The 48 confirmed cases in western Kent County are among members of the Amish community.
There is no information on the ages of the cases, or whether any required hospitalization.
The first symptoms of pertussis usually develop within 7 to 10 days after being exposed, however, it can take up to 6 weeks.
The early signs can include runny nose or congestion, sneezing, and a mild cough or fever. Severe coughing can begin 1 to 2 weeks after the illness starts.
In infants, there may be no cough, though they might develop apnea - a pause in their breathing.
The Delaware DPH is working closely with the Amish community to offer vaccinations, treatment, and information.
It is also advising the general community to be on alert for whooping cough, because of an increase in cases across the country in recent years.