Cats are usually infected by ingesting the organism present in the tissues (meat) of another infected animal known as an ‘intermediate host’ which is usually a rodent. The Toxoplasma organism replicates first locally in the intestinal tract of the cat, and is often contained there. The replication in the intestinal tract results in shedding of oocysts in the faeces. The oocysts represent a hardy form of the organism that can survive in the external environment for many months or even years. Other animals can become infected by ingesting these oocysts, but disease will result only if large numbers are ingested.
In some cats, particularly if their immune defenses are compromised, the Toxoplasma organisms can invade beyond the intestine and spread into various organs of the body. There, they may cause enough damage to cause signs of disease or may become dormant in a tissue cyst. This is not the same as the oocyst form. Such tissue cysts can be infective if the infected tissue is eaten by another animal.