Your finding is not surprising. I'll explain:
Re: "Zika virus was detectable in a pregnant woman's serum samples for more than 3 and a half months after symptom onset."
Our public health authorities (WHO and CDC) apparently ignored a Eurosurveillance Rapid Communication: "Detection of Zika Virus RNA in Whole Blood of Imported Zika Virus Disease Cases Up to 2 Months After Symptom Onset, Israel, December 2015 to April 2016." Source: Eurosurveillance, Volume 21, Issue 26, 30 June 2016
More recently (Dec. 2016): "detection of Zika virus RNA in vaginal secretions up to day 14 and in erythrocytes up to day 81" (almost 3 months in blood). Source: December 14, 2016 CDC post called Prolonged Detection of Zika Virus in Vaginal Secretions and Whole Blood.
Re: "ZIKV viremia is normally detectable only within the first week ..."
I wonder if this is because of the highly flawed Trioplex assay (which fails to detect 40 percent of Zika cases and all four strains of dengue). The ZIKV Detect IgM Capture ELISA test, made by InBios International has been deemed "too inaccurate" by the FDA, as well.
As of November 12th, 2016, a whopping 564,571 donations have tested positive on the cobas Zika test.
Dr. Jay S. Epstein, Office of Blood Research and Review director at the FDA, said in a statement:
"Zika virus is a transfusion transmitted disease which can cause potentially severe consequences including microcephaly and Guillain-Barré syndrome. The requirement to test blood donations for Zika virus has already resulted in interdicting contaminated collections confirming the value of testing."
Source (with citations): http://www.infobarrel.com/Zika_Virus_Our_Tainted_Blood_Supply
Also, UC San Francisco researchers have identified fetal brain tissue cells that are targeted by the Zika virus and determined that azithromycin, a common antibiotic regarded as safe for use during pregnancy, can prevent the virus from infecting these cells.
Hope this information helps,
Canadian freelance writer and activist
Education: nursing, orthotics/prosthetics, ophthalmology (as an assistant)