Cat - Lungworm (Aelurostrongylus abstrusus)

Although lungworm disease occurs in both dogs and cats, different types of roundworms are responsible. They both belong to a family of relatively small roundworms known as metastrongyles. Most of the metastrongyles are endoparasites, living in, or next to, the lungs of the host. Canine lungworms live mainly in the pulmonary (lung) blood vessels, adjacent to rather than in the lung tissue itself whereas Aelurostrongylus abstrusus, the common feline lungworm lives in the air spaces in the lung tissue. These are small worms, the females which are the larger rarely measure more than 10 mm whereas the males are approximately half this size (5 mm in length).

Lungworms in both dogs and cats have one feature in common. In order to complete the life cycle the immature worms (larvae) when shed by the host cat (or dog) have to enter a molluscan (slug / snail) intermediate host.

In the case of the cat lungworm, the intermediate host (slug or snail), then has to be eaten by another host, either a bird or a rodent. This in turn has to be eaten by the cat in order that the life cycle can be completed.

How common is lungworm infection in cats?

Lungworm infection in cats is recognised in most parts of the world. In areas where cats normally live outside, catching rodents and birds, the chances of lungworm infection are obviously much greater. Nevertheless, many infections take place without the infected cat showing any signs, in other words it is asymptomatic. Usually the cat will build up its own natural immunity so that after three or four months the parasite is expelled from the body and because of the size of the worm, only a maximum ½ to 1 cm (¼ to ½ inch), any worms expelled are usually unnoticed.

Once the host cat has built up sufficient immunity and the worms expelled, it is then strongly resistant to any reinfestation. However during the period that this immunity is developing the lungs of an affected cat can be literally riddled with worms, larva and eggs, all to the detriment of the unfortunate cat.

How is lungworm in cats diagnosed?

Once infected the problem can be positively diagnosed by identification of immature larvae in the faeces. These first appear at about 5 weeks after the cat has been infested with the parasite. Many cases however are totally asymptomatic with the cat developing an immunity after 3 or 4 months.

Life cycle of the parasite

The adult worms lay their eggs in the lung tissue or the walls of the blood vessels and the first stage larvae (L¹) released from the eggs will start migrating up the respiratory air passages and eventually reach the pharynx (back of the mouth). From here they are either swallowed and passed into the digestive tract or coughed out of the mouth on to the ground. Some of the larvae that are swallowed develop further within the digestive tract of the host cat and then burrow into blood vessels, to be transported by the blood through the capillaries to the lung tissue where they finally develop into adults and complete their life as before.

Those larvae that are either coughed out or reach the ground in the faeces are either eaten by or penetrate the 'foot' tissue of suitable slugs or snails in which they develop. The molluscs are in turn eaten by the paratenic hosts (birds or rodents), which in turn are caught and consumed by the cat, thus passing the parasite on so completing the life cycle.

Why has the problem worsened in recent years?

This is due to our warmer and more humid summers which, as any gardener will know, have been very conducive for the slug and snail population to increase.

What are the signs?

When they do occur, signs can be very acute. Coughing following exertion is usually the first sign together with laboured or difficult breathing (dyspnoea). Sometimes there is marked lack of condition. All these signs are similar to those of chronic 'cat flu' and this may be one of the reasons why really acute lungworm cases are only positively diagnosed on post mortem examination.

Is there any effective treatment?

Lungworms in the cat are very susceptible to many of the modern feline roundworm preparations including some multi-purpose 'spot-on' drops that are available. If you have any concerns, and particularly if your cat spends a lot of time outside hunting, please feel free to discuss the problems with us.

If you live in a known lungworm prevalent area it is certainly worthwhile regularly worming your cat, particularly for the first 2 or 3 years of its life since it is young cats that are more prone to pick up the parasite. We will be happy to offer advice regarding routine medication.