Increasing numbers of British pets are being given Prozac to help them battle against depression, a leading veterinary expert has revealed.

Tropical birds such as parrots are the worst affected by depression, according to television vet Romain Pizzi.

Mr Pizzi, who presents Creature Clinic on BBC3 and is a specialist in zoo and wildlife medicine for the Royal College of Veterinary Surgeons, said pets are affected when owners leave them alone for hours on end.

He said: "Contrary to some people's expectations parrots are very intelligent and sensitive animals.

"Typically if people go out to work all day their parrot will get very bored and frustrated and eventually develop depression.

"Symptoms often include plucking out their feathers or self-harming, which is obviously very dangerous.

"When Cockatoos in particular are depressed they can start to self-mutilate and peck their own legs to the bone."

Mr Pizzi, the director of Edinburgh-based consultancy Zoological Medicine, said that the number of tropical birds requiring anti-depressants such as Prozac was rising.

However he added that the drug was only prescribed in "the most extreme cases".

He said: "Firstly we will change the environment of the animal and make sure it has more stimulation and toys.

"When we have ruled out underlying medical problems, we try to break the cycle by using Prozac.

"The Prozac is given to the parrots in liquid form, which is often flavoured.

"It doesn't cure all animals, but around two-thirds respond to the treatment. In a small number of cases things will go well until we wean them off Prozac and the problems return."

He warned owners to think carefully before buying a tropical bird.

"Unfortunately there is a big proportion of people who buy these birds because they are pretty and they talk.

"They are not thinking it through in terms of their lifestyle. Parrots require a lot of care and stimulation."

Birds are not the only pets that get depressed - recent research suggested that as many as 632,000 cats and dogs in the UK area affected.

Symptoms include attacking furniture, loss of appetite, incessant scratching and aggression.

Vet and small animal specialist Mark Johnston said: "A dog can't sit on the couch and discuss his worries but he can howl the house down, chase his tail or chew everything to pieces."

The market for animal medication is booming. Last year the makers of Prozac, US pharmaceutical firm Eli Lilly, launched its first anti-depressant for dogs - a once-a-day chewable tablet flavoured with beef.

The drug, branded as Reconcile, can be taken by puppies as young as six months old.

New York-based Pfizer has also created Slentrol, a diet drug for dogs, and Cerenia, a motion-sickness remedy for pets.

In January Slentrol was approved for use by British vets in a bid to tackle Britain's growing dog obesity crisis.

Vets believe up to 40 per cent of Britain's estimated nine million dogs are grossly overweight, compared with only around 10 per cent in the 1970s.

The drug was criticised by the RSPCA, which claimed that the best way to keep a dog slim is through a healthy diet and regular exercise.