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Local Risk for Zika Virus is Very Low

Alameda County Public Health Official and Washington Hospital Infectious Disease Specialist Note that Zika Cases in California are all Associated with Travel Outside the U.S.

“While the Zika virus has become a public health issue of international concern, the risk for contracting Zika virus disease in Alameda County remains very low,” says Erica Pan, MD, MPH, FAAP. Dr. Pan is the director of the Division of Communicable Disease Control and Prevention and deputy health officer of the Alameda County Public Health Department.

Zika virus was first discovered in 1947 and is named after the Zika forest in Uganda. Zika virus disease is spread primarily by Aedes aegypti mosquitoes (also known as yellow fever mosquitoes) or Aedes albopictus mosquitoes (also known as Asian tiger mosquitoes) that are infected with Zika virus. These are the same mosquitoes that spread dengue and chikungunya viruses. Zika virus is not spread through casual contact such as touching or hugging an infected person, but the Centers for Disease Control and Prevention (CDC) has cited evidence that men can spread Zika virus to their sex partners.

“So far, all of the confirmed cases of Zika in California have been related to travel outside the United States,” Dr. Pan notes. “The Zika virus is much more common in tropical and subtropical areas in Africa, Southeast Asia and various islands in the Pacific Ocean. Recent outbreaks of Zika in the Caribbean and in Central and South America have increased the concern about the disease, particularly because there is a possible association between Zika and potential birth defects, including microcephaly – an abnormally small head and brain. More studies are needed to evaluate this potential relationship between Zika and microcephaly and other adverse fetal and infant outcomes.”

As of March 23, 2016, the CDC reported 273 cases of Zika in the United States, none of which were acquired via mosquito bites in the U.S. Of those cases, 19 are pregnant women, and six of the cases were sexually transmitted.

The California Department of Public Health (CDPH) reports that as of March 25, there have been 22 cases of people in California with confirmed diagnoses of Zika virus disease in 2015 and 2016. All of the cases have been travel-associated, according to the CDPH. The majority of the cases (14) have been in southern California counties. In northern California, there were three cases in Contra Costa County, one case in Alameda County, one case in Napa County, one in San Francisco, one in San Joaquin County and one in Yolo County.

One case in San Diego reportedly is a woman who was infected via sexual contact with an infected partner who returned from traveling in the South American country of Colombia. The case in Napa County is a pregnant woman who contracted the virus while traveling in Central America.

“The number of confirmed cases in California is expected to increase as people become more aware of Zika and seek testing,” Dr. Pan says. “Zika is actually a very mild disease for adults, and only about 20 percent of people with Zika will have any symptoms. Our main concern is for pregnant women or women who want to become pregnant because of the potential risks to the fetus.”

The CDC reports that the most common symptoms of Zika include fever, rash, joint pain and conjunctivitis (red eyes). In most known cases of likely sexual transmission, the men had Zika symptoms, but the virus can be transmitted before, during and after symptoms develop. The virus remains present in semen longer than in blood.

“Aedes mosquitoes have been found in 12 California counties, mostly in the southern part of the state and the Central Valley,” says Dianne Martin, MD, an infectious disease specialist and co-chair of the Infection Prevention and Control Committee at Washington Hospital. “Fortunately, here in the Bay Area, we do not have the climate for these types of mosquitoes to thrive because of our cooler weather. Nevertheless, it makes sense to take steps to avoid mosquito bites, especially because West Nile virus – which also is transmitted by infected mosquitoes – is still a concern in the Bay Area.”

CDPH recommendations for protecting yourself from mosquito bites include:

  • Use insect repellents containing DEET, picaridin, IR3535, oil of lemon eucalyptus, or para-menthane-diol for long-lasting protection
  • If you use both sunscreen and insect repellent, apply the sunscreen first and then the repellent • Pregnant women and women who are breastfeeding can and should choose an EPA-registered insect repellent and use it according to the product label
  • Wear long-sleeved shirts and long pants
  • Use air conditioning or window/door screens to keep mosquitoes outside
  • If you are not able to protect yourself from mosquitoes inside your home or hotel, sleep under a mosquito bed net
  • Help reduce the number of mosquitoes outside by emptying standing water from containers such as flowerpots or buckets

According to the CDC, most mosquitoes bite in the early evenings or near dawn, but Zika-carrying mosquitoes bite mostly during the day. In addition, these mosquitoes do not breed out in the open near ponds, lakes or wetlands like most mosquitoes. Instead, they are “city-slickers” that prefer to breed in manufactured containers such as birdbaths, buckets, barrels, kiddie pools and old tires.

In addition to avoiding mosquito bites, Dr. Martin recommends restricting your travel to areas where the Zika virus is spreading rapidly, if possible.

“If you have traveled to areas where Zika virus is common, such as Brazil, Puerto Rico, the U.S. Virgin Islands and American Samoa – especially if you are pregnant – you should ask your doctor whether you should be tested for the virus,” she emphasizes. “Men who have traveled to those locations and have experienced Zika symptoms also should be tested if they have female partners who are pregnant or intend to become pregnant. We really don’t know yet how long the virus remains in the blood, but most sources say about 28 days.”

Previously, blood tests for Zika had to be sent to the CDC for processing, and it could take two to three weeks to get results. As of March 11, however, the CDPH lab in Richmond has been authorized to process Zika tests, producing results more quickly for residents of the Bay Area. A physician referral is required for testing, and currently all testing is routed and approved through local public health departments.

“Microcephaly is a rather nonspecific condition, and it could be due to a variety of causes,” says Dr. Pan, who is also a pediatric infectious disease specialist. “There is increasing evidence, however, that points to the Zika virus being a potential cause of microcephaly and other problems for the fetus throughout gestation. A recently released report on nine pregnant travelers who acquired Zika showed that two of them had early miscarriages, and three had babies born with microcephaly. Obviously, there is increasing evidence for concern, and further study is needed.”

Dr. Martin echoes the concerns expressed by Dr. Pan.

“We do know that Zika virus can be present in semen longer than in blood, but we do not know how long the virus is present in the semen of men who have had Zika,” says Dr. Martin. “Therefore, out of an abundance of caution, the CDC recommends avoiding pregnancy and to abstain from sex or use condoms during sex for at least six months after the onset of symptoms if the man has tested positive for Zika to help avoid transmitting the Zika virus to female partners. In addition, if the man’s partner is a pregnant woman, he should use condoms consistently and correctly or abstain from sex for the duration of the pregnancy.”

Learn More

For more information about the Zika virus from the Centers for Disease Control and Prevention, visit

Information about the Zika virus from the California Department of Public Health is available at The CDPH updates its report on the number of laboratory-confirmed positive Zika cases in California every Friday.