The prohibition of consuming worms found inside a living creature is discussed in the following passage from the Gemara in Chulin (67b):

Rav Sheishes the son of Rav Idi said: Kukiani are forbidden. Why? Because they originate outside [the creature in which they were found]. Rav Ashi challenged him: If they originate outside, they should also be found in the digestive tract!

The Gemara cites an alternative version of the same statement:

Rav Shisha the son of Rav Idi said that kukiani are permitted because they originate inside the animal, and Rav Ashi responded: That is obvious, because if they originated outside the animal, they would be found in the digestive tract.

The Gemara sums up:

The halacha is that kukiani are prohibited. Why? Because while the animal sleeps, they enter through its nostrils.

In this passage, the Gemara deals with a type of worm known as kukiani, which Rashi identifies as worms found in the liver and the lungs of an animal. (In Maseches Shabbos 119b, Rashi defines kukiani as worms found inside the intestines, but the Gemara here indicates that it is referring specifically to worms found in the liver and lungs but not in the intestines. A third view, that of Rabbeinu Gershom, maintains that kukiani are found in the animal’s head.) Tosafos maintains that the sugya is referring to worms found in a fish. Even according to Rashi, says the Beis Yosef (Yoreh Deah 84), the definition of kukiani as worms found in animal liver and lungs is only an approach to this sugya, but Rashi agrees that the prohibition would apply equally to worms found in fish.

The underlying principle of the Gemara is that the Torah prohibits the consumption only of worms that originate outside of the animal, but those that originate inside an animal or fish are permitted. Rav Ashi attempts to prove that kukiani must originate within the animal, for if they enter from outside, that would mean that they are ingested along with the animal’s food. Since kukiani are not found in the digestive tract, it is clear that they are not ingested, which means that must originate within the animal and are therefore permitted. The Gemara ultimately rules that they are prohibited, however, and disregards their absence from the digestive tract because they can enter the animal through its nose (or ‘ear’, according to a different version, which would apparently mean its gills, since fish do not have ears) and proceed from there to the trachea, lungs and liver.

The Gemara goes on to discuss another type of worm:

Darni [worms found between the skin and flesh—Rashi] are prohibited in an animal and permitted in fish. Ravina said to his mother, “Hide them within the fish so I cannot see them, and I will eat them” [i.e., they are permissible for consumption, but since Ravina was disgusted by them, he asked her to conceal their presence so he would not be in violation of “bal teshaktzu” when he ate them; see Pri Chadash, Yoreh Deah 83:3].

Rav Mesharshia the son of Rav Acha asked Ravina: “How is this different from the Beraisa that states that the verse ‘You shall detest their carcasses’ includes ‘darnim’ that are found in an animal [i.e., why are they permissible when found in fish]?

He answered: “It is not comparable. An animal becomes permitted when it is slaughtered, and the slaughter does not affect the worms, so they remain prohibited. Fish, however, are permitted when they are gathered, and the worms thus grow in a place where they are permitted [i.e., they grow inside the fish rather than originating outside of it].”

Based on these passages, we can identify three types of worms: (1) Darni, which are found between the skin and the flesh of a fish, are permitted for consumption because they grow within the fish. (2) Worms that are found in the intestines of a fish, on the other hand, are definitely prohibited, because we assume that they originated outside of the fish and were ingested along with its food. (3) The Gemara adds that kukiani are also prohibited, even though they are not found in the digestive tract, because they are assumed to have entered through the nostrils or ears

Is migration between organs possible?

From a careful examination of the Gemara, it should be clear that the Gemara is not referring at all to the type of worms known as Anisakis worms. The sugya focuses on worms that are found only in other internal organs and not in the digestive tract at all, which is why the Gemara had to develop the explanation that kukiani enter a fish or animal through orifices other than its mouth. The Gemara implies, however, that worms that are found in the digestive tract are definitely prohibited even if they are also found in other organs. The Anisakis worm is found both in the intestines of the fish and in its other internal organs. The Anisakis worm should therefore be prohibited even if there are no grounds to assume that it enters the fish through other orifices—because its presence in both places indicates that it enters the digestive tract and then travels to other organs.

The notion that a worm can migrate from organ to organ within a fish has been heavily challenged by those who wish to permit fish infested with Anisakis. But the Gemara itself is a clear indication that migration is possible! The Gemara states that kukiani are permitted only because they are not found in the digestive tract, which implies that if they did appear in the digestive tract, even kukiani found in other organs would be prohibited. If migration was impossible, how would they be found in other organs?

The matirim argue that the Gemara’s question is that kukiani should be found exclusively in the digestive tract since they are ingested along with food, and the Gemara’s answer, that they enter through the ears or nostrils but not by migration between the various internal organs, seems to indicate that they cannot travel within the body of the fish. If migration was possible, why would the Gemara need to explain that they enter through other orifices?

This definition contradicts the facts on the ground, as is obvious to anyone with experience in the field of kashrus, all of whom know that it is possible for worms to travel from one internal organ of a fish or animal to another. Sefer Temunei Chol describes the various passageways within the body of a fish through which a worm might travel even without piercing a noticeable hole in its flesh, and the Anisakis worm is particularly thin and flexible, making it even more capable of moving from organ to organ. Furthermore, we often encounter Anisakis worms that are partially or completely embedded in the flesh of a fish, and when we remove the worms, the flesh closes upon itself, leaving no indication that the worm was ever there. This also supports the supposition that the worms travel between different parts of a fish without leaving any marker.

It appears, then, the Gemara was asking simply how it is possible that kukiani are not found in the digestive tract at all, to which it responds that they enter the fish through a different orifice.

If it is possible for worms found in the flesh to have entered the fish through one of its orifices and then penetrate the flesh, why would the Gemara permit worms found in the flesh of a fish? Perhaps the species that the Gemara calls kukiani, which is not a known species today, is incapable of traveling out of the digestive tract of the fish. Furthermore, the worms that the Gemara permits are found between the skin and the flesh, and even Anisakis worms are not found there. It appears, then, that the Gemara permitted only specific worms when it is clear that they could not have originated outside the fish.

In order to avoid the conclusion that Anisakis worms travel between organs of the fish, some have advanced the possibility that the Anisakis worms found in other parts of a fish may be a different species than those found in the digestive tract, and are therefore permitted. The Shevet HaLevi and other gedolei hador have written that since the worms that are found in the digestive tract appear identical to those found in other parts of the fish, we can conclude that they are the same species of fish, and therefore prohibited.

Worms found outside the digestive tract

Another argument advanced by the matirim is that only a worm found in the digestive tract is considered to come from outside the fish, but those found in the cavity outside the digestive tract can be assumed to have originated there and are therefore permissible. This approach is very difficult. First of all, since we have observed that worms travel freely throughout the digestive system or circulatory system of a fish, there seems to be no grounds to make such a distinction. Secondly, the Shut Beis Ephraim (Yoreh Deah sec. 25) infers from the wording of the Rashba in Toras HaBayis that even worms that are found in the area surrounding the digestive tract, but not actually in the tract itself, are prohibited.  (See Shut Lehoros Nassan vol. 9 YD sec. 24, who cites the Beis Ephraim and concurs with his understanding, citing additional support from the Or Zarua.) My good friend Rabbi M.M. Karp shlita also pointed out that the Lechem Yehudah on the Rambam (Maachalos Asuros 2:17), as well as the Shut Mateh Reuven (167) have the same understanding.

In fact, the consensus of the Acharonim is that worms found on the spine of a fish or on its eggs are prohibited. Certainly, worms such as the Anisakis, which are found in the digestive tract of a fish, should be prohibited even when they are found in other parts of the fish. Regardless of where they are found, the worms can be assumed to have entered the fish along with its food. Even secular scientists agree with this assumption.

Further support for this position can be drawn from the words of the Beis Yosef (Yoreh Deah 84, s.v. kol hatolaim): “It appears that Rabbeinu Tam prohibited only [worms] found in the intestines of fish and similar organs, since they can be assumed to have originated outside.” The Beis Yosef’s reference to “similar organs” appears to allude to any part of the fish that is comparable to its intestines, or that the worms could have penetrated from its digestive system into those “similar organs”.

An even more compelling inference appears later in that passage of the Beis Yosef: “It seems to me that according to Rashi’s version, worms found in the intestines of fish are also prohibited, because they might have come from outside and entered through its nostrils.” On the surface, the Beis Yosef’s words are puzzling. If the worms entered the fish through its nostrils, why would they be found in the intestines? Worms that are found in the intestines are likely to have entered through its mouth! It appears, then, that the Beis Yosef maintains that the actual stomach cavity of the fish is also viewed like its intestines, and any worms found there are prohibited since they are likely to have entered through the fish’s nostrils.

In short, the Gemara indicates that worms that are found between the skin and the flesh of a fish are permitted because they are assumed to have originated there, whereas worms that are found in the internal organs are prohibited because they are assumed to have entered the fish from without. If a certain species of worm is found both in the fish’s digestive system and in other internal organs, it is prohibited. (In truth, the Rambam appears to prohibit even worms that are found between the fish’s skin and flesh, but the rest of the Rishonim permit those worms and prohibit only those that are found in the digestive system itself, or are of the same species as worms found in the digestive tract.) In my book Tolaas Shani, I cited the wording of various Rishonim who emphasize that worms found in the flesh of a fish are permitted only on the grounds that they are assumed to have originated outside of the fish, and not due to some blanket principle that permits any worms found in that part of the body. This is also the reasoning behind the rulings of the Shulchan Aruch (Yoreh Deah 84:16) and the Rema (ibid. and in Toras Chatas, Dinei Tolaim 47-48.) Thus, if a species of worm (such as the Anisakis) is found both in the flesh and in the digestive tract, indicating that it originated outside the fish, it should be prohibited.

A Halachic Anomaly

According to those who maintain that Anisakis worms are permitted for consumption, there appears to be a halachic anomaly: the very same worms that are prohibited while they are in the digestive tract become permitted merely by virtue of the fact that they have moved to a different part of the fish! One of the rabbanim who permits the worms justified this anomaly with the contention that even the Anisakis worms that are in the digestive tract are prohibited only due to a safek as to whether they are truly a prohibited species of worms. He adds that only worms that grow inside the fish can penetrate different parts of its body and travel from one body part to another. Consequently, when Anisakis worms are found in other parts of a fish, their very presence there indicates that they are not a prohibited species of worms.

This contention is difficult to accept. First of all, the Gemara’s debate revolves around the question of whether the worms originate inside or outside the fish, not whether they have the ability to penetrate the layers of its flesh. Furthermore, the Gemara entertains the possibility that the worms enter through the ear of the fish. If they do enter through the ear and end up in the liver, they would have to penetrate various organs in order to get there. Thus, according to the Gemara’s ruling that worms found in the liver are prohibited, the inescapable conclusion is that prohibited worms also have the ability to bore through the organs and flesh of the fish.

Other Arguments of the Matirim

Some of the matirim question the assumption that the worm originates outside the fish rather than within it. In truth, since the worm is found in the actual digestive system of the fish, it should be evident that it is ingested along with whatever the fish eats and that it actually originates in the sea. Furthermore, in the Norwegian sea that is heavily populated with Anisakis-infested fish, experiments have shown that fish bred in closed cages within that sea are found to be completely free of Anisakis worms. This is a clear indication that Anisakis worms originate in the sea itself and enter the fish from without.

Another issue that has been raised is the size of the worm when it enters the fish, because only worms that can be detected with the naked eye are prohibited.

First of all, since the worm is found in the fish’s digestive tract itself, its size upon entry seems to be immaterial. Although a sheretz must be large enough to be visible in order to be prohibited, it stands to reason that the worms should be prohibited since they are visible upon consumption and it is clear that they entered the fish from without. The size of those worms when they enter the fish should therefore be irrelevant.

Furthermore, scientific tests have revealed that Anisakis worms are larger than one centimeter. On rare occasions, they are found to be smaller (such as in very small sardines), but they are usually no smaller than half a centimeter. Even on the extremely rare occasions when they were found to be even smaller than that, they have never been found to be of microscopic size. (These scientific tests are generally conducted in a laboratory, where microscopic worms would be discovered.) Thus, there is ample evidence, which is accepted by scientists as well, to indicate that the worms are not microscopic in size even when they enter the fish—although, as we have already established, their size at that time is not relevant with regard to the halacha.

The matirim of the Anisakis worm raise another issue: at what point in time does it enter the flesh? This issue, as well, really has no bearing on the halacha. As long as we know that the worm enters the digestive tract from outside, and from there penetrates the flesh, it is irrelevant when the penetration takes place. Nevertheless, scientific observation, along with our own experiments, have shown that a fish that is cleaned and filleted as soon as it is caught and removed from ice water tends to be free of infestation, while fish that are processed more slowly have a greater tendency to be infested. This is an indication that in fact, the worms penetrate the flesh of the fish only once the fish has died. Thus, while the worms would be prohibited even if they penetrated the fish while it was still alive, the fact that they enter the flesh after its death makes for an even more powerful argument that they should be prohibited.

Which Worms are Permitted

As we mentioned, the kukiani of the Gemara are not the Anisakis worm, nor is the worm itself mentioned in the Gemara. The Shulchan Aruch clearly rules that worms found in the flesh of a fish are permitted, without making a distinction between those that are also found in the intestines and those that are not. Some have cited this as evidence to permit the Anisakis worms found in the flesh. However, the Anisakis worm is not found in Africa or in Bavel, and is extremely rare in Eretz Yisrael. Thus, its omission from the Gemara and Shulchan Aruch should not be surprising, since neither the Amoraim nor Rav Yosef Caro could have been aware of its existence. Furthermore, due to the fact that the methods with which fish are caught and processed today are dramatically different from those of yesteryear, it is much more likely to find worms in the flesh of a fish than it was in previous generations, even in locales where the worms did exist. Nor should one wonder which worms are permissible according to the Gemara and consider the possibility that the Anisakis fit into that category, because scientists are familiar with species of worms that exist exclusively in the flesh of fish and do not penetrate its internal organs. One example is the kudoa (Microsporidium feiferi), which resembles a grain of rice and is primarily found in cod in the area of Argentina.

Scientific evidence also contradicts the argument raised by some matirim that it is impossible for there to be two distinct species of worms that inhabit the flesh of fish, and there should therefore be a blanket heter for all worms found in fish flesh. Scientific observation has ascertained that there are indeed two distinct species, one of which also appears in the intestines while the other does not.

Others suggest that a DNA test would reveal that the Anisakis worms found in the flesh are not identical to those found in the digestive tract, and that those found in the flesh are therefore permissible because they originated there and did not migrate from the digestive tract, we have observed the actual penetration in fish such as herring, cod, sea bass, and flounder. As for salmon, although we never actually observed the worm traveling from a salmon’s stomach to its flesh, we have encountered hundreds of such worms that are embedded halfway into the flesh, which supports the contention that they do enter the flesh from the stomach. Furthermore, the worms found in salmon are found almost exclusively in the portion of its flesh covering the intestines, not in the upper part of the fish. According to those who maintain that the Anisakis worms originate within the flesh, why would they not travel throughout its body, even to the outer layers? It appears, therefore, that the worms enter the fish through its digestive system, and they succeed in penetrating only the softer part of the flesh surrounding the stomach.

And even if we speculate that the worms found in the salmon’s flesh might be a different species – and this hypothesis could potentially be corroborated by scientific testing – as of now the evidence indicates that they are members of the same species, just as in other fish. The default assumption must therefore be that the worms are prohibited, and any contention that they should be permitted has to be supported by evidence. Even if a greater quantity of worms is found in the flesh than in the stomach, there are many logical explanations to account for that, and it should therefore not be used as basis for a heter.

Canned Salmon

Some rabbanim contend that canned salmon should be permissible in any event, because Anisakis worms are never found in canned salmon, leading to the conclusion that any worms in the salmon are ground during the canning process or disintegrate when they are cooked. While there is room to discuss the halachic validity of their argument, we conducted our own experiment in which we found entire whole Anisakis worms (albeit somewhat softer than normal) in cans of salmon. Furthermore, we conducted another experimented in which we cooked pieces of salmon and still found whole worms even after they had been cooked. Furthermore, attempts to cook an Anisakis worm by itself have revealed that it does not disintegrate. When we cooked a worm-infested salmon for a much longer time than it is normally cooked before eating, we discovered that the salmon itself had crumbled, making it exceedingly difficult to check for worms, but a whole worm was still discovered in the fish.

The Need to Suspect Infestation

Regardless of the debate over the kashrus of the worms themselves, I have often been asked why the fish are themselves not permitted due to a sfeik sfeika, since there is a doubt as to whether any particular fish is infested, and even if it is infested, the worms themselves are prohibited only due to a safek as to whether they entered it from outside (as per the Sifsei Daas). I responded that the Sifsei Daas is referring only to the Gemara’s prohibition of worms that are found in other organs, which might have entered through the fish’s nostrils or ears. Only those worms are considered a safek, whereas worms that are found in the digestive tract, such as Anisakis, are definitely prohibited.

Furthermore, the second safek is also not always applicable, since fish that are brought from certain locales are invariably infested, such as wild salmon, fish from the area of Alaska, and most fish that are caught in Norway. If a fish is thoroughly cleaned and its intestines removed soon after being caught, there is room to debate whether it can be considered a safek. Some fish, such as salmon and sole, are invariably assumed to be infested because more than fifty percent of such fish contain worms. Others are given the status of miut hamatzui (frequently infested), or an even lower degree of infestation, such as Argentinean cod, filleted herring, and so forth. Such fish may be considered a safek, and there is room to view them leniently when that safek is coupled with another safek, such as if the fish is sliced into small pieces and the worms might no longer be whole. This is often the case with the herring that is marketed today, and since the process of checking herring for worms is an arduous process that must be conducted with a particular type of light, there is room for leniency in this respect.

Rabbi Shneur Zalman Revach heads the Machon L’mitzvos Hatluyos Ba’Aretz, and a leading figure in researching many halachic issues. This article is based on tshuvos penned by Rabbi Revach in Hebrew, which were translated by Kolmus staff.