Hydatid Disease

Organism Echinococcus granulosus, a cyclophyllidean tapeworm of canines.

Clinical features Hydatid cysts, the intermediate stage of the parasite, have been recorded from all parts of the human body. The commonest site is the liver, with lung, abdomen, kidney and brain in descending order of frequency. As the cysts increase in size, they can cause serious problems some times fatally. The cyst contents are infective so if it ruptures either accidentally or at operation, then numerous new cysts are formed. The liberation of so much foreign protein in the body can result in a severe anaphylactic reaction.

Diagnosis of the disease is clinically, from enlargement of liver or discovery of a cyst on chest X-ray. Immunological methods are useful. A diagnostic aspiration of the cyst must never be made.

Transmission Eggs passed in the dog faeces contaminate pastureland and when eaten by sheep, pigs, goats, cattle, camels and horses develop into hydatid cysts (Fig. 17.1). The hydatid cyst is a fluid-filled sack containing enormous numbers of scolices, any of which can become an adult worm in the dog. The common means of infection is for dogs to be fed the offal of domestic animals. In the wild, jackals, wolves and wild dogs become infected by killing and eating infected herbivores.

Humans enter this cycle accidentally by swallowing the eggs either:

• through food items (e.g. fruit or vegetables contaminated by dog faeces);

• drinking water contaminated by dog faeces;

• close contact with dogs (e.g. by touching their fur or being licked by them; when a dog licks itself, it can spread eggs all over its body as well as sticking to its tongue).

Primary infection commonly occurs in childhood, with symptoms developing in adult life.

Incubation period Generally a period of several years.

Table 17.2. Post-exposure and anti-rabies guide. This is only a guide and should be used in conjunction with local knowledge of rabies endemicity and the animal involved.

Type of animal Status of animal

Type of exposure

Treatment

Domestic

Dog Cat Cow

Wild

Fox Wolf Raccoon Mongoose Bat, etc.

Healthy and remains so Any for 10 days Signs suggestive of rabies. Retain animal for 10 days Rabid or becomes so during retention

Escaped, killed or unknown

Regard as rabid if unprovoked attack

Mild, scratch or lick Bite

Mild, scratch or lick

Bite

Mild, scratch or lick

Bite

Bite

None

Rabies vaccine (discontinue if animal not rabid by day 5)

Rabies vaccine

Anti-rabies serum plus vaccine (discontinue if animal not rabid by day 5)

Rabies vaccine

Anti-rabies serum, plus vaccine

Serum immediately followed by course of vaccine

Eggs swallowed by human

Eggs swallowed by human

Dog eats infected animal offal

Fig. 17.1. Hydatid disease, the life cycle of Echinococcusgranulosus.

Dog eats infected animal offal

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Fig. 17.1. Hydatid disease, the life cycle of Echinococcusgranulosus.

Period of communicability Dogs are often repeatedly infected so continue to be a source of infection, especially to children.

Occurrence and distribution The disease is widespread, but occurs in concentrated pockets, such as sheep-rearing areas or where dogs live in close proximity to humans. A very high rate of infection is found in the Turkana people of northern Kenya where dogs are trained to care for young children.

Control and prevention can be implemented at several points in the life cycle:

Infected material should not be fed to dogs or if this cannot be avoided, it must be well cooked. Dogs can be treated with prazi-quantel to remove any adult worms. Measures should be taken to reduce faecal contamination, such as fencing water sources and food gardens or the general training of dogs. Ultimately though control will depend upon human attitude to dogs by keeping them in an appropriate place, not touching them or feeding them at meal times, destroying unwanted animals and observing personal hygiene. Children, in particular, should be taught to wash their hands before eating and after touching dogs.

Treatment Albendazole and mebendazole are effective, but where necessary surgical removal may be required, taking care to remove the entire cyst or if rupture seems likely to sterilize the contents with formalin. Praziquantel will prevent the development of secondary cysts if rupture of a primary cyst has taken place, so is a useful precaution during surgery.

A similar, but rare infection is E. multilocularis, which as its name suggests forms multi-loculated lesions rather than single cysts. These invade the body much in the same way as a neoplastic growth, including producing metastases. It is found in the colder regions of the world (Siberia, Alaska and northern Canada), a parasite of foxes and dogs, with voles, lemmings and mice being intermediate hosts.

Another variety, found in Colombia, Ecuador and Brazil, is E. vogeli, caught by dogs eating agoutis, pacas and spiny rats. A polycystic hydatid cyst results.

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