Heartworm in Cats


Feline heartworm disease is a condition which for some years was unknown as a diagnosis as most authorities thought that only canids (the dog family) and a few other carnivores or omnivores such as the badger could suffer from it.  Angiostrongylus vasorum is the European equivalent of the North American heartworm, also known as French heartworm. It is not overly common in domestic cats, although it is relatively common in feral populations and mixing can cause problems. As its secondary hosts include snails and frogs, cats with peculiar tastes may come in contact with it. Cats in the UK are generally considered as fairly safe from heartworm, but it is always a good idea to be alert to the dangers. Heartworm has gone from being rare or unknown in the United States to being diagnosed reliably in all of the contiguous states; in other words, all but Alaska and Hawaii.

Signs of heartworm in cats

Heartworms spend part of their lifecycle in a mosquito (European ones require a snail or slug) and a bite from just one infected insect can be enough to pass on the disease. This means that if an infected cat is brought in to the country, the heartworms will not be spread via mosquito, as the essential part of the lifecycle will not be present. Heartworms cannot be caught from the host, even if they cough on another animal or human. This is not how the disease is spread. A cat with heartworm is essentially suffering from fluid around the pleural cavity as well as local irritation and so the symptoms are coughing and difficulty breathing. Some animals are given an early diagnosis of asthma, but later symptoms include lack of appetite and vomiting, which usually leads to more investigations being carried out. In a non-heartworm area it is unlikely to be on the vet’s list of things to check, but it is anyway an extremely difficult disease to diagnose, as there is no reliable serological test as antibodies don’t always form and echocardiography doesn’t always show the worms, which can be tiny in the cat and often in relatively small numbers. Sometimes the heartworm infestation will suddenly reach critical numbers and the cat will experience seizures, blindness and sudden collapse and death.

What is heartworm?

The species depends on where the animal lives, but it is a nematode or roundworm, which is one of the most diverse animals on earth. It has been estimated that there are probably more than a million different species of nematode worm, of which most are still to be discovered. They are tricky beasts to deal with, as they can be very hard to diagnose and can be any size from microscopic to quite a few inches long. The parasitic ones tend to be the largest, and this is no surprise as they don’t have to provide their own food as their host does that for them. Most need an intermediate host for the development of their eggs and in this way they can spread far from the original host or location without any effort. In short, they are easy to contract, much more difficult to dislodge. They try not to kill their hosts of course, as they will then die themselves, but when a nematode crosses the species boundary, as heartworm seems to be doing with cats and dogs, sometimes they are not as successful in this and heartworm is a killer, more often than it would choose, left to itself.

Treatment and prognosis of heartworm

It is rare for cats in the UK to get heartworm at the moment, but as the North American heartworm has crossed the species barrier it is something which many vets believe should be kept in mind for the future. There are specific anti-helminth (anti-worm) medications available in heartworm areas, and these are used as a spot-on treatment year round and are really the only line of defence. There is really nothing that can be considered a cure for heartworm, so treatment is mainly an attempt to try and keep the heart and lungs working as healthily as possible and also to boost the cat’s wellbeing and natural defences. Most vets will give doses of prednisolone in the first instance, as well as bronchodilators to improve breathing.

If your cat will tolerate it and you can become comfortable with its use, this can be given by inhaler, which is a much better method of administering the drug. Overdosing is less likely this way and also it gets to the problem more directly if the animal is in respiratory distress. Drugs to stabilise the heart may be given and in extreme cases, oxygen. If the cat has become very dehydrated with vomiting and lack of appetite, intravenous fluids might be necessary. In some cases, a vet may attempt removal of some of the worms by surgical intervention, but this is not often very successful and is attempted only in extreme cases.

So, does my cat have heartworm?

If it has lived all its life in the UK, it is unlikely. Even so, the symptoms of heartworm are indicative of many other serious conditions, so any cat exhibiting them should be taken to the vet as soon as possible. There is a lot of vigilance behind the scenes to make sure that problems such as heartworm do not spread worldwide or cross species and your vet will be up to date on all the latest news, so don’t worry that he might miss any new development.


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