Parasites in dogs

“A parasite is an organism that lives in or on an organism of another species (its host) and benefits by deriving nutrients at the others expense.”


Fleas are common parasites in dogs and they can act as vectors for a number of infections, including some that affect humans.

The flea life cycle

  • A flea jumps onto an unprotected dog
  • It starts feeding immediately on the dogs blood by biting them (this happens within minutes of the flea acquiring the host).
  • Female fleas start laying eggs within 24-48 hours – up to 50 per day!
  • Eggs fall off into the environment and hatch into larvae within 1-10 days.
  • Larvae feed on flea dirt and crawl to find dark crevices and corners.
  • They then turn into pupae – a hard shelled structure, which is very hard to kill.
  • Pupae hatch into adult fleas but this can take up to six months!
  • Once hatched, a flea will look again for a host and a meal! An adult flea can live for up to 160 days on one or more hosts.

Fleas are a major cause of irritation and skin disease in dogs and they are also a vector for the flea tapeworm Dypilidium caninum. This is an important reason for making sure your dog has protection against fleas, since human ingestion of an infected flea can transmit this worm to the human gut where it can thrive. Yuk!


Ticks are blood sucking parasites that can infest our dogs. They will even crawl onto and bury themselves in human skin!

Life cycle

Each life cycle stage feeds once on a new host individual after actively seeking or “questing” for their hosts by climbing up stalks of tall plants such as blades of grass. They hatch from an egg into a small six-legged nymph. This nymph will find a suitable host such as a mouse to feed on. The next stage nymph may find a larger mammal, then eventually the nymph moults into an 8 legged tick which looks out for a large mammal (i.e. your dog!) onto which to feed and mate. This can take about 3 weeks.

Tick borne diseases

Ticks themselves are rather horrid, irritating and painful as they attach themselves by burrowing their heads into the skin. If this happens, the skin often reacts leaving a nasty inflamed sore which can get infected. In addition, and more worrying, is that they can transmit some very nasty diseases including Lyme disease and Babesiosis.



This is caused by the worm Angiostrongylus vasorum. It is sometimes called French heart worm as the mature worms live in the blood vessels of the heart. This worm causes serious health problems in dogs and can be fatal if not diagnosed and treated. It particularly affects young dogs and there are pockets of the country where the parasite is more prevalent than others.

  • Dogs become infected by ingesting slugs, snails or even just their slime
  • The larvae develop into adults in the dogs heart and arteries
  • These adults lay eggs which hatch in the lungs and the larvae are coughed and swallowed
  • These larvae pass out in the faeces and are eaten by slugs and snails completing the cycle.

Lungworm causes many different clinical signs involving coughing and bleeding disorders. Sadly, dogs can die from these parasites. For further advice on lungworm >

Round worms

The dog roundworm is very common! It is the worm that looks a bit like small spaghetti with adults measuring as much as 15 cm in length. Most puppies are born with some worms as they can get them via their mother’s milk. If they have a lot of worms it can make them poorly with tummy upsets and failure to thrive. Later on in life for protection against these parasites, dogs still need to be wormed regularly as they never develop an immunity to them, although they are less likely to be poorly as a result.

The simple life cycle means that eggs are passed out in faeces into the environment, where they can survive for years until they are eaten by an intermediate host such as a mouse. Dogs are then infected by eating the eggs direct from the environment or by eating under-cooked meat. This is why it is so important to pick up your dog’s poo.

Humans can be infected by ingesting infective eggs from the environment which has been contaminated by dog, cat or fox faeces. These hatch into larvae which can cause serious diseases and even blindness – although this is rare.


Echinococcus species

It is really important to treat dogs regularly for tapeworms as although even infected dogs can remain free of clinical disease, they act as carriers for the life stages of the worms that can seriously affect animal and human health. The worms mature and live in the dogs’ intestine before eggs pass into the environment. Once in the environment they infect an intermediate host such as a sheep, which can then develop life threatening cysts in its organs. This can also happen to humans and is called hydatid disease.

For further advice on how to check your dog for parasites >