State health officials say whooping cough is taking a stronger hold in Iowa, but they’re not sure why. Dr. Cort Lohff is assistant state epidemiologist. He’s asking more doctors across Iowa to be vigilant and prevent more cases from occurring.Dr. Lohff says there’ve been nearly 400 cases of whooping cough reported across Iowa in the past three years. That’s more cases than in any three-year period since the 1960s — though the cause of the increase is still unclear. He says it’s most severe in young infants.Lohff says sometimes doctors don’t think of whooping cough, known as pertussis, when a patient has a persistent cough.At least 67 confirmed cases are reported statewide this year with more than 50 unconfirmed.