Comparison of Detection Rates of Toxoplasma gondii among Five Host Tissues and Two Primer Sets in Three Bird Species

Toxoplasma gondii is a globally distributed parasite that infects a wide range of warm-blooded animals and requires felids as definitive hosts. Although birds are recognized carriers of T. gondii, in New Zealand species morbidity and mortality events have been sporadically reported, and systematic data are lacking. The objective of this study was to determine the prevalence and tissue distribution of T. gondii in three common aquatic birds in New Zealand: the native Red-billed Gull (Chroicocephalus scopulinus) and Black-backed Gull (Larus dominicanus), and the introduced Mallard (Anas platyrhynchos). Birds were collected between September 2022 and April 2025 and screened using nested PCR with two commonly used primer sets (B1, targeting the B1 gene, and FOOD, targeting the pppk-dhps region). Five organs (liver, lung, heart, brain, and spleen) were tested to compare detection rates across tissues. Overall, prevalence was low but consistent across primers and tissues in all three species. Black-backed Gulls and Mallards showed higher prevalence than Red-billed Gulls, probably reflecting differences in diet, habitat, and behavior. Brain and heart tissues yielded the highest detection rates, and the FOOD primers were approximately twice as sensitive as the B1 set. These findings provide practical guidance for primer and tissue selection in avian T. gondii studies and represent the first assessment of infection in these three bird species in New Zealand. They also highlight potential ecologic differences among species that may influence exposure to T. gondii.