It is definitely possible to spread herpes from one partner’s mouth to another partner’s genitals during oral sex, which would result in the uninfected partner contracting genital HSV-1. Transmission is more likely when sores are present, but sometimes herpes can be transmitted without any symptoms – we call this “asymptomatic viral shedding.” This shedding can happen in both oral and genital herpes infections. Shedding is most common in the first few months after getting a new infection, and the frequency of shedding depends on which type of HSV you have and where the infection is. For example, oral HSV-1 (what we call “cold sores”) sheds about 12% of days, and genital HSV-2 sheds about 20% of days (https://academic.oup.com/jid/article/198/8/1098/879583). Another study showed shedding of genital HSV-2 on 10% of days (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3144252/). Since this shedding is invisible, its not possible to know for sure when your partner is potentially contagious.
Even though your partner has herpes, it is not a guarantee that you will contract the infection. One study showed that transmission between couples is about 10% per year, and about 70% of these transmissions happened when the infected partner had no symptoms (https://annals.org/aim/article-abstract/705283/risk-factors-sexual-transmission-genital-herpes?volume=116&issue=3&page=197).
Avoiding contact with visible sores will reduce your risk of getting oral HSV-1 when your partner goes down on you.
At City Clinic, we perform herpes tests by swabbing sores. This is the most accurate way to test, and this kind of test can distinguish between HSV-1 and HSV-2. There are blood tests available, but we do not routinely use these for our visits. City Clinic does not have blood tests for HSV-1 – if you would like this test, please talk with your primary care provider.