By Melissa Chan
May 12, 2016

Dogs bit more than 6,000 on-duty mail carriers in the country last year, the U.S. Postal Service said Thursday, noting an increase in attacks from the year before.

Most of the incidents in 2015 took place in Houston, San Diego, Cleveland, Chicago and Dallas, according to the Postal Service, which released a ranking of the top 30 cities where attacks were reported.

About 6,500 employees were attacked by dogs last year as they walked their route, U.S. Postal Service spokesman Mark Saunders said, up from about 5,800 in 2014. The city with the most dog attacks was Houston, with 77. Those attacks represented a 22% spike in dog attacks in the city from last year, officials said. In other cities like Columbus, Ohio, dog attacks on mail carriers almost doubled.

Read More: The Post Office Releases its List of Worst Cities for Dog Bites

“Dogs are protective in nature and may view our letter carriers handing mail to their owner as a threat,” Saunders said in a statement. “With more packages in the mail, our letter carriers are more exposed to attacks when they bring parcels to your front door.”

There are about 4.5 million dog bite incidents each year in the U.S., although half of them occur inside homes, according to the Centers for Disease Control and Prevention.

To protect its workers, the Postal Service advises pet owners to temporarily place dogs in a closed room before opening the front door to receive packages.

Contact us at editors@time.com.

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