Internal Parasites

Last week we focused on external parasites which can be a huge pain for our pets and us. This week it’s time for us to look at internal parasites. Internal parasites can cause major problems for our pets including anaemia, poor immune function, bleeding or potentially could be fatal. We’ll talk through the major categories, signs to watch out for and how to prevent these parasites in your pets. 

Internal Parasites

Internal parasites or endoparasites are terms we use for any parasite living inside your pet’s body. These can include lungworm, heartworm and intestinal worms. In this section we can also discuss a type of infection in the gastro-intestinal system which is passed on by a very small type of parasite called protozoa. Protozoal infections may mimic an internal parasite problem but can if left untreated, become quite serious for the animal. 

There are many ways to treat and prevent internal parasites. Treatments can vary on how you give them, how often they are needed, how long the treatment lasts and the cover over different parasites for your pet. At different stages of your pet’s life there will be different requirements from an internal parasite or worming treatment. It is well worth discussing with your vet what the best option is for your individual pet’s needs. 

Gastro-intestinal worms

Gastro-intestinal worms are worms that inhabit the stomach or intestines. They are normally passed on through the faecal-oral route meaning they are passed out in the animal’s faeces and then consumed by the same or another animal. Gastro-intestinal worms include roundworms, tapeworms, whipworms and hookworms.  We’ll discuss below the main features of each of these parasites.


Roundworms are the most common form of internal parasites. These worms can pass onto puppies and kittens from their mother either during pregnancy or via the mother’s milk. They can also be passed on via the faecal-oral route.  Animals may not always show signs of having these parasites but in young animals a pot belly, weight loss, vomiting and diarrhoea may be noted. Roundworms can also lead to a cough or breathing problems if they pass through the lungs. It is important to remember that roundworms can also affect humans. Children are commonly affected by this problem when handling soil where faeces have been passed. One of the worst side effects of these parasites can be permanent blindness.  


Tapeworms are segmented worms that can reach up to and above 15cm in length. These don’t tend to cause major issues for our pets but we may see general weight loss and potentially some vomiting or diarrhoea if the parasite is causing irritation. When these worms pass out of the animal they normally look like grains of rice. It should be noted that fleas can carry tapeworm larvae so this can be another route of infection.


Whipworms are relatively rare in the UK. If a heavy infestation occurs then diarrhoea with blood and anaemia may be noted. If left untreated this could become severe and prove fatal.  They are called whipworms due to their characteristic appearance. 


Hookworms act in a similar way to roundworms and can cause diarrhoea with blood due to their feeding on the animal’s intestines. Young and old animals are particularly at risk. Humans can be affected and may develop burrowing tracts where the worms can be seen. 


Lungworm is a growing problem in the UK. Lungworm can be passed on by dogs eating slugs or snails. Foxes can carry and transmit lungworm too. The worms affect the small vessels of the lungs. They pass through the lungs to be coughed up and swallowed where they are next passed out in the faeces. Lungworm can pose a serious risk to our dogs causing bleeding problems due to a lack of clotting. The first sign may be a cough or a reduction in the animal’s energy. Lungworm can also lead to death so it is very important that our pets receive preventative treatment and are safe from this parasite.


Whilst heartworm is not currently seen in the UK commonly, it can be seen in animals that have travelled abroad. Heartworm is passed on by mosquitoes and can be found in southern and eastern Europe. Initially dogs will tire easily when exercising but in the latter stages of the disease, dogs will be tired even at rest. If left untreated death can occur. Speak to your vets before going abroad to ensure your pet is covered to prevent this parasite. 

Treatment options – Prevention is better than cure!

For all the internal parasites, prevention is much better than cure. Routine worming treatment is the gold standard in dogs and cats. For higher risk animals e.g. young or old animals or animals that hunt a lot, preventative treatments should be given more frequently. Puppies and kittens require more regular treatments due to the risk of transmission of parasites from their mothers. Treatments now come in different formulations including spot ons, tablets and liquids. Speak to your vet about your pet’s individual needs to make sure they are covered.

The Extras:

Protozoal Infections

Protozoa are microscopic and include infectious agents like Giardia. These organisms act in a parasitic way and can lead to diarrhoea with blood or mucus which normally has a strong smell. There are a few options for treatment of this disease but all need to be prescribed by your vet. 


We can’t forget our rabbits when talking about internal parasites. As we discussed in the rabbit health weeks E.cuniculi is a protozoal infection of rabbits which can lead to head tilt, loss of balance, inappetence and weight loss. Sometimes cataracts can develop. The rabbits are usually disorientated depending on the stage of disease. Treatment is usually possible, depending on the stage of the disease, but your rabbit may be left with a permanent head tilt. Treatment normally requires 28 days of oral paste to be given to the affected rabbit. 


Whilst ringworm isn’t a true worm we include it here because it is often mistaken for one. Ringworm or dermatophytosis is caused by a fungus which spreads spores. All animals are susceptible including humans. The lesions appear in rings but can vary between red areas and scaled crusts. Ringworm is highly infectious so treatment should be sought as soon as possible. Your vet will need to prescribe the most appropriate course and can discuss how to prevent further spread of the fungus. 

We hope you have found our Pesky Parasites blogs useful. Remember to always check with your vet how best to meet the needs of your pet when it comes to external and internal parasite prevention.

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