With Zika virus making its presence known in the United States and local transmission occurring in Florida, there is a growing concern about the risk of Zika. But we have to ask the question: WHY is it spreading so rapidly, and why now? Once Zika virus was identified in Brazil, with knowledge from previous outbreaks, it was obvious that the disease would spread across the Americas and was here to stay. The BIG question was when it would emerge as a locally-transmitted disease in the U.S.
There are four main factors that are needed for the Zika virus to spread:
1) Zika-infected mosquitoes (the vector),
2) A rise in severe drought, flooding, heavy rains and temperature (El Niño), which can result in an increase in the mosquito population due to favorable breeding sites,
3) Humans (the reservoir), who once infected (regardless of how they became infected), can transfer the infection to other non-Zika carrying mosquitoes or transfer the disease via sexual or other routes, and
4) The human mobility (travel) between and within countries.
All of these factors have created a very favorable environment for the Zika virus to jump from one country to another and spread within a country. Before the World Health Organization (WHO) declared Zika virus an international health emergency, I was following the disease, knowing that viruses do not observe geographic borders and was certain that the disease would emerge in the U.S. Through this Zika blog series, my intention is to inform the scientific, as well as non-scientific, community about the disease: what is the current situation of this epidemic, who is at risk, what are the necessary precautions, and what is being developed by the global community to counter this viral threat?
Zika: The Current Situation
Discovered in Uganda in 1947, Zika virus has been circulating amongst humans in Africa and Asia for over half a century. It spread to the Western Pacific in 2007, and was first confirmed in the Americas in May 2015, when cases were diagnosed in Brazil . Since then, the situation has changed dramatically as the virus continues to spread in the Americas and has been linked to rare, but severe, health effects.
As of today, WHO has reported 65 countries and territories with evidence of mosquito-borne Zika virus transmission . According to the Centers for Disease Control and Prevention (CDC), local, mosquito-borne transmission of Zika virus reached the United States for the first time on 29 July 2016 .
Disease Effects of Zika Virus
The disease affects all age groups and is typically mild, resulting in only a small number of adults seeking medical care. Most patients recover completely, and hospitalization and death are very rare. However, in pregnant women, the virus can cause devastating effects on the fetus, resulting in serious birth defects. The virus has been linked to microcephaly (unusually small heads and brain damage) in newborns. Children born to mothers who are infected by Zika can also develop blindness, deafness, seizures, and other congenital defects. In adults, the disease can cause a form of temporary paralysis called Guillain-Barré syndrome .
Despite the possibility of severe health effects, only 20-25% of infected individuals will develop symptoms, meaning up to 80% of people with Zika virus will not even know they are infected. The asymptomatic population presents a challenge from the perspective of surveillance and control.
Predicting the Spread of the Disease
It is believed that by the end of 2016, the disease will spread across southern United States. According to the CDC, from January 1, 2015 to August 10, 2016, there have been 1,955 travel-associated cases of Zika virus in the U.S. Three U.S. Territories (American Samoa, Puerto Rico, U.S. Virgin Islands) have reported a total of 6,618 cases (Guillain-Barré syndrome: 20) out of which 6,587 were through local transmission and 31 were travel-associated .
Local, mosquito-borne transmission within the United States was first confirmed in one neighborhood of Miami, Florida, in late July . Initially a small local cluster, the number of Zika cases has now risen to 30 cases . Recently, the first case linked to travel within the contiguous U.S. has surfaced in El Paso, Texas. This individual had traveled to Miami, where he acquired the disease .
The outbreak of mosquito-borne transmission in Florida and recent case of travel-associated Zika within the U.S. highlights that we not only have to worry about transmission of the virus through international travel routes (endemic countries), but also through local travel within the U.S. With no vaccine currently available to fight this virus, we have to be vigilant in taking precautionary measures such as vector (mosquito) control strategies, limiting or avoiding transmission through travel to Zika-affected regions, and personal protection. At the state and local level, active vector control and surveillance programs will be critical.
Here at IEM, we have public health, GIS and modeling experts working to identify high risk areas within the U.S. through overlaying the mosquito distribution with travel database information, cases of travel-associated as well as local transmission of the disease, and climate data (temperature and precipitation). Our objective is to develop a model that can provide alerts to the communities so they may take personal safety precautions against the disease.
In the next blog, I will discuss who may be at risk for Zika virus infection, as well as how and why factors such as the mosquito population, climate, and travel contribute to risk.
Author: Rashid Chotani, Senior Scientist
. Mary Kay Kindhauser et el. Zika: the origin and spread of a mosquito-borne virus. Bulletin of the World Health Organization 2016. Published online: 09 February 2016. http://dx.doi.org/10.2471/BLT.16.171082
. CDC. Florida investigation links four recent Zika cases to local mosquito-borne virus transmission. http://www.cdc.gov/media/releases/2016/p0729-florida-zika-cases.html
. CDC. Zika Virus. Symptoms. https://www.cdc.gov/zika/symptoms/symptoms.html
. CDC. Case Counts in the US. http://www.cdc.gov/zika/geo/united-states.html
. The Hill. Florida governor: Local Zika cases rise to 30. http://thehill.com/policy/healthcare/291493-florida-governor-local-zika-cases-rise-to-30
. Texas Department of State Health Services. Texas Zika Illness Linked to Miami Travel. http://www.dshs.texas.gov/news/releases/2016/Texas-Zika-Case-Linked-to-Miami.aspx