When Edward Kaplan’s kids were young, he created a chart to keep track of their encounters with strep throat.

Starting when the three siblings were 2, 6 and 8 years old, every sore throat launched their physician father into action. Using a supply of wooden tongue depressors and a stack of gel-filled petri dishes in the refrigerator, he would swipe cells from the sick child’s throat and take the sample to his lab for analysis.

His kids hated the experiment. “We joke about it now,” says Kaplan, an emeritus professor of pediatrics at the University of Minnesota Medical School in Minneapolis. “But they haven’t forgiven me for it.”

The results of his work, which appeared in the Pediatric Infectious Disease Journal in 1994, helped Kaplan make a case that doctors and public health officials should track cases and outbreaks as part of a coordinated strategy against strep.

The findings also offered a window into the patterns of strep infection. Between 1972 and 1976, Kaplan’s middle child had strep the most, with 11 cases over four years. His youngest had it the least, with just three episodes. Sometimes but not always, a case would ricochet from one child to the next — illustrating how common, and variable, strep can be during childhood.

Two decades later, some things haven’t changed. Strep throat remains common enough to be considered “an occupational disease of schoolchildren,” Kaplan says. And it continues to drive parents crazy.

My motivation to look into strep trends came from some recent reports by friends about seemingly strange strep experiences: strep in the summer, strep among parents, strep that seemed to come back again and again. One of my boys had strep — not in his throat but around his anus, a skin infection caused by the same type of bacteria that infects throats.

Could there be some underreported epidemic of strep?

Probably not, experts say. Although strep is common, there’s no evidence that it’s becoming more (or less) so. And even though strep tends to appear more often from late fall to early spring, it can hit anytime. Adults and very young children do get it, even as most cases occur in kids between ages 5 and 15.

Strep throat is caused by Group A streptococcus, a common type of bacteria that also causes scarlet fever, impetigo and other illnesses. It is responsible for as many as 30 percent of sore throats within the 5-to-15 age group and up to 15 percent of sore throats in everyone else, though cases are rare in children younger than 3.

Several million cases of strep throat and related illness occur in the United States each year, according to the Centers for Disease Control and Prevention. And the average American gets three cases of strep throat by age 13, according to one often-quoted estimate, Kaplan says.

One September, Kaplan started testing kids in a suburban Minneapolis third-grade classroom and found that by January, more than 80 percent had had a positive result for Group A strep.

Strep throat can cause symptoms such as fever, sore throat, painful swallowing and sometimes nausea, vomiting and abdominal pain. But penicillin still works: Strep has yet to develop resistance to antibiotics.

The real concern comes when the bacteria get into blood, lungs or muscles, leading to invasive illnesses such as necrotizing fasciitis and streptococcal toxic shock syndrome, or when it causes acute rheumatic fever, an inflammatory reaction that can lead to rheumatic heart disease.

Every year, according to the CDC, Americans get 11,000 to 13,000 cases of invasive Group A strep, causing between 1,100 and 1,600 deaths.

The developing world suffers a greater burden, and serious streptococcal disease kills about a million people worldwide every year, says James Dale, chief of the division of infectious diseases at the University of Tennessee Health Sciences Center in Memphis.

Because of the potential for complications, U.S. doctors recommend antibiotics after a positive strep test, even as many European doctors recommend waiting it out — a difference in practice, Dale says, that should provide helpful information as data on complication rates accumulate.

In the meantime, his group and others are working to develop a strep vaccine that could save lives and reduce use of antibiotics. It has been a challenging task, in part because strep is a complex enemy.

Studies have revealed more than 200 subtypes of strep A, each producing its own immune reaction. That variety makes it hard to develop a single vaccine, and it explains why a child might test positive for strep not long after recovering from a recent case: They may simply become infected with another type. Certain subtypes occur more frequently than others in different geographic regions and even in different schools, Kaplan says.

Strep’s ability to infect and reinfect is not the only reason that some kids face the dreaded throat swab again and again. Strep A also tends to stick around, and modern tests are sensitive enough to come out positive even if they detect just a few organisms.

When Dale and colleagues analyzed periodic throat cultures and blood samples from school-age kids over a two-year period, they found that kids commonly carry some strep organisms in the mucus of their throats for months after a 10-day course of antibiotics has cleared symptoms.

“Most experts think that if there is a recurrence of symptoms within a short period of time following treatment,” he says, “that represents a viral infection that just happens to be in a child carrying Group A strep.”

Further complicating matters, it is common to get strep but show no symptoms. In the same study, Dale’s team found that, in more than half of cases, infections occurred without any sign of illness.

People who get strep without symptoms aren’t necessarily safe from complications. After an outbreak of acute rheumatic fever in Utah in the 1980s, Dale says, only half of patients remembered having had a sore throat, even though that illness shows up only after a strep A infection.

Repeated exposure to strep A, with or without symptoms, seems to help build immunity and probably explains why adults get strep less often than kids do, Dale says. In unpublished results from previous vaccine trials, he and colleagues found that adults have developed antibodies to an average of seven or eight subtypes of strep A, even if they did not recall being sick with strep that many times.

Until a vaccine becomes available, Dale’s advice to strep-weary parents is to err on the side of caution.

“Maybe I took it too far when I had a young child,” he says, “but if I was concerned, I took the child to the doctor.”

Advice for parents

Not all sore throats are caused by strep. If throat pain is accompanied by a runny nose and cough, chances are that a virus is at work and it’s usually best to wait it out. But strep may be to blame if your child’s sore throat comes on suddenly and painfully along with symptoms such as fever, headache, stomachache, pain with swallowing or small, red spots on the roof of the mouth.

When in doubt, doctors say, bring the child in. Only a throat swab can show for sure if it’s strep.

For more, go to health.clevelandclinic.org and search for “strep.”