Separate fact sheets are available on roundworms, tapeworms and heartworms but this fact sheet will give an overview of the problem.

Are there different sorts of worms?

Broadly there are two types of worms that cause problems in the dog. These are Nematodes or roundworms, of which Toxocara canis, the common intestinal roundworm and Diarofilaria immitis, the heart worm, are the main examples and also Cestodes or tapeworms of which Dipylidium caninum, Taenia species and Echinococcus species are important examples. Hookworms (Ancylostoma species) are also of importance in some areas but thankfully not in Britain.

Are these endoparasitic infections serious in the dog?

In adult dogs intestinal worms, both roundworms and tapeworms, are only occasionally life threatening, usually in debilitated animals or those that are immuno compromised.

Heartworm disease, which is a condition that does not at present occur in Great Britain except in imported animals can be a major problem. Indeed heartworm disease is considered to be one of the most important conditions seen in small animal practice in the United States. It is prevalent in the eastern states, especially Florida but also in parts of southern Europe and eastern Australia.

With the relaxation of quarantine in Britain under the PETS initiative heartworm could be seen more frequently in animals that have visited countries where diarofilaria is endemic and then return to Britain. Since transmission depends upon mosquitoes at present not indigenous in the UK, it is unlikely that the condition will become endemic in Britain.

In young animals gastro intestinal worms can be a problem.

Roundworms – no intermediate host required

Nematode (roundworms) are free living in the bowel. If in large numbers in the growing puppy they can stunt growth, cause serious digestive upsets and result in a lot of gas formation. This presents with a typical ‘pot bellied’ appearance of the puppy. Roundworms can be spread directly from dog to dog via infective eggs.

What about tapeworms?

An intermediate host is required.
Dipylidium caninum, the common tapeworm of the dog both in the UK and the US causes few problems in the adult host but in the growing puppy can result in digestive upsets and stunting. Diplydium is spread by the flea as intermediate host.

Echinococcus spp, another type of tapeworm, is important because it is zoonotic, meaning we can be infected. Sheep and sometimes man can act as the intermediate hosts in which the immature forms of Echinococcus develop inside hyadatic cysts in various organs. In man this can be lungs or brain.

Control measures involving regular deworming of the dog together with avoidance of feeding raw or under cooked offal are sensible precautions.

Cestodes, tapeworms, on the other hand are usually found in adult dogs where they cause few problems. However occasionally puppies are infested and depending on the type of worm involved, by sheer volume can cause serious blockages to the bowel.


Hookworms, particularly Ancylostoma spp are one of the most pathogenic parasites of the dog. Although not a major problem in Britain except in those dogs undergoing quarantine, this tropical hookworm, approximately 1-2 cm (1/2 to 1”) long, attaches itself to the lining of the bowel and as a result of blood sucking activities can cause severe anaemia. In addition the infective larvae can enter the host either by mouth or through the skin, particularly of the feet. Eczema and secondary bacterial infection can result due to irritation as they burrow through the skin.


These are a major problem in parts of southern Europe and the United States. They are large worms, adults reaching 16 cm (5.5”). They are chiefly located in the right ventricle of the heart and adjacent blood vessels and heart worm disease is considered to be one of the most important conditions seen in small animal practice in the United States.

The period of development to the adult stage is about six months after infection following a bite by a mosquito carrying the infective larvae (Microfilariae). The typical signs are fatigue on exercise, coughing and poor condition.

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