Chapter 3 – Epidemiology of infectious diseases (Parasites & Scabies)
201
“” Hydatid disease (echinococcosis) “”
Epidemiology
Infectious (causitive) agent:
Echinococcus granulosus (dog tapeworm)
Incubation period:
varies from months to years.
Public health significance & occurrence
Hydatid dis. occurs worldwide and is mainly associated with sheep farming. People who
trap wild dogs are similarly at risk
Reservoir
The domestic dog and other canids, definitive hosts for E. granulosus, may harbour
thousands of adult tapeworms without being symptomatic.
Felines and most other carnivores are normally not suitable hosts for the parasite.
Intermediate hosts include herbivores, sheep, cattle, goats, pigs, horses, kangaroos,
wallabies and camels. Sheep are the major intermediate hosts. Sheep eat the worm eggs
from pasture contaminated with dog faeces.
These hatch inside the sheep, forming cysts. The life cycle is completed when dogs are
infected through eating the offal of infected livestock or wild animals, particularly the
liver and lung.
Mode of transmission
Human infection occurs by hand-to-mouth transfer of tapeworm eggs from dog faeces.
The larvae penetrate the intestinal mucosa, enter the portal system and are carried to
various organs where they produce cysts in which infectious protoscoleces develop.
The important life cycle is dog-sheep-dog. A dingo-wallaby-dingo (or wild dog) sylvatic
cycle also occurs. A dog-wild pig-dog cycle has been recognized and poses a special risk
for wild pig-hunters.
Period of communicability
Hydatid disease is not transmitted from person to person.
Dogs pass eggs approximately seven weeks after infection. In the absence of reinfection
this ends within one year
Susceptibility & resistance
Young children are more likely to be infected as they are more likely to have closer
contact with infected dogs and they are less likely to have appropriate hygiene habits.
There is no evidence to suggest children are more susceptible to infection than adults.
Notification and school exclusion are not required.
Clinical features
Hydatid disease in humans is produced by cysts that are the larval stages of the tapeworm
Echinococcus. Brood capsules are formed within cysts,cysts containing 30–40
protoscoleces. Each of these is capable of developing into a single tapeworm. Symptoms
depend on the location of the cyst within the body and develop as a result of pressure,
leakage or rupture. The most common site for the cysts is the liver. Less commonly brain,
lungs and kidneys are affected. The heart, thyroid and bone are uncommonly affected
Cysts in the body may remain viable or die and calcify. They may be detected on routine
X-rays. The prognosis is generally good & depends on the site & potential for rupture &
spread. Sudden rupture of the brood capsules& liberation of daughter cysts may cause
fatal anaphylaxis. Persons who have calcified cyst detected on X-ray may still have active
infection.
Chapter 3 – Epidemiology of infectious diseases (Parasites & Scabies)
202
Method of diagnosis:
Dx may be made by plain X-ray, ultrasound or CT scan. If a cyst ruptures, appropriate
examination for protoscoleces, brood capsules and cyst wall in sputum, vomits, faeces or
urine should be undertaken. The Casoni skin test has now been replaced by serological
tests for hydatid disease. These include fluorescent antibody (FA) and indirect
haemagglutination antibody testing.
Control measures
Preventive measures:
Basic hygiene such as washing hands with soap after gardening
or touching the dog and washing vegetables that may have been contaminated by dog
faeces, are important in prevention of this disease.
Control of case:
Surgery is often the treatment of choice for infection with
Echinococcus granulosus, sometimes combined with prolonged high-doses of the drug
albendazole. Percutaneous drainage with ultrasound guidance plus prolonged high-dose
albendazole therapy has been effective for liver cysts.
Praziquantel followed by prolonged high-dose albendazole theraphytherapy is used if
there is cyst spillage from trauma or surgery.
Control of contacts:
Persons carrying the infection are not contagious to others.
Encourage contacts to practice appropriate hygiene & to report early any compatible
symptoms.
Control of environment
Dogs kept in and around the case’s house may require veterinary screening for infection
for Hydatid infection. In general, dogs should be treated with an anti-tapeworm
medication such as praziquantel every six weeks in rotation with a broad spectrumbroad-
spectrum de-worming preparation to prevent disease in dogs and break the life cycle of
the parasite.
Review practices that may have led to infection. In particular, restrict dog access to raw
offal from infected sheep or kangaroos to prevent the life cycle continuing. Incinerate or
deeply bury infected organs from dead intermediate animal hosts.
Outbreak measures:
Not applicable.