Question 600: In the act of Intiqal where if a creature lands on you and sucks your blood, you can whack them and for some animals, the blood (Insect Blood) doesn’t make your skin najis, is that the same thing for lice?
Answer 600: There are number of things that clean other unclean things. 1. Water 2. The Land, 3. The Sun 4. Transformation (Istihala) 5. 2/3 Evaporation 6. Transfer (Intiqal) 7. Islam 8. Dependency (Taba’iyat) 9. Removal of the Actual Unclean Substance 10. Disappearance of a Muslim 11. Purification (Istibra) of animal which feeds on najasat 12. Draining of the usual quantity of blood from the slaughtered body of an animal.
As it is previously mentioned, transfer (Intiqal) is a cause of cleanness of an unclean substance. This means that if the blood of a human being, or of an animal whose blood gushes forth when its large vein is cut, is sucked by an insect, normally known to be bloodless, and it becomes part of its body, the blood becomes Pak. This process is called Intiqal. But when a blood-sucking leech sucks human blood during some treatment, it will be najis, because it is not considered as part of its body it is considered as human blood.
If one kills an insect (mosquito, lice or …) which has sat on one’s body, and blood which it has sucked comes out, it will be considered Pak, as it was destined to be its part.
If the time gap between its sucking and it being killed is very small that people saying it is the blood of human being or there is doubt about whether it is its blood or blood of human bineg it would be considered as Najis, according to maraja. But, Sayyid Sistani (ha) says: Even if the time gap between its sucking and it being killed be very small it was destined to be its part and then considered as Pak, however, as a recommended precaution, one should avoid such blood.
In Tahrir al-Waliah Imam Khomeini (ra) says: Transfer is a cause of cleanness of an unclean substance when transferred and added to another thing and is considered a part of it, as the blood of blood spurting animal is transferred to an animal not having a spurting blood, (when the blood of a human being is transferred into a mosquito).
The same rule shall apply if what is transferred is something other than blood and the object to which it is transferred is other than an animal, like a plant, etc.
If it is known that there has been no addition of the transferred thing being added, or there is doubt about it as it has not taken place in the stomach of the animal (whom it has been transferred), for example, in a way that it could be attributed to that animal, in such case the blood sucked by a leech shall continue to be unclean.
 . Tahrir al-Wasilah of Imam Khomeini (ra), Vol. 1, Rules conderning Mutahhirat; The official website of the office of Sayyid Sistani (ha), Mutahhirat » Introduction, issue 149.
 . The official website of the office of Sayyid Sistani (ha), rules concerning Mutahhirat » Transfer (Intiqal), issue: 210.
 . Tawzih al-Masael of Maraja (annotated by Imam Khomeini), Vol. 1, Pg. 126, Issue: 206.
 . The official website of the office of Sayyid Sistani (ha), rules concerning Mutahhirat » Transfer (Intiqal), issue: 211.
 . Tahrir al-Wailah of Imam Khomeini (ra), Vol. 1, chapter concerning things that clean other things (Mutahhirat).