Toxoplasmosis in Small Animals

Toxoplasma gondii, an obligate intracellular coccidian protozoan parasite, is the cause of Toxoplasmosis and exposure to this organism is widespread among the human and domestic animal population (an estimated 30-40% of adults in the United States have antibodies to T. gondii).

  Toxoplasma gondii is a tissue protozoan with three life stages – tachyzoites, bradyzoites and sporozoites.  Tachyzoites is the rapidly dividing stage of the organism that disseminate in the blood or lymph during active infection and that can infect almost all tissues.  The tachyzoites replicate intracellularly until the infected cell is destroyed, releasing the organism.

  Clinical signs develop as a result of inflammation in infected tissues.  If organism replication is attenuated by immune response, tissue cysts may develop that contain the more slowly dividing bradyzoites.  Bradyzoites can later be reactivated under conditions of immunosuppression to divide rapidly as tachyzoites, potentially resulting in clinical disease.  Bradyzoites can persist in affected tissues for the lifetime of the host.

 Cats are the only species able to complete the coccidian life cycle of T. gondii in the intestinal tract.  After cats ingest bradyzoites (that are encysted in tissues of prey animals) intestinal epithelial cells are infected and several rounds of asexual replication occur followed by the sexual cycle.  Sporulated oocysts are passed in the feces; at this stage they are non-infectious.  Sporozoites develop in the oocysts after one to five days of exposure to oxygen in conjunction with appropriate environmental temperatures and humidity.

  Toxoplasmosis can be spread by ingestion of encysted bradyzoites in tissue, ingestion of food or water contaminated with feces that contain sporulated oocysts, or transplacental transmission.  Cats are the key animal species in the epidemiology of Toxoplasma gondii because they are the only hosts that excrete the infective, environmentally resistant oocysts in their feces.

  The intestinal schizogonous replication cycle in cats rarely causes clinical signs, but vomiting and diarrhea have been reported.  In kittens, severe enteric disease can occur if concurrent disease is present (e.g., viral respiratory infection).

  Clinical disease in the cat and other species is most commonly associated with the dissemination and replication of the organism during the extra intestinal cycle.  Asexual reproduction occurs intracellularly in all body tissues except red blood cells and results in destruction of the infected cells plus subsequent clinical signs that vary depending on the organ systems most severely affected.

  Tissue cysts rarely result in clinical signs but may serve as sources of antigen in immune-mediated diseases.  Immuno-suppression can reactivate the bradyzoite cysts and lead to rapid dissemination, tachyzoite replication, and location-dependent clinical signs.

  Respiratory tract involvement is common and is manifested by dyspnea and coughing.  Anorexia, malaise, lameness, icterus, fever, tonsillar enlargement, lymphadenomegaly, splenomegaly and evidence of encephalitis are often observed depending on the site affected.  Muscle discomfort from myositis is frequently noted during physical examination and neurological signs can be present.

  Retinochoroiditis caused by organism replication is the primary ocular lesion, but other ocular manifestations include anterior and posterior chamber changes involving either one or both eyes.  Secondary changes include vitreal hemorrhage, vitreal opacity, retinal detachment, iritis, iridocyclitis, hyphema, cataracts and corneal precipitates.

  Diagnostic tests for toxoplasmosis include hematology, clinical chemistry, and urinalaysis.  Although there are no laboratory findings pathognomonic for toxoplasmosis, suggestive clinical history and the following laboratory abnormalities raise the index of suspicion: non-regenerative anemia, neutrophilic leukocytosis, monocytosis, eosinophilia, elevated creatine kinase, elevated alkaline phosphatase, elevated alanine transferase, elevated lipase, hyperbilirubinemia, hyperproteinemia, proteinuria and bilirubinuria.

  Radiographic findings may include a diffuse interstitial to alveolar pattern with mottled lobar distribution in the thorax.  In cats with CNS involvement, mass lesions may be detected by myelography, computed topography, or magnetic resonance imaging.

Cytological examination may reveal tachyzoites in blood, CSF, transtracheal wash fluid, peritoneal effusion, or pleural effusion from clinically ill animals.

  The short period of oocyst shedding, combined with the difficulty in demonstrating the oocysts, makes fecal examination a poor procedure for determining the status of T. gondii infection.   However, due to the potential zoonotic risk, fecal examination should be performed in any cat with clinical signs suggestive of toxoplasmosis.  When present, the oocysts are typically observed in the plane of view just beneath the coverslip.  T. gondii oocysts are 10x12 mm in diameterand can be demonstrated microscopically in feline feces following flotation using solutions with specific gravity of 1.18 (sugar centrifugation is the preferred technique to demonstrate oocysts.)

  Toxoplasma gondii specific antibodies and antigens can be detected using a range of tests that are commercially available including ELISA for both antibodies and antigens.  Antigens are released intermittently from the tissues of cats up to one year post-infection.  Because of the intermittent shedding, antigen detection cannot be used to differentiate infection from clinical disease or to predict oocyst shedding.  Other commercially available techniques include immunofluorescent antibody assay, western blot immunoassay and the Sabin-Feldman dye test.  ELISA, immunofluorescent antibody assay and western blot immunoassay have been adapted to detect IgM, IgG and IgA antibody responses.  T. gondii DNA has been detected in the aqueous humor of cats with uveitis by polymerase chain reaction (PCR).

  Tissue biopsy sections can be assessed for the presence of T. gondii by H&E staining or immunohistochemical staining.  Immunohistochemical staining is superior to H&E staining because it is specific for

T. gondii.  It can be difficult to document the organism in the tissue of some clinically sick cats because of the small percentage of tissues evaluated histopathologically and because the pathogenesis of the disease in some cats may be immune-mediated.

  Definitive diagnosis of clinical feline toxoplasmosis requires demonstration of the organism in the tissue in association with inflammation.  This usually is achieved at necropsy in cats with overwhelming tachyzoite replication although a definitive diagnosis of clinical feline toxoplasmosis is occasionally made antemortem by demonstrating the bradyzoites and tachyzoites in tissue or effusions.  Since

T. gondii-specific antibodies can be detected in the serum, CSF and aqueous humor of normal as well as clinically affected animals, it is not possible to make a diagnosis of clinical toxoplasmosis based on those tests alone.  However, a presumptive antemortem diagnosis of clinical feline toxoplasmosis may be used on the following combination of findings

• Demonstration of antibodies in serum, aqueous humor or CSF (documented exposure to T. gondii.)
• Demonstration of an IgM titer of above 1:64, a fourfold or greater increase in IgG titer
• Clinical signs disease referable to toxoplasmosis
• Exclusion of other common etiologies
• Positive response to appropriate treatment 

Research is currently in progress to develop

simple, inexpensive tests to detect active    T. gondii infection.  The proposed test would involve detection of two partially characterized secreted antigens called H4 Nd P18.  Preliminary studies suggest that    T. gondii tachyzoites actively secrete H4 and P18, suggesting that these antigens may circulate in the blood during acute infections.  The antigens would be detectable by a simple antigen-capture blood test.  This study subsequently could lead to the development of a commercially available, rapid test for diagnosis of acute toxoplasmosis of cats.

-by Roman Arteaga, ECFVG Student

-edited by Dr. Theresa Boulineau, ADDL Graduate Student

References

1. Dubey JP and Carpenter JL: 1993.     Histologically confirmed clinical toxoplasmosis in cats: 100 cases (1952-1990).  JAVMA 203: 1556-1565

2. Dubey JP: 1994.  Toxoplasmosis.  JAVMA 207:205.

3.   Dubey JP, Lappin MR, and Thuyllies P: 1995.  Diagnosis of induced toxoplasmosis in neonatal cats.  JAVMA 207: 179-185.

4.   Hilsenroth R: 2000.  Developing new tests to detect acute toxoplasmosis.  Feline Practice 28:3.

5.  Lappin M: 1999.  Feline toxoplasmosis.  In Practice 21: 578-589.

6.   Lappin M: 1989.  Toxoplasmosis.  Current Veterinary Therapy   1112-1115.

7.   Leighty JC: 1990.  Strategies for control of toxoplasmosis.  JAVMA 196: 281-285

8.   Client  Information Series: 1991.  Toxoplasmosis.  Feline Practice 19:19-22.