THE DIFFERENT TYPES OF WORM
There are many species of worm that can affect dogs and cats in this country, but for practical purposes they can be divided into 3 main groups: tapeworms, roundworms and hookworms.
The correct identification of the type of worm is important when selecting appropriate treatment and a description of all 3 types follows:
TAPEWORMS
A tapeworm consists of a small head, provided with hooks and suckers for attaching itself to the gut wall and a long segmented body. Fully grown tapeworms may vary from less than an inch to several feet in length depending on the species, but complete worms are rarely seen by the pet owner. Adult tapeworms, however, continually shed segments from the end of the body and these are commonly noticed in the faeces or in the coat round the animal's anus. The individual segments are alive and moving when they emerge and are flat, white and ½-1 inch long. When they die and dry out, they look like grains of rice and these may be seen in the animal's bedding.
The Tapeworm Life Cycle
Tapeworms have a complicated life cycle involving what is known as an intermediate host. The segments which are passed out by an infested dog or cat are full of eggs and these are eventually dispersed into the environment when the segment dies and disintegrates. However, these eggs are no directly infested to another dog or cat and will only develop if eaten by a different animal, for example, a rodent, rabbit or most commonly a flea or louse. This second creature is termed the intermediate host and the species involved varies according to the type of tapeworm concerned. In rural areas, farm livestock, particularly sheep, can be important intermediate hosts for some species of tapeworm.
Tapeworms are thus most likely to occur in pets which roam freely and hunt, are fed raw meat or offal, or have fleas.
What Tapeworms Do?
The symptoms of heavy tapeworm infestations are occasional colic and chronic diarrhoea accompanied by a voracious appetite.
Listlessness, a shaggy coat and loss of weight are also frequent signs. Mature segments present in the rectum often produce irritation in dogs, but rarely in cats.
However, cats and dogs often carry light infestations of tapeworms without showing visible signs, although the characteristic segments or chains can often be seen in the faeces or in the fur around the anus.
Tapeworms and People
There is really only one species of dog tapeworm that can infest people; this normally uses sheep as the intermediate host. If people eat the eggs then the larvae may migrate through the body and form very large cysts. Fortunately this is comparatively rare and the problem is mainly confined to areas with high sheep populations.
ROUNDWORMS
Roundworm infestations are very common in dogs and cats and can cause significant disease in young animals. They can also cause a condition known as "visceral larva migrans" in man and so have a public health importance. It is therefore worthwhile considering this type of worm in detail.
What Are Roundworms?
Adult roundworms are yellowish white in colour, range up to about 4 inches in length and have long cylindrical bodies usually pointed at the ends. Of the 3 worm types mentioned, roundworms bear most resemblance to a small earthworm in size and shape.
The Roundworm Life Cycle
The life history of the roundworm is complex and the different species do vary, but a basic understanding is necessary to appreciate the principles of control. The most important roundworm species is Toxocara canis, which infests the dog and the life cycle of this worm is described below.
Adult roundworms in the gut of an infested dog produce vast numbers of eggs which pass out in the faeces. These eggs are very resistant to heat and cold and may survive in the soil for many years. They are very sticky and can attach themselves to the coat; the animal subsequently ingests the egg while grooming.
Once the eggs are swallowed they hatch in the gut to become larvae. In the most direct form of the life cycle, the minute larvae burrow through the gut wall and migrate to the lungs. There they undergo further development, are coughed up, swallowed and return to the gut to become adults.
This cycle of infestation occurs predominantly in young puppies. In older dogs the situation is different in that a large proportion of the larvae burrowing out of the gut are carried in the bloodstream to tissues such as the liver, kidney and muscle, where they form cysts and remain dormant. Over a period of years, an adult dog can accumulate a large number of these encysted larvae and they are particularly important in the spread of infestation to puppies.
During pregnancy, hormonal changes in the bitch cause some of the inhibited larvae in the tissues to revive. The larvae then continue their migration, but this time end up inside the developing puppies in the uterus. Thus nearly all puppies already have worms when they are born.
In addition to the above, the bitch's own tolerance to worms falls during pregnancy and lactation and ingested eggs develop rapidly. Puppies may be infested by eggs passed out in the batch's faeces and larvae present in her milk.
Taking all these aspects of the life cycle together, it is no wonder that puppies can easily acquire massive roundworm infestation.
Roundworms, however, are not restricted to puppies, but can infest dogs of all ages. Although adults often tolerate infestation unless resistance is low, they will nevertheless pass numerous eggs ensuring a continuing source of infestation.
Roundworms and Cats
There are several possible variations to the above life cycle, one of which occurs when the eggs are eaten by another creature, for example a mouse or earthworm. The cycle then involved an intermediate host as already described by the tapeworm. This type of infestation is particularly important in the main can roundworm, Toxocara cati.
The eggs eaten by the intermediate host hatch and the larvae migrate through the tissues to form cysts. These cysts will develop further only if the intermediate host is eaten by the main host, in this case the cat.
When roundworms have gone through an intermediate host, they do not migrate again in the main host, but develop entirely in the gut.
What Roundworms Do?
Small numbers of roundworms, particularly in the adult animal, cause no signs. Though they will be producing large numbers of eggs and adding to the level of potential infestation in the environment.
Heavy infestations, which occur particularly in puppies and kittens, result in digestive upsets, general listlessness and failure to grow. Frequently, the animals have a pot bellied or tucked-up appearance and look "poor". Migrating larvae in puppies can damage a variety of tissues, especially the lungs, resulting in a cough and possible secondary pneumonia.
Roundworms and People
People may accidentally ingest roundworm eggs from their fingers, from plates which dogs have licked, from furniture, or, particularly in the case of young children, from contaminated soil.
The important species for human health is Toxocara canis, the eggs of which will hatch in the human gut. As in the dog, the larvae burrow out of the intestine, migrate to the tissues and form cysts, resulting in a condition known as "visceral larva migrans". However, further development does not occur and adult pet roundworm species are not found in the human intestine.
The health problems associated with Toxocara canis in man arise from damage caused by the worm cysts. The most dramatic symptoms involve the eye and have mainly been seen in children. The condition is very rare, but has led to considerable publicity, rightly stressing the importance of roundworm control.
HOOKWORMS
Hookworms are classified as a type of roundworm, but form a distinct group when compared to the "ascarid" roundworms described below. In other countries they can be a major problem, but in the United Kingdom they are generally only significant in groups of kennelled dogs, such as foxhounds or greyhounds. The worms themselves are very small, up to about ½ inch in length and are bent into a hook shape.
The Hookworm Life Cycle
Infested animals pass out large numbers of eggs in the faeces which hatch to become larvae. These larvae can infest another dog or cat in 2 ways, either by being ingested or by penetrating the skin.
In the case of ingestion, the swallowed larvae pass into the intestine and burrow into the gut wall. They spend some time developing in the gut wall tissue before re-emerging into the gut to become adults.
If infestation occurs via the skin, the larvae migrate through the tissues and eventually enter the bloodstream. They are then carried in the blood to the lungs where they lodge in the small vessels. Once in the lungs, the larvae develop, migrate into the airway, are coughed up, swallowed and arrive in the gut where they develop into adults.
With the species of hookworm occurring in Britain, ingestion is thought to be the main method of infestation. Although skin penetration occurs, many of the larvae fail to reach the intestine. There is some debate about whether infestation can be spread to puppies via the bitch's milk, but if this does happen it is probably not of great significance.
What Hookworms Do
Heavy hookworm infestations can lead to digestive upsets and anaemia, through the blood sucking activities of the parasite. The skin penetration and tissue migration may cause a variety of symptoms, including irritation of the feet. However, they vast majority of infestations are mild and cause no obvious signs, though they may occasionally be suspected as a cause of poor performance in hunting or racing dogs.
How to Protect Against Worms
The first thing is to accept that worms are a potentially serious problem and they are present at any time. The objective is to keep worm levels constantly low and not just to treat pets when the adult worm is seen in the faeces.
Apart from regular and accurate worming, there are other sensible precautions which should be taken:
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Train your dog to defecate in street gutters and not in areas used by children, such as parks.
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Remove dog faeces, especially those from pregnant bitches, from the garden and public places.
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If possible, do not allow your dog to roam unsupervised in public places.
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Forbid children to handle lactating bitches or cats and sucking puppies and kittens.
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Discourage dogs from licking human hands and faces.
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