Pinworm infection is usually benign, and 1/3 of those infected are asymptomatic. By far the most common clinical sign of pinworm infection is perianal or perineal itching, and the itching is usually the most severe at night. This is caused by the migration of female pinworms to the anus to lay eggs (specifically, insertion of the tail pin into the mucosa for ovideposition), and scratching leads to finger contamination and subsequently the spread of ovum to others. The scratching may also excoriate the skin and lead to secondary bacterial infections. Secondary symptoms, which are due to disturbed sleep caused by pruritis, include anorexia and irritability.
However, pinworm infection is not always harmless, as it has been implicated in causing appendicitis (as high as 2.39% of cases in developing countries), intestinal obstruction, intestinal perforation, enterocolitis mimicking Crohn's disease, and eosinophilic ileocolitis.
Extra-intestinal infections are also possible, and while most involve the female genital tract, infection of other sites has been documented, including the lungs, breast, liver, and spleen. These extra-intestinal infections may lead to pruritis vulvae, urinary tract infections, postmenopausal bleeding, epididymitis, pelvic mass, chronic sialoadenitis, and unilateral salpingitis, among others. The dead parasites and eggs deposited in ectopic sites can also be responsible for the formation of granulomas and abscesses.
One theory for the mechanism of infection of the female genital tract is that gravid female worms migrate from the perianal region to the vagina, where it may ascend through the fallopian tubes to the peritoneum. This theory is supported by the presence of only female worms and eggs via cervical smears, as well as in peritoneal granulomas.Another theory states that the pinworms may pass through intact intestinal wall to produce pelvic peritoneal granulomata.