Toxoplasmosis in Cats
Dr. Anne Marie Manning
A complete blood count (CBC) is done to determine if your pet is anemic (low red blood cell count) and to assess the white blood cell count. Pets with toxoplasmosis are commonly anemic, but the white blood cell count often remains unchanged.
The diagnosis of toxoplasmosis can be very difficult. In addition to obtaining a complete medical history (including your pet's travel history) and performing a complete physical examination, your veterinarian will likely perform many of the following diagnostic tests.
A serum chemistry profile allows your veterinarian to evaluate the internal organs such as the liver, pancreas and kidneys. Because toxoplasmosis often causes elevations in liver enzymes and total bilirubin (causing jaundice), this test is essential to evaluate involvement of the liver. Additionally, elevations in blood sugar, total protein levels and globulin levels may be present.
Examination of a fecal (stool) sample may be performed to look for toxoplasma oocysts (one of the life cycle stages of toxoplasma). Because infected cats shed oocysts intermittently, a fecal sample may be negative despite serious infection. Thus, a negative fecal sample does not rule out toxoplasmosis.
An ocular (eye) examination should be part of the physical examination performed by your veterinarian to look for evidence of uveitis (inflammation of the interior of the eye), hemorrhage or lesions in the back of the eye (fundic lesions) that are commonly present with toxoplasmosis. The finding of these lesions does not definitively diagnose toxoplasmosis, but it does raise the index of suspicion for the disease and should prompt your veterinarian to submit additional tests to look for toxoplasmosis.
Immunoglobulin G (IgG) and Immunoglobulin M (IgM) antibody testing with the Immunoflourescence Assay (IFA) is the most common method of diagnosing toxoplasmosis. IgG-IFA antibodies do not develop until 2 weeks post-infection and may remain high for several years. Therefore, high IgG-IFA antibody titers do not definitively diagnose an active infection. Instead, the diagnosis of active infection using IgG-IFA antibody titers requires that the titers increase fourfold over a 2-3 week period. Your veterinarian would need to draw two blood samples 3 weeks apart to demonstrate the rise in titers. An IgM-IFA antibody titer can be measured and can detect active infection. A positive IgM-IFA titer along with a negative IgG-IFA titer is diagnostic of an active infection. The reverse (positive IgG-IFA with negative IgM-IFA titers) would indicate a chronic infection.
An ELISA test may also be used to detect IgM antibodies. A high IgM-ELISA titer (>1:256) with a negative or low IgG-IFA titer indicates an active infection. Interpretation of titers is difficult and this information should be used in conjunction with other laboratory tests and the patient's clinical condition to render a diagnosis.
Other tests that may aid in the diagnosis of toxoplasmosis include:
Thoracic (chest) x-rays are helpful to rule out other causes of respiratory compromise such as pneumonia, cancer, and fungal infection.
Analysis of cerebrospinal fluid obtained by a cerebrospinal fluid tap is used to help differentiate toxoplasmosis from other diseases causing central nervous system problems. The fluid is analyzed for red and white blood cells (none should be present) and antibody titers may be measured in the fluid as well. Test results must be assessed in conjunction with other findings in the patient as CSF analysis alone is not sufficient to provide a diagnosis.
On rare occasions, tachyzoites (one of the life cycle stages of toxoplasma) may be found in pleural (chest) or abdominal (belly) fluid that is obtained from a pet infected with toxoplasmosis. A sample for analysis is obtained by removing fluid from the pet's chest or abdomen by sterile aspiration. Because all pets with toxoplasmosis do not experience the escape of fluid into the chest or abdomen (effusions), this test is limited to those that do.
Additionally, tachyzoites may be found in fluid obtained by aspirating a sample pf bronchial exudate (transtracheal aspirate) in infected pets with pulmonary (lung) lesions. A transtracheal aspirate is performed with the pet under sedation. A sterile endotracheal tube is placed in the pet's trachea (windpipe), sterile saline is squirted down the tube via a long sterile catheter, and the pet is induced to cough by gentle percussion of the chest wall. Fluid is then aspirated into a sterile syringe and examined under a microscope.
Because the results of some diagnostic tests take several days to return, your veterinarian may need to initiate supportive therapy before a definitive diagnosis is made.
Antibiotics such as clindamycin, trimethoprim-sulfa, sulfonamides and pyrimethamine are administered to pets with toxoplasmosis. Clindamycin is the antibiotic of choice and is generally administered twice a day for 4 weeks. Trimethoprim-sulfa can be used alone or in combination with pyrimethamine. Both drugs are given twice daily for 4 weeks. Trimethoprim-Sulfa and pyrimethamine in combination may cause bone marrow suppression in cats. Bone marrow suppression can be corrected by supplementing the pet's diet with folinic acid or yeast.
Anticonvulsant medications such as diazepam (Valium®), phenobarbital or pentobarbital may be used to control seizures.
Intravenous fluids and/or intravenous nutrition may be necessary for pets that are dehydrated or severely debilitated due to infection with toxoplasmosis. Nutritional support is particularly important while a patient is fighting an infection. If the pet is not vomiting but refuses to eat, your veterinarian may elect to place a naso-esophageal tube to permit feeding of a liquid diet. If the pet is vomiting, nutrition can be provided via a sterile intravenous catheter. Your pet may be referred to a 24- hour care practice or to a specialist for nutritional support.