Cystic echinococcosis (hydatid disease)

What it is

Cystic echinococcosis or hydatid disease is caused by the Echinococcus granulosus sensu lato complex, a group of cestodes that includes E. granulosus sensu stricto (the dog–sheep cycle, responsible for most human cases), E. ortleppi, associated with cattle, E. canadensis and other species. Its life cycle needs two hosts:

• Definitive host: the dog and other canids. The adult tapeworm, a few millimetres long, lives in the small intestine and sheds eggs in the faeces. The eggs are infective as soon as they are passed and survive a long time in the environment.
• Intermediate hosts: sheep, cattle, goats, pigs, camelids and other herbivores, which ingest the eggs with pasture or water. The oncosphere crosses the intestinal wall, reaches the liver through the portal blood and, if it passes the liver, the lungs, where the hydatid cyst develops: a fluid-filled vesicle surrounded by a fibrous capsule from the host, whose germinal layer produces protoscoleces.
• The dog is infected by eating raw offal with fertile cysts. The cycle is maintained mainly in rural areas, when animals are slaughtered at home and condemned livers and lungs are fed to dogs.

People are accidental intermediate hosts: they are infected by ingesting eggs through contact with infected dogs or with contaminated food, water or soil, and develop cysts, mainly in the liver and lungs, that may require surgery. WHO includes it among the neglected tropical diseases. It is endemic in South America (Argentina, Chile, Peru, Uruguay and southern Brazil), the Mediterranean basin, Central Asia, China and East Africa. It is notifiable in many countries and its control follows official programmes with a One Health approach.

Who it affects

Dogs with access to raw ruminant offal: farm and sheepdogs, dogs roaming near abattoirs or places with uncontrolled slaughter, and dogs in endemic rural areas. What determines the risk is diet, not breed.

Anatomy involved

• Small intestine: adult tapeworms attach to the mucosa with the hooks and suckers of the scolex, between the villi and deep in the crypts. There may be thousands in a single dog and still no relevant lesions. Their location explains why the dog sheds eggs in its faeces for months and why diagnosis is made on faeces.

Clinical signs

The infected dog usually shows no signs, even with very heavy burdens. That is why it goes unnoticed and becomes a silent source of eggs for livestock and for the people in the household.

Diagnosis

• Faecal examination: the eggs are taeniid and cannot be distinguished from those of Taenia under the microscope, so this test is not enough.
• Coproantigen ELISA: detects parasite antigens in faeces; it is useful for population screening, but identifies the genus and its accuracy is moderate.
• Faecal PCR (copro-PCR): identifies the species and is more sensitive and specific; it is used for confirmation.
• Arecoline purgation: historically used in control programmes and epidemiological studies to expel and count the tapeworms, but it is impractical and has been gradually replaced by faecal tests.
• Necropsy of the small intestine is the post-mortem reference test.

Treatment and control

Praziquantel eliminates the adult tapeworms from the dog. It does not destroy the eggs, so faeces passed in the days after treatment must be collected and disposed of carefully. If the dog keeps eating raw offal it is reinfected at once, which is why control programmes treat dogs repeatedly, at intervals shorter than the time the parasite takes to produce eggs again.

Control relies on several measures at once:

• Regular deworming of dogs with praziquantel.
• Preventing dogs from eating raw offal: not feeding them condemned organs, disposing of them safely (burial, incineration or boiling) and controlling home slaughter.
• Meat inspection at the abattoir and condemnation of organs with cysts.
• Vaccinating sheep with the EG95 vaccine, which protects the intermediate host and is already used in programmes in Argentina and Chile.
• Dog population management, health education and hygiene: washing hands after handling dogs and washing vegetables well.

In cattle

Cattle are an intermediate host. They are infected by ingesting eggs with pasture or water contaminated by dog faeces, and develop hydatid cysts that grow slowly over months or years. In cattle many cysts are sterile (they produce no protoscoleces), although the proportion varies with the Echinococcus species; E. ortleppi, adapted to cattle, does form fertile cysts.

• Liver: the first filter after intestinal absorption and one of the two main sites. Cysts form rounded vesicles with whitish walls and fluid contents, on the surface and within the parenchyma, and lead to condemnation of the organ.
• Lungs: the second filter. In cattle lung cysts are very common and, depending on the region, may outnumber liver cysts.

Infection almost never causes signs and is detected at meat inspection, by viewing, palpating and incising the liver and lungs; cysts must be distinguished from abscesses, tuberculous granulomas and other lesions. Serological tests are not reliable in livestock. It is not treated. Losses come mainly from condemnation of offal. Prevalence at the abattoir is a good indicator of transmission in the area and of the effectiveness of programmes, and the key point is that condemned organs are disposed of so that dogs cannot eat them.

Key studies

• Possenti et al. (2016), systematic review and meta-analysis of 37 studies (8 case-control and 29 cross-sectional) on risk factors for human cystic echinococcosis. In the case-control studies, risk was increased by having free-roaming dogs (OR 5.23; 95% CI 2.45–11.14), feeding dogs offal (OR 4.69; 95% CI 3.02–7.29), home slaughter (OR 4.67; 95% CI 2.02–10.78), dog ownership (OR 3.54), living in rural areas (OR 1.83) and low income (OR 1.68). The studies are observational and heterogeneous, but they confirm the basis of the cycle.
• Hartnack et al. (2013), Bayesian latent-class evaluation of three diagnostic tests in dogs on the Tibetan plateau, without assuming a perfect reference test. For E. granulosus, copro-PCR had a sensitivity of 84% and a specificity of 83%; coproantigen ELISA, 57% and 69%; and arecoline purgation a sensitivity of 85% assuming 100% specificity. It is a single region, where E. multilocularis also occurs.
• Amarir et al. (2021), four-year field trial in the Middle Atlas, Morocco. Of 402 lambs vaccinated with EG95 and 395 controls, at about four years of age 9.37% of vaccinated sheep had viable cysts versus 72.82% of controls, and the number of viable cysts fell by about 97%. Treating 91% of owned dogs with praziquantel every 4 months lowered the proportion of infected dogs from 35% to 12%, but the reinfection risk was 4% per month and it did not reduce transmission to sheep or add to the effect of the vaccine, probably because unowned dogs and wild canids were not treated. Deworming only owned dogs at that interval was not enough. It is a single trial in one region.
• Tamarozzi et al. (2020), systematic review of 79 publications on cystic echinococcosis in European Mediterranean and Balkan countries between 2000 and 2019. Average annual human incidence by region ranged from 0.10 to 7.74 per 100,000 population and prevalence ranged from 0.003% to 64.09% in cattle, from 0.004% to 68.73% in sheep and from 0% to 31.86% in dogs, with the highest values in southern and insular Italy, central Spain, Romania and Bulgaria. Cattle data resembled human data more closely than sheep and dog data did. The data were highly heterogeneous, with substantial under-reporting.
• Larrieu and Zanini (2012), critical analysis of control programmes in South America between 1974 and 2010. After 30 years of praziquantel use no endemic area had achieved eradication and the impact on human disease had been slight; the main obstacle was sustaining deworming of rural dogs 8 times a year for many years. Some areas of Argentina reduced transmission to people with simpler strategies based on continuous community fieldwork. It is a narrative review of programmes; it is included because there is no systematic review for the region.