Toxoplasmosis of the Eye
Information from Visual Impairment Scotland, Scottish Sensory Centre, The University of Edinburgh
What we see is made in the brain from signals given to it by the eyes.
What we see is in fact made in the brain. The brain makes sight from signals given to it by the eyes.
What is the normal structure of the eye?
The eye is made of three parts.
- A light focussing bit at the front (cornea and lens).
- A light sensitive film at the back of the eye (retina).
- A large collection of communication wires to the brain (optic nerve).
A curved window called the cornea first focuses the light. The light then passes through a hole called the pupil. A circle of muscle called the iris surrounds the pupil. The iris is the coloured part of the eye. The light is then focused onto the back of the eye by a lens. Tiny light sensitive patches (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 film known as the retina. The retina lies on a layer of nerves and blood vessels. This layer is called the choroid. The retina and all the photoreceptors need a healthy choroid to work. 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 special ‘vision’ parts of the brain. All parts of the brain and eye need to be present and working for us to see normally.
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 faeces of these infected animals. A person can catch an infection if they eat food dirtied by faeces with the toxoplasma germ. Foods that can lead to an infection include 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 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 but their own visual world. At first they assume that everyone else has vision the same as their own. 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 bit. If this happens then the central part of the vision will also be missing. The child will not usually notice if a part of the retina away from the centre is scarred.
Toxoplasma can affect one or both eyes. If the central bit 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 still be OK. This vision is useful for getting around and not bumping into things. The child will however have difficulty reading and recognising faces. Sometimes fast ‘to and fro’ movements of the eyes occur. This is called Nystagmus. Squint may also develop.
Sometimes Toxoplasma can cause other eye conditions
Toxoplasma usually causes inflammation and scarring of the retina and choroid. It can however 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)
Toxoplasma may cause other conditions to develop
Other conditions can also develop because of a Toxoplasma infection. These include:
- Epilepsy
- Difficulty with hearing
- Learning difficulties
- Blockage of the flow of fluid in the brain (hydrocephalus)
Fortunately most children do not develop these other problems.
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 the 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. If treated quickly the blurred vision and floaters usually settle within a few weeks. In some cases the jelly in the eye (vitreous) can become inflammed. 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.
How can parents, family, friends and teachers make a difference?
There is a small risk of a further patch of infection occurring. If it is causing visual problems then vision will be reduced in the affected eye and the child should see an eye doctor as soon as possible. Recurrent inflammation rarely if ever affects both eyes at the same time.
If vision is reduced in both eyes it is important to understand how the child sees and to make up 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 special problems with vision.
Problems at school may be due to some of the reading books being hard to see. This often means it takes longer and more effort to do the work. If the size of print is increased most children will find 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 facial expressions can be seen and responded to. Then always try to talk and smile from within this distance. This helps a child to learn what facial expressions mean and to copy them.
Even if a child has very poor vision many useful and practical things can be done to help. See the ADVICE section on the VI Scotland website.
http://www.ssc.education.ed.ac.uk/resources/vi&multi/eyeconds/Toxop.html
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Imperial College London news release
Under strict embargo for
1700 hours British Summer Time
Thursday 10 May 2007
Scientists have provided new insight into how the parasite which causes toxoplasmosis invades human cells, says new research published today in The EMBO Journal.
Toxoplasmosis is a parasitic disease, primarily carried by cats. It is transmitted to humans by eating undercooked meat or through contact with cat faeces. It is particularly dangerous for pregnant women, whose foetuses can be infected via the placenta, and those with a weakened immune system, such as people infected with HIV. In severe cases, toxoplasmosis can cause damage to the brain and eyes, and even death.
Now researchers from London and Geneva have determined, for the first time, the atomic structure of a key protein which is released onto the surface of the parasite just before it invades host cells in the human body. They found that the protein known as TgMIC1 binds to certain sugars on the surface of the host cell, assisting the parasite to stick to, and then enter the human cell.
Using a novel carbohydrate microarray the team were able to identify the precise sugars to which the parasite protein binds. Following this the team used a combination of NMR spectroscopy and cellular studies to characterise the behaviour and interactions of the parasite protein and host cell sugars. This means that the team have a more detailed picture than ever before of exactly how the parasite recognises and attacks host cells in the body.
Professor Steve Matthews from Imperial College London's Division of Molecular Biosciences, one of the paper's authors, explains the significance of the research, saying: "Understanding the fundamental, atomic-level detail of how diseases like toxoplasmosis pick out and invade host cells in the human body is vital if we want to fight these diseases effectively.
"Now that we understand that it's a key interaction between a protein on the parasite's surface and sugars on the human cell which lead to the cell's invasion, there is potential to develop therapeutics that are targeted at disrupting this mechanism, therefore thwarting infection."
Toxoplasma gondii, the parasite that causes toxoplasmosis, is one of the world's most common parasites. Around a quarter to half of the world's population is thought to be infected, and around 1 per cent of people in the UK catch toxoplasmosis each year. In the majority of cases, those affected don't have any symptoms. But for those with weakened immune systems, and unborn babies, toxoplasmosis can cause very serious health problems.
-Ends-
For more information please contact:
Danielle Reeves, Imperial College London Press Office,
Tel: +44 (0)20 7594 2198
Mob: +44 (0)7803 886248
Email: Danielle.reeves@imperial.ac.uk
Notes to Editors:
1. 'Atomic resolution insight into host cell recognition by Toxoplasma gondii', The EMBO Journal, 10 May 2007.
Tharin MA Blumenschein (1), Nikolas Friedrich (2), Robert A Childs (3), Savvas Saouros (1), Elisabeth P Carpenter (1), Maria A Campanero-Rhodes (3), Peter Simpson (1), Wengang Chai (3), Theodoros Koutroukides (4), Michael J Blackman (5), Ten Feizi (3), Dominique Soldati-Favre (2) and Stephen Matthews (1).
(1) Division of Molecular Biosciences, Imperial College London, London, UK.
(2) department of Microbiology and Genetics, Faculty of Medicine, university of Geneva CMU, Geneva, Switzerland.
(3) Glycosciences Laboratory, Division of Medicine, Imperial College London, Middlesex, UK.
(4) Medical Science division, Fox Chase Cancer Centre, Philadelphia, PA, USA.
(5)Division of Parasitology, National Institute for Medical Research, London, UK.
2. About Imperial College London
Rated as the world's ninth best university in the 2006 Times Higher Education Supplement University Rankings, Imperial College London is a science-based institution with a reputation for excellence in teaching and research that attracts 11,500 students and 6,000 staff of the highest international quality.
Innovative research at the College explores the interface between science, medicine, engineering and management and delivers practical solutions that improve quality of life and the environment - underpinned by a dynamic enterprise culture.
With 62 Fellows of the Royal Society among our current academic staff and distinguished past members of the College including 14 Nobel Laureates and two Fields Medallists, Imperial's contribution to society has been immense. Inventions and innovations include the discovery of penicillin, the development of holography and the foundations of fibre optics. This commitment to the application of our research for the benefit of all continues today with current focuses including interdisciplinary collaborations to tackle climate change and mathematical modelling to predict and control the spread of infectious diseases.
The College's 100 years of living science will be celebrated throughout 2007 with a range of events to mark the Centenary of the signing of Imperial's founding charter on 8 July 1907.
Website: www.imperial.ac.uk
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