(CNN) The Philadelphia Department of Health has declared a public health emergency due to a surge in Hepatitis A, Dr. Thomas A. Farley, Health Commissioner of the City, said Thursday.

So far this year, 154 people have tested positive for the highly contagious liver disease, with 85% of diagnoses occurring since May, the department said. Typically, the city sees between two and nine cases each year.

Preliminary information indicates 67% of people who have been diagnosed with Hepatitis A say they have used drugs, while 26% report homelessness, according to health officials. All but four of the confirmed cases have occurred in adults.

"While there's not an exact cause that we can pinpoint, Philadelphia had been in the grip of the opioid crisis, which the Health Commissioner had called the worst epidemic here in more than a century," James Garrow, spokesman for Philadelphia Department of Public Health, told CNN in an email.

Garrow said the bulk of cases have occurred among at-risk populations in the northern neighborhood of Kensington.

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