A Tour around Hydatid Cysts (inside is Hydatid Sand)

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http://www.allaboutmedicalsales.com/medical_briefings/hydatid_cysts_260905.html

 

 

A Tour around Hydatid Cysts (inside is Hydatid Sand)

(Date of publication 26 September 2005)

 

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.....