Echinococcal (Hydatid) Disease

DEFINITION
Echinococcal disease.
AKA Cystic hydatid disease of the liver.
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EPIDEMIOLOGY

Endemic around Mediterranean, middle East, Central Asia and South America
Eliminated in NZ / Australia through control programs.
- some residual disease in older people
And travelers.

Risk factors
Rural / farming
Travel / immigrants
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AETIOLOGY

Of four known species these are mostly associated with human disease:
Is a tapeworm
Echinococcus granulosis (most commonly).
Echinococcus multilocularis.

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BIOLOGICAL BEHAVIOUR

Life Cycle and Natural History

Definitive host is canids
- dogs, wolves, jackals etc.
- 'definitive' means that the adult (breeding) form of the parasite lives in these animals

Intermediate host is
- typically herbivore livestock eg sheep, deer
- humans are an accidental intermediate host.
- intermediates are where laval non-breeding form is found.

Life Cycle
Live ova are excreted in the stool of the definitive hosts.
- these are ingested by intermediate hosts / humans
- penetrate intestine and pass via portal vein to liver (75%), lung (15%) and other tissues including spleen (6%)
Cysts grow in the organs of the intermediate host
--> then these organs are ingested by the definitive host.
i.e. Dogs must eat infected offal ... hence control programs have targeted de-worming dogs and stopping offal eating.

1. Adult breeds attached to dog intestine.
2. Eggs are shed in feces
3. Egg hatches in intermediate host intestine and releases an 'oncosphere'. 
4. The oncosphere penetrates the intestinal wall and moves to liver and lungs. 
5. Invades these organs and develops a cyst.
6. The cyst slowly enlarges, growing daughter cysts and protoscolices and daughter cysts.
7. Definitive host eats the cyst and therefore the protoscolices
8. Protoscolices attach to intestinal wall.
9. Develop into adult worms and cycle starts over.

Pathology

Only 60% are hepatic - also in the lung or bone.
The cyst has 2 layers

Inner cyst

Germinal layer, containing actual echinococcal scolices.
The cyst is usually filled with clear fluid.
Contains hundreds to thousands of protoscolices.
'Daughter cysts' (germinal layered but no outer layer) develop and float in the cystic fluid.
Also 'brood capsules' of budded germinal layer.

Outer cyst

AKA pericyst, 2-5mm thick
Comprised of compressed host tissue and reactive fibrous layer
Contains calcification in 50%
Look white and soft like skinned grapes.

Complications
Expansion of main cyst may erode into bile ducts
- 1/3 of longstanding
May become secondarily bacterially infected
May rupture into body cavities and chest / abdo
Many chronic cysts become calcified and inactive.
Mature cysts are often asymptomatic and have false-negative serology testing.

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MANIFESTATIONS

Symptoms

Non specific
- e.g. abdo pain, discomfort from expansion, symptoms of infection or obstructive jaundice
Diagnosed with a high index of suspicion wrt epidemiologic data, serologic tests, noninvasive imaging.
Patients remain asymptomatic for a long time.
Pts usually diagnosed before symptoms occur.
Extremely large cysts can present with vena cava compression or portal hypertension.
Rupture (rare);
- acute abdominal pain, anaphylaxis and shock


INVESTIGATIONS

Biochemistry

Transaminases may be normal, even in very large cysts
- cholestatic mildly elevated in 1/3

Haematology
Nonspecific eosinophilia in 25% only
WCC only elevated if cyst secondarily infected

Microbiology
Specialised diagnostic techniques
3 tests of serologic tests are available
1. ELISA - sensitivity 64-100%
2, Immunoelectrophoresis; very accurate for hepatic cysts.
3. Hydatid antigen blotting

Imaging
US
Preferred first option.
Specificity ranges around 90%
Standardized US criteria from a WHO working group.
- signs include cyst wall, hydatid sand, multivesicular cysts.
- if starting to degenerate, see decreased intracystic pressure, no daughter cysts, ultimately a thick calcified wall (inactive cyst)
- if cyst has inactive old burnt out features, usually no living protoscolices.
CT
Structural detail.
Show depth and position of cyst.
Unilocular or complex, thick walled, often with calcification, presence of daughter cysts.
MRI
Can compliment CT but not a first line investigation.

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MANAGEMENT

Principles
Eliminate parasite and prevent recurrence.
Minimize morbidity and mortality.
Options are systemic chemo, surgery, and PAIR (puncture, aspiration, injection and re-aspiration)
- surgery is the mainstay of therapy
- chemo and PAIR for those who cannot tolerate or refuse surgery.
--> hence, depends on health of patient, number, size, location.

Conservative

Old scarred hydatid cysts may need no treatment.

Antimicrobials
Benzimidazole
E.g. mebendazole, albendazole.
- chemo alone associated with a 90% treatment failure.
Preoperative administration of albendazole 1-4d
- 3m for mebendazole; much longer.
Postoperative Rx controversial, not generally indicated when no spillage / completely removed.

PAIR
Minimally invasive.
Theory is that it reduces size and volume, cyst thickens and shrinks, then solidifies
Benzimidazole for 1w before then 4w after.
Different scolicidal agents are used: 20% hypertonic saline most commonly.
- hypertonic saline has a high density, allowing evaluation of filling and cyst contact.
Cyst fluid evaluated for viability of remaining protoscolices.
Complications include infection and leak; can result in fever or anaphylaxis.
Indicated for WHO classification 1-3 cysts (active cysts); infected, multiple accessible cysts, or refusal or contraindication to surgery.
Is safe and effective; complications 14.7; recurrence only 1.6%.
Only 0.25% required surgery after PAIR
- further work still needed to define role as surgical replacement.

Operative
Remains treatment of choice in surgical candidates.
Options include simple cyst drainage, partial cystectomy
To radical procedures i.e. pericystectomy, liver resection and rarely transplantation.
Preferred approach quite controversial.

Conservative surgery

Best for cysts on periphery of liver
Before entering cyst, pack operative field with scolicidals.
- minimize risk of peritoneal soilage and contamination.
Cyst contents are then aspirated; system infused with scolicidals.
- if cyst fluid bile stained, avoid scolicidals in the cavity.
Then unroof cyst and fully explore cavity; clear debris and fill with an omental pedicle.

Radical surgery
Complete removal of cyst and pericyst.
Including any exocyst and adjacent parenchyma.
Is really the best treatment for all forms of hydatid but especially large biliary-cyst fistulae.
Either open or closed technique
- if open, cyst contents removed and scolicidals infused; then excision.
- contents must not be spilled.
- pericyst wall removed by electrocautery and dissector.
--> recommended for large cysts, thin wall and high risk of rupture.
- if closed, whole cyst removed en-bloc; no healthy liver tissue; plane outside pericyst.
--> preferred when cysts peripheral with thick wall and not involving major structures.
- control vessels / biliary structures with clips and sutures
- can use intraop USS useful to define relationship with vascular and biliary structures.
Closed is more demanding as parenchymal resection.

Resection and Transplantation.
Nonanatomic wedge resection, if less cut liver surface than pericystectomy.
No ischaemic pedicle.
If large multilocular cysts, formal liver resection.
Remnant should be clear.

Liver Transplantation
Unusual circumstances, transplantation may offer only curative approach.
E. multilocularis (multilocular by name) --> more likely reqd.

Biliary-cyst communication.
Investigate.
Pre-op ERCP can help identify; or cholangitis or numerous segments involved = high risk.
Finding of bile-stained fluid in cyst cavity should alert.
Leave a dry pad on inner aspect of cyst while applying pressure on gallbladder --> if stains, highly suggestive.
<5mm communication --> close with sutures.
- transcystic dye study after excision.
>5mm = more difficult.
- can close over a T-tube to decompress the biliary tree

Lap?
Same principles.
Disadvantages of greater risk of spillage.

Outcome
Low morbidity and mortality.
Shock due to protoscolice spillage is rare.
Morbidity in range of 20%; mortality 1%.

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REFERENCES
Cameron 10th