Derrick Garwood, Freelance Medical Writer and Editor

It is difficult to imagine that the Golden Retriever puppy in this toe-curlingly cute photograph could soon harbour a parasite capable of causing significant morbidity, even death, to its adoring owner. The potential culprit is Echinococcus granulosum, a tiny tapeworm only 3 to 6 mm long that is endemic in most regions of the world and has only three or four segments, as can be seen from this photograph.

There are two biologically and ecologically distinct forms of E. granulosum: a sylvatic or wild form, for which the definitive hosts are carnivores such as wolves, foxes, dingoes and jackals, and a pastoral form for which the definitive host is the domestic dog. The organism's life cycle also requires an intermediate host that is a herbivore, such as a sheep, cow, deer or kangaroo. Unfortunately, humans sometimes fall unwittingly into this category. The adult tapeworm resides asymptomatically in the small intestine of the carnivorous host and eggs are shed via mature segments with the faeces (up to 1,000 every 10 days for 2 years). The eggs contaminate vegetation and are ingested by the intermediate host. Humans can become infected by eating contaminated food, contact with canine faeces or simply by handling infected dogs, whose indiscriminate licking can transfer eggs to their coat.

The eggs hatch into embryos in the intermediate host's intestine, penetrate the gut wall and travel via the bloodstream or lymph system to lodge somewhere in the body's tissues. Here they develop into large, fluid-filled bladders – hydatid cysts – over a period of one or two years. This photograph shows some cysts, or metacestodes, which have just been surgically removed from a patient. They contain protoscolices, each of which is an invaginated precursor of the scolex, or head of a tapeworm, that bears hooks and suckers for attachment. The structure can be clearly seen in the photomicrograph on this page. In the normal course of events, when the intermediate host dies its tissues are eaten by another definitive host. The cyst wall is then digested, the protoscolices evaginate, attach themselves to the gut wall and develop into adult worms in about 7 – 9 weeks. As each cyst may contain many protoscolices, comprising the 'hydatid sand', definitive hosts may be infected with many tapeworms – try counting the white, hair like structures attached to this section of dog intestine.

In humans, the effect of hydatid cysts depends upon their location and size. They have the potential to grow to 30 cm or more in diameter, as in this photograph, and the pressure exerted on adjacent tissue can be extremely serious, particularly if the brain is affected. The rupture of a cyst can prove fatal, not only because it may contain tens of thousands of protoscolices, each of which is capable of evolving into another cyst (secondary infection), but also because the contents are highly allergenic and the patient may succumb to anaphylactic shock.

In each anatomic site, the cyst is surrounded by host tissue (the pericyst) which encompasses the parasitic endocyst. Here is a diagram and here a microscope slide of the cyst wall, showing its characteristic laminated structure. Inside is a germinal layer that gives rise to brood capsules, protoscolices and daughter cysts. The growth rate is highly variable, but averages about 1 to 1.5 cm per year.

As they can occur virtually anywhere in the body, the clinical features produced by hydatid cysts are highly variable and occasionally bizarre. Liver lesions may present as cutaneous abscesses, but other symptoms can include loss of vision, dyspnoea, abdominal pain, fever and a swollen thigh! Unusually for parasitic infections, diagnosis is primarily serological, while radiography and ultrasonography are also useful. CT scanning is precise, but the cost may be prohibitive in developing countries where infection rates are high.

Until recently, surgery was the only available treatment and it is still considered the therapy of choice, although it is associated with considerable mortality (up to 2%), morbidity, and recurrence rates (2%-25%). It may take the form of radical excision or a minimally invasive procedure termed PAIR (puncture, aspiration, injection of a scolecoidal agent, re-aspiration). Despite the fact that some trials have shown PAIR to produce a complete cure effectively and safely, its use remains controversial and some clinicians are unconvinced of its benefits.

The only drugs effective against hydatid cysts are mebendazole and albendazole, the latter being significantly more effective. It is usually given in 4-week cycles, separated by 1 – 2 weeks without drugs, when it can produce an apparent cure in up to 30% of patients and an observable response in 40% – 50% more. Caution must be exercised, however, because this compound has been shown to cause birth defects in animal studies. Consumer information about albendazole can be found on the Medline Plus site.

The UK Health Protection Agency gives some useful advice about preventing hydatid disease, such as washing hands after contacting dogs, avoiding canine faeces and not feeding raw sheep meat or offal to dogs. In essence, dog owners should practise common sense hygiene and not be overly affectionate towards their pets. No mention of sheep farmers.....

Derrick Garwood, a Freelance Medical Writer and Editor submitted this tour

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