Toxocariasis is a rare infection caused by roundworm parasites. It is spread from animals to humans via their infected faeces.
Roundworm parasites are most commonly found in cats, dogs and foxes and usually affect young children. This is because children are more likely to come into contact with contaminated soil when they play and put their hands in their mouths.
However, cases have been reported in people of all ages.
Signs and symptoms
For most people, an infection with these roundworm larvae causes no symptoms and the parasites die within a few months.
However, some people experience mild symptoms such as:
a high temperature (fever)
In rare cases, the roundworm larvae infect organs such as the liver, lungs, eyes or brain and cause severe symptoms such as:
loss of appetite
blurred or cloudy vision in one eye
See your GP as soon as possible if you think you or your child have symptoms that may be caused by toxocariasis.
A blood test can usually detect toxocariasis, although you may need an eye examination to look for parasites if your eyes are affected.
Why it happens
The roundworm parasites responsible for toxocariasis (called Toxocara) live in the digestive system of dogs, foxes and cats. The worms produce eggs, which are released in the faeces of infected animals and contaminate soil.
If small particles of contaminated soil get into someone's mouth, they may develop toxocariasis.
It's not possible for someone with toxocariasis to pass it directly to another person.
Reducing your risk
The best way to reduce the chances of developing toxocariasis is to practise good hygiene.
For example, washing hands with soap and warm water after handling pets or coming into contact with soil or sand.
If you have a pet cat or dog, they should be regularly de-wormed and their faeces should be disposed of immediately.
How it's treated
If you have no or mild symptoms, treatment is not usually necessary.
However, you will need medication if you have a severe infection affecting your organs. Specifically, a type of medication called an anthelminthic is used to kill the parasites.
If your eye is affected by toxocariasis, anthelminthics are not used, but steroid medication (corticosteroids) may be required to suppress inflammation. Surgery may also be needed, for example if you develop retinal detachment.
Most cases make a full recovery and don't experience any long-term complications. However, there is a risk of permanent vision loss if one of the eyes is affected.
Toxocariasis is caused by roundworm parasites (toxocara)
How common is toxocariasis?
Toxocariasis is uncommon in the UK, although it's hard to determine exactly how many cases occur every year, as the condition is often misdiagnosed or undiagnosed.
The Health Protection Agency (HPA) only recorded 30 cases in England and Wales between 2000 and 2010.
However, it's likely many more people have been exposed to the parasites without knowing it.
Symptoms of toxocariasis
Many people infected with the roundworm larvae responsible for toxocariasis don't have any symptoms.
Symptoms only tend to develop if you're infected with a high number of larvae, or if your body is particularly sensitive to them.
If there are any symptoms, these are often mild and can include:
a high temperature (fever) of 38ºC (100.4ºF) or above
As well as the symptoms above, signs of a severe infection may include:
loss of appetite
blurred or cloudy vision, usually only affecting one eye
a very red and painful eye
If one of your eyes is affected by toxocariasis, there is a risk of permanent vision loss. However, prompt treatment can help reduce the chances of this happening.
When to see your GP
You should see your GP as soon as possible if you think you or your child may have toxocariasis.
Causes of toxocariasis
Toxocariasis is caused by an infection of roundworm parasites.
Specifically, roundworms called Toxocara canis and Toxocara cati are responsible for most cases.
These parasites live inside the intestines of animals (often dogs, cats and foxes that appear healthy) and their eggs are passed in the faeces of these animals.
However, the eggs only become infectious after 10-21 days, so there is no immediate danger from fresh animal faeces. Once the eggs are passed into sand or soil they can survive for many months.
After the eggs become infectious, humans can become infected if contaminated soil gets into their mouth. Toxocariasis cannot be spread from person to person.
Once the eggs are inside the human body, they move into the bowel before hatching and releasing larvae (the earliest stage of development). These larvae can travel to most parts of the body.
However, as humans aren't the normal host for these larvae, they can't develop beyond this stage and they don't produce eggs. This means the infection cannot spread between humans.
In most people, treatment for toxocariasis is not required as the condition often improves on its own.
Treatment is usually only needed in severe cases where the infection causes organ damage.
Medication is the main treatment in these cases. You can usually be treated at home, but you may need to be admitted to hospital if your symptoms are particularly severe.
A type of medication called an anthelminthic is the main treatment for severe cases of toxocariasis. These medicines kill the parasite larvae responsible for the infection.
Albendazole is most often used and mebendazole is an alternative.
These medicines don't usually cause side effects, although some people may experience headaches or stomach pain.
In addition to anthelminthics, steroid medications (corticosteroids) are often given to reduce any inflammation caused by a severe infection.
Steroid medication is used instead of anthelminthics for toxocariasis affecting the eye.
Surgery may be recommended in severe cases of toxocariasis where one of your eyes is infected
The main types of surgery used include:
vitrectomy - where the jelly-like substance that fills the inside of the eye is removed and replaced with a gas or liquid substitute
laser photocoagulation - where a laser is used to kill the parasites in the eye
Both these techniques can be carried out using local anaesthetic eye drops to numb your eye and the surrounding area.
Like all surgical procedures, these techniques carry a risk of complications. For example, there is a risk of some degree of permanent vision loss (such as reduced side and night vision). However, the risk of total vision loss in the infected eye due to toxocariasis usually outweighs the risk of surgical complications.
Practising good hygiene can help prevent toxocariasis.
Some of the steps you can take are listed below.
Wash your hands well with soap and warm water after handling pets or coming into contact with soil or sand.
Teach children always to wash their hands after playing with dogs or cats, after playing outdoors and before eating.
Wash food that may have come into contact with soil.
Try to avoid letting children play in areas where there is a lot of dog or cat faeces.
Teach children that it's dangerous to eat dirt or soil.
Advice for pet owners
Parents and children should be aware of the dangers associated with puppies, kittens and older dogs and cats.
Many puppies are infested with the roundworm parasites from birth, as a pregnant dog can pass the parasites to her puppies before they're born.
All dogs and cats require regular de-worming with anti-worm medicine. See your vet for regular check-ups and for specific advice on how to treat your pet.
The parasite eggs responsible for toxocariasis can survive for many months in soil or sand, so all pet faeces should be collected and disposed of in the garbage. There is no immediate danger from fresh faeces, as the eggs only become infectious after a few weeks.
Pets should be kept away from children's sandpits, which should be kept covered when not in use.
Your pet's living area should be cleaned at least once a week.
Some areas within public parks in the UK have been set aside as designated dog exercise areas. Dog owners should ensure that their dogs use these areas to minimise the risk of other park users getting toxocariasis.