Yesterday, the U.S. Centers for Disease Control (CDC) announced that “CDC Concludes Zika Causes Microcephaly and Other Birth Defects.” Press release here. Several researchers from the CDC also wrote a paper published yesterday in the New England Journal of Medicine explaining their thought process on this conclusion of causality.

Zika virus has spread to 42 countries since the first recorded outbreak in Brazil in early 2015. The illness itself is mild— the most common symptoms include fever, rash, joint pain, and conjunctivitis. Most people do not experience symptoms. The virus is most commonly transmitted through mosquito vectors, but sexual transmission and transmission from a pregnant woman to her fetus are also possible.

Increased rates of neurological complications have been seen following increases in Zika infections. These include increased rates in adults of Guillain-Barre Syndrome (GBS) and an auto-immune disorder similar to multiple sclerosis a few weeks after infection. It also includes increased rates of microcephaly and other fetal neurological effects in babies born to women infected with Zika virus during pregnancy.

Establishing causality is a challenging undertaking, especially during an emerging infectious disease event. The amount and type of evidence available during epidemics makes it hard to know exactly what is happening. There are theoretical frameworks available that can help to categorize and weigh evidence to understand whether a causal relationship is supported. The CDC paper published yesterday uses two such frameworks to discuss the available evidence: Shepard’s Criteria for Proof of Teratogenicity in Humans and Bradford Hill Criteria for Evidence of Causation. After weighing the available evidence, the authors conclude that Zika infection causes microcephaly and other fetal neurological effects. Read the paper here.

In other news, a report was released today describing the first documented case of male-to-male sexual transmission of Zika. Not unexpected as several accounts of sexual transmission from an infected male to his female sexual partner have already been reported. Current CDC guidelines recommend using condoms or abstaining from sex for 8 weeks after a male partner returns from travel to an affected area and 6 months if the male partner experienced symptoms consistent with Zika during/after travel. Read all CDC guidelines here.