The fox tapeworm (Echinococcus multilocularis) is a parasite that normally infects foxes and other animals such as dogs and cats (primary/ definitive hosts). It is approximately 2–7 mm in size. The tapeworm eggs are excreted in the faeces of the primary hosts and ingested by small rodents, usually mice (intermediate hosts), or rarely by humans (accidental hosts).
In the intestines of the intermediate or accidental hosts, the eggs develop into larvae, which enter the liver or other organs via the bloodstream and develop into cysts (larval stages). Head structures (protoscolices) develop inside the larvae. If an infected mouse is eaten by a fox, dog or cat, the head structures develop into adult tapeworms in the small intestine of the main host – thus completing the cycle.
Humans become infected via eggs that are excreted in the faeces of infected foxes or dogs. These eggs can stick to food such as vegetables, berries, mushrooms, windfall fruit or wild garlic. Contact with infected animals or their fur – for example, if a dog has rolled in faeces – or with contaminated footwear can also lead to transmission. The microscopic eggs are not visible to the naked eye. People with a fox tapeworm infection are not contagious.

Frequency and risk factors

Infections with fox tapeworm are rare, although there has been a significant increase in new diagnoses in Switzerland in recent years. Risk factors include agricultural activities, living in rural areas, gardening, eating unwashed vegetables and berries, and close contact with dogs.
With the elimination of rabies – which had severely decimated the fox population – the number of foxes has risen significantly again. In addition, their territories have increasingly expanded into conurbations and cities. After a certain time lag, the number of fox tapeworm infections in humans in Switzerland, Austria and Germany also rose significantly. In Switzerland, around 40–50 new cases are registered each year.

Prevention

Thoroughly washing hands, berries, vegetables and mushrooms, and heating collected food are the most effective ways to protect against infection with fox tapeworm. Regular deworming of dogs – especially if they hunt mice – and keeping foxes away from household waste also help to minimise the risk.

Symptoms

In humans, the larvae usually infect the liver, less commonly other organs. The resulting disease is called alveolar echinococcosis. The larvae destroy liver tissue in a similar way to a malignant tumour. The disease can remain undetected for years – it often takes 10–15 years for symptoms to appear. At first, the symptoms are non-specific, such as upper abdominal pain or, later, jaundice. The disease is often discovered by chance during an ultrasound or other imaging examination.
If left untreated, alveolar echinococcosis is fatal. Today, however, it can be treated surgically and with medication (see below).

Diagnosis

Diagnosis is challenging and is carried out using ultrasound, computed tomography (CT) or magnetic resonance imaging (MRI) as well as special laboratory tests, for example to detect antibodies against the pathogen.

Treatment

A complete cure is only possible through surgical removal of the affected tissue – but only in the early stages of the disease. After the operation, at least two years of follow-up medication is required to prevent a relapse.
In inoperable cases, drug therapy with benzimidazoles (anthelmintics), usually albendazole, significantly improves the prognosis. These drugs do not kill the parasite, but inhibit its growth so that the disease does not progress further. As a rule, treatment must be continued for life.
Since the drugs act throughout the body, side effects may occur, such as a reduction in white blood cells, hair loss or elevated liver enzymes.