Lead toxicity may be acute, due to ingestion of a lead object, or it may be chronic due to repeated chewing of lead paint or grooming the dust of lead paint from the skin and fur. The most common route of exposure is ingestion but lead paint dust may also be inhaled.

Lead toxicity primarily affects the nervous system and gastrointestinal system. Common neurological symptoms include sudden onset of seizures, blindness, behavior changes, hysteria, chomping, muscle spasms and circling. Lead crosses the placenta and is excreted in milk, so it can affect unborn fetuses and nursing young.

Gastrointestinal signs include abdominal pain, loss of appetite, vomiting, and constipation or diarrhea. Some animals develop megaesophagus (enlargement of the esophagus and decreased ability to move food from the mouth to the stomach) resulting in regurgitation. Aspiration pneumonia may result from regurgitation. Lead toxicity may also suppress the immune system making cats more susceptible to infections.

With chronic exposure, lead is deposited in the tissues of internal organs (liver, kidneys) and in the bones. Lead that is deposited in the bones serves as a "reservoir" and can cause lead levels to remain high despite treatment in some patients. These cats often require long-term therapy.

Lead inhibits normal remodeling of the long bones of the body such as the femur, tibia, humerus and radius. As a result, specific areas in these bones look denser (whiter) on x-rays and these areas are referred to as "lead lines." Lead lines do not represent lead deposits in the bone.

The symptoms of lead toxicity are similar to the symptoms of the following diseases or toxicities:

Rabies virus can cause behavior changes and seizures. Cats with rabies often have a history of exposure to a wild animal or a history of bite wounds of unknown origin.

Epilepsy (seizures due to an abnormality in the brain) is typically seen in cats between the ages of 1-7 years. Epilepsy is idiopathic (of unknown cause), genetic, or secondary to trauma or another disease that caused structural changes in the brain. Epilepsy is diagnosed after exclusion of all other possible causes of seizures.

Thallium poisoning is rare but can cause gastrointestinal and neurological problems.

Strychnine poisoning can lead to seizures. Strychnine products are used to kill rodents but these products are rarely used in the United States.

Zinc poisoning can cause depression, loss of appetite, vomiting, diarrhea and anemia. The most common source of zinc is pennies minted after 1983, nuts, bolts and batteries. Zinc toxicity is differentiated from lead toxicity based on x-rays of the intestinal tract and measurement of blood zinc levels.

Chlorinated hydrocarbon poisoning (DDT, Chlordane, Dieldrin, Endrin, Aldrin, Chlordecone, and Endosulfan) is rare but can cause depression, vomiting, muscle tremors and seizures which look similar to symptoms caused by lead poisoning. Chlorinated hydrocarbons are used as insecticides.