We saw a little dog called Lenny this week, his gums as white as a cricketer’s trousers - Julian Norton

When presented with a case of anaemia, it is essential to work out the cause, even though, at times, this can be complicated.

Essentially, it is either due to a reduced rate of production or an increased rate of destruction of red blood cells or their loss from the body through internal or external haemorrhage.

Discovering which of these processes is going on is the first step in solving the riddle of anaemia.

Of all the causes of anaemia in dogs and cats, the one that has always stuck in my mind is “Heinz-body anaemia”. The red cells are destroyed prematurely because of exposure to a particular type of toxin.

You would think, given the name, that the toxin would be something to do with baked beans.

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It’s not though (baked beans are not, to my knowledge, toxic to dogs, although grapes and raisins didn’t used to be either, until a few years ago. Now, if the internet is to be believed, they are amongst some of the most deadly little foodstuffs around.)

Heinz bodies are found in the red cells of dogs that have been eating onions. Onions are toxic to dogs, and the oxidative damage they cause to the red cells results in the formation of Heinz bodies and subsequently anaemia as those red cells are destroyed. In some cases, if not diagnosed promptly, it can be fatal.

So when we saw a little dog called Lenny this week, his gums as white as a cricketer’s trousers, and with a history of possibly having eaten some onions, Heinz body anaemia was high on the list of possible causes.

His owners had been on the internet and were sure this was the case. However, we could not jump straight to that conclusion and set about some tests. X-rays, abdominal scanning and blood tests quite quickly confirmed that the onions were, in fact, red herrings. Lenny had no Heinz Bodies in his cells.

He had a bleeding growth on his spleen and before long was in theatre having the offending mass removed.

James, the three-month-old pug, was the next patient this week to present with suspicious signs. The youngster was walking backwards, a very unusual thing for an animal to do. It can indicate severe central brain disease.

Could James have eaten something poisonous or were the signs due to some sort of abnormal development of the young brain?

An X-ray showed something in the stomach. James’s owner thought he could have eaten a party popper. James, though sad and walking backwards, was not being sick – the usual sign associated with things stuck inside the stomach, so was this another red herring?

He, too, was wheeled into theatre and it would again be a scalpel and not an antidote that came to the rescue.

But there was another twist to James’s story. The foreign body in his stomach turned out to be not a party popper, but a huge length of unchewed dog chew, swallowed whole.

The working hypothesis was that the chew that was jammed across James’s stomach, was causing so much pain that he couldn’t walk properly or stretch his head down to eat from his bowl. It was an odd theory, but one which turned out to be correct.

The following day, James was bombing about the surgery – forwards. For the second time in a week a possible toxicity had proved to be definitely non-toxic.

■ Julian Norton’s new book, A Yorkshire Vet: The Next Chapter, published by Hodder and Stoughton and costing £16.99, is out on February 6.