PHILADELPHIA (CBS)—Terminix has released their top 20 bed bug cities and Philadelphia has made the top 10.

The list includes cities across the United States and features the following five top cities: Cleveland, Cincinnati, Detroit, Las Vegas, and Denver. Four states scratched their way onto the list with more than one city: Ohio, Arizona, Pennsylvania and California.

Terminix says bed bug infestations have significantly increased since the late 1990s, and while researchers are unsure of the exact causes, factors such as increased international travel and insecticide resistance likely bear some responsibility.

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“Bed bugs are notoriously difficult to eradicate,” said Paul Curtis, director, technical services at Terminix, “and they can travel to new locations very easily—hitching a ride on clothing, handbags, suitcases and taxis, which can create substantial infestations in no time.”

Signs of an infestation include reddish-brown blood spots on sheets or mattresses, a strong musty odor, or sightings of the pests themselves, which are the size, shape and color of an apple seed.

The top 20 cities for bed bug infestations in the United States include:

  1. Cleveland, Ohio
  2. Cincinnati, Ohio
  3. Detroit, Mich.
  4. Las Vegas, Nev.
  5. Denver, Colo.
  6. Houston, Texas
  7. Phoenix, Ariz.
  8. Indianapolis, Ind.
  9. Oklahoma City, Okla.
  10. Philadelphia, Pa.
  11. Baltimore, Md.
  12. Pittsburgh, Pa.
  13. Washington, D.C.
  14. Tucson, Ariz.
  15. San Francisco, Calif.
  16. St. Louis, Mo.
  17. Atlanta, Ga.
  18. Tampa, Fla.
  19. Memphis, Tenn.
  20. 20. San Diego, Calif.

This list was created by compiling services data from more than 300 Terminix branches across the country.