What stage of Toxoplasma is associated with transplacental or transmammary transmission?

The stage of Toxoplasma that is associated with transplacental or transmammary transmission is the tachyzoites, which are the rapidly replicating form of the parasite. Tachyzoites are capable of invading host cells and proliferating rapidly within the body; they are the form responsible for the acute phase of infection. When a pregnant individual becomes infected with Toxoplasma, tachyzoites can cross the placenta and infect the developing fetus, leading to congenital toxoplasmosis.

Similarly, these same tachyzoites can also be present in the mammary glands of nursing animals and can be transmitted to the offspring through milk. This highlights the significance of tachyzoites in both vertical transmission of the parasite (from mother to offspring) and the immediate health impacts it can have on young animals.

Other stages of Toxoplasma do not facilitate the same type of transmission. Cysts, for instance, are not actively transmitted but remain dormant forms found within host tissues. Sporozoites, found in oocysts, are involved in the initial infection process when the oocysts are ingested, but they don't directly transmit through the placenta or milk. Bradyzoites are also slow-growing forms encased in