Shih Tzu - Flea Control Products

Pyrethrins:
Pyrethrins are insecticides derived from the chrysanthemum plant. They have good "knockdown" and are relatively safe when properly applied. Pyrethrins are found with enhancers, typically a product called piperonyl butoxide, to reduce the chance of resistance by the flea. This product is available in powders, shampoos, sprays, foggers, etc. It is widely used and only moderately toxic.

Pyrethroids (Synthetic Pyrethrins):
Pyrethroids are synthetic products, derived from the pyrethrins. Pyrethroids generally have better residual action and are less toxic than pyrethrins. Permethrin is the most commonly used pyrethroid. Pyrethroids are found in shampoos, rinses, foggers, pour-ons, and an assortment of household sprays.

Citrus derivatives:
Fipronil: Fipronil is a member of the phenylpyrazole class of insecticides. This compound is very specific for invertebrates (e.g., insects) and acts basically as a neurotoxin killing fleas on contact. The product is sold as Frontline®; (Merial) as a spray to be applied topically once monthly or as a topical "pour-on" (Top SpotTM;). Gloves should be worn by humans applying this product. If the spray is used, it should be applied in a well-ventilated area.

Advantages:
This product claims to be effective even when the animal becomes wet by bathing or swimming. 2) The product can be used on dogs and cats down to 8 weeks of age. Disadvantages: None known at this time. The product has only been available for a relatively short time in the United States, but based on reports from overseas, is quite effective.

Selemectin:
Selemectin is a member of the avermectin class of insecticides. (Similar compounds include ivermectin and milbemycin oxime.) This product is sold as Revolution TM; by Pfizer Animal Health. Selemectin, like most avermectins, works by inducing a form of neuromuscular paralysis. Mammals are less susceptible than are nematodes and arthropod parasites to the specific mechanism of action, therefore these agents have a wide margin of safety to mammals. This product is labeled for parasite control against a large number and type of parasitic agents.

Labeled uses of selemectin include:
Flea control, tick control (Dermacentor variabilis), heartworm prevention, treatment of scabies (aka, sarcoptic mange) and otoacariasis (aka, ear mites), and treatment of intestinal roundworm and hookworm infestations in cats. It is considered safe in ivermectin-sensitive collie dogs and in breeding male and female dogs and cats, and pregnant and lactating dogs and cats. If ingested by cats, it may result in salivation and vomiting.

Insect growth regulators and development inhibitors:

General comments:
Insect growth regulators (IGRs) and insect development inhibitors (IDIs) are key agents for long term flea control. However, the agents do not kill adult fleas and therefore, other traditional flea control products will be necessary in an appropriate flea control program. In addition, the IGRs and IDIs appear to be very safe.

Insect growth regulators are juvenile hormone analogs (resemble the natural growth factor found in the flea) that work by interfering with egg development and molting from various life stages of the flea. The two most commonly available IGRs are methoprene (Precor®) and fenoxycarb. IGRs are found in sprays, foggers, and flea collars. They may be used on the pet or applied to the environment. An advantage of the IGRs is their high margin of safety. These are products that would be among the most safe for application in a household with infants or other people intolerant to insecticides. Methoprene is broken down by ultraviolet exposure, so fenoxycarb is the agent of choice for the outdoors. These products do NOT kill adult fleas, and therefore, are generally combined with an insecticide in most products.

Note: As of March 1996, CIBA (now Navartis Animal Health) has voluntarily withdrawn fenoxycarb from the market after government tests with exaggerated doses given to laboratory animals indicated that the product may be carcinogenic. As labeled the product appears to be very safe. Fenoxycarb may or may not return to the veterinary market when current supplies are exhausted.

Pyriproxyfen (Nylar®,McLauglin Gromley King Co.) is an insect juvenile hormone analog, similar to methoprene and fenoxycarb. Pyriproxyfen is available in premise sprays/foggers (e.g. EctoKyl®,DVM; Knockout®,Virbac), rinses for application to dogs (EctoKyl®,DVM), and sprays for topical application to dogs in products (e.g., Knockout®, Allerderm/Virbac) combined with adulticides.

Advantages: This insect growth regulator is stable in UV light and is extremely safe. Pyriproxifen is proported to bind to hair and skin, which allows it to maintain efficacy even when an animal becomes wet (e.g., bathing or swimming)! If true, this would be a tremendous advantage in a flea control product. Disadvantages: KnockoutTM; is currently only labeled for use on adult dogs.

Insect development inhibitors work by interfering with a particular aspect of development. Most of these products interfere with the synthesis of chitin, a protein necessary for maturation and function of the flea exoskeleton. Chitin inhibitors include lufeneron, pyriproxyfen, and cyromazine. Cyromazine is not commercially available in the United States, but work in Australia has shown lufeneron (in a combination product with diethylcarbamazine) to be very effective in reducing the number of fleas in a controlled environment.

Lufenuron (Program®,Norvartis): This product is administered once monthly as a tablet for dogs and as a suspension for cats. Serious adverse effects of the product have not been reported. As with all insect growth regulators, lufeneron is NOT appropriate as the only flea control product if fleas are present in a household. IF lufenuron is used as the only flea control product in a household with flea infested animals, it will take 12-16 weeks to reduce the flea population to a minimal level. Therefore, insecticides effective against adult fleas should be applied to the animals and environment to maximize flea control and damage.

Lufenuron is available in an injectable (subcutaneously) form for cats. It provides up to 6 months of activity for flea control. The product is available only by prescription from a licensed veterinarian.

Sentinel tm; is a new product by Novartis Animal Health US Inc. that combines lufenuron with a drug called milbemycin oxime. Milbemycin oxime is available (and has been for years) as a once monthly oral medication to control heartworm infection AND roundworm (Toxocara canis & Toxascaris leonina), hookworm (Ancylostoma caninum), and whipworm (Trichuris vulpis) infestations (Interceptor®). The combination of these two compounds provides a convenient preventive medication for a variety of parasitic problems seen in dogs. Sentinel is now available in flavored tablets to make it easier to administer to dogs.

Advantages: Products containing lufenuron are given orally once monthly, making it extremely convenient. 2) Lufenuron is a very effective insect growth regulator. Disadvantages: Fleas from outside sources can still bite the pet causing varying degrees of skin problems, IF the animal is exposed to an outside source of fleas (e.g., the park, neighbor animals, boarding).

The administration of lufenuron or use of other insect growth regulators is an important part of a flea preventive program.

Special Considerations

Safety: Product safety is affected by:
Toxicity of the insecticide. This varies from agent to agent, and is measured by the LD50 of an insecticide...which is the amount of agent required to kill 50% of the animals when given at that dose. Insecticides in increasing order of toxicity (based on the LD50 in rabbits) are: microencapsulated methoprene (an IGR), microencapsulated pyrethrins, cirtus derivatives (d-limonene), pyrethroids, pyrethrin, malathion, carbaryl, bendiocarb, chlorpyrifos, propoxur, lindane, and dichlorvos. From this data, we see that microencapsulated pyrethrins are relatively safe, while chlorpyrifos is relatively toxic!

Amount of exposure: Obviously, the more insecticide to which the animal (or human) is exposed, the greater the risk.

Summation effects: Drugs in the same class cause toxic effects by the same mechanism. Therefore, is it inappropriate to administer different drugs of the same class (e.g., organophosphates) to a pet. For example, an organophosphate flea powder should not be applied to an animal receiving weekly organophosphate rinses!

Microencapsulation: This is a process by which the insecticide or other agent is incorporated into microscopic spheres. These spheres release their contents under specific situations. Microencapsulation has two effects on a drug.

First, microencapsulation will reduce the availability of the drug to quickly kill a flea. It reduces the knockdown ability of a drug, but enhances the residual activity, because it "protects" the drug from breakdown. For example, pyrethrin has good knockdown but is rapidly inactivated by UV light so it has poor residual action. When microencapsulated, pyrethrins have poor knockdown, but good residual action.

Second, microencapulation increases the safety of the product. For example, the LD50 of pyrethrin is 1500 mg/kg, but when microencapsulated, the LD50 is greater than 34,000 mg/kg. Clearly, the process is a desirable one considering safety to animals and humans exposed to the drug!

Insecticides currently available as microencapsulated products include pyrethrins, chlorpyrifos, and diazinon.