Fleas – three cases of control breakdown and how to rectify it

The flea most commonly seen on dogs and cats is the “cat flea”, Ctenocephalides felis.

After introduction into households, persistent infestation can establish quickly with cat fleas having the capacity to reproduce rapidly.

Adult fleas lay eggs within 24 hours of feeding and produce 40 to 50 eggs per day. The eggs hatch in one to six days and the larvae can then pupate in as little as 10 to 20 days under warm humid conditions.

Adult fleas can then emerge from pupae in as little as three weeks.

This means that under household conditions the life cycle can be completed in less than eight weeks (Blagburn and Dryden, 2009) and, at any one time, 95% of the flea population exists in the environment as eggs, larvae and pupae.

Although adults can emerge quickly from pupae under favourable conditions of warmth and high humidity, they can also survive for up to two years if conditions are unfavourable.

Adults emerge from pupae in response to heat, movement and atmospheric gas variations, in response to the presence of possible hosts.

Principles of flea control

The most important aspect of flea control is to ensure adult fleas are killed on the pet before they can initiate egg production.

Treatment of the environment with insect growth regulators, insecticides, hot washing of bedding and daily vacuuming is also important to reduce environmental larvae and eggs.

Reducing pupal numbers in the environment is difficult, however, making effective, rapid killing of adults essential.

For effective flea control to occur, choice of adulticide, how to effectively treat the environment and aiding client compliance all need to be considered.

However, even when all of these measures are employed, flea control may still fail. This can happen for a number of reasons and all of them need to be considered if flea control is to be re-established.

Flea resistance to insecticide

Despite numerous large-scale studies into the efficacy of flea treatments, no published evidence of flea resistance to products containing fipronil or imidacloprid in the field has emerged.

The presence of resistance genes in a flea population may lead to a need for increased treatment frequency, but this would appear to be comparatively rare compared with other reasons for flea control breakdown occurring (Coles and Dryden, 2014). Therefore, although the possibility of drug resistance as a cause of flea control breakdown should not be ignored, it should only be considered as a possibility once other causes have been eliminated.

Insufficient frequency of application

It can be easy for owners to miss flea treatment doses or not apply them on time. The use of sebum stripping shampoos may also increase the required frequency of application of spot-on products containing fipronil and imidacloprid.

Not all pets in the house being treated

Cat fleas are highly adaptable, and capable of infesting and reproducing on a variety of mammals as well as cats, including dogs, rabbits, and ferrets. Clients may not realise rabbits and ferrets may be infested with cat fleas; therefore, it is important to ensure all pets in the house have been treated with an appropriate product. Clients may also not mention stray or owned cats that visit the house from elsewhere.

Poor compliance

If clients are having difficulty administering a product or are not shown how to administer a new product effectively, then correct dosage and frequency of drug treatment may not occur. If finances are an issue, then owners may compromise on dosing frequency recommendations beyond licence claim statements and/or veterinary advice.

Discussing client treatment preferences and demonstrating to clients how to administer products will help to improve compliance. Practice plan schemes where cost of flea treatments are spread over time, and reminder systems to help ensure clients do not forget to administer treatments, are also beneficial.

Infestations being caused by types of flea other than cat fleas

It is important to identify what type of flea is causing an infestation if control is being lost. Cat fleas have both genal and pronotal combs, and a characteristic elongated head, with the head being twice as long as it is tall (Figure 1). Flea control advice for household pets is centred around C felis and treatment of all susceptible pets with an effective adulticide at the correct frequency, combined with treatment of the environment, should be sufficient for control.

Pulex irritans, the “human flea”, can be easily distinguished from Ctenocephalides species as it has no genal or pronotal combs and a rounded head, giving it a “bald” appearance.

Although described as the human flea, it is primarily a flea of wildlife, but may also infest cats, dogs and people. Spilopsyllus cuniculi, the “rabbit flea”, possesses both genal and pronotal combs, but adults are smaller than cat fleas, with adults only reaching 1mm in length. They characteristically congregate around the ear pinna, and fleas found predominantly in this location should raise suspicion of S cuniculi infestation (Figure 2).

Reproduction in the flea is controlled by rabbit hormones to ensure flea mating and egg production occurs in the presence of young rabbits, so is unlikely to establish in people’s homes.

Cats and dogs can become incidentally infested when hunting rabbits or investigating warren entrances. It may appear in these circumstances that household flea control is failing when it is outdoor repeated exposure taking place. Bird fleas have a pronotal, but no genal, comb.

Adult fleas live in nests and jump on birds using the nest to feed. They overwinter as pupae and then feed again on the birds using the nest the following year. If the nest is not reused, however, fleas will vacate the nest and seek out new hosts. In this situation, control will centre on treatment of the environment and eradication of the unused nest if it is in the rafters, eaves or attic space of the building.

Case one – existing infestation

An owner of two cats presented with concerns over a possible flea infestation in her home. The owner had recently moved into a new house and had started receiving bites on her ankles (Figure 3). The cats were both being treated routinely with fipronil from a pharmacy every four weeks to prevent flea infestation, and had no skin lesions, alopecia or signs of pruritus.

The owner, therefore, believed her cats were not the source of the infestation and had treated the house with cans of permethrin/pyriproxyfen spray. The bites had subsequently continued, leading the owner to believe that the fleas infesting the house were “human fleas” living on her. She noticed fleas jumping from the carpet, and so had positioned fly paper at carpet level to catch the fleas. Flea combings of the cats found no adult fleas, but small amounts of flea dirt were present.

Possible causes of flea control breakdown

The presence of a flea infestation in the face of ongoing treatment, which should be effectively applied every four weeks, means one or more of five factors may have been responsible.

• Lack of compliance. The owner had not experienced a breakdown in flea control in the previous 12 months that she used fipronil at this treatment frequency in her cats. This does not imply, however, that correct application of the product had taken place up to this point, as the cats may not have been exposed to fleas and infestation in this time. The owner also had not had correct application demonstrated by a veterinary professional, so this was demonstrated to the client. It was also confirmed with the client that the cats were not shampooed and did not swim.
• Presence of fleas other than C felis. The owner was requested to bring in samples of fleas caught on the fly paper for identification to establish which genus of fleas were present.
• Existing household flea infestation. C felis pupae may have already been present when the client moved into her new property, leading to adult emergence in response to heat, and movement from both cats and owners. Although fipronil applied every four weeks and environmental treatment would successfully control this reactivation, it would take at least three months and possibly significantly longer (Dryden et al, 2000).
• Other pets present in house or visiting household. The owner confirmed that no other pets were present in the house and the owner was not aware of other cats in the neighbourhood entering the home.
• Insecticide resistance. This should be considered if the other four factors could not explain the loss of flea control.
• Flea identification

The client was able to bring fly paper into the practice with fleas attached to it.

They were examined under the microscope and pronotal and genal combs identified, confirming Ctenocephalides species.

Flea control programme

A flea control programme was discussed with the owner and implemented.

This consisted of the following.

• Continued treatment of the adult cats with fipronil. The cats were treated every four weeks as the owner had a supply at home.
• Treatment of the environment. The environment had already been treated with a pyrethroid insecticide and an insect growth regulator. Daily vacuuming of cat bedding and areas where large numbers of fleas had been seen was advised in an attempt to reduce pupal numbers.
• Preventing exposure of fleas to other potential hosts. The client was advised to prevent other pets from entering the household while control was being achieved. The client advised friends to only bring pets to the house if adequately flea treated, and stray cats were prevented entry by using a microchip-activated cat flap.

Results of the flea control programme

The client continued to use fly paper and no adult fleas were caught two months after first presentation at the practice. Flea bites on the owner reduced over this two-month period and ceased after three months.

The cats were checked every four weeks with a flea comb for signs of continuing infestation, and no adult fleas or flea dirt were found from three months after initial presentation.

It was, therefore, concluded that flea control had been re-established.

Existing flea infestation was thought to be the source of the bites.

Case two – cat fleas not responsible

Two domestic cats had been treated continuously with selamectin in a household every four weeks for a period of 12 months.

During this time period, owners had received suspected flea bites around the ankles and fleas had been found sporadically in the house.

Various causes of flea control breakdown were investigated.

• Compliance. Treatment was applied every four weeks, originally by the owners and then, in the face of possible flea control breakdown, by a veterinary nurse to rule out misapplication.
• Untreated pets. No other pets entered the household, including stray cats or those from other households.
• Lifestyle. The cats went outside, presenting the possibility of clients seeing fleas due to exposure before the fleas were killed by the selamectin.

This did not explain the bites, however, as household control would still be maintained.

This made the possibility of fleas other than cat fleas being the source of infestation a significant possibility, and fleas were requested for identification.

Flea identification

Two adult fleas were sent in a sealed container and subsequently identified as bird fleas by the absence of a genal comb (Figure 4).

Bird fleas can invade homes from abandoned nests in attics, guttering and house walls, but in this case it was found that the source of the bird fleas was from an abandoned nest in an attic space.

Flea control programme

A flea control programme was discussed with the owner and implemented. This consisted of the following.

• Continued treatment of the adult cats. This was to ensure protection against cat fleas was also maintained.
• Treatment of the environment. The environment was treated with a pyrethroid insecticide and an insect growth regulator. The nest was removed from the attic.

Results of the flea control programme

Flea bites on the owner reduced over a one-month period. The cats were checked every four weeks with a flea comb for signs of infestation and no adult fleas or flea dirt were found.

After one month, no further fleas were seen or bites occurred, and it was concluded that the flea infestation had been controlled.

Case three – the visiting cat

A domestic cat had been treated continuously with an isoxazoline tablet product since kittenhood.

Over the previous month, owners had received suspected flea bites around the ankles and fleas had been found sporadically in the house. The owners believed drug resistance had developed to the product.

Various causes of flea control breakdown were investigated.

• Compliance. Treatment was applied monthly and the owners were confident a break in treatment had not taken place.
• Untreated pets. After accurate history taking, the owners revealed that a neighbour’s cat had been visiting the property intermittently for the past two or three months, but had not made a possible connection and failed to mention this previously.
• Lifestyle. The cat went outside, presenting the possibility of clients seeing fleas due to exposure before the fleas were killed by the isoxazoline. This did not explain the bites, however, as household control would still be maintained.

Fleas were requested for identification to eliminate other types of flea as a possible cause, but the owner was unable to acquire any and none were found on the cat when flea combed in the surgery.

Flea control programme

A flea control programme was discussed with the owner and implemented. This consisted of the following.

• Continued treatment of the owned cat. This was to ensure ongoing cat flea protection.
• Treatment of the environment. The environment was treated with a pyrethroid insecticide and an insect growth regulator.
• Preventing entry of the neighbour’s cat. A microchip-activated cat flap was used.

Results of the flea control programme

Flea bites on the owner reduced over a one-month period. The cat was checked every four weeks with a flea comb for signs of infestation, and no adult fleas or flea dirt were found.

After one month, no further fleas were seen or bites occurred, and it was concluded that the flea infestation had been controlled, with the neighbour’s cat having been the source of the infestation.

Conclusions

Flea control is important in terms of human nuisance and revulsion, reducing vector-borne disease and the clinical signs associated directly with flea infestation.

These cases demonstrate that flea control can temporarily be lost despite adequate adulticides being used. In all three cases, drug resistance was found not to be the cause of flea control breakdown, and it should not be assumed it is until all other possible causes have been investigated.