Although the Mishnah (daf, or page, 42a) teaches that an animal that is found to have a hole in its lung will be deemed a terefah, there are a number of circumstances where we may assume that there are other circumstances that caused the hole to appear after the animal had already been slaughtered. If we are convinced that there is a good likelihood that the hole was caused by such factors, the animal will be declared kosher. Thus, if we know that the butcher had dealt with the lungs in a manner that would likely have torn them, we can assume that the animal is kosher. The Rema (Shulhan ArukhYoreh De’ah 36:5) adds that this is only true if the appearance of the hole corroborates that assumption. If, however, the indications are that the hole existed while the animal was alive, e.g., it was perfectly round or the flesh around the hole was discolored, then we could not make an assumption that would permit the animal.
Another such situation is when we see that the hole in the lung was caused by a parasite, like a morena. Must we assume that the parasite ate its way through the lung while the animal was still alive, rendering it a terefah – or can we rely on the fact that the parasite first made the hole after the animal had been slaughtered? The Gemara concludes that we can rely on the fact that such a hole was made after the animal had been killed. Rabbenu Yehonatan and the Me’iri explain that in this case we know that these parasites do not leave the body of their hosts until they have died, which is why we can say with some level of certainty that the hole first appeared after slaughter.
It is common for animals to act as hosts to a number of different types of parasites, many of whom live as adults in the intestines of the animal, where they lay eggs. From the digestive system the eggs or newly hatched worms make their way to other internal organs of the animal where they develop. Such parasites make their way from the intestines to the lungs by way of the stomach and/or blood vessels. The morena – or, in Latin, muraena – is one such parasite.