August 10, 2017

Executive Summary

The mosquitoes capable of transmitting Zika virus are not present in most US states; therefore, local Zika virus transmission is not possible in much of the country. However, these mosquitoes are common in southern states, and these areas could see local Zika virus activity (map), or a possible recurrence of Zika transmission observed in Florida and south Texas during 2016. New local transmission would likely be quickly identified and addressed appropriately by authorities. Overall, the risk of ongoing Zika virus transmission within the US remains low.

Zika Virus: What You Need to Know

Zika virus is primarily transmitted by the bites of infected Aedes aegypti mosquitoes. However, in rare cases the virus can be transmitted by contact with infected blood or through sexual contact with an infected individual. Aedes aegypti is a tropical mosquito that thrives in urban areas and feeds primarily on people; these mosquitoes are typically more active during warmer, wetter months, when environmental conditions are ideal for mosquito breeding – typically May to September in the continental US.

Symptoms of Zika virus infection include fever, reddening of the eyes (conjunctivitis), rash, and temporary pain or swelling of joints - typically the hands and feet. These symptoms are usually mild and last four to seven days. Research suggests Zika infection can lead to neurological conditions in any patient. Transmission can also occur between currently infected pregnant women and their fetuses; research suggests Zika virus infection during pregnancy may result in “congenital Zika syndrome,” which describes a pattern of conditions found in babies infected with Zika virus in utero. These include microcephaly (an abnormally small head), and damage to the developing brain, eyes, muscles, and joints. Babies born with congenital Zika syndrome may not display all these conditions. No vaccine or specific treatments are available for Zika virus.

Zika Virus Transmission in the US

Most Zika virus cases in the continental United States are identified among travelers returning to the US from countries with ongoing Zika virus transmission. However, the US territories of American Samoa, Puerto Rico, and the US Virgin Islands continue to be significantly more affected by Zika virus due to their tropical climates. Those returning to the US unknowingly acquire the disease while abroad and often do not feel ill until arriving home in the US. Furthermore, the majority of Zika virus infections present with few or very mild symptoms and last only four to seven days, making it especially difficult to diagnose.

Despite concerns about these imported cases, the Aedes mosquitoes that transmit Zika virus are not present in most US states, making local Zika virus transmission impossible in much of the US. However, Aedes mosquitoes are common throughout the southern region, including the states of Alabama, Arizona, California, Florida, Georgia, Louisiana, Mississippi, New Mexico, and Texas, and these areas could see locally acquired cases (map). A total of 224 local mosquito-transmitted Zika virus cases occurred nationwide during 2016, including Miami-Dade County, Fla. (218 cases), and Brownsville, Texas (six cases).

Many states already have mosquito control programs. However, most of these plans focus on the specific mosquitoes that transmit West Nile virus, which are different from those capable of transmitting Zika virus. Therefore, the plans and methods used for detecting and controlling West Nile virus mosquitoes are now being modified by states within the estimated range of Aedes aegypti.

Protecting Yourself from Zika Virus and other Mosquito-borne Diseases

To prevent mosquito-borne illnesses, diligent use of precautions to prevent them are crucial regardless of destination in the US, and are especially important in southern states where Aedes mosquitoes occur. The Health Intelligence Team’s Insect Bite Precautions Information Sheet and Zika Virus Information Sheet include additional details regarding symptoms, risks, and prevention strategies.