The Dr. Bill Takeshita Foundation

Last Modified: Sat, 18 Aug 2007

View other Common Causes of Vision Impairment

Toxoplasmosis is a term used to describe vision impairment due to a parasite called toxoplasma gondii. This parasite is often found in cat feces and raw meat. The infection of the parasite can damage the retina and cause blurred sight, sensitivity to glare (photophobia), and blind spots in the central and peripheral fields of vision. There is no medical treatment to reverse the damage to the retina but most patients respond extremely well to low vision aids. Toxoplasmosis is generally a stable condition but the infective parasite can “flare up” and cause additional damage to the retina. Such relapses are more common among those with weak immune systems.

Adults and children are most often infected by Toxoplasmosis gondii when they come in contact with cat feces while gardening, cleaning litter boxes, or when touching raw meat and pork. Many people are infected with the parasite but have no symptoms. The parasite can attack the eyes and brain, causing vision impairment and cognitive dysfunction. Pregnant women and those with weak immune systems are at risk of infection and must be very careful when preparing meat and working near cat feces. In fact, Toxoplasmosis is a leading cause of miscarriages among expecting women.

The retina is the light sensing tissue that lines the internal surface of the eye. It consists of millions of rod cells and cone cells. The cone cells are located in the central region of the retina and are responsible for detailed sight, color vision, and vision during the daylight. The rod cells are located on the periphery of the retina and provide peripheral vision, night vision, and visual clues for walking. When the toxoplasmosis parasite infects the eye, it can affect any region of the retina. When the central retina is affected, blind spots develop in the central field of vision and patients complain of blurred sight. The level of sight is often 20/200 (able to read a 4 inch letter from 20 feet). Conversely, when the peripheral retina is infected, the clarity of sight may be excellent but blind spots in the peripheral vision may exist.

Children with toxoplasmosis often have nystagmus, the horizontal shaking of the eyes. They generally do not perceive the world as moving or shaking. Eye muscle surgery, rigid contact lenses, and eye exercises are sometimes recommended to attempt to reduce nystagmus. However, it is rare for nystagmus to be completely eliminated. Children with nystagmus will often turn or tilt their head to position their eyes to reduce the nystagmus.

Most patients with toxoplasmosis have a high degree of functional vision and their vision is generally stable. They respond extremely well to low vision aids, which can improve their clarity of sight, reduce problems with glare, and improve their peripheral vision. Most students with toxoplasmosis are able to access printed information at school and many are able to take their drivers license examination when they wear bioptic spectacles.

Recommendations

• Infants will benefit from vision stimulation to maximally develop the visual cortex of the brain. For specific details, please see Developing Your Child’s Vision.
• A consultation by a low vision specialist and low vision aids are strongly recommended. Low vision aids such as bioptic glasses, handheld telescopes, and video magnification systems can improve distance sight to 20/20 while specialized reading glasses, hand magnifiers, and closed circuit televisions can allow patients to read small print.
• Tinted contact lenses, specialized melanin filters, and the use of a hat with a dark brim can greatly improve visual function during the day for patients who are severely sensitive to glare and bright light.
• Students will benefit from being positioned in the front portion of the class with their backs facing windows, doors, and glare sources. The use of a black chalkboard and bold architecture chalk are generally easier to see than dry erase boards due to the glare created by the white board. In the event that a dry erase board must be used, it is very helpful to only use bold black dry erase markers rather than colored markers.
• Students will benefit from having copies of material normally written on the board or presented on overhead transparencies. If PowerPoint presentations are used, a dark background with white letters typed in Arial or Tahoma fonts will improve readability for students with low vision.
• Some students with toxoplasmosis may have difficulty participating in sports such as tennis and baseball.
• Computer users will benefit from changing the background of the display to a black or blue background with white letters. Enlargement of the font size will also increase readability. For information on how to modify the computer screen, see “Solutions to Help You Use Your Computer.
• Students may benefit from using specialized computer software and technology such as Zoom Text magnification software, scanning programs such as Open Book and Kurzweil, and Video Magnification systems. For more information, see Computer and Assistive Technology.
• Teenagers with toxoplasmosis who are interested in learning whether they have sufficient vision for driving should have a low vision examination by a low vision specialist who specializes in fitting bioptic-driving glasses.
• Specific low vision aids that are often helpful to patients with toxoplasmosis include:
  • 4x and 6x Ocutech bioptic spectacles
  • DVI clear image aspheric reading glasses
  • Prescription CPF, Transitions, Melanin, and NoIR filters
  • Pocket magnifiers such as 5x Packet for reading small print
  • Flipper video magnifier and Clarity deskmate to assist with reading the chalkboard.
  • Zoomtext 9.0 software magnification program for accessing information from the computer.
• For more information, please see Low Vision Rehabilitation