America's North Shore Journal

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Zika Is Coming: Establishing Base Levels of Microcephaly in the US and the Gulf Coast States



The spread of Zika viral illnesses and the reports, from Brazil, correlating microcephaly and other birth defects with prenatal illness in the mother suggest that the United States in the near future may experience locally acquired outbreaks of the illness and an increased incidence of microcephaly. A base level for this birth defect should be established before that occurs. The presence of the mosquito vector for Zika in the states along the Gulf Coast suggests a need to establish an incidence rate for each of the states. Using data collected by the National Center on Birth Defects and Developmental Disabilities, a national incidence of microcephaly of 7.02/10,000 births was calculated. Four of the five Gulf Coast states have data that provides an incidence. Those rates range from 16.0/10,000 in Mississippi, 11.5/10,000 for Texas, 11.1/10,000 for Louisiana, to 6.4/10,000 in Florida. The existing data suggests that the incidence of microcephaly in the Gulf Coast states is elevated above the national incidence. Any future correlation between the arrival of Zika and cases of microcephaly in that region should take into account the existing high levels.


Microcephaly is a fetal condition diagnosed in pregnancy or at birth and is linked to a variety of genetic causes, maternal behaviors and a number of infections contracted by the pregnant mother which pass to the fetus. The incidence of microcephaly can vary wildly because of the complex medical, social and economic factors involved. It is often, though not always, accompanied by a variety of other birth defects.

The Zika virus was first detected in the Western Hemisphere by Brazilian public health authorities in April, 2015. (1) As of March 17, confirmed autochthonous cases have been confirmed in 33 nations and territories in the Region of the Americas as designated by the Pan American Health Organization. (2) The U.S. Centers for Disease Control and Prevention (CDC) have received reports of 258 travel-associated Zika viral illnesses in the 50 states, and 283 locally contracted illnesses in Puerto Rico and the U.S. Virgin Islands as of March 16. (3)

Since the introduction of Zika in Brazil, that nation has noted an increase in the number of cases of microcephaly. That increase correlates to the arrival of the virus and a number of studies have found evidence of Zika infections in deceased newborns with that and other neurological conditions. A PubMed search for “Zika” and “microcephaly” for 2015 and 2016 reveals 77 items.

The vector for the Zika virus is the Aedes aegypti or Yellow Fever mosquito. It is active year-round in south Florida and sub-tropical parts of the Texas Gulf Coast. As the temperatures warm, the mosquito’s activity moves north into those areas where the weather is warm and wet enough. A recent study found that a population of Ae. Aegypti may be overwintering in Washington, D.C. (4) In the early years of the United States, yellow fever outbreaks were recorded as far north as Boston, Mass. (5)

It appears nearly certain that locally acquired Zika viral illnesses will appear in the continental United States. Determining the incidence of microcephaly before the arrival of Zika viral illnesses requires some effort. In the United States, microcephaly is not a notifiable condition as set by the CDC. The same is true for many state governments. Determining the pre-Zika base level for the condition, especially in the Gulf Coast states where the vector is presently active, is important in order to determine if there are any increases correlating with locally acquired Zika viral illnesses.


Multiple Internet searches were done for government data on the incidence of microcephaly at the state level and from the federal government. In addition, PubMed and Google Scholar were searched for any published studies on the topic. Various investigators have discussed the incidence with respect to some of the known causes such as phenylketonuria but we located no studies of the combined, overall incidence.

There is no accepted national incidence rate for microcephaly. Various authorities cite a wide range of numbers. The CDC states that microcephaly incidence tracked at the state level ranges from 2/10,000 to 12/10,000 live births. (6) The State of Virginia Department of Health states that the incidence is 4/10,000 to 6/10,000. (7) The Cleveland Clinic describes the incidence on its website as from 1/8,500 births to 1/6,200 births. (8) Boston Children’s Hospital uses the highest estimate of all, 25,000 cases per year in the United States. (9)

Using the CDC’s estimated of live births for 2014 of 3,988,076 (10), the seven authoritative incidence rates for microcephaly in the United States can be compared in Table 1.

Table 1
Comparison of Microcephaly Incidence Estimates for the United States
Source Incidence per 10,000 Estimated Cases












Cleveland Clinic



Cleveland Clinic



Boston Children’s



The National Center on Birth Defects and Developmental Disabilities, Centers for Disease Control and Prevention, collects data from a number of U.S. states and from the U.S. military on birth defects. It produces an annual report with this data. The last report which contained data on the incidence of microcephaly was produced in 2013 and contained data from the five year period 2006 through 2010. (11) At that time, 33 states and the Department of Defense provided data on microcephaly. The reports for 2014 and 2015 do not have such data.

Data for three states not included in the original data set was located during searches. This data is not from the same time period as the larger report. It will be discussed separately.

A spreadsheet was constructed, with columns for the state, the incidence, the number of live births in the period and the number of cases of microcephaly in the period. The average number of cases per year was calculated. Also calculated for each state was the percentage of live births and the percentage of cases with respect to the total live births and total cases.

The data from the Department of Defense was not excluded. The geographically widespread nature of DoD medical facilities suggests that its data could be broadly representative of the national incidence.

A final column of calculations was done for each state. The percentage of the total of live births for that state was divided by the incidence rate for the state, providing a weighted proportion which totals to an estimated national incidence.
Base levels of microcephaly in the US


Gross analysis found that the 33 states and the DoD had an average incidence of microcephaly of 6.7/10,000. The median value was 5.75/10,000. The reported incidence for the entire data set ranges from a high of 19/10,000 for Alaska to a low of 0.5/10,000 in Maryland. Weighting the reported incidence of microcephaly for each state and the Department of Defense by the proportion of live births provides an estimated incidence for the entire data of 7.02/10,000.

The live births reported in the data set represent 63 percent of all live births in the United States for the five year period. (12) Applying that proportion and using the reported numbers of microcephaly cases, the national microcephaly case total for 2006 through 2010 is 17,736 or an average of 2,947 yearly.

Table 2 shows the data for the four Gulf Coast states that participated in the program and reported on microcephaly cases. Alabama did not track that condition. The four states reported 3,089 microcephaly cases, an average for the four of 617.8 per year. The four states represent 23.5 percent of the total reported live births in the data set and 33.3 percent of reported microcephaly.

Table 2
Microcephaly Data Reported by Gulf Coast States 2006-2010
State Incidence per 10,000 Live births Microcephaly cases
















The CDC has finalized the national total live births for 2014. The total for each of the four Gulf states was located, as well. (13) Table 3 shows the estimated number of microcephalic births for 2014 for the nation and four of the states at immediate risk of Zika viral illnesses.

Table 3
Estimated Cases of Microcephaly 2014
State Incidence per 10,000 Live births Microcephaly cases
















United States




Additional data from three states not included in the larger report was obtained from Arizona, Connecticut and Hawaii. A 1995 report from Arizona revealed an incidence rate that year of 11.1/10,000 and 81 cases of microcephaly. (14) The state of Connecticut has a report from 2001 to 2004, showing an incidence rate of 4.0/10,000 and 68 cases. (15) Hawaii has data available from 2001 to 2005, showing 62 microcephaly cases in the five year period and a calculated incidence of 6.7/10,000. (16)


The loss of data on microcephaly from the national report on birth defects beginning with 2014 is regrettable. The data set used herein appears to represent the best collection of both national and state data available at this time. That stated, applying incidence rates from 2006-2010 to 2014 live births does carry a risk of error.

The incidence rates of microcephaly for Texas, Mississippi and Louisiana during 2006-2010 are in the top ten for all locations. Florida is number 16 on the list. This suggests that microcephaly is more common than average along the Gulf Coast of the United States. That is a key finding with import to measuring the effects of Zika viral illnesses in the region.

News reports have noted that the vector mosquito that transmits the Zika virus is already active in South Florida. (17) Aedes aegypti are breeding and biting. The state of Florida has has issued a Declaration of Public Health Emergency for 12 counties and reports 72 imported Zika illnesses through March 23, 2016. (18) The majority of these travel-associated Zika cases at this time are in Miami-Dade.

No one can predict when the first case of locally acquired Zika viral illness will be announced. All of the pieces are in place for that to occur. The vector species is active in South Florida. Imported cases of the illness are appearing with regularity in South Florida. When Zika arrives in the United States, this analysis provides a basis for determining if there is a correlation between an increase in cases of microcephaly and Zika viral illnesses.

References and Citations

  1. Kindhauser MK, Allen T, Frank V, Santhana RS & Dye C. Zika: the origin and spread of a mosquito-borne virus. Bulletin of the World Health Organization. E-pub: 9 Feb 2016. doi:
  2. Zika Epidemiological Update – 10 March 2016. PAHO.
  3. Zika virus disease in the United States, 2015–2016. CDC.
  4. Lima A, Lovin DD, Hickner PV, Severson DW. Evidence for an Overwintering Population of Aedes aegypti in Capitol Hill Neighborhood, Washington, DC. Am J Trop Med Hyg. 2016 Jan 6;94(1):231-5. doi: 10.4269/ajtmh.15-0351
  5. Blake JB. Yellow fever in eighteenth century America. Bulletin of the New York Academy of Medicine. 1968;44(6):673-686.
  6. Facts about Microcephaly. CDC.
  7. Microcephaly. Virginia Genetics Program, Virginia Department of Health.
  8. Microcephaly in Children. Cleveland Clinic.
  9. Microcephaly Symptoms & Causes. Boston Children’s Hospital.
  10. Births and Natality. CDC.
  11. Major Birth Defects Data from Population-based Birth Defects Surveillance Programs in the United States, 2006-2010. National Center on Birth Defects and Developmental Disabilities, Centers for Disease Control and Prevention.
  12. data center,,38,35,18,17/5/12703
  13. Number of Births by Race. Kaiser Family Foundation.
  14. 1995 Arizona Birth Defects Monitoring Program Report. Arizona Department of Health Services.
  15. Birth Defects in Connecticut 2001-2004. Connecticut Birth Defects Registry.
  16. Hawai‘i Birth Defects Surveillance Report. Hawai‘i State Department of Health.
  17. Mosquitoes are ready for Zika in South Florida.
  18. Surgeon General Dr. John Armstrong’s Daily Zika Update. Florida Department of Health.
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