Cats become infected most commonly through ingestion of bradyzoites in infected tissues. They shed oocysts only after ingestion
of infected meat for a maximum period of two weeks, although the parasite remains encysted in their muscle for life. Re-shedding
of oocysts does not occur under most circumstances. Oocysts are difficult to locate and distinguish from other Coccidia in
fecal preparations, so this is not used as a definitive means.
Clinical signs in the acute disease are varied, depending on the tissue of encystment. Fever, lymphadenomegaly, hyperesthesia
and uveitis have been seen. Therefore, specimens for diagnosis may be chosen depending on the system of involvement. Neonates
or immunosuppressed cats develop systemic infection.
In older cats with disseminated encystation, infection of the lungs, CNS or muscle, hepatic, cardiac and ocular tissues are
most common. In older or chronically infected animals, the clinical signs may be more insidious. CNS or ocular signs are most
common in chronic reactivated infections associated with stress or immunosuppression.
Clinical signs in dogs also can involve the respiratory, neuromuscular or GI systems. Reactivated infections may be more chronic
while disseminated disease involving parenchymal organs, such as the lung and liver, can be more rapid and fatal. Myocardial
involvement may be more insidious and lead to arrhythmias or eventual myocardial failure. Chronic polymyositis is a syndrome
observed in dogs with reactivated infections. Neurologic dysfunction is seen also with chronic infections and the signs observed
depend on the location of the inflammation within the CNS.
Hematologic and serum chemistry profile abnormalities usually are nonspecific in acute illness and reflect inflammatory changes
in many organs. Hypoalbuminemia may be observed in the acute infections and hyperglobulinemia in more chronic infections.
Increased hepatic transaminases or creatine kinase activities are found in animals with acute hepatic or muscle necrosis.
Tachyzoites can rarely be detected in tissues and body fluids during acute illness. Radiographic findings may indicate an
alveolar to interstitial infiltrate in the lung fields. Intestines or mesenteric nodes may be enlarged.
Detection of oocysts in the feces of cats is an inaccurate means of confirming a diagnosis. These oocysts are very small approaching
the size of erythrocytes. The prevalence of oocysts is low and the excretion interval is 14 days or less. Only some cats develop
diarrhea during the period of oocyst shedding and so indications for submitting specimens for fecal examination are absent.
A number of serologic test methods have been used and no method is in itself definitive. Methods allowing for distinguishing
between IgG and IgM antibodies have been valuable for indicating multisystemic infection in cats and for determining their
public-health risk. Once infected, animals harbor Toxoplasma cysts for life.
Young kittens acquire maternal antibody in colostrum that can persist for up to 12 weeks. Serologic testing shows exposure
to Toxoplasma is prevalent worldwide and increases with the age of the animal. A number of serologic assays are available; however, none
by itself is definitive. Agglutination, IFA and ELISA assays are most commonly used.
Documentation of a positive agglutination or IgM titer generally indicates active or recent infection and may correlate with
the period of oocyst shedding in cats. A change in IgG titer with paired sera can be used also. A single increased IgG titer
can be associated with chronic or reactivated infections. A positive IgG titer in a cat actually confirms its acute infection
period has passed. In other words, there is little risk of the cat shedding oocysts. For ocular or CNS infections, measurement
of ratios of serum to aqueous humor or CSF levels has been used.
Organism detection is definitive for infection by Toxoplasma; however, finding the organism does not always confirm its role in clinical disease. Encysted forms may be quiescent while
circulating tachyzoites often indicate acute systemic spread of infection. Cytologic identification can be helpful, but very
insensitive. Toxoplasma antigens have been shown to be released from encysted muscle during chronic infection and this release is exacerbated by
immunosuppression. PCR has been used to verify the presence of T. gondii in biologic specimens. Unfortunately, it is so sensitive that false-positive results may be caused by quiescent-encysted
organisms in tissues. Therefore, antibody methods should still be considered the mainstay of diagnosis for this infection.
Dr. Hoskins is owner of DocuTech Services. He is a diplomate of the American College of Veterinary Internal Medicine with
specialities in small animal pediatrics. He can be reached at (225) 955-3252, fax: (214) 242-2200 or e-mail:
J ohnny D. Hoskins