A reader, Joseph, submitted the following comment, which I thought worthy of publishing as a post:
Regarding the claim by R. Bleich that there are no poskim who subscribe to R. Slifkin's thesis regarding halachos based on mistaken factual premises, this is certainly mistaken. Just to give one example, Rav Hershel Schachter, in a recent Q&A session in London (in front of at least 50 people), discussed this exact case, and made reference to the Dor Revi'i. I asked him about the anisakis question, and he said that Chazal believed in spontaneous generation, in accordance with the common belief in that era, and that this is the basis for why they allowed anisakis worms.
He went on to say that despite the fact that we now know this belief to be false the halacha stands, because, as the Dor Revi'i explains, Chazal had the authority to establish the halacha for all generations. He said that the position some report in the name of Rav Elyashiv, namely that worms in the past used to spontaneously generate, and hence were muttar, but have since stopped doing so, and are now assur, is a 'joke'.
Rav Schachter also mentioned that when R. Moshe Feinstein was asked this question, he refused to discuss it, and said that it was ridiculous that anyone should even ask about something that the Shulchan Aruch explicitly permits.
I would add that there are numerous areas of halacha that are based on scientific premises that are now know to be invalid, such as the various se'ifim in Shulchan Aruch allowing certain worms in fruit or cheese for consumption, or the rules of shabbos which allow putting uncooked food in a kli sheni full of boiling water, or many of the dinim of 'belios' in kashrus, and yet we generally do not suggest we should change the established halacha.
To go back to the example of killing lice on shabbos, if one does not accept the apologetics that Chazal would have allowed this even if they understood how lice reproduce (which R. Slifkin provides cogent reasons for rejecting), then there is no different between this case and that of the anisakis. In both, one is subscribing to the codified rule to do something which would be considered forbidden according to the principles that Chazal were using to propagate that law.
A parallel for this can be found in the case of 'okimtos' that the gemara makes on tannaitic statements, even though these explanations are often plainly not in accord with the underlying reasoning behind a given Tanna's ruling. To quote R. David Foldes' elucidation of Rav Shlomo Fisher's drasha on the topic, "the kabalah that later generations would not argue on the Sages of the Mishna, which is mentioned by the Kesef Mishneh, is only regarding the formal halacha but not regarding content and rationale. The amoraim frequently add svaros (rationales) and derashos (homiletics) to the teachings of the tannaim, and similarly they can argue with the legalistic logic applied by the tannaim. The amora is thus bound by the formal teaching of the Mishna, which he cannot dismiss completely. In case he disagrees with the tannaitic rationale (which he is allowed to) he may accept the ruling of the Mishna in a very specific case, and maintain his own ruling as the principle. As with the biblical covenant regarding mitzvos derabanan, which differentiated between biblical and rabbinic law, so too here the kabalah extends only as far as the terms of the original agreement, namely the formal acceptance of the final rulings of the tannaim."