Document Type

Thesis - Open Access

Award Date


Degree Name

Master of Science (MS)

Department / School

Biology and Microbiology

First Advisor

Clyde A. Kirkbride


Toxoplasma gondii is a coccidian parasite which produces a disseminated infection and reproductive failure in sheep. It is a major cause of abortion in sheep in New Zealand, Australia, and England. Within the last 5 years, T. gondii has been reported to cause ovine abortion in the United States. Individual flocks in New Zealand may lose 15-20 percent and sometimes as high as 50 percent of the lamb crop as a result of T. gondii induced abortion. The prevalence of T. gondii and the economic impact of the disease in U.S. sheep flocks is unknown. Several approaches have been employed to diagnose toxoplasmosis in ovine abortions. Inoculation of mice with fetal tissues can confirm the diagnosis but may require 3-8 weeks to detect the infection or isolate the organism. Microscopic examination of fetal tissues for lesions of toxoplasmosis is useful, but lesions are not always present, or they may not be identifiable in aborted lambs because of autolytic changes. The immune status of the ewe at the time of the abortion may indicate prior exposure of the ewe to T. gondii. However, this provides little information concerning the cause of abortion, because high antibody titers persist through several pregnancies. Ewes experimentally infected with T. gondii often developed serum antibody titers to this organism after abortion or parturition. The serologic status of the aborted fetus appears to be useful in diagnosing T. gondii-induced abortions. Ovine maternal antibody normally does not cross the placental barrier. Therefore, antibody present in the fetus is of fetal origin, and detection of toxoplasma antibody in ovine fetal sera or body fluids is diagnostic. In contrast, the absence of specific antibody does not rule out the possibility of toxoplasmosis. The fetus may become infected prior to the age when fetal antibody production normally begins, or the strain of T. gondii may be so virulent that fetal death may occur before an immunocompetent fetus can produce an immune response. Serologic kits are available for detecting antibody to T. gondii in human serum. These kits can be modified for veterinary use with some reagent substitutions. These modified kits are convenient to use in veterinary diagnostics because the antigens and other reagents are standardized by the manufacturer and are consistent from lot to lot. This eliminates the need for diagnostic laboratory personnel to propagate antigen in mice or cats. The objectives of this research were to evaluate and compare the efficacy of 3 serologic techniques (i.e. direct agglutination, enzyme-linked immunosorbent assay, and indirect fluorescent antibody) for detecting antibody to Toxoplasma gondii in aborted ovine fetuses submitted to the South Dakota Veterinary Research and Diagnostic Laboratory, and to determine the prevalence of toxoplasma-induced ovine abortions.

Library of Congress Subject Headings

Toxoplasmosis in animals


Sheep - Parasites -- Control



Number of Pages



South Dakota State University