Survival Rate of <i>Toxoplasma gondii</i> Tachyzoites in Cow's Milk at Different Temperatures

Toxoplasma gondii is the causative agent of toxoplasmosis, an obligate intracellular parasite of warm-blooded animals; the definitive host is cats and felines. Transmission of this parasite in herbivorous intermediate hosts occurs through contaminated water and forage by the oocyst stage of the parasite, and in cats and humans, it occurs through eating contaminated meat and milk by the cystic stage, tachyzoite and oocyst of the parasite. Some people consume the milk of various animals, including cows, sheep, goats, camels and donkeys. Such kinds of milk are supposed to be a good source of protein and vital minerals. Nonetheless, they are frequently ingested uncooked or not heated with high temperatures to destroy dangerous microbes. Hence, the potential viability of T. gondii tachyzoites in the milk of these animals under various temperatures needs to be investigated, as this could be a significant risk indicator of human infections. For this purpose, purchase 1000.0 mL of cow's milk from milk supply centres and then divide it into different parts, and after adding T. gondii tachyzoites of the RH strain (10⁷ × 5) to each milk sample, they are subjected to rapid pasteurization temperatures (75°C for 15 s), slow pasteurizations (60°C for 15 min), temperatures of 25°C and 35°C for 3, 6 and 12 h and refrigerator temperatures (4°C for 6, 12 and 24 h), and after centrifugation, the resulting precipitate was injected three times into three mice. One mouse without parasite tachyzoite injection was considered a negative control, and three mice injected with parasite tachyzoites were considered as positive controls. Our results showed that in the first trial, T. gondii tachyzoites treated at 75°C, 60°C, 35°C, 25°C and 4°C survived after 15 s and caused the death of mice. However, in the second trial, at 75°C, the parasite tachyzoites were completely eliminated after both 15 min and 3 h, and all treated mice survived. In contrast, in the third trial, at 4°C, 25°C and 35°C, after 15 min, 3, 6, 12 and 24 h, the parasite survived and caused the death of mice. Statistical analysis showed that the mortality of mice treated at 75°C and 60°C during the treatment periods was significant. This study showed that milk possibly contaminated with parasitic tachyzoites can survive in rapid and slow pasteurization and refrigerator temperatures, suggesting heating milk at 60°C for more than 15 min is recommended.