This article introduces a common parasite found in cats. Toxoplasmosis is a common disease found in nearly all warm-blooded animals, caused by a single-celled parasite called Toxoplasma gondii (T gondii).
The life cycle of T gondii is complex and involves two types of host: definitive and intermediate. Cats, both wild and domestic, are the only definitive hosts, meaning that the parasite can only produce oocysts (eggs) when infecting a cat. When a cat ingests an infected prey (or other infected raw meat) the parasite is released into the cat's digestive tract.
The organisms then multiply in the wall of the small intestine and produce oocysts during what is known as the intra-intestinal infection cycle. These oocysts are then excreted in great numbers in the cat's faeces. Cats previously unexposed to T gondii will usually begin shedding oocysts between three to ten days after ingestion of infected tissue, and continue shedding for around ten to 14 days, during which time many millions of oocysts may be produced. Oocysts are very resistant and may survive in the environment for well over a year.
During the intra-intestinal infection cycle in the cat, some T gondii organisms released from the ingested cysts penetrate more deeply into the wall of the intestine and multiply as tachyzoite forms. These forms then spread out from the intestine to other parts of the cat's body, starting the extra-intestinal infection cycle.
Eventually, the cat's immune system restrains this stage of the organism, which then enters a dormant or "resting" stage by forming cysts in muscles and brain. These cysts contain bradyzoites, or slowly multiplying organisms.
Other animals, including humans, are intermediate hosts of T gondii. These hosts can become infected but do not produce oocysts. Oocysts passed in a cat's faeces are not immediately infectious to other animals. They must first go through a process called sporulation, which takes one to five days depending on environmental conditions.
Once sporulated, oocysts are infectious to cats, people, and other intermediate hosts. Intermediate hosts become infected through ingestion of sporulated oocysts, and this infection results in formation of tissue cysts in various tissues of the body.
Tissue cysts remain in the intermediate host for life and are infectious to cats, people and other intermediate hosts if the cyst-containing tissue is eaten. Most cats infected with T gondii will not show any symptoms. However, clinical disease toxoplasmosis will occasionally occur. When disease does occur, it may develop when the cat's immune response is not adequate to stop the spread of tachyzoite forms.
The disease is more likely to occur in cats with suppressed immune systems, including young kittens and cats with feline leukemia virus (FELV) or feline immunodeficiency virus (FIV). The most common symptoms of toxoplasmosis in cats include fever, loss of appetite, and lethargy. Other symptoms may occur depending on whether the infection is acute or chronic, and where the parasite is found in the body. In the lungs, T gondii infection can lead to pneumonia, which will cause respiratory distress of gradually increasing severity.
Toxoplasmosis can also affect the eyes and central nervous system, producing inflammation of the retina or anterior ocular chamber, abnormal pupil size and responsiveness to light, blindness, lack of coordination, heightened sensitivity to touch, personality changes, circling, head pressing, twitching of the ears, difficulty in chewing and swallowing food, seizures, and loss of control over urination and defecation.
Toxoplasmosis is usually diagnosed based on the history, signs of illness, and the results of supportive laboratory tests. Measurement of IgG and IgM anti-bodies to T gondii in the blood can help diagnose toxoplasmosis.
The presence of significant IgG antibodies to T gondii in a healthy cat suggests that the cat has been previously infected and now is most likely immune and not excreting oocysts. The presence of significant IgM antibodies to T gondii, however, suggests an active infection of the cat. The absence of T gondii antibodies of both types in a healthy cat suggests that the cat is susceptible to infection and thus would shed oocysts for one to two weeks following infection.
Sometimes the oocysts can be found in the faeces, but this is not a reliable method of diagnosis because they look similar to some other parasites. Also, cats shed the oocysts for only a short period of time and often are not shedding the oocysts when they are showing signs of disease. A definitive diagnosis requires microscopic examination of tissues or tissue impression smears for distinctive pathologic changes and the presence of tachyzoites.
Most cats that have toxoplasmosis can recover with treatment. Treatment usually involves a course of an antibiotic called clindamycin. Other drugs that are used include pyrimethamine and sulfadiazine, which act together to inhibit T gondii reproduction. Treatment must be started as soon as possible after diagnosis and continued for several days after signs have disappeared. In acute illness, treatment is sometimes started on the basis of a high antibody titer in the first test. If clinical improvement is not seen within two to three days, the diagnosis of toxoplasmosis should be questioned. Remember that only your veterinarian can diagnose toxoplasmosis and prescribe the appropriate treatment.
No vaccine is as yet available to prevent either T gondii infection or toxoplasmosis in cats, humans, or other species.
Next, we will look at the transmission of this parasite to humans and what precautions you should take to avoid becoming a host.
Copyright © Kristel-Marie Ramnath 2012. For further information contact Kristel-Marie Ramnath at 689-8113 or bestpetsbehave@ hotmail.com
