Toxoplasma gondii is a tiny organism, slightly bigger than a bacterium, called a protozoa. This parasite has a worldwide distribution, except in the absence of cats. Cats are the only animals capable of completing the life cycle of this organism. Other warm blooded animals, including cats can serve as intermediate hosts for the parasite. The organism has a very high prevalence, but rarely causes clinical disease in dogs and cats. This is an important parasite to be aware of due to the fact that it is an important zoonosis, meaning it is an animal disease that can be transmitted to humans.
Cats are the only hosts capable of passing oocysts, or eggs, of the Toxoplasma parasite in their stool. These eggs are environmentally resistant and can stay in the environment for an extended period of time. They are even resistant to multiple disinfectants. Once these eggs are ingested by a cat, they penetrate the gut lining and start replicating inside cells. More eggs will form and be released into the cat’s stool. Sometimes the eggs will penetrate deeper into the gut wall and develop into a more active form called tachyzoites that replicates quickly and may cause a slight small intestinal diarrhoea. In other animals like the dog, eggs will not be excreted, and the active parasite will form tissue cysts inside cells of the body. The active parasites replicate rapidly in the cells of the liver, lungs, pancreas and nervous system. These cells are destroyed by the rapid replication and this leads to clinical disease and even death of the host animal.
When the immune response is adequate it slows down parasite replication and they go into a dormant, or resting phase by forming cysts inside tissues of the host’s body. Parasites inside the cysts divide slowly. These cysts usually form in tissue of the central nervous system or muscle. Animals either get infected by ingesting eggs shed in stool of cats or by ingesting cysts in the tissues of infected animals. Cats do not commonly eat their own stool (called coprophagia), and are usually infected through ingestion of tissue cysts of infected animals when they hunt. It has been noted that mice infected with tissue cysts become fearless of cats and therefore become easy prey. Kittens can be infected through the placenta or through the milk when they are suckling from the infected mother. The infection in kittens is often severe and they can die acutely from infection involving the liver or lungs.
Symptoms of Toxoplasma
Most cats infected with Toxoplasma will not show any signs of the disease. Cats that do show signs of disease often suffer with underlying infections of other viruses like feline leukaemia virus, feline immunodeficiency virus or feline infectious peritonitis. These infections impair their immune systems and prevents it from controlling the disease in the early stages. This is also true for immuno-compromised dogs such as those infected with canine distemper. The first clinical signs or symptoms that cats will show include depression, loss of appetite and fever. Later on as the disease progresses and depending on where the organisms are located they can become hypothermic (low body temperature), the abdomen can fill with fluid, they can become jaundiced (yellow) and may battle to breath.. Dogs and cats can become chronically infected and will show signs of eye infections (uveitis), muscle twitching, balance problems with swerving of the hindquarters (ataxia), seizures, pancreatitis and jaundice otherwise known as icterus. The neurological signs depend on where the tissue cysts form and how big they get. Muscles may also be affected together with the central nervous system and can lead to a stiff gait, weakness of voluntary movement (paresis), total loss of muscle function (paralysis), and seizures.
Diagnosis of Toxoplasma
Toxoplasma is difficult to diagnose due to infected animals often now showing any signs of the disease and because of the non-specific symptoms infected animals that do show signs have. Basic laboratory tests of the blood will give non-specific results like low red blood cell count or anaemia, increased or decreased white blood cells counts, protein in urine, increased liver and kidney enzymes. None of these results are specific to this disease. The only way a definite diagnosis can be made is by detecting the antibodies to the organism in tissue fluids like lung fluids and cerebrospinal fluid. Sometimes the eggs can be found in a cat’s stool, but this is not a very reliable way to make a diagnosis as the eggs cannot easily be distinguished from worm eggs and a cat will not always be shedding eggs at the same time he or she is showing clinical signs. Measurement of antibodies called IgG and IgM in the blood can help support the diagnoses of Toxoplasma when the patient is showing clinical signs related to Toxoplasma infection but due to the fact that healthy cats can also show high levels of both IgG and IgM, it is not a reliable method of diagnosis.
The most reliable method to make a diagnosis currently is to show high levels of IgM in cerebrospinal fluid of a patient showing related clinical signs.
Treatment of Toxoplasma
Supportive care for very ill patients is important and treatment should be started as soon as possible. The treatment of choice is an antibiotic called clindamycin but there are other combinations of antibiotics that can be used as well. Patients usually respond quickly to treatment, and failure of improvement of clinical signs within a few days usually means the diagnosis should be questioned.
Spread to humans
The most common way humans get infected is through contaminated soil. Unwashed fruit and vegetables can play a big role, as well as eating uncooked meat, especially lamb and pork. Another way of getting infected is by ingesting unpasteurised dairy products, especially goat’s milk. Mothers infected with Toxoplasma while pregnant can lead to clinical Toxoplasmosis in the foetus through transplacental infection. This can cause stillbirths, central nervous system disease and eye disease. Pregnant woman should not clean cat litter boxes or do gardening or if they have to, always wear rubber gloves. They should only eat cooked meat and wash fruit and vegetables well before use. It is also useful to keep children’s sandboxes covered to prevent cats soiling it. Due to the high prevalence of the disease it is difficult to prevent infection in cats. The only way will be to prevent cats from roaming and hunting, which is an almost impossible task! Clinical disease is also noted in immuno-compromised patients such as those with HIV and cancer. In these patients nervous system may be affected and people can die from an infection with Toxoplasma.
As with all matters related to pets, the general principles of hygiene must always be observed and regular visits to the vet for annual health check-ups and vaccinations where and when relevant, will help detect potential problems early on, so one can act pro-actively if need be.
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