Toxoplasmosis

The purpose of this resource is to describe:

• The way the eyes and brain normally work to give ‘vision’
• The reason why vision may become impaired by toxoplasmosis
• The cause of toxoplasmosis
• The effects of toxoplasmosis on a child’s vision
• How toxoplasmosis is diagnosed
• What can be done to help

This resource is not a substitute for a consultation with a health professional and should not be used as a means of diagnosing a condition.

What is the normal structure of the eye?

The eye is made of 3 parts.

• the light focussing bit at the front (cornea and lens)
• the light sensitive film at the back of the eye (retina)
• the large collection of communication wires to the brain (optic nerve)

The cornea first focuses the light which then passes through a hole called the pupil. The coloured part of the eye is a circle of muscle that surrounds the pupil and is called the iris. The light is then focused onto the back of the eye by a lens. Tiny light sensitive areas (photoreceptors) cover the back of the eye. These photoreceptors collect information about the visual world. The covering of photoreceptors at the back of the eye forms a thin layer known as the retina. Each photoreceptor sends its signals down very fine wires to the brain. The wires joining each eye to the brain are called the optic nerves. The information then travels to many different ‘vision’ parts of the brain. All parts of the brain and eye need to be present and working for normal vision.

What is toxoplasmosis of the eye and what causes it?

A germ called toxoplasma can cause infection within the eye. This is known as toxoplasmosis of the eye. The infection causes damage to the eye that can lead to visual impairment.

Many animals are infected with this germ, including cats. The germ is found in the poo of these infected animals. A person can catch an infection if they eat food dirtied by poo with the toxoplasma germ.

This germ can also come from infected foods such as some uncooked meats, unpasteurised milk and raw vegetables. The toxoplasma germ does not survive cooking or boiling.

Most adults do not know they have caught an infection. Most adults do not develop any problems from a toxoplasma infection and do not get toxoplasmosis of the eye. In fact, most adults in the UK, if tested, have had a toxoplasma infection at some time in their lives but have normal eyes.

Toxoplasmosis in babies

Toxoplasmosis of the eye is mainly a problem for unborn babies

If a pregnant mother becomes infected with toxoplasma the germ can also infect the unborn child. A toxoplasma infection in an unborn child is more serious than in an adult.

The infection can cause inflammation and damage to many parts of the body. The eye is often affected (toxoplasmosis of the eye). The most common part of the eye to become affected is the retina and choroid. This is called retinochoroiditis.

When the inflammation settles, a scar is usually left on the retina.

How does toxoplasmosis of the eye affect the way a child sees?

Most young children will feel their vision to be ‘normal’ as they have never known anything else. They do not realise that other people see things differently.

The toxoplasma germ can cause scarring of any part of the retina. But it tends to cause scarring of the central part. If this happens then the central vision will be missing. The child will not usually notice if part of the retina away from the centre is scarred.

Toxoplasma can affect one or both eyes. If the central part of the retina in both eyes is scarred, then the child will have blurred vision with the central part missing. The vision around the sides will not be affected. This vision is useful for getting around and not bumping into things. The child may have difficulty reading and recognising faces. Sometimes fast ‘to and fro’ movements of the eyes occur. This is called nystagmus.

Toxoplasma can cause other eye conditions

Toxoplasma usually causes inflammation and scarring of the retina and choroid. It can also cause other eye conditions. These include:

• clouding of the lens (cataract)
• the eye can be smaller than usual (microphthalmia)
• loss of some of the communication wires from the optic nerve (optic atrophy)
• damage to the ‘vision’ parts of the brain (cerebral visual impairment)
• inflammation of the eyes (uveitis)

Toxoplasma may cause other conditions to develop

Other conditions can also develop because of a toxoplasma infection. Fortunately, most children do not develop these other problems. They include:

• epilepsy
• difficulty with hearing
• learning difficulties
• blockage of the flow of fluid in the brain (hydrocephalus)

How is the diagnosis made?

An eye doctor can recognise the typical pattern of scarring at the back of the eye caused by toxoplasma. Blood tests may also help.

What can be done to help my child’s eyes?

The scarring and damage caused by the toxoplasma germ does not get better. The retina cannot grow back and fix itself. Usually, the scarring does not get any worse.

However, the toxoplasma germ can stay ‘asleep’ in the retina for many years and ‘wake up’ at any time. If it ‘wakes up’ a new infection can start. Blurred vision and floaters are common symptoms of a new infection.

It is important to treat any infection quickly to prevent further damage to the retina. Antibiotics and steroid tablets are often used to treat it. If treated quickly, the blurred vision and floaters usually settle within a few weeks.

In some cases, the jelly-like substance in the eye (vitreous) can become inflamed. This can cause annoying floaters that get in the way of vision. In most children the floaters gradually go away with treatment.

Vision can be improved with the use of low vision aids and increasing the size of print for reading.

How can I help my child?

There is a small risk of a another infection occurring. Your child’s vision may be reduced in the affected eye, so if you notice anything, you should take your child to see an eye doctor as soon as possible.

Regular eye tests in young children with the condition are important to identify reduced vision. Recurrent inflammation rarely if ever affects both eyes at the same time.

If vision is reduced in both of your child’s eyes, it is important to understand how your child sees and to adapt for this. We use our vision to get around, learn new things and to meet other people and make friends.

Most children with toxoplasmosis have few problems getting around. The way they act can give the impression that their vision is normal. It is important however to be aware of their own individual problems with vision.

Problems at school can include difficultly reading books which means your child may talk longer to do their work, and is a lot more effort for them. Ask your child’s teacher to increase the size of text to help make schoolwork easier.

It is worth watching carefully to find out what the smallest toys are that a child can see and play with. Then try to only play with toys that are the same size or bigger.

Recognising facial expressions can often be difficult. It is worth trying to find out at what distance your child can recognise facial expressions and responded to them. Then always try to talk and smile from within this distance. This will help your child to learn what facial expressions mean and to copy them.

Even if your child has very poor vision, many useful and practical things can be done to help. Support and advice will be available within and offered by the Eye department.

This information is intended to describe most aspects of toxoplasmosis, but each child is different and there will always be exceptions.