Fleas: A Worldwide Pest

When we think of fleas, we usually think of pets. Cats and dogs can regularly be seen scratching because of these tiny parasites, and preventing or checking for them is a regular part of pet care. But fleas can bother livestock animals too. With over 2,500 species, fleas are diverse and worldwide pests. On top of that, they can carry a host of diseases. Historically, rat fleas were responsible for the spread of bubonic plague across the world in the middle ages, causing millions of deaths. These days, the risk of plague is significantly lower, but it can still be a problem in some areas of the world like Madagascar and the Democratic Republic of Congo.

What are fleas?

Fleas are small, wingless insects which feed on blood.
Fleas are small, wingless insects which feed on blood.

Fleas are small, flightless insects of the order Siphonaptera. The adults are obligate parasites, which means they must feed on a host to survive. Adults tend to have powerful legs, which allow them to jump onto (and occasionally off) their hosts, and strong claws to hold themselves in place. Some are nest-living species which will jump onto a host from a nest, feed from them, and then jump off again. Other species attach onto a host and then remain there, sometimes for life. A sharp proboscis lets the flea pierce through tough skin and drink the blood of its host. Fleas generally parasitise mammals, but some species specialise on birds.

While adult fleas have study bodies which protect them from extreme force, for example scratching, larvae are worm-like and much less durable. These young fleas live in the environment and can feed on a wider variety of food than adults. They can consume many types of organic matter, from dead skin to the faeces of adult fleas. Each female flea can produce hundreds of eggs in her lifetime.

Fleas are found everywhere in the world, but tend to follow seasonal population patterns in colder areas. Although most flea species tend to only prey on one species of host animal, some are generalists. These generalist species are usually the main pests, as they can spread between animals and humans easily.

Fleas and animals

Flea infestation presents with a number of different symptoms. The primary sign of a flea infestation is scratching. Flea bites can be extremely irritating – for animals and people – particularly as some animals and humans become sensitised to the bites. In severe cases scratching can cause wounds which are then susceptible to infection. Very large numbers of flea bites can increase the risk of anaemia in most animals. This is a particular risk for young animals, such as puppies and kittens, and can even cause death.

Once an animal becomes a host for fleas, the population can multiply rapidly, with eggs falling into the environment where they develop and then spread to other animals around them. At this time of year in the UK, the relatively warm conditions of barns and sheds, coupled with dry bedding like straw and hay, provide a perfect environment for the fleas to multiply. These parasites can even then feed on livestock, such as pigs and cattle, although this is rare in the UK.

Some species, for example the cat flea, Ctenocephalides felis, can carry parasites which then infect the flea’s host. Cat fleas are known to transmit Dipylidium caninum tapeworm larvae, with infection occurring when the dog or cat accidentally eats a flea.  

Rabbit fleas

An unlikely target.
An unlikely target.

Similar to the cat flea, rabbit fleas, Spilopsyllus cuniculi, are generalists. This means that, while their primary hosts are rabbits and hares, they can and will infest other animals. Many wild rabbits across the world are hosts to these fleas. Although the fleas themselves are not a significant health issue, they can carry the Myxoma virus, which causes myxomatosis. Rabbit fleas are able to survive for long periods of time as adults, and as a result, rabbit burrows often have flea populations even when there are no longer rabbits living there. There have been reports of burrowing seabirds, such as puffins and shearwaters, becoming hosts for these fleas. This is likely due to the birds adopting rabbit burrows and picking them up from there. Care should be taken when pet rabbits are kept close to wild rabbits as it’s possible for fleas to transfer to the pet rabbits.

Control measures

In pets, flea treatments are usually chemical in nature. There are various methods and specific insecticides used, and these vary by region and by individual preference. The most common methods are spot-on treatments, oral medications, flea collars, and flea shampoos. Insecticides, like fipronil, are highly effective at killing adult fleas and are a common part of anti-flea treatments. Methoprene in contrast, is an insect growth regulator and prevents immature stages in the environment from developing. A method that works well for one species can be harmful to others, so management strategies vary. As such, it is always important to check whether a flea treatment is appropriate for use on the target animal before applying it. If in doubt, vets can provide advice.

Medicated shampoos are an effective way to remove an active flea population.
Medicated shampoos are an effective way to remove an active flea population.

Similarly, if a house becomes infested with fleas, removing them can be extremely challenging. Flea cocoons are sturdy and can stay dormant for a long period of time. They are also very small, so can easily remain hidden in floorboard cracks, or in carpets and the like, for a long time. Vacuuming, steam treatments, soap and water, and chemical insecticides can be a great help.

While there is currently no known route of biological control for fleas, there are some non-chemical alternative treatments. Diatomaceous earth, a naturally occurring sedimentary rock, can be used in its powdered form to kill fleas in bedding and barns, although it should not be used directly on animals. Organic extracts from plants such as garlic, cedar, and lemon are traditional remedies or fleas. Although the validity of these treatments has not been experimentally determined, they are often used by organic producers.