By Betsy Todd, MPH, RN, CIC, clinical editor
Zika virus is now being actively transmitted in 42 countries, primarily in the Americas and on islands in the South Pacific. As of April 13th, there had been 358 travel-associated cases reported in the U.S., including 31 pregnant women.
While there are as yet no locally acquired U.S. cases, local transmission has been established in several U.S. territories (primarily, Puerto Rico). Travel-associated cases are expected to continue in the U.S., almost certainly leading to eventual limited local transmission.
Transmission. Most cases of Zika virus infection have been vector-borne—that is, they resulted from the bite of an infected mosquito. Other modes of transmission have been documented, including intrauterine, perinatal, sexual, and via exposure to lab specimens
Transmission may be possible during blood transfusion, organ or tissue transplantation, breast feeding, or fertility treatments, but thus far there have been no reports of infection acquired in this way.
Immunity after infection with Zika virus is currently thought to be lifelong. There have been no known cases of reinfection after a primary Zika infection, nor of “relapsing” cases of the disease (as seen after Ebola virus infection).
Serologic testing can be done via reverse transcription–polymerase chain reaction (RT-PCR) for viral RNA, or immunoglobulin M (IgM) testing for antibodies. Information regarding the accuracy of tests done on infected but asymptomatic people is limited, and Zika virus disease should not be ruled out on the basis of a negative test. As with other infectious diseases, the decision to test is guided by the patient’s symptoms, travel history, and current geographical location.
Sexual transmission of Zika. The virus has been detected in semen. All reported cases of sexual transmission involved men who had or developed symptoms of infection, and who did not use condoms. Both female and male partners have been infected. There has been no known transmission from infected women to their partners.
The CDC’s newest recommendations regarding sexual transmission of Zika virus suggest time periods during which men potentially exposed to or diagnosed with Zika infection abstain from sex or consistently use condoms in order to prevent transmission. The length of time varies from eight weeks to six months, depending on the circumstances.
The CDC does not recommend testing semen for Zika virus in an effort to determine the risk of sexual transmission. These tests are not widely available and it’s still not clear how to interpret the results.
There are still many unanswered questions about sexual transmission, and studies of semen, vaginal fluids, and saliva are ongoing.
- Can women transmit the infection to their sex partners?
- Can asymptomatic infected individuals transmit the disease sexually? (The majority of infected people have no symptoms.)
- How long does the virus remain infectious in semen? Live virus has been cultured from semen at least 14 days after the start of symptoms; virus particles in semen have been detected at least 62 days after symptom onset.
- Can other fluids in addition to semen transmit Zika virus?
- Does sexual transmission pose a different risk than mosquito-borne transmission for microcephaly and other congenital conditions.
Zika and pregnancy. The CDC has posted updated recommendations about Zika for women trying to conceive during the epidemic (including couples seeking infertility treatment).
In addition, the CDC offers ideas for a “Zika prevention kit” for pregnant women and a “Doctor’s Visit Checklist” for pregnant women who have travelled to an area where there is active Zika transmission.
For clinicians, the CDC has devised algorithms to assist in determining whether a pregnant woman (or, eventually, her new baby) should be tested for Zika virus, and in order to monitor pregnancies and outcomes during the epidemic, the CDC has instituted a Zika Pregnancy Registry. Clinicians should help to ensure that the following patients are referred to the Zika Pregnancy Registry:
- pregnant women with lab evidence of Zika virus infection
- babies born to these infected women, even when the babies are born with no evidence of Zika infection
- babies with evidence of Zika virus infection or complications, even when their mothers were not known to be infected with Zika during the pregnancy (mothers of babies in this last category should be added retrospectively to the Registry)