oral dosing with methoprene (an insect growth regulator) [71], while the commonest clinical signs of toxicity seen in companion animals exposed to indoxacarb are anorexia, emesis, diarrhoea and lethargy [17]. Moreover, amitraz (a formamidine insecticide, mentioned in Table 1) can cause temporary pruritus, urticaria and oedema after the initial administration to pets [18]. In addition, a brief sedation has been recorded in dogs after an amitraz bath that may last for one day or three days in puppies.

4. Conclusion

This chapter review presented information on the benefits and risks of the applications of pesticides, mainly insecticides, to pets. Even though pests are harmful to companion animals and their owners, they should be controlled cautiously with the use of appropriate pesticides approved by Veterinarians and relevant regulatory agencies in different countries. This will ensure that the hazards inherent in the pesticides are adequately mitigated. Also, there is a need for researchers, Veterinarians, related health care professionals and pesticide manufacturers to collaborate and find out innocuous methods for the prevention and control of pests in pets. This effort can improve human, animal and ecosystem health and integrity.

Acknowledgments

The authors are thankful to the staff of the Faculty of Veterinary Medicine and the Veterinary Teaching Hospital, University of Abuja, Nigeria, for their support.

Conflict of interest

The authors declare that there is no conflict of interest.