DIAGNOSIS
A presumptive antemortem diagnosis of toxoplasmosis can be made in the live animal by demonstrating either an elevated immunoglobulin (Ig) M titre in conjunction with a low or negative IgG titre (acute disease) or rising/ elevated IgG and IgM titres (chronic disease), combined with supporting clinical signs and history.
A definitive postmortem diagnosis can be made using a combination of histopathology and immunohistochemistry with or without molecular diagnostic techniques.Serological tests available for the antemortem diagnosis of toxoplasmosis include the MAT and direct agglutination tests (DAT), Sabin-Feldman dye test, the latex agglutination test, the indirect haemagglutination test, the immunofluorescent antibody test (IFAT) and various ELISAs. However, given the often acute course of the disease, a serological response may not be evident, making antemortem diagnosis of acute cases challenging. An elevated IgM titre, as determined by the DAT, with a low or absent IgG titre, as determined by the MAT, is considered to be indicative of acute toxoplasmosis (Johnson et al. 1989; Lynch et al. 1993; Obendorf et al. 1996; Miller et al. 2000) Elevations in both IgM and IgG occur after the acute phase of infection. High specificity of the MAT for diagnosis of toxoplasmosis in southern brown bandicoots (Isoodon obesulus) and common brush-tailed possums (Trichosurus vulpecula) has recently been demonstrated using Bayesian latent class analysis (Hillman et al. 2017).
On the basis of histology alone, T. gondii tissue cysts cannot be differentiated from those of other cyst-forming coccidia such as N. caninum or Hammondia hammondi. Molecular diagnostic and immunohistochemical techniques can be used on tissue samples to confirm toxoplasmosis. Immunohistochemistry using a variety of anti-T. gondii antibodies can be performed on fixed tissue (Bowater et al. 2003; Donahoe et al. 2014; Donahoe et al. 2015; Carossino et al. 2021). PCR has recently been used to diagnose toxoplasmosis in bare-nosed wombats, various macropod species and a long-nosed fur seal (Para- meswaran et al. 2010; Pan et al. 2012; Donahoe et al. 2014; Donahoe et al. 2015; Guthrie et al. 2017; Carossino et al. 2021). Next-generation sequencing based on the 18S rRNA gene has also been used to diagnose and characterise the T. gondii genotype involved in a case of toxoplasmosis in a Risso’s dolphin (Cooper et al. 2016).
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