Ask the rabbi: May one kill lice on Shabbat?

Matters become more complex on Shabbat, when it is biblically prohibited to kill any creature unless it poses an acute health danger.

Rabbi 311 (photo credit: MCT)
Rabbi 311
(photo credit: MCT)
Treating hair infested with lice is never fun, yet parents remain responsible to promptly and thoroughly clean their children (even if this includes school absence!) and homes to prevent it from infesting others.
Matters become more complex on Shabbat, when it is biblically prohibited to kill any creature unless it poses an acute health danger.
Lice, however, might be an exception to the rule, and actually present us with a fascinating case of the tension between Jewish law and contemporary science.
After recording the general prohibition of killing creatures, the Sages (Shabbat 107b) asserted that one may kill lice since they “do not reproduce” but instead generate spontaneously from sweat (MB 316:38). The Sages adopted the widespread ancient belief, articulated by Aristotle and others, in “spontaneous generation,” which claimed that some life does not emerge from seeds, eggs or parents. Belief in spontaneous generation remained widespread until Louis Pasteur entirely debunked the theory in 1859, although skeptics had questioned aspects of the theory for two centuries beforehand.
The Sages asserted that the prohibition of killing on Shabbat only applied to sexually reproduced creatures. They similarly exempted asexually reproduced organisms from the biblical prohibition of consuming insects (Leviticus 11:10-11), and subsequently permitted consuming worms found within the flesh of fish or fruit (YD 84:16).
With regard to non-legal matters, many scholars, like Maimonides’s son, Rabbi Avraham (d. 1237), have stated that despite our reverence for the talmudic sages, their statements regarding medicine, natural science and astronomy remain entirely non-authoritative (Ma’amar al Aggadot Hazal). Matters become more complex, however, when faulty scientific assumptions were the basis for legal declarations.
No consensus has emerged regarding the status of these laws, with acrimonious debate occasionally dividing the scholarly world.
This controversy recently exploded over the widespread discovery of the Anisakis worm in the flesh of many popular kosher fish, including wild salmon, flounder, sardines and herring.
These worms are parasites whose eggs regularly travel through the aquatic food chain until they ultimately emerge in the organs and flesh of a large host fish.
Since the worms also appear in the cavity of the fish, and their origins are well-documented, contemporary decisors like Rabbis Yosef Elyashiv and Shmuel Wosner (Shevet Halevi YD 4:83) have asserted that one must assume these worms are not identical to the permissible creatures mentioned in the Talmud.
As such, their followers have produced lists of regularly contaminated fish to carefully inspect or avoid. This approach, in practice, accepts contemporary scientific analysis, while avoiding any claim that the Sages held errant scientific beliefs. Elyashiv has reportedly adopted a similar approach with lice, thereby prohibiting killing them on Shabbat (Teshuvot Shevet Hakehati 3:126).
Others, including Rabbi Yisrael Belsky of OU Kosher, have defended the continued consumption of these fish by citing numerous historical sources (Teshuvot Imrei Yosher 2:11) which indicate that decisors have been aware of such worms, yet continued to maintain the talmudic rule that insects found within the flesh do not affect the kashrut status of the fish (Shulhan Gavoha 84:51). Nothing has changed, asserts Belsky, and there is no reason to depart from the historical practice.
Of course, this approach does not address the fact that the entire dispensation for worms in fish flesh was based on a now-debunked scientific theory. Historically a few rabbinic figures denied contemporary science because they believed that was necessary to defend this talmudic law. This approach was criticized by Rabbi Yitzhak Lampronti (Italy, d. 1756), who asserted that the Sages were limited by the knowledge of their times. If Jewish law does not reform from its incorrect scientific assumptions, then people might violate Shabbat by killing lice (Pahad Yitzhak, Tzeida).
While Rabbi Yosef Kapach adopted this approach, Rabbi Eliyahu Dessler countered that the unalterable talmudic law was based on ancient traditions and not solely on scientific assumptions. He and Rabbi J. David Bleich (Tradition 38:4) have further suggested that the Sages based their assessment on the fact that the sexual reproduction of lice is unperceivable to the eye, and that contemporary findings do not alter that legal criterion.
Rabbi Natan Slifkin (Sacred Monsters) has recently advocated a different approach found in the writings of Rabbis Moshe Glasner (Dor Revi’i, Hakdama) and Isaac Herzog (Heichal Yitzhak OC 29). These figures acknowledged that contemporary science was correct and that the Sages errantly believed in spontaneous generation, but asserted that talmudic law had become canonized and should not be altered. Thus the dispensation to kill lice remains authoritative, even as we acknowledge that the initial scientific assumption behind the law is no longer true.
Even if one adopts a lenient position regarding killing lice, other prohibitions, such as pulling out hair or squeezing water from it, might preclude intensive treatment of lice infestation on Shabbat, and one should consult with their rabbi for further guidance.
The writer, online editor of Tradition and its blog Text & Texture (, teaches at Yeshivat Hakotel.