Crab lice usually cause intense itching, due mainly to an allergic reaction to the saliva of the louse. There also may be tiny red or blue marks where the insect has been biting, and the eggs, or nits, may be seen attached to the pubic hair close to the skin. Sometimes, more severe reactions occur, such as the development of pustules or intense skin reactions where the insects have been feeding. Scratching the area may cause skin infection. Crab lice, themselves, do not transmit infections.