Hydatid Cysts in the Liver and Lungs: Why the Dog Is the Link to Break

Every Eid al-Adha the same scene repeats itself: the sheep is opened and white bubbles full of clear fluid appear in the liver or lungs, so the butcher cuts them out and throws them to the dog waiting at the door. That small gesture is exactly what keeps the disease alive in the village year after year.

What are these cysts?

These bubbles are hydatid cysts, which many people simply call “water cysts”, and the disease they cause is echinococcosis. According to the World Organisation for Animal Health, the cyst is the larval stage of a small tapeworm only a few millimetres long, Echinococcus granulosus. The adult worm lives in the dog's intestine — the “definitive host” — while the cysts form in the organs of the “intermediate host”, most often in the liver and lungs.

The World Health Organization explains that the strain behind most human infections is maintained mainly in a dog → sheep → dog cycle, and other animals can be involved, including goats, cattle and camels. According to the World Organisation for Animal Health, a cyst in sheep is usually 2 to 6 centimetres across, and can reach 10 to 20 centimetres in long-lived animals such as camels, and in humans.

An infected animal usually does not look sick: the cysts grow slowly, and many animals are slaughtered before the cysts cause them any problem. But the loss is real, and the World Organisation for Animal Health lists among it poorer growth, lower production of milk, meat and wool, and organs condemned at post-mortem inspection. The World Health Organization notes that the highest prevalence is found in rural areas where older animals are slaughtered.

The full cycle: from the liver to the dog to the pasture

1. A dog eats a raw organ containing cysts, or scavenges the carcass of an infected animal.
2. The larvae are released in its intestine and grow into adult worms, and eggs then pass out in its faeces after 25 to 80 days depending on the species and strain, as the World Organisation for Animal Health states.
3. The eggs contaminate pasture, soil and water, and can survive for up to a year in cool, moist conditions, though they are susceptible to drying out.
4. A sheep swallows them while grazing, cysts form in its organs, and the liver goes back to the dog on slaughter day.

The trouble is that nobody sees any of this happening: the worm causes few ill effects in the dog's intestine, according to the World Organisation for Animal Health, and the World Health Organization notes that the infection shows no signs in livestock or dogs. A dog that looks perfectly healthy may itself be the source of the eggs.

How do people get infected? Not from meat

According to the World Health Organization, people are infected in the same way as sheep: by swallowing the worm's eggs in contaminated food, water or soil, or after direct contact with infected animals. The Terrestrial Animal Health Code of the World Organisation for Animal Health adds that infection occurs through contact with infected dogs, or by eating food or drinking water contaminated with eggs from dog faeces. The organisation's disease page explains that the eggs can contaminate fruit, vegetables and water, or stick to an animal's fur and be carried on the hands to the mouth, and that dog owners, veterinarians and people who handle animals are at higher risk.

That is why meat is not the route. People, in the World Health Organization's words, are “accidental” intermediate hosts: they pick up the infection from eggs and pass it on to no one, and the cyst can only complete its cycle in the intestine of a carnivore such as a dog. The Code treats skeletal muscle meat (the red meat itself) and its products, milk and milk products, and hides and skins as safe commodities for which no conditions relating to this worm should be required on import. The organ that carries the cysts is a different matter, as explained below.

In people the cysts usually settle in the liver and lungs, and may stay silent for many years before they grow large enough to cause abdominal pain, nausea and vomiting when they are in the liver, or a chronic cough, chest pain and shortness of breath when they are in the lung. The World Health Organization describes treatment as often expensive and complicated, and it may require extensive surgery, prolonged drug therapy, or both.

What do you do with the infected organ?

The World Organisation for Animal Health Code states that all slaughtered livestock should undergo post-mortem veterinary inspection, including inspection of the offal for hydatid cysts, and that offal containing cysts should either be disposed of under the rules for disposing of dead animals, or be treated by one of two methods that kill the larvae:

• Heating until the core of the piece reaches at least 80°C for ten minutes, or an equivalent time and temperature.
• Freezing to minus 20°C or below for at least two days.

The Code states plainly that dogs should not be fed offal unless it has been treated by one of these two methods. Applying this to home and Eid slaughter is simple:

• Do not cut out the visible cyst and give the rest of the liver to the dog; the Code speaks of the organ that contains cysts, not just the cyst you can see.
• Do not throw the organ into an open bin or onto waste ground, where stray dogs will reach it. Dispose of it where animals cannot get at it, for example by burial or burning, both of which the Code lists among methods for disposing of dead animals, and keep animals away from the disposal site.
• If you do want to feed offal to a dog, cook it until its core reaches 80°C for ten minutes, or freeze it to minus 20°C for two days — and make sure your freezer actually reaches that temperature.
• Keep dogs away from the whole slaughter area and from the carcasses of animals that have died; the Code recommends preventing dogs from reaching raw offal during on-farm slaughter, and from scavenging on dead animals.

The dog: the link you can control

The World Health Organization says that prevention programmes focus on deworming dogs, because they are the definitive host. It states that periodic deworming of dogs with praziquantel, at least four times a year, together with better hygiene at slaughter, proper destruction of infected offal and public education, has lowered transmission — and in high-income countries prevented it.

The World Organisation for Animal Health Code is stricter for high-risk dogs: a dog known to be infected, or suspected of having access to raw offal, or living in contact with livestock, should be treated with praziquantel or another tapeworm-killing product of comparable efficacy at least every 4 to 6 weeks. It recommends that the dog's faeces passed during the 72 hours after treatment be disposed of by burning or burial where possible.

There is a second tool at the other end of the cycle: the sheep vaccine known as EG95. The World Health Organization states that it is produced commercially and registered in China and Argentina, that trials in Argentina showed the added value of vaccinating sheep, and that a programme combining vaccination of lambs, deworming of dogs and culling of older sheep could eliminate the disease in humans in less than ten years. Availability differs from country to country, so ask your national veterinary services about it.

Protecting the household

The World Organisation for Animal Health says the infection passes from hand to mouth, so frequent hand washing is an important preventive measure. In practice:

• Wash your hands with soap and water after touching or playing with a dog and after working in the soil, and before eating or preparing food.
• Wash vegetables and fruit well, especially those eaten raw or grown on land that dogs can reach.
• Drink from a protected water source that dogs cannot reach.