How an animal acquires a worm infection depends not only on whether it is a nematode or tapeworm but also differs with each type of worm.

It is almost impossible to prevent worm infections as cats and dogs are exposed to challenge wherever they go. Local parks are a prime source of infection of Toxocara canis for your dog, T. cati eggs can be found in the soil where your cat scratches to go to the toilet and if your pets are infested with fleas then it is quite likely that they are also infected with Dipylidium caninum. It is therefore very important to treat your pets for worms and fleas on a regular basis. There are primarily three methods of infection:

Oral infection

This is where an animal becomes infected by taking the eggs or larvae in through the mouth. This can take several forms:

• Ingestion of infective eggs
  from the environment such as a cat cleaning its feet after scratching in the dirt (Toxocara cati, Toxascaris leonina) or a dog snuffling around in the park (T. canis, T. leonina).

• Ingestion of an Intermediate host
  such as if an animal eats a flea while grooming itself it may be exposed to the tapeworm Dipylidium caninum. Sheep dogs that are allowed to scavenge sheep carcasses may become infected with Echinococcus granulosus. Cats that hunt and eat mice may become infected with Taenia taeniaeformis. All tapeworm infections occur by exposure to an intermediate host.

• Ingestion of Larvae
  cats and dogs can acquire hookworm infections from the ingestion of larvae from the environment. Dogs can be infected by Filaroides species from larvae passed in faeces; in addition bitches can infect their puppies when grooming them. See 'Maternal Infection'

• Ingestion of a Paratenic host
  hunting cats are also likely to be exposed to infection from Toxocara cati.

• Suckling from Mother
  puppies and kittens can be infected with Toxocara canis and cati respectively from their mother's milk. See 'Maternal Infection'
  The difference between an intermediate and a paratenic host is that the larva does not develop into the next larval stage in a paratenic host whereas it does in an intermediate host. It is usually the case that nematodes have paratenic hosts and tapeworms have intermediate hosts.

Percutaneous infection

Ancylostoma caninum, a hookworm, infects its host by penetrating the skin of the dog's feet. The larvae of the hookworm Uncinaria stenocephala also uses this method but once inside the body is usually unable to reach maturity. High levels of challenge however can lead to skin problems in dog's feet.

Maternal infection

There are three types of maternal infection:

• Intrauterine
  Toxocara canis larvae cross from the mother to the puppies via the placenta during pregnancy.

• Lactogenic
  After birth, puppies and kittens can be infected by Toxocara species from their mother's milk. In addition, the pups could re-infect their mother while she is cleaning around their back end as some larvae may pass straight along the pups' digestive tract to be swallowed by the bitch.

• During Grooming
  Bitches can infect their puppies with Filaroides species as larvae brought up from the lungs can be present in saliva.