Introduction....Life Cycle...Insecticides...Organophosphates...Pyrethrins...Insect Growth Regulators...Dessicants...Biologicals....Central Nervous System Modulators Flea Control

Introduction: Fleas are external, blood-sucking parasites of dogs (and other canids), cats (and other felids), pigs, rodents, rabbits, people, birds. Ctenocephalides species infest dogs and cats, naturally, but can also jump on and bite other animals and people. Adult fleas bite the host animal, sucking blood and sometimes, transmitting diseases and/or other parasites (e.g. plague, cat scratch fever, peliosis (both from Bartonella henselae), human and cat/dog tapeworms, other worms, blood parasites (Hemobartonella felis). Bites are painful, cause local itching, produce intense discomfort and at times secondary infection (due to excessive self trauma from scratching) , especially in animals particularly sensitive (allergic) to flea saliva. Additionally, infestation of very young, small animals can result in severe anemia (from blood sucking)...and even death. The cat flea, Ctenocephalides felis, is believed to transmit the blood pathogen, Hemobartonella to cats.

Life Cycle: The flea metamorphosis consists of a cycle of eggs to larvae to pupae to adult stages....more pictures of these are shown below...
Insecticides for Controlling Fleas: Insecticides are used in area sprays, dips, shampoos, spot-ons, oral and topical preparations, and flea collars whose purpose(s) is to manage fleas in companion animals by preventing, controlling and/or obliterating these nasty parasites. Classes of insecticides include organophosphates, carbamates, pyrethrins, insect growth regulators (IGR's) and insect development inhibitors (IDI's), dessicants, biologicals, and selective central nervous system (CNS) modulators. In some instances, products are formulated to enhance initial "knockdown" effect by quickly killing the egg stage, or by acting synergistically to enhance or prolong the initial insecticidal activity of the primary product. These classes of product are discussed separately:
Organophosphates & Carbamates: These insecticides affect fleas (and other ectoparasites) via preventing the natural degradation of the neurotransmitter (acetylcholine) at neural junctions (synapses). The result is persistent neurotransmitter-mediated neural stimulation, resulting in hyperactivity, respiratory paralysis (due to muscle exhaustion), convulsions and death. Unfortunately, the same mechanism of activity can also result in toxicity and death to animals. Since these insecticides can be absorbed through the skin under certain circumstances, they are potentially dangerous...especially in certain breeds of dogs (e.g. Whippet and Greyhounds) and to cats. In addition to the neurological mechanism of toxicity, excessive exposure to organophosphates in cats can lead to fatal, necrotizing pancreatitis. Organophosphates, in this author's opinion, should not be used on cats.

Example of Organophosphates used in flea products applied to animals (or to flea collars) OR as area/premises treatments include Chlorpyrofos, Coumaphos, Cythioate, Diazinon, Dichlorvos, Dioxathion, Malathion

Example of Carbamates include Carbaryl, Methomyl and Propoxur

Pyrethrins: Pyrethrins are botanical insecticides derived from plant material, particularly the plant Chrysanthemum cinerariaefolium. They also act on the nervous system, but are generally less toxic to animals than the organophosphate-carbamate insecticides. Their effect is also shorter lived than the latter. Synthetic pyrethrins are called pyrethroids, and are more potent than their natural counterparts. Both kinds of products are used in various topical and area/premises treatments

Examples of Pyrethroids include allethrin, permethrin. The latter is not so well tolerated by cats and toxicity is all too common.

Insect Growth Regulators/Insect Development Inhibitors: These act to inhibit the development of flea cycle at the larval or egg stages, respectively.. They do not kill adult fleas...their purpose is to control the population of fleas in the environment! Pupae are resistant to insect growth regulators.

Examples of Insect Growth Regulators:

Methoprene or derivatives of methoprene are added to many area treatment products. Examples include Precor ® and Altosid SR10®. Methoprene competes with endogenous insect develop hormones, preventing the breakdown of juvenile hormone, which when present prevents the development of eggs to larvae to pupae. Methoprene does not kill adult fleas, but is otherwise very safe.

Pyriproxifen is commercially available as Nylar®. It is found in animal and area sprays, spot-ons, some flea collars. It directly sterilizes eggs

Example of Insect Development Inhibitor: Lufenuron (Program®,  Sentinel®). Lufenuron inhibits the formation of chitin...the external skeleton of fleas. It does NOT kill the adult fleas, and the exact mechanisms of inhibition are not known (the manufacturer has not explained these). Adult female fleas ingest the product during a blood meal; the flea's eggs are then rendered sterile...they can not develop into other stages. The product is given orally (pill or liquid) monthly or as a long-acting (6 month duration) injectable preparation. It is considered very safe for puppies and kittens 6 weeks or older. The combination oral antiparacidal (Sentinel®) is approved for animals 4 weeks and older.

Dessicants:in preparation...
Biologicals: in preparation...
Central Nervous System Modulators: The current primary products in this category are fipronil(Frontline® , Frontline Plus®,Top Spot®), imidacloprid (Advantage®), Selamectin (Revolution®) and Spinosad (Comfortis®, Trifexis®). Frontline® and Advantage® work on the flea's (adult and larvae stages) central nervous system, but in slightly different manners. Fipronil affects the movement of chloride ions across sensitive nerve cells...essentially paralyzing these nerves. Imidacloprid interferes with the functionality of specific neurotransmitter receptors. By thus interfering with the signals between nerve cells and the essential functions they perform, the effect of either product is death of the adult flea.  Frontline Plus® also contains an insect growth regular (IGR) prevent development of eggs into adult fleas. The chemicals used in these products are selectively toxic to fleas...not to animals. Revolution®, affects selected chloride ion movement across nerve and muscle cells of invertebrates, resulting in paralysis. It is applied similarly to the other two products but is absorbed systemically and is parasiticidal to a variety of internal and external parasites in this way (round worms and hook worms (cats only), mites; it is also inhibitory to the development of heartworm larvae to adults) in addition to killing adult fleas. Unlike plain Frontline®,or Advantage®, the product also inhibits development of the flea egg into adult fleas.

Fipronil: Animals must be at least 8 weeks of age. The product is applied topically to skin (not to fur). Within 24 hours, the product spreads via natural oils to all areas of the body, remaining in the oil (sebaceous) glands and hair follicles for up to three months (dogs) or one month (cats). It is not absorbed into the blood. Fipronil is also toxic to ticks for up to one month after application.

  • do not bathe for 48 hours before or after applying, then....
  • the manufacturer claims that the product remains effective even after bathing! However according to some, it is not clear how much subsequent bathing affects efficacy, so bathing is not recommended.
  • side effects include hypersensitivity...local irritation at the site of administration
  • should not be applied more than once every 30 days
  • probably should not be applied to debilitated or pregnant or nursing animals...though there is no evidence that treatment of these animals is harmful
  • incidental, unsubstantiated reports of possible toxicity of fipronil when applied to some species other than dog and cats (e.g. rabbits). The product is ONLY approved for dogs and cats!

Imidacloprid: The product is applied exactly as for fipronil. It, too, spreads over the entire oiled skin and is also not absorbed systemically. Imidacloprid has NO activity against ticks, though the manufacturer has recently made available a supplementary formulation that kills ticks. The product must be reapplied at 30 day (or more frequent) intervals in the dog and cat.

  • do not bathe for 48 hours before or after applying
  • Unclear as to how product is affected by bathing...some is probably removed but whether there is significant reduction in efficacy or duration is unclear. Avoid bathing if possible.
  • the manufacturer states that there are no adverse reactions of hypersensitivities to imidacloprid
  • can be applied to puppies 7 weeks or older and to kittens 8 weeks or older (earlier information indicated that animals had to be 16 weeks old..but the manufacturer has since restated the age-related safety limitations).
  • needs to be applied at 30 day intervals for maximum efficacy. Sometimes, when flea exposure is severe, more frequent application is necessary and apparently safe.( e.g 3 week intervals...but do not apply more often than once weekly). Imidacloprid should probably not be applied to debilitated or pregnant or nursing animals, though there is no evidence that treating these animals is at all harmful.

Selamectin (Revolution®, Pfizer Animal Health):

  • The product is applied as Frontline® or Advantage® to the skin of the back of the neck of animals 8 weeks of age. If bathing the animal, wait at least two hours afterwards and be sure the animal is dry.
  • This product must be reapplied on a monthly basis to remain effective
  • According to the manufacturer, the product was without noticable problems when administered to pregnant or lactating dogs.
  • In a study of Collie dogs (Some Collies and other herding breeds [e.g. Old English Sheep Dog, Australian Shepherds and outcrosses]) that are sensitive to other avermectin products (e.g. Ivermectin)), one dog became ataxic (unstable gait) eight hours after receiving twice the recommended first dose but had no noticable reactions to subsequent treatments. More information about this product is here (in Adobe Acrobat [.pdf] format).

Nitenpyram (Capstar® Novartis Animal Health): This is an oral insecticide that also acts on central nervous system of the adult flea causing paralysis and death.

  • The product has a very wide margin of safety (even at 10 times the recommended dose, no harmful effects were reported). There are no known harmful drug interactions.
  • Can be used in puppies and kittens 4 weeks of age and older and weighing at least 2 pounds.
  • Reaches peak plasma concentration by 30 minutes after oral administration.
  • Down sides of using this product:
    • Does not kill eggs, larvae or pupae, though...and does not kill ticks
    • Initially the pet may show an increase in scratching behavior as fleas begin to die
    • It must be given daily if sustained protection is needed. A single dose is only useful for acute flea infestations and does not prevent subsequent infestation on successive days
    • Daily usage is expensive, in comparison with the more common flea protection products already described
  • May be useful in conjunction with other flea protection profucts, such as insect growth regulators (e.g. lufenuron)

Metaflumizone (Promeris® Fort Dodge) this was a topical, non-acqueous application that, that was also uniquely neurotoxic to fleas and, according to the company, highly efficacious.

However, in 2011, Fort Dodge made a decision to withdraw this product from the market, because there were data showing that some animals treated with Promeris developed a serious, immune-mediate skin condition known as phemphigus

Spinosad (Comfortis®, Trifexis®) is a monthly tablet that acts at the level of the flea's central nervous system causing rapid paralysis and death. It is now approved for use in dogs and cats. This drug is contradicted in animals on extra-label (high) dose ivermectin therapy (e.g. for demodectic mange). Most common side effects are gastrointestinal upset.

    Flea Control:

    Much of the flea life cycle is spent off of the animal, as illustrated below (click the "Home" button).

    Once the adult flea lands on a host animal, egg production begins 24-48 hours later. About 40-50 eggs per day are layed and fall off the animal into the environment almost immediately. Since reproduction occurs on the animal, control of reproduction on the animal is the first step in controlling the flea population on the premesis.

    In general, eliminating viable eggs on the animal, which can occur with products such as Lufenuron, Selamectin and selected topical IGRs will control the adult population of fleas in the environment, though elimination of all adult fleas in this manner may take one or more months. Advantage® and Frontline®, in theory, kill adult fleas before the 24-48 hour post-infestation egg-laying begins; hence these should also control the environment. However, flea survival and reproduction do, on occasion, occur beyond 48 hours post-application with these products. Thus the addition of an topical IGR ovacide may also be efficaceous in halting reproduction and elimination of fleas from the environment

    Efforts to decrease the number ofdeveloping eggs already present in the environment should also be considered. Fleas eggs are deposited wherever the animal moves. The greatest concentration is usually where the animal spends most of it time. Hence, blankets/bedding, sofas/chairs, carpets etc. should be washed/vaccumed frequently. Application of stong area insecticides and/or area IGRs can be considered when basic cleaning and animal flea control measures prove inadequate because the flea/developing flea populations are overwhelming.

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