How to Treat Common Parasites Safely

CHAPTER 37 How to Treat Common Parasites Safely



Marcia Schwassman, Dawn Logas



CHAPTER OUTLINE


Fleas 390
Insecticides/Adulticides 392

Insect Growth Regulators 395

Insect Development Inhibitors 395

Environmental Flea Control 395

Mosquito and Culicoides Hypersensitivity 396

Ticks 397

Demodicosis 397
Demodex gatoi 397

Demodex cati 397

Ear Mites (Otodectes cynotis) 397

Feline Scabies (Notoedres cati) 397

Cheyletiellosis 398

Cat Fur Mite (Lynxacarus radovsky) 398

Pediculosis 398

Great advances in feline ectoparasite control have been made in the past decade. In particular the development of many new flea control products has made killing fleas without harming their host cat easier than ever before. These new products have both great efficacy and safety. Some of these new flea control products also are effective against other ectoparasites including ticks, mites, and lice.



FLEAS


The cat flea, Ctenocephalides felis felis, is the most important ectoparasite of domestic cats. In addition to causing flea-allergy dermatitis, fleas are the intermediate host for a tapeworm (Dipylidium caninum), and can serve as vectors for some rickettsial organisms, as well as Bartonella henselae. Severe flea infestations can lead to anemia in affected cats, as well as annoyance for the human members of the household.


A basic understanding of the life cycle of the cat flea is essential to developing an effective flea control program. Adult cat fleas spend their entire life on their host. Female fleas will mate as soon as 8 hours after initiating feeding and will begin laying eggs as soon as 24 hours after initiating feeding. Female fleas are prolific egg layers, capable of laying 40 to 50 eggs daily. Eggs and flea feces fall off the host cat into the environment, with the greatest concentration of eggs found in areas where the pet sleeps or rests. Eggs usually hatch within 2 to 10 days. The larvae feed on adult flea feces, nonviable eggs, and other larvae as they progress through three larval stages. Flea larvae prefer higher ambient temperatures and relative humidity and can be ready to pupate in 8 days when reared at 32° C and 75 per cent relative humidity. The majority of flea larvae inside the home develop at the base of carpets. In the outside environment, flea larvae develop best in warm, moist areas (e.g., in leaf litter or mulch under trees and shrubs). Once the larvae are ready to pupate, they spin a cocoon. The cocoon surface is sticky and environmental debris will adhere to its surface. The preemergent adult inside the cocoon can emerge in as few as 5 days, or given favorable conditions, can remain dormant for 200 days if no host is available. Heat and mechanical pressure that indicate the presence of a host stimulate adult emergence from the cocoon. The newly emerged adult flea jumps onto a mammalian host and the cycle starts again.


It is important to note that the cat flea has broad host specificity and also will readily accept dogs, ferrets, rabbits, opossums, and raccoons as hosts.1 Under ideal conditions the entire flea life cycle can be completed in as little as 14 days, or can be delayed by 6 months. It also is important to note that adult fleas represent only 5 per cent of the entire flea population, whereas the immature life cycle stages (eggs, larvae, and pupae) represent the vast majority (95 per cent) of the population.


The concept of integrated pest management is advocated as the most effective way to eliminate existing flea populations quickly as well as prevent reinfestations. Integrated pest management involves the routine use of flea adulticides on the pet to eliminate the most visible part of the flea infestation, as well as control of egg and larval populations in the environment. Both on-pet and premise products are available to control immature stages of the flea life cycle. The preemergent adult fleas within their protective cocoons are the stage most resistant to destruction by any means. Integrated pest management also should provide the added benefit of delaying the development of resistance to the newer insecticides that are so valuable to us. The convenience and outstanding effectiveness of the newer insecticides may tempt the clinician into forgoing good client education and the principles of integrated pest management, but we must avoid this trap if we wish to preserve the effectiveness of these new insecticides (Table 37-1).



Table 37-1 Summary of Commonly Used Flea Control Products in Cats

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In addition to knowledge of flea biology, proper client education concerning the new flea control products also is important. Misconceptions regarding these newer flea control products are common. Many pet owners think that these products repel adult fleas, and therefore assume fleas have become resistant to a product if they see any fleas on their pet during the month after application of the product. In reality they are not seeing resistant fleas, but rather newly hatched adults that have developed from already existing immature stages in the environment. Complete eradication of the entire flea population takes time. Current extensive monitoring efforts have not detected any field collected strains of fleas resistant to imidacloprid,2 although the KS1 laboratory strain of fleas has demonstrated some reduced susceptibility to various insecticides, including imidacloprid and fipronil.3,4 Only one isolate of fleas collected in 1998 from Florida has shown some reduced susceptibility to fipronil.4


Flea control programs also must take into account whether or not the patient is flea allergic. Control of flea populations is desirable to prevent flea-induced anemia and transmission of parasites and infectious diseases; however, complete eradication of fleas from a cat’s environment may be necessary if a cat is extremely flea allergic. Products that stop flea feeding quickly and/or kill adult fleas quickly will be the most beneficial. The amount of effort required will vary, given the degree of hypersensitivity of each individual patient, as well as differences in local environment.



INSECTICIDES/ADULTICIDES



Imidacloprid


Imidacloprid (Advantage) was the first newer generation insecticide released for control of fleas in 1996. It is a topical spot-on that spreads over the skin by translocation and is not absorbed systemically. It is a neonicotinoid (chloronicotinyl nitroguanidine) that acts as an agonist on the postsynaptic nicotinic acetylcholine receptors. By keeping the receptor channels open, it causes constant neuronal stimulation and therefore paralysis and death of adult fleas. Imidacloprid has a very high margin of safety as a result of the affinity of the compound for insect acetylcholine receptors over mammalian acetylcholine receptors. Imidacloprid is absorbed through the flea intersegmental membranes and does not require ingestion by the flea to be effective.5


Imidacloprid kills adult fleas rapidly, with 100 per cent efficacy achieved within 12 to 24 hours after application.6 Efficacy decreases slightly by 28 days after application, with 90 to 96 per cent of newly applied fleas killed in 24 hours.7 One earlier study showed that imidacloprid’s efficacy decreased to below 100 per cent by approximately 3 weeks after application, and that some viable eggs were produced after this time period.3 Therefore initial application of imidacloprid every 3 weeks may be beneficial for the very flea-allergic cat. Imidacloprid also has good larvicidal activity,8 although the principles of integrated pest management would discourage the use of a single chemical as both an adulticide and a larvicide. Imidacloprid is labeled for use in cats 8 weeks of age and older, and can be administered as often as once a week. The label recommends following veterinary advice for use on debilitated, aged, pregnant, or lactating cats.


Proper application of these topical parasiticidal products is essential for optimal effectiveness. The product must be applied to the skin, not the haircoat, to ensure adequate dispersal over the entire body. The product must be applied at the base of the skull. If the product is applied further down the neck incorrectly, the cat may be able to remove much of the product by grooming itself. The authors have seen a few localized reactions consisting of erythema, alopecia, papules, and crusts at the site of application, as well as a few cats who exhibit generalized pruritus following application of the product. Should any adverse reaction be seen, thorough bathing with a degreasing shampoo should remove most of the product. It is unknown if the reactions are to the imidacloprid itself or to the vehicle in the product.


Imidacloprid is available by itself (Advantage) or in combination with moxidectin (Advantage multi). Moxidectin is an avermectin that acts at the γ-aminobutyric acid (GABA) and glutamate-gated chloride channels, causing paralysis of parasites. In addition to controlling fleas, Advantage multi also prevents heartworm disease caused by Dirofilaria immitis, ear mite (Otodectes cynotis) infestations, and controls intestinal hookworms (Ancylostoma tubaeforme) and roundworms (Toxocara cati). Advantage multi is labeled for use in cats 9 weeks of age and older as a once-monthly spot-on. Advantage multi for dogs is labeled for use against lice and Sarcoptes scabiei, so it would be expected to have activity against the cat louse (Felicola subrostratus) as well as Notoedres cati.



Fipronil


Fipronil (Frontline) was introduced shortly after imidacloprid in 1996. It is a phenylpyrazole insecticide with activity against adult fleas and ticks. It binds at the GABA-gated chloride channel, causing excessive neuronal stimulation, paralysis, and death of adult fleas. Some evidence suggests it is the glutamate-gated chloride channels that are affected rather than the GABA-gated chloride channels. Fipronil’s high affinity for insect receptors over mammalian receptors is responsible for its safety in animals. Fipronil kills insects by contact or by ingestion.9 It is available as a spray (Frontline spray), as a spot-on formulation (Frontline Top Spot), and in combination with the insect growth regulator methoprene as the spot-on formulation Frontline Plus.


Fipronil spreads over the skin by translocation via the surface lipids on the skin and is stored in the sebaceous glands from where it is constantly resecreted onto the skin and hair. It is not absorbed systemically. Because of its dependence on the skin surface lipid layer, many dermatologists recommend that fipronil be applied 24 to 48 hours before or after bathing, because bathing can disrupt the surface lipid layer. Fipronil reaches 100 per cent flea kill by 24 hours after application; however, like imidacloprid, efficacy decreased to below 100 per cent by 3 to 4 weeks after application, allowing some viable egg production.3 Viable egg production is prevented by the methoprene in Frontline Plus. These data indicate that some extremely flea-allergic cats may not have adequate flea control with fipronil alone for the full 4 weeks. Unfortunately fipronil is an EPA-registered product, and clinicians are advised that it is a violation of federal law to deviate from the label instructions stating the product should not be reapplied before 30 days. The authors are aware of cases in which fipronil has been used more frequently (e.g., every 3 weeks) without any adverse effects. Clinical trials have demonstrated the effectiveness of fipronil in reducing the clinical signs of flea-allergy dermatitis in cats.10 It is labeled for use in cats 8 weeks and older, and is safe for use in breeding, pregnant, and lactating cats.


Proper application of the product to the skin and not the haircoat at the base of the skull is important for adequate dispersal of the product. Some cats may develop inflammation at the site of application, and the authors have seen a few cats who developed generalized pruritus following application of the spot-on formulation of fipronil. Should any adverse reaction occur, thorough bathing with a degreasing shampoo should remove the majority of the product. As with imidacloprid it is not known if the reaction is to the fipronil itself or to its vehicle. Frontline for dogs also is labeled for use against Sarcoptes scabiei and chewing lice, and appears to have activity against N. cati, cat lice, and O. cynotis.



Selamectin


Selamectin (Revolution) was introduced in 1999. It is a semisynthetic avermectin derived from a bioengineered strain of Streptomyces avermitilis. Selamectin binds to receptors on glutamate-gated (and possibly GABA-gated) chloride channels, leading to flaccid paralysis and death of parasites. Selamectin’s greater affinity for insect chloride channel receptors accounts for its safety in animals. Selamectin is a topical spot-on formulation that is absorbed through the skin into the blood and then is redistributed to the sebaceous glands. Selamectin achieves 100 per cent flea kill by 24 hours after application6; however, by 28 days after application, flea kill has decreased to less than 100 per cent.6,11 Selamectin also has ovicidal and larvicidal activity, but as with imidacloprid, principles of integrated pest management would discourage use of a single product as an adulticide and larvicide. Selamectin has been shown to decrease clinical signs in flea-allergic cats when applied monthly.12 Although some reports demonstrated slower flea kill by selamectin compared with other flea adulticides, selamectin was shown to decrease flea feeding more than imidacloprid or fipronil.13


In cats selamectin is labeled for the control of flea infestations, prevention of heartworm disease caused by D. immitis, control of ear mite (O. cynotis) infestations, and control of the intestinal parasites A. tubaeforme and T. cati. Selamectin can be used in cats 6 weeks of age and older. It also is safe for breeding, pregnant, and lactating cats. Adverse events are infrequent. As with all topical spot-on flea control products, local irritant reactions at the site of application have been reported. Rare instances of vomiting, loose stool, anorexia, lethargy, tachypnea, and muscle tremors also have been reported. Selamectin has been reported to be effective against Cheyletiella spp. when used once monthly for 3 months,14 and for Notoedres cati when used every 2 weeks for three applications.15

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Aug 6, 2016 | Posted by in INTERNAL MEDICINE | Comments Off on How to Treat Common Parasites Safely

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