The Epidemiology and Transmission of the Zika Virus are Not Fully Understood and Accurate Information on the Incidence is Lacking

While several countries have reported outbreaks of Zika virus disease,  unanswered questions remain regarding the epidemiology and the  transmission of the disease. Many factors, including a large number of  asymptomatic patients, mild symptoms, a lack of a consistent  international case definition of Zika virus disease, as well as of  microcephaly, and a lack of validated diagnostic tests complicate our  understanding of the virus and may hinder our response to the current  outbreak. Questions also remain regarding the strength of the association  between Zika virus infection and microcephaly or Guillain-Barré  syndrome. As of February 24, 2016, 107 cases of continental  U.S. travel-associated Zika virus disease have been reported, according  to CDC. Zika virus can also be transmitted through blood transfusion, according to  U.S. Food and Drug Administration (FDA) documents. Mild Symptoms and Asymptomatic Cases May Lead to Underestimation

Researchers have reported that an estimated 80 percent of the  individuals infected with the Zika virus are asymptomatic, that is, they  have the virus but do not manifest clinical symptoms. Lack of Consistent International Case Definitions May Lead to Inaccurate Count

An accurate count of the number of Zika virus disease cases requires a  consistent case definition, or set of uniform criteria to define the disease  for public health surveillance and to determine who is included in the  count and who is excluded. When detecting antibodies, diagnosing cases of Zika virus disease and  differentiating it from other diseases caused by other flaviviruses, such as  dengue or yellow fever, is difficult if someone has been infected by  another flavivirus. For example, a person previously infected with  another flavivirus such as dengue could be falsely identified as also  having been exposed to the Zika virus (and vice-versa). According to the CDC, while there are no commercially-available  diagnostic tests for Zika, an antibody-based test for the Zika virus (Zika  MAC-ELISA) was recently authorized for Emergency Use by the FDA. Various Methods of Mosquito Control Exist and Novel Approaches Are Underway

Because Zika virus disease cannot yet be prevented by drugs or  vaccines, vector (mosquito) control remains a critical factor in mitigating  risks associated with this disease. There are both large scale and personal methods for mosquito control. We provide a brief preliminary overview of some population-scale control  methods identified in the literature, agency documents, and interviews  with industry officials and academicians, which include three potentially- overlapping categories: (1) standing water treatment, (2) insecticides, and  (3) emerging technologies. Emerging Technologies

Emerging technologies include (1) use of biological control methods, (2)  genetically-modified mosquitoes, and (3) auto-dissemination traps. Scientific literature has identified another approach—using bacteria to  reduce disease transmission from mosquitoes to humans. NIH has identified areas of high priority including:
Basic research to understand viral replication, pathogenesis, and  transmission, as well as the biology of the mosquito vectors; potential  interactions with co-infections such as dengue and yellow fever  viruses; animal models of Zika virus infection; and novel vector control  methods; and
Pursuing Zika virus research to develop sensitive, specific, and rapid  clinical diagnostic tests; drug treatments for Zika virus as well as  broad spectrum therapeutics that treat multiple flaviviral infection; and  effective vaccines and vaccination strategies. The CDC has identified priority areas of research focus on Zika including:
Determining the link between Zika virus infections and the birth defect  microcephaly and measuring changes in incidence rates of the birth  defect over time; Improving diagnostics for the Zika virus, including advanced methods  to refine tests and support advanced developments for vector control;  and
Enhancing international capacity for virus surveillance, expanding  laboratory testing, and health care provider testing in countries at  highest risk of Zika virus outbreaks. The priority research areas identified by NIH and CDC are ambitious and  agencies may face some challenges in implementing this agenda,  including:
Given that there are few known cases in the United States, NIH and  CDC may have to rely on the cooperation of other countries with  sufficient number of cases in order to carry on the proposed research. Biosurveillance: Challenges and Options for the National Biosurveillance  Integration Center. This is a work of the U.S. government and is not subject to copyright protection in the  United States.
Why GAO Did This Study

Emerging infectious diseases constitute a clear and persistent threat to the health and well-being of people and animals around the world. The Zika virus, which at present appears to be primarily transmitted to humans by infected mosquitos, can cause symptoms including fever, rash, and joint pain. A large ongoing outbreak is occurring in Brazil that started in May 2015. As of February 24, 2016, over 100 cases of U.S. travel-associated Zika virus disease cases have been reported.
Due to concerns about its potential impact, you asked GAO to present preliminary observations on the Zika virus. This statement addresses (1) the epidemiology and transmission of the Zika virus disease, including reporting on the incidence of disease and what is known about its link to microcephaly; (2) detection and testing methods; (3) methods for mosquito control; and (4) the proposed federal research agenda as it relates to the Zika virus and Zika virus disease.
To report on these questions, GAO reviewed relevant peer-reviewed scientific literature, epidemiological alerts, agency documents, and prior GAO work from 2003-2016 on related topics; consulted experts in the fields of virology, infectious diseases, and vector control, including industry representatives; and interviewed officials of the CDC and NIH.

What GAO Found

While several countries have reported outbreaks of Zika virus disease—which appear to be primarily transmitted to humans by mosquitos—unanswered questions remain regarding the epidemiology and transmission of the disease. Many factors—including a large number of asymptomatic patients and patients with mild symptoms, and a lack of a consistent international case definition of Zika virus disease—complicate understanding of the virus and may hinder responses to the current outbreak. For example, an estimated 80 percent of individuals infected with the Zika virus may not manifest clinical symptoms. As a result, incidence of the infection may be underestimated. Questions also remain regarding the strength of the association between Zika virus infection and two other conditions: microcephaly and Guillain-Barré syndrome.
A lack of validated diagnostic tests, consistent international case definitions, and trend information may also contribute to difficulty in estimating the prevalence of the virus. The United States uses two diagnostic tests for Zika, and according to the U.S. Centers for Disease Control and Prevention (CDC), while there are no commercially-available diagnostic tests for Zika, an antibody-based test for Zika virus was recently authorized for Emergency Use by the U.S. Food and Drug Administration. Diagnosing Zika virus infection is also complicated because it is difficult to differentiate it from other similar diseases, such as dengue or yellow fever. For example, a person previously infected with dengue could be falsely identified as also having been exposed to the Zika virus (and vice-versa). Moreover, the World Health Organization has acknowledged the need for a consistent case definition—that is, a set of uniform criteria to define the disease for public health surveillance and to determine who is included in the count and who is excluded. Additionally, a lack of pattern and trend data has made surveillance challenging.
Because Zika virus disease cannot yet be prevented by drugs or vaccines, vector (mosquito) control remains a critical factor in preventing and mitigating the occurrence of this disease. There are three methods for mosquito control: (1) standing water treatment, (2) insecticides, and (3) emerging technologies. Mosquito control has been achieved in some locations by methods such as reducing or chemically-treating water sources where mosquitoes breed or mature, or by insecticide dispersal. Emerging technologies, including biological control methods—such as infecting mosquitoes with bacteria— genetically-modified mosquitoes, and auto-dissemination traps, show some promise but are still in development and testing phases.
The National Institutes of Health (NIH) and the CDC have identified several high priority areas of research. Research priorities include basic research to understand viral replication, pathogenesis, and transmission, as well as the biology of the mosquito vectors; potential interactions with co-infections such as dengue and yellow fever viruses; linkages between Zika and the birth defect microcephaly; improving diagnostic tests; vaccine development; and novel vector control methods. These efforts are ambitious, and agencies may face challenges in implementing this agenda.

What GAO Recommends

GAO is not making recommendations at this time.