False-Negative Serologic Tests for Toxoplasma in Birds

Epidemiologic studies for Toxoplasma infection often rely unquestioningly on serologic tests as if necessarily they were sensitive indicators of past or present infection. To compound the problem, the sera are often screened at higher dilutions than are generally found positive in birds. In a recent study of animals in Florida (Burridge et al., 1979, AJVMA 175: 964-967) the indirect hemagglutination test was used at at dilution of 1:64. Positive tests were found in 8.4% of 2,179 mammals, ranging between 11 and 19% in opossums, rats, raccoons, and armadillos. However, only 0.6% of 1,279 bird sera were reactive. This was interpreted that . . birds appear to have a minor role in the maintenance of T. gondii infection in Florida. In a similar survey in California, though using a kit of a different manufacturer, antibody reactivity was found in 14.4% of 1,132 mammals but only in 4.4% of 125 birds (Franti et al., 1975, JAVMA 167: 565-568). There are several isolated observations on finding low, transient or unmeasurable antibody titers in experimentally infected chickens using dye, complement-fixation or fluorescent antibody tests (Jones et al., 1959, J. Parasit. 45: 31-37; Boch et al., 1966, Berl. Munch. Tierarztl Wchsch., 79: 352-356). A recent study in an epidemiologic setting illustrates the insensitivity of usual serologic methods to detect Toxoplasma infection in chickens and sparrows (Ruiz and Frenkel, 1980, Am. J. Trop. Med. Hyg. 29: 1161-1166). Toxoplasma was isolated from brain or skeletal muscle from 54% of 50 chickens from Costa Rican farms and from 16% of 106 sparrows all of which were seronegative in the dye test at 1:2. By comparison, 11% of 107 rats had antibody and Toxoplasma was isolated from 12.5%; also, 6% of 202 mice had antibody and Toxoplasma was isolated from 3.5% by passaging brain. Whereas serologic tests in rats and mice were reasonably sensitive in predicting infection, this was not so in chickens and sparrows which actually had a higher infection rate than the mammals tested.