Note for Pet Owners

This information is provided by Provet for educational purposes only.

You should seek the advice of your veterinarian if your pet is ill as only he or she can correctly advise on the diagnosis and recommend the treatment that is most appropriate for your pet.

Note for Animal Owners:
Your animals may appear to be normal to you but in fact they may be carrying a large number of parasitic worms inside their stomach or intestines. Infective stages of the common parasites survive for a long time in the environment and after treatment reinfection occurs frequently. In addition, some worms are transmissible to man (see Zoonosis). It is therefore important to give routine treatment against the common worms.

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Worms are classified as helminths and they are parasites that live inside their host's body. They are therefore known as endoparasites. Worms can be surprisingly large - the common roundworm of dogs (Toxocara canis) can grow to 18cm in length !

There are three main types of helminth
of clinical importance:

  • Roundworms (nematodes)
  • Tapeworms (cestodes)
  • Flukes (trematodes)

Worms have a life-cycle which involves stages outside the main host. Sometimes the life-cycle requires that a  parasite stage passes through another (intermediate) host. Sometimes the infective stage of a worm can accidentally infect an unusual host - such as happens 

Breed Occurrence
Most species of animal can be infected with worms including mammals (dogs, cats, rabbits, horses, sheep, cattle etc), fish, birds and reptiles.

The distribution of parasites in our domestic animals varies worldwide depending upon many factors including climate. However the common worms are :




  • Roundworms
  • Lungworms
  • Tapeworms
  • Flukes


Animals with parasites may show no signs at all. However, if a host has a large number of worms it may find it difficult to maintain body condition and it will lose body weight. In some cases sufficient injury can be caused to the host to produce signs of disease (eg vomiting, diarrhoea). 

Occasionally heavy worm burdens can cause death.

Worms sometimes have complex  life-cycles which involve a period of existence and development outside the primary host. In some cases a stage of development may require passage through an intermediate host

Understanding the life-cycle of a specific parasitic worm is important so that strategies for treatment and prevention can be designed and implemented.

Diagnosis is usually made by identifying typical eggs in faecal samples passed by an animal. 

Sometimes, as in Toxocara canis or Trichinella spp dormant larval stages may be recognised in muscle biopsy samples. In some cases eg the heartworm Dirofilaria immitis larvae can be found in blood sample.

For some worms (eg D.immitis) there are also serological diagnostic tests to confirm exposure.  

If an animal passes whole worms in faeces they can be identified by their characteristic anatomical features.

Routine treatment of young animals and regular worming of adult animals is important to reduce the incidence of infection and to reduce environmental contamination. 

A wide variety of treatments are available for the various worm infections that occur in our domestic species. Under certain circumstances natural resistance can develop so rotation of products is often recommended - particularly for farm animals.

The prognosis is good for infected animals - except for individuals with a heavy worm burdens  when damage to the intestine wall can lead to ulceration and even perforation. 

Long term problems
Re-infection is a common problem - particularly in animals that are in contact with a heavily contaminated environment eg pasture, parkland.


Updated October 2013