Methylmercury poisoning is brain and nervous system damage from the chemical methylmercury.

Methylmercury is a type of mercury ("quicksilver"), a metal that is liquid at room temperature. Most compounds containing mercury are poisonous. Methylmercury has been used to preserve seed grain, which is fed to animals. Methylmercury may also form in water when other forms of mercury in the water react with certain bacteria. Methylmercury poisoning has occurred after people have eaten meat from animals fed seedgrain or fish from waters contaminated with methylmercury (such as Minamata Bay in Japan.)

Unborn babies and young infants are very sensitive to methylmercury's effects. Methylmercury causes central nervous system (brain and spinal cord) damage. How bad the damage is depends on how much poison gets into the body. Many of the symptoms of mercury poisoning are similar to those seen in cerebral palsy. In fact, methylmercury is thought to cause a form of cerebral palsy.

The FDA has warned women who are pregnant or may become pregnant and nursing mothers to avoid fish that may contain unsafe levels of methylmercury. Such fish includes swordfish, king mackerel, shark, or tilefish. (Young infants should not be given these fish, either.) You should not eat any type of these fish caught by friends and family. Check your local or state health departments for warnings against locally caught, noncommercial fish.

Some health care providers have raised concerns about ethyl mercury (thimerosal), a chemical used in some vaccines. However, research shows that childhood vaccines do not lead to dangerous mercury levels in the body. Vaccines used in children today only contain tracy amounts of thimerosal. Thimerosal-free vaccines are available.

• Cerebral palsy
• Growth problems
• Microcephaly (small head)
• Deafness
• Blindness

Tests will vary depending on the symptoms that occur.

Methylmercury damage is irreversible. Treatment is determined by the severity of the condition and is similar to that given for cerebral palsy.

The symptoms are irreversible; however, they do not usually worsen unless there is a new exposure to methylmercury.

Complications depend on the severity of the condition, and the specific symptoms manifested (such as blindness or deafness).

Call your health care provider if you have symptoms of this type of poisoning or if you know have eaten foods that may have been contaminated with methylmercury.

The National Poison Control Center (1-800-222-1222) can be called from anywhere in the United States. This national hotline number will let you talk to experts in poisoning.

This is a free and confidential service. All local poison control centers in the U.S. use this national number. You should call if you have any questions about poisoning or poison prevention. It does NOT need to be an emergency. You can call for any reason, 24 hours a day, 7 days a week.

See National Poison Control center.

Strict avoidance of any foods contaminated with methylmercury will prevent poisoning. Because of manufacturing, mercury has become so common in the environment that trace amounts of methylmercury are present in many foods derived from the ocean, including deep-sea tuna. Fortunately, the levels are low enough that most of these foods remain safe. Contact Poison Control if you believe you may have been exposed.