EL MONTE, Calif. — Los Angeles County health officials said Wednesday they are investigating a fifth locally acquired case of the mosquito-borne illness dengue, this time in a resident of El Monte.

The latest patient does not appear to have any connection with the four previous reported infections, according to the county Department of Public Health.

Health officials again stressed that locally acquired cases of dengue are extremely rare in areas where it has not been previously transmitted by mosquitoes. The county previously reported a cluster of three infections in the Baldwin Park area, and last week a fourth case was confirmed in a Panorama City resident. Health officials called the spate of local cases "unprecedented."

None of the local patients had any history of travel to areas where dengue is endemic.

In October and November of last year, single locally acquired cases of dengue were confirmed by health officials in Pasadena and Long Beach.

"This case further indicates that dengue can spread in our community," Dr. Muntu Davis, Los Angeles County health officer, said in a statement Wednesday. "Preventing mosquito bites and mosquito breeding is the best way stop local transmission of dengue. Mosquitoes thrive in hot weather, increasing the risk of bites and mosquito-borne diseases."

He urged residents to use insect repellent, remove standing water that can attract mosquitoes and install or repair window screens to prevent mosquitoes from entering.

Dengue is generally spread by a bite from an infected Aedes mosquito. Most patients never exhibit any symptoms, but one in four will develop signs including fever, nausea, vomiting, rash and aches and pains to the eyes, joints or bones.

The symptoms generally last no more than a week, and people usually fully recover within two weeks. Deaths from the virus are extremely rare, health officials said.

Health officials noted that the symptoms of dengue often mirror those of other viruses, and they urged health-care providers to be "vigilant for dengue fever in patients with acute febrile illness and test for and report such cases of mosquito-borne diseases."