Pathogenicity of selected Toxoplasma gondii isolates in young pigs

Abstract

The pathogenicity in 7-week-old pigs to five different Toxoplasma gondii strains of various host species origin was compared after i.v. inoculation of 10⁴ tachyzoites. Additionally, one group of pigs was inoculated i.v. with 10⁶ tachyzoites of the reference strain, SSI 119. In response to the infection a significant effect of T. gondii tachyzoite inoculation dose as well as differences among strains could be observed in several parameters. The 10⁶-dose inoculated pigs showed variable degrees of clinical illness and recurrent episodes of fever 4–17 days p.i., while pigs of four of the 10⁴ tachyzoite inoculated groups experienced a short-lived rise in body temperature from day 6–8 p.i. without any apparent illness or inappetence. Control pigs and pigs infected with the least pathogenic strain had normal body temperature throughout the experiment. In all inoculated pigs, T. gondii-specific IgM and IgG antibodies appeared from day 8–10 and 10–17 p.i., respectively. Serum levels of alkaline phosphatase and the acute phase protein haptoglobin were decreased or increased, respectively, in response to the infection. Differential leukocyte count on peripheral blood revealed a significant lymphocytopenia on day 6 p.i. equal to both CD4⁺ and CD8⁺ T-cells, but shifting towards a reduced ratio of CD4⁺/CD8⁺ T-cells from day 8–14 p.i. In the 10⁶-dose inoculated pigs a considerable increase in zymosan induced and spontaneous oxidative burst capacity of peripheral blood leukocytes was observed from 6 days p.i. compared with control pigs. Oxidative burst capacity was not examined for other pigs. In conclusion, several useful parameters to identify differences in T. gondii pathogenicity other than mortality were identified. Furthermore, even at low doses, significant differences between recently collected Danish T. gondii field isolates were demonstrated after i.v. inoculation in young pigs.

Introduction

The ubiquitous, obligate intracellular protozoan Toxoplasma gondii has cats as the only definitive host, but is capable of infecting most warm-blooded vertebrates with formation of infective tissue cysts. There are well-known differences in susceptibility to T. gondii among various species with hares, marsupials and new world monkeys as the most vulnerable1, 2, and cattle being very resistant3, 4. Species of intermediate susceptibility include sheep, where Toxoplasma induced abortions can constitute severe problems5, 6, and pigs, which are considered to be the most important meat source for human T. gondii infections[3]. The role of naturally occurring transplacental infections in pigs is yet unresolved.

Toxoplasma gondii strains of human and animal origin have been subjected to an array of typing techniques and a common nomenclature clustering the highly mouse virulent strains into group A and the mouse avirulent strains into groups B and C has been proposed[7]. Experimental studies characterising differences in pathogenicity among strains in pigs are very limited, but a characterisation based on mortality is undesirable for ethical animal welfare reasons. In a recent survey, 38 Danish T. gondii strains, collected within a short time span from a variety of animal and human sources were typed by their reaction pattern to four mAb, by which discrimination between strains of the A, B and C groups was possible[8]. All 38 Danish isolates were identified as one group with the same reaction profile as group B strains and the SSI 119 strain originally isolated from a Danish sow in 19689, 10.

In the present experiment, young pigs were i.v. inoculated with selected strains of the Danish T. gondii collection and various biological parameters were used to evaluate differences in pathogenicity. The experimental infection with T. gondii by i.v. inoculation was also used as a parasite infection model for the study of the porcine acute phase haptoglobin response (Heegaard P, Nielsen JP, Jensen L, Lind P. Haptoglobin as an acute-phase protein in swine during experimental infections with Toxoplasma gondii and with Salmonella. 4th International Veterinary Immunology Symposium. 1995 p. 163). This parameter could be attractive as a measure of pathogenicity in pigs as the haptoglobin response to T. gondii infection is highly correlated with the inoculation dose as well as with strain virulence in mice[11], and in hares and rabbits[12]. Furthermore, because the virulence of intracellular pathogens may be related to circumvention of cellular defense mechanisms, we also studied the effect of the infection on the leukocyte oxidative burst capacity examined in the high dose inoculated group of pigs.