In Chullin 52b:
Or, in English:מר רב עמרם אמר רב חסדא דרוסת חתול ונמייה בגדיים וטלאים דרוסת חולדה בעופות מיתיבי דרוסת חתול נץ ונמייה עד שתינקב לחלל
Why the abdominal cavity? Rashi explains:An objection was raised: The clawing by a cat or a hawk or a marten [does not render trefah] unless the claw actually penetrated into [the abdominal] cavity.17
עד שתינקב לחלל - כקוץ בעלמא ועד שניקבו הדקין ומדשוי להו כקוץ בנקובה מכלל דארס שלה אין מזיק אפילו לעופות וכל שכן לגדיים וטלאים:It has to do with the venom, and its strength. Read through the whole gemara to get a better sense of it. But for a (seeming) explicit statement that there is venom involved, see this:
הא ההיא תרנגולת דהואי בי רב כהנא דרהט חתול בתרה ועל לאידרונא ואיתחיד דשא באפיה ומחייה לדשא בסיחופיה ואשתכח עלה חמשה קורטי דמא
הצלת עצמה נמי כהצלת אחרים דמי ורבנן זיהרא אית ליה ולא קלי זיהריה
But it once happened that a hen belonging to R. Kahana was being pursued by a cat and it ran into a room. The door shut in the face of the cat so that [in its fury] it struck the door with its paw. There were then found on it five spots of blood!21— When the attacked animal tries to save itself it is the same as when others are present to save it. But [does not this incident contradict the view of] the Rabbis?22— They maintain that it has venom, but the venom does not burn.23The "blood" is red venom, as Rashi explains -- ארס של חמש אצבעות:
(I might be able to "save" Chazal from maintaining this by distinguishing between the Amoraim and the setama de-gemara, but I will leave this alone for this post.)
Could the "venom" be bacterial infection, and 'cat scratch disease'? I don't see how this would account for finding five spots of 'blood' (=venom) on the door, in the case of Rav Kahana. And furthermore, on the next amud, 53a, we have:
Abaye said: We have it on tradition that clawing is only with the fore-leg, thus excluding the hind leg; that clawing is only with the claws, thus excluding the teeth; that the clawing must be intentional, thus excluding an unintentional act;5and that the clawing must be by a living animal,thus excluding the clawing by a dead animal.6But since you have already said it must not be unintentional, is it then at all necessary to say that it must not be by a dead animal? — It is indeed necessary for the case where the animal struck with its claw and it was immediately amputated. Now you might have thought that it discharges the poison at once when it strikes with the claw, we therefore learn that it discharges the poison only when it withdraws the claw.Does this strike you as Chazal discussing bacterial infection? It is speaking of discharging poison on withdrawal of the claw. Should the bacteria not infect the wound as it enters, but only as it exits? No. This is, rather, a conflict between a gemara and the metzius as we presently know it.
The Kuzari, in maamar 4, section 31, brings this as proof to Chazal's deep knowledge of science:
It is interesting how a 'proof' can so swiftly turn into a 'disproof'.
We have another "out" besides saying that Chazal erred in science. In other contexts, some folks try to claim that conflicts with Chazal's descriptions demonstrate that another animal must be under discussion. Or, they say this about a statement by a Rishon. This occurs in apologetics, such as in identifying the shafan, where it cannot be a hyrex, since a Rishon who never encountered a shafan described a feature which instead belongs to the, common in that place, rabbit. Or people reject the best candidate for the chilazon, because an aggadic statement of Chazal or a position of a Rishon does not fit with what we presently know about murex trunculus. So, in like manner, Chazal must have been speaking about other animals. It could not be a cat or a hawk, because we know that they don't have venom. It must, instead, be an otherwise unknown species.
Perhaps there are other resolutions out there. This is not something I have spent a good time researching.
Note: Right before publishing this to the web, I saw that Rabbi Slifkin posted this morning on this very topic, on Rationalist Judaism.