31 Mar 2020
Ian Wright
Job Title
The cat flea Ctenocephalides felis remains at high prevalence on UK cats and dogs, with a milder climate and centrally heated homes allowing year-round activity.
Fleas are a source of revulsion, distress and irritation to pet owners, but can also cause disease in pets and owners alike. Flea control is, therefore, vital, but can be difficult to achieve, with flea control breakdown common. This leads to pet owner frustration, increased morbidity and questions arising regarding treatment efficacy.
It is essential for veterinary professionals to assess why flea control is failing, to be able to put effective corrective measures in place.
Fleas are highly prevalent on domestic cats and dogs, with 28.1% of cats and 14.4% of dogs found to be infested in the UK (Abdullah et al, 2019).
The vast majority of fleas on cats and dogs are cat fleas, although poultry fleas, hedgehog fleas and dog fleas have also been found (Abdullah et al, 2019).
Cat fleas can live on a wide variety of mammalian hosts – including cats, dogs, rabbits, and wildlife, such as foxes and hedgehogs (Clark et al, 2018). They are also well-suited to living in the humidity and temperatures maintained in most UK homes, with 95% of a typical flea infestation existing in the home as eggs, larvae and pupae.
This combination of factors leads to increased flea challenge on domestic pets – and without routine preventive treatment, a high risk exists of flea infestations establishing (Coles and Dryden, 2014).
Fleas are a cause of allergic dermatitis and vectors for a variety of infections – including Bartonella henselae (cause of cat scratch disease), Rickettsia felis (cause of spotted fever), haemoplasma species (cause of feline infectious anaemia) and Dipylidium caninum tapeworms.
A total of 11.3% of flea infestations on UK cats and dogs have recently been found to be positive for Bartonella species (Abdullah et al, 2019). Humans are thought to be exposed to this pathogen, primarily through flea faeces – making flea control vitally important for preventing zoonotic exposure.
Flea control is, therefore, essential to reduce disease risk, and maintain a strong, healthy relationship between pet and owner. Successful flea control strategies require consideration of the flea life cycle and prevention of further flea multiplication within the home. Failure to do so will lead to flea control breakdown, and subsequent owner frustration and pet morbidity.
The most important aspect of flea control is ensuring adult fleas are killed on the pet before they can initiate egg production.
Treatment of the environment with insect growth regulators and environmental insecticides – as well as hot washing of bedding and daily vacuuming – are also important to reduce environmental larvae and eggs.
However, reducing pupal numbers in the environment is difficult – making effective, rapid killing of adults essential.
Adult fleas can lay eggs within 24 hours, so the adulticide chosen must kill fleas at least within that time. They must also be administered frequently enough to continue to prevent flea egg laying.
The time after application of the adulticide at which fleas survive long enough to lay eggs is known as the reproductive break point. If the reproductive break point is reached, flea control will fail.
Frequent swimming or shampooing may affect the efficacy and duration of action of some imidacloprid and fipronil spot-on solutions, so this should be considered when choosing a specific product. Many flea products are also effective against other parasites, so other parasite control requirements will also influence product choice.
The use of broad-spectrum parasiticides and endectocides will reduce the number of treatments required for owners to administer where broad-spectrum parasite cover is required.
Environmental treatment to reduce eggs, larvae and pupae will decrease the time required to bring an infestation under control.
Spray cans containing a larvicide/ovicide and an insect growth inhibitor – used in household environments – are useful to reduce flea and larval numbers, while also preventing development into pupae.
When treating the environment directly, all areas where the pets frequent – such as cars, furniture and bedding – must be treated. Effective penetration of chemicals from aerosol-based products can be poor, unless used from the correct distance and after removal of obstructing objects, such as children’s car seats, pillows and cushions. These should be treated separately.
Care must be taken to ensure instructions are followed carefully, and that fish, birds, invertebrate pets and cats are removed while treatment is taking place.
Systemically acting products containing lufenuron can be used in pets to prevent flea eggs from hatching.
Some adulticides – such as imidacloprid and selamectin – are shed into the environment after the pets have been treated, reducing environmental egg and larval contamination. Some spot-on flea products also contain growth regulators to achieve a similar effect.
Daily vacuuming of areas frequented by flea‑infested pets has also been demonstrated to reduce pupae numbers in the environment, as has hot washing of pet bedding to at least 60°C.
Although an adulticide alone used on all pets frequently enough will break the flea life cycle, without treatment of the environment some flea infestations will take many months to eliminate (Dryden et al, 2000).
Household flea infestations take at least three months to eliminate, even when environmental treatment is used (Dryden et al, 2000).
If this is not effectively communicated to clients, they may quickly become disillusioned with the flea control programme and compliance will be reduced. Therefore, it is important to not only stress the three months’ minimum treatment time, but also the variety of reasons why duration of time to achieve control may be prolonged.
Failure to consider any of the following factors may lead to flea control breakdown. Even when all of these measures are employed, however, 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.
Despite numerous large-scale studies into the efficacy of flea treatments, no published evidence has emerged of flea resistance to products containing fipronil or imidacloprid in the field.
Even where resistance genes are known to exist in laboratory strains of fleas, fipronil, selamectin and spinosad have all been shown to be highly efficacious at three weeks post‑application (Bass et al, 2004; Dryden et al, 2013).
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 the reproductive break point being reached (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, more common, causes have been eliminated.
If other factors are not considered and resistance incorrectly blamed, then lack of control may continue, even if the flea product used is changed. This is likely to lead to frustration on the part of the client and possibly result in poor compliance.
Many pet owners live busy lifestyles – and it can be easy for flea treatment doses to be missed or not applied on time.
Apps and text message reminders help to remind clients when products are due to be applied. Practice plans and postal services can also be useful to prompt clients as to when products are due to be applied, and ensure they are using the correct amount of product.
The use of sebum-stripping shampoos is likely to increase the required frequency of application of spot-on products containing fipronil and imidacloprid.
Potential flea hosts can be present in households that owners have not considered may also require treatment. These include stray cats entering houses, and domestic cats visiting and spending time in multiple households.
Clients may also not realise rabbits and ferrets can infested with cat fleas.
Wildlife casualties brought into homes may harbour cat fleas. A study showed 130 wildlife species to be harbouring cat fleas, including hedgehogs (Clark et al, 2018). This demonstrates the potential for fleas on rescued wildlife casualties to establish infestations in homes and practices.
It is important to identify what type of flea is causing an infestation if control is being lost.
The cat flea has 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 Ctenocephalides 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 humans. Due to the predominance of the cat flea, and climatic and wildlife distribution changes, it is now uncommon in northern Europe. However, its presence in a household means medical advice must be sought by pet owners as treatment of the owner – as well as pets in the household – will be required.
Spilopsyllus cuniculi – the “rabbit flea” – is smaller than other fleas infesting household pets, with adult females typically only reaching 1mm long and adult males being smaller.
They characteristically congregate around the ear pinna – fleas found predominantly in this location should raise suspicion of S cuniculi infestation (Figure 2).
They are more sedentary than other species of flea found on domestic pets, and household rabbits are uncommonly infested.
Reproduction in the flea is controlled by rabbit hormones to ensure flea mating and egg production occurs in the presence of young rabbits. S cuniculi will, therefore, not establish household infestations, but cats and dogs can become incidentally infested when hunting rabbits or when investigating warren entrances.
Treatment with an adulticide will eliminate infestation, but hunting or warren investigation may need to be avoided to prevent repeated infestation.
It may appear in these circumstances that household flea control is failing when outdoor repeated exposure is taking place.
Ceratophyllus gallinae – the “European chicken flea” (Figure 3) – has a pronotal, but no genal, comb. They are fleas of birds, with adult fleas living in nests and jumping on birds using the nest to feed.
They overwinter as pupae, 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.
If nests are situated close to chicken coups or domestic pigeon housing, these may be infested. Houses may also be invaded if nests are adjoining buildings, and owners and pets subsequently bitten.
In this situation, control will centre on treatment of the environment and eradication of the unused nest if it is in the rafters, eves or attic space of the building.
If clients are having difficulty administering a product or not shown how to administer a new product effectively, correct dosage and frequency of drug treatment may not occur.
If finances are an issue, 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 improve compliance. Practice plan schemes, where cost of flea treatments are spread over time, may also be beneficial.
Flea control is important in terms of limiting zoonotic disease, improving animal health and strengthening the human-animal bond.
Flea control continues to remain a challenge and it is vital all aspects of control are considered. Understanding the reproductive break point, effective environmental control, compliance and management of owner expectation are all essential if flea control plans are to succeed.
By considering frequency of product application and compliance, flea egg laying can be suppressed and flea control maintained. When flea control fails, it is important to consider all the reasons this may be – and only when all other factors have been addressed should flea resistance be considered a possibility and investigated.
