Killing adults in the war on fleas

Adult fleas, living on your pet, are only 5% of the total population but killing them gives pet owners immediate satisfaction, and pets relief.  The benefits can be short-lived, however, if the dead are swiftly replaced by their offspring. Traditionally, ‘adulticides’ were shampoos and rinses, but nowadays pet owners are presented with an ever increasing range of products offered by big pharma; each jockeying for market share, and questioning the accuracy of their competitors claims. Vets  and owners are often left feeling confused, and sometimes ripped off, and there’s many who throw-in-the-towel with newfangled agents, and resort to old school insecticides they grew up with. Let’s look at the options available.


the evil eye

Old-school topical insecticides:

Shampoos are cheap, old-fashioned, and only kill the fleas on your pet at the time of use, having no residual activity. You’re only killing 5% of the population. If done once weekly, you wont make much progress;  if 3 times a week, you may get somewhere, slowly. There are some that are ok in young puppies.

Rinses are the old-school, cheap, smelly chemicals that you remember pouring on your dog after their bath. Containing  Organophosphates (OP’s) or synthetic pyrethroids, they’re too toxic for your cat and puppy, but more effective than shampoos as they continue killing fleas that jump on the dog for 3-5 days.  Once weekly treatment will make progress; every 5 days, more so. Avoid in the young, elderly or liver compromised.

Permoxin® spray: like rinses above, this product is a synthetic pyrethroid. It can be used in a spray bottle, applied directly to fleas as you would cockroaches in your kitchen: alot easier than trying to catch them. Sprayed daily to the rump, this product still has a place in treating flea allergic animals when owners cannot afford newer agents, and don’t mind chemicals.

Flea rinses and Permoxin seem to have more of a flea repellent activity than the newer spot-ons, and hence can be more effective in highly flea allergic animals.  I often recommend these products  to pet owners who are jack of forking out $60 for a pack of spot-on they don’t believe works, and don’t care about chemical residues: nostalgic for the days when their childhood pet stank like an industrial chemical.

Herbal products vary from those containing natural insecticides like neem and pyrethrin, to essential oils with only a deterrent activity. They are not without toxic effects of their own. If you have a mild flea problem they may suffice; ineffectual with multipet households and flea-allergic pets. See borate salts in Breaking the breeding cycle if you favour natural over industrial.

Flea Collars don’t seem to work at all. Vets see lots of animals with fresh flea collars and heaps of happy, healthy fleas. Don’t waste your money here. Tick collars are a different matter.

Oral insecticides – flea bath in a pill:

These agents are great for pet owners who cant cope with environmental chemicals – concerned about the potential harm to themselves and their family. Given orally to their pet, these agents enter the circulation and poison the flea when they feed on blood.  Toxicity to pet is an obvious and valid concern, especially with Proban. Capstar and Comfortis seem very safe in the short term, with effects unknown in the long.

Dogs who swim regularly, or those regularly shampooed for dermatologic problems, are good candidates for these products as they cant be washed off.

Proban® is an old-fashioned OP insecticide given as a tablet to your cat or dog. This agent is effective if given every 2-3 days at high enough doses. The problem is toxicity to your pet. OP’s are toxic to both insects and mammals and it’s a matter of getting the dose right, so it kills the parasite but not the host. Avoid in the young, elderly and liver compromised. It’s registered for use in cats, but can make some unwell – avoid if you can.

Capstar® can be thought of as a shampoo in a pill. It’s cheap and kills fleas feeding on the dog or cat for the following few hours; beyond this there is no residual action. If you give it every few days for long periods, like flea shampooing above, it may work well.

Comfortis® is the latest weapon to be released in the war on fleas. No vets in Australia will yet have extensive personal experience with this agent, but the technical data looks promising. Within 4 hours of giving the pill to your dog, 100% of fleas that suck blood are dead, which only drops to 97% by the 4th week of the month. Competitors are suggesting this may not be matched in real life situations where meals and individual dog’s metabolism may affect results. The cost works out as about $15 /month.

Newfangled sprays and spot-on agents:

The problem with the old school OP’s, described above, is their toxicity to insects AND mammals. When used to control fleas, a dose needs to be selected that is high enough to kill the insect but not enough to kill the host: your loved pet. Seeing the toxic writing on the wall,  for the last 30 years big pharma has been working hard to develop new insecticidal agents to replace OP’s, which specifically target insects and hence are non- or minimally toxic to man and animals.

Frontline Plus®, Frontera®, Advantage®, Revolution®, and their combination cousin, Advocate® , are modern insecticides which appear less toxic, and don’t require bathing, so especially good on cats.  They’re expensive, but you’re paying for convenience..

Effectiveness and frequency of application

As toxicity falls, higher doses can be applied, leading to longer residual activity; most promote themselves as a monthly application. These new long-acting agents have transformed flea control over the last 20 years. Many pet owners are lazy or forget to apply short-acting agents; if just 5-10% of fleas get past a short-acting shampoo or rinse, they can produce enough young to make it seemingly ineffectual.

In laboratory studies, kill percentages for most of these products is 100%  for 2 weeks, then dwindle to around 95 to 98% by the end of the month. In the real world, with dogs that are shampooed and swim, these figures may be considerably lower.  If it’s a flea emergency, a stubborn colony, or a flea sensitive pet, and you want hard-core carnage, application every 2 weeks is safe, although this can get costly in multipet households.

Maximise the time between application and washing/swimming as these agents bind to oils in the coat, so the greasier the better. If you bathe the dog every Sunday, apply on a Wednesday.

If using some medicated shampoos, such as Malaseb®, which strip these oils from the coat, spot-ons may be a waste of money, and oral agents may be better.


Product failure

Vets occasionally see failure of these products, which may be incorrect application; frequent swimming or bathing; massive infestations with fleas hatching as fast as they’re killed; or flea resistance.

Heavy selection pressure should result in resistance, and many owners report it, but it’s usually hard for vets to pick  the cause of failure with certainty. Pharmaceutical companies sometimes investigate claims of resistant fleas, but always seem to disprove it.

Chemical interaction between fipronil and the newly added (l)-methoprene, was rumoured to be a cause of decreased efficacy of Frontline Plus® in the early noughties, but this seems to have been corporate muckraking by competitors. In retaliation, the addition of an extra chemical to Advantage®, to make Advocate®, saw similar claims of interactions and reduced efficacy. Vets are left uncertain what to believe.

One unique circumstance, where vets do see almost experimental test conditions for a flea product, is the housebound pet in a tropical environment.  A closed biosphere with ideal breeding conditions, makes it possible to trial various spot-ons, in sequence. Belinda, a vet nurse, describes her experience here.

It’s important to emphasise these agents are not flea repellent. For a flea to be killed it must jump on the dog, possibly bite, and then it dies, sometimes upto 24 hours later. Before you leap to the conclusion that the presence of a flea is proof that a spot-on isn’t working and demand a refund, have a closer look. Sick and dying fleas are easy to catch.

If a product doesn’t seem to be working, try a different brand, as they have different actives, or ditch these altogether and try an oral agent.


These products can either be used  all year round, or as required during the flea season. In cases where finances are restricted, chemical residues are a concern, or in colder climates where flea seasons are short, it is reasonable to monitor your dog for fleas and start treatment as required.  It’s true that if it’s a cold winter and you don’t centrally heat your house, few pupae will be hatching, and early spring treatment may be more strategic. Monthly cost is about $15 per animal; double this if using fortnightly.


In reality, these are new compounds that haven’t been around long enough to really know for sure there aren’t long-term toxic effects. Once we’ve experimented with these drugs for long enough on our pets, they may eventually be deemed sufficiently safe to use with our own parasite problems: head and body lice.

With the exception of the alarming report that Frontline can kill rabbits, so far, they these products appear very safe in animals. Veterinary staff sometimes report skin reactions, and some pets experience similar localised discomfort or hair loss at the site of application. This is not necessarily the insecticide, and can be the carrier agents that help dispersal through the coat.

Key Points

  • Relying too heavily on killing adult fleas alone, and ignoring the breeding cycle, may cost more and be less effective than a multi-targetted, holistic approach.
  • If you have a massive infestation, with thousands of pupae hatching each week, it may take a month or two, and some discomfort, for you and your animal, before you finally see results.  It may be better to apply a spot-on every 2-3 weeks for the first few months, to really nail the bastards, then drop to monthly.
  • If your dog is bathed or swims regularly, oral insecticides may be more effective than topical.
  • Advocate®, Revolution® and Advantix® are combination products that also prevent ticks or worms, making direct price comparison difficult.
  • Many people don’t notice fleas on their pets until the population has grown sufficiently large. Given their exponential population growth, failing to notice the first crop of spring fleas can result in tens of thousands of fresh eggs laid, and curse you for the rest of summer. If you are saving yourself the expense of winter treatment, buy a flea comb so you can swiftly detect a repeat invasion.
  • If you live in a tropical or sub-tropical climate, the argument for continuous use, all year, is strong. Definitely don’t wait to see skin disease before starting treatment, as money saved on flea control will just end-up spent on treatment for dermatitis.  If it seems too costly to use all year round, have a look at Splitting Vials: Harm Minimisation
  • You can avoid the expense of newer, long-acting insecticides, by resorting to old-school products, but you will need to use them with religious frequency, and put up with the smell.
  • Cats, the young, and animals with liver problems can be sensitive to the toxic effects of many insecticides and pet owners should always scrutinise labels to ensure a purchased flea product is safe for the use they intend. Be careful to observe your pets for adverse effects when using Proban®, Rinses, Advantix® and tick collars.

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2 Responses to “Killing adults in the war on fleas”

  1. Paul Beckett says:

    Environmental controls are an area of flea and sand fly control, important where I live, we also have a large cane toad population and many unique native insects. I dislike using all insecticides on my dogs and cat but after resorting to the leading brand oral and topical products and getting poor results for 3 months following a move to a new home, I thoroughly swept, hosed and blew down all hard areas and mowed the lawns, before applying generous amounts of calcium, lime, and dolomite to the lawns and gardens and then using builders or hydrated lime diluted in a watering can to cover all concreted areas, especially under the house and presto, the sand flies where gone overnight and the fleas are under control too. There’s a Melaleuca swamp just down the road and we get heaps of mossies, so heartworm is also an issue. So very good to have access to a site that provides considered alternatives to toxic products when alternatives are available. Thanks Dr Matt.

  2. VOS says:

    but how to stop the SHELL-BACKS???

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