Toxoplasma gondii

Test code: X0002 - Qualitative detection of Toxoplasma gondii by PCR

Toxoplasmosis is the most common parasitic infection worldwide affecting humans and a number of domestic animals. The organism that causes toxoplasmosis is Toxoplasma gondii - a single-celled organism.

The cat is the only animal in which sexual reproduction of the organism occurs. Thus, cats are the only domestic animal that has the potential to shed the organism’s eggs. Although there is generally a high prevalence of infection in cats, most surveys show a less than 1% incidence of oocyst shedding. This is to be expected as infected cats generally do not re-shed oocysts following their first exposure to Toxoplasma gondii.

Dogs may transmit Toxoplasmosis to humans by rolling in foul-smelling substances and by ingesting fecal material. The fact that 50% of stray dogs and cats carry T. gondii antibodies suggests that they have been infected with the parasite. Reports show that dogs in shelters, dogs living in close contact with wild birds and rodents in rural areas, and dogs fed raw meat are at much higher risk for being infected by T. gondii.

Despite the high prevalence of Toxoplasma gondii infection, significant clinical disease in cats (and other species) appears to be very rare. This implies that many infected animals can be carriers of the parasite with unnoticed symptoms. When disease does occur, it may develop either following primary infection due to an inadequate immune response to stop the invasive tachyzoites, or as a result of reactivated infection due to compromised immunity. Clinical disease appears to be most common in young cats (less than 2 years of age), and this may be due, in part, to a poorly developed immune response in these cats. Reactivation of infection in older cats may be linked to co-infection with feline leukemia virus or feline immunodeficiency virus in some cats. The most commonly reported clinical signs associated with feline toxoplasmosis are anorexia, weight loss, lethargy, dyspnea (due to pneumonia), ocular signs (iritis, chorioretinitis) and pyrexia. Other less common features include gastrointestinal signs (vomiting and diarrhea), neurological signs, lymphadenopathy, jaundice, myositis and abortion.

Toxoplasmosis presents a serious health risk for people living in close contact with these animals. Infection is especially dangerous for people with suppressed immune system, such as AIDS and cancer patients, and for pregnant women. Swollen glands and fever are the most common findings in those who have any symptoms. Infected infants may show various symptoms including jaundice, encephalitis, mental defects, and eye disease.

The diagnosis of toxoplasmosis is problematic and a definitive diagnosis rests on demonstration of the active form of the organism in tissues taken at post mortem examination or in biopsy samples. Laboratory tests may also be used as diagnostic aids. The “gold standard” for the detection of T. gondii in clinical specimens is mouse inoculation and then the detection of T. gondii-specific antibodies. This method is sensitive and specific but very time-consuming, taking up to six weeks to obtain a diagnosis. Cell culture detection of this parasite is also slow and lack sensitivity. An ELISA test may be used, but is difficult in animals with severe immune dysfunction. Furthermore, serological tests cannot detect cats that are shedding the parasite in their feces. Molecular detection by PCR method overcomes these difficulties and provides a highly sensitive and specific determination of an animal’s infection status.

Utilities:

• Confirm the disease causing agent
• Ensure that animal groups are free of T. gondii
• Early prevention of spread of this parasite among a group of animals
• Minimize human exposure to this parasite

References:
Dubey, J.P. (1993) Toxoplasma, Neoplasma, Sarcocystis, and other tissue cyst-forming coccidian of human and animals. pp1-56. In: Parasitic protozoa (Kreier, P.J. ed), vol. 6, 2nd ed., Academic Press, Inc., San Diego, California.
Inoue, M. (1997) Acute toxoplasmosis in squirrel monkeys. J. Vet. Med. Sci. 59:593-595.
Ruch, T.C. (1959). pp.297-299, 313-318, 423-424. In: Diseases of laboratory primates. W.B. Saunders Co., Philadelphia.
Cunningham, A.A., Buxton, D. and Thomson, K.M. (1992) An epidemic of toxoplasmosis in a captive colony of squirrel monkeys (Saimiri sciureus). J. Comp. Pathol. 107:207-219.
Furuta, T., Une, Y., Omura, M., Matsutani, N., Nomura, Y., Kikuchi, T., Hattori, S. and Yoshikawa, Y. (2001) Horizontal transmission of Toxoplasma gondii in squirrel monkeys (Saimiri sciureus). Exp. Anim. 50:299-306.
Grover, M.C., Thulliez, P., Remington, J.S. and Boothroyd, J.C. (1990) Rapid prenatal diagnosis of congenital Toxoplasma infection by using polymerase chain reaction and amniotic fluid. J. Clin. Microbiol. 28:2297-2301.
Dupouy-Camet, J., de Souza, S.L., Maslo, C., Paugam, A., Saimot, A.G., Benarous, R., Tourte-Schaefer, C. and Derouin, F. (1993) Detection of Toxoplasma gondii in venous blood from AIDS patients by polymerase chain reaction. J. Clin. Microbiol. 31:1866-1869.
Ho-Yen, D.O., Joss, A.W.L., Balflour, A.H., Smyth, E.T.M., Baird, D. and Chatterton, J.M.W. (1992) Use of the polymerase chain reaction to detect Toxoplasma gondii in human blood samples. J. Clin. Pathol. 45:910-913.
Johnson, J.D., Butcher, P.D., Savva, D. and Holliman, R.E. (1993) Application of the polymerase chain reaction to the diagnosis of human toxoplasmosis. J. Infect. 26:147-158.
Cristina, N.H., Pelloux, C., Goulhot, J.P., Brion, P., Leclercq, P. and Ambrosis-Thomas, P. (1993) Detection of Toxoplasma gondii in AIDS patients by the polymerase chain reaction. Infection 21:150-153.
Farmley, S.F., Goebel, F.D. and Remington, J.S. (1992) Detection of Toxoplasma gondii in cerebrospinal fluid from AIDS patients by polymerase chain reaction. J. Clin. Microbiol. 30:3000-3002.
Hussein, A.H., Nagaty, I.M. and Fouad, M.A. (2002) Evaluation of IgM-ELISA versus PCR in diagnosis of recent Toxoplasma gondii infection. J Egypt Soc Parasitol. 32:639-46.

Specimen requirement: 0.5 ml whole blood in EDTA (purple top) or ACD (yellow top) tube, or 1 ml cat feces, or 0.5 ml amniotic fluid, CSF or tissue, shipped overnight at room temperature; or tissue shipped frozen.

For specimen types other than those listed here, please call to confirm specimen acceptability and shipping instructions.

For all specimen types, if there will be a delay in shipping, or during very warm weather, refrigerate specimens until shipped and ship with a cold pack unless more stringent shipping requirements are specified. Frozen specimens should be shipped so as to remain frozen in transit. See shipping instructions for more information.

Turnaround time: 2 business days

Methodology: Qualitative polymerase chain reaction

Normal range: Nondetected