CDC: Zika Virus Will Hit U.S. Again as Weather Warms
Standard precautions, needle safety will prevent healthcare spread
Any hopes that Zika virus would strike and disappear like SARS and other episodic infectious disease outbreaks have been dashed by the CDC. Zika is fully expected be a mosquito-borne infection threat in the U.S. as the warmer months arrive, primarily in the form of a horrific panoply of birth defects to infants born to infected mothers.
“This is the first time a mosquito-borne disease has ever caused birth defects in humans,” Lyle R. Petersen, MD, MPH, director of the CDC’s Division of Vector Borne Infectious Diseases, said at a recent two-day Zika summit at the CDC. “The last time an infectious pathogen — rubella virus — caused an epidemic of congenital defects was more than 50 years ago.”
Infection preventionists know the drill. Though the primary threat is to pregnant women and unborn children, adherence to standard precautions and injection safety should block transmission if patients with Zika are hospitalized or treated in other healthcare settings. But there is an element of unpredictability to the virus that was underscored last year by the surprising routes of transmission, suggesting the virus has mutated since it was first found in a rhesus monkey in the Zika Forest of Uganda in 1947.
The CDC recently reported Zika-affected pregnancies with birth defects in the U.S. were about 20 times higher than pregnancies occurring before the virus emerged as an epidemic in the Americas last year.
“Defects and other early brain malformations, eye defects, and other central nervous system problems, were seen in about 3 of every 1,000 births in 2013-2014,” the CDC reported.1 “In 2016, the proportion of infants with these same types of birth defects born to women with Zika virus infection during pregnancy was about 6%, or nearly 60 of every 1,000 completed pregnancies with Zika infections.”
Aggregated data reported to the U.S. Zika Pregnancy Registry lists 47 live births with defects and 5 pregnancy losses as of Feb. 21, 2017.2
The birth defects include microcephaly, with the critical risk period to the fetus occurring in the first trimester of pregnancy, Petersen said. The virus attacks the brain before the cranial plates of the skull are fully set, causing them to collapse in to form the small head, he said, pausing to describe the nightmarish anguish of families living with a Zika baby.
“The excessive crying and irritability with this syndrome is really devastating to families because these children basically just incessantly cry 24 hours a day almost,” he said.
That — in stark human terms — is what is at stake as the weather warms, and female Aedes aegypti mosquitoes again dart in and out at all hours and locales, seeking blood meals to produce offspring of their own.
Flying Cockroach
“Aedes aegypti has been called the cockroach of mosquitoes,” Judith Monroe, MD, FAAFP, president of the CDC Foundation, said at the Zika summit. “These mosquitoes live indoors and outdoors and lay their eggs in small amounts of water. They bite during the day and night and they bite multiple people during their life span, creating the potential to spread diseases if the mosquito is infected.”
Aedes aegypti mosquitoes are prevalent in much of the world, and Zika virus has been reported in 61 countries and territories, she added. The other viral diseases spread by the same mosquito predict that Zika will be back and essentially become an endemic threat.
“Zika, chikungunya, yellow fever, dengue — they are not going away. We know that for a fact,” Petersen said. “Chikungunya was introduced and it is still circulating three years later — doing quite well. Yellow fever is an emerging problem in South America right now. Dengue incidence has increased 30 times in the last three decades and is a continuing problem. I think one theme of all of this is that new pathogens are likely to emerge. Nobody expected Zika, and who knows what’s next?”
There is also another troubling aspect that makes Zika disturbingly different. “This is also the first mosquito-borne virus that has shown to be sexually transmitted in humans,” Petersen says.
Conventional Wisdom
The prevailing consensus is that most Zika infections are largely asymptomatic and inconsequential unless the infected person is pregnant or has had unprotected sex while the virus was circulating in the blood or persisting in a human reservoir like semen. Thus, we have seen the tragic birth defects, failed or terminated pregnancies, transmission to sexual partners both male and female, and Zika infection following a needlestick.
A recently published study3 of Canadian travelers challenged the current dogma to some degree, noting that Zika was a more severe infection than dengue or chikungunya in a cohort of returning travelers. Of 1,118 travelers returning from the Caribbean and Central and South America, 41 (3.7%) acquired Zika virus. Three pregnant women were infected, resulting in two cases of congenital infection. Two other travelers developed Guillain-Barré paralytic syndrome, meaning that overall 10% of the Zika cases had severe complications. Symptoms in travelers with Zika included rash (88%), fever (80%) during the acute phase, and about half complained of muscle pain, joint pain, or headaches. About 1 in 6 travelers with Zika virus developed conjunctivitis.
“Even in this small cohort, we observed the full clinical spectrum of acute Zika virus, including adverse fetal and neurologic outcomes,” the authors conclude. “Our observations suggest that complications from Zika infection are underestimated by data arising exclusively from populations where Zika is endemic. Travellers should adhere to mosquito-avoidance measures and barrier protection during sexual activity.”
In addition, 2016 saw the strange case of a 73-year-old patient in the U.S. who apparently transmitted Zika to a visiting acquaintance — possibly through tears — before dying with an incredibly high level of circulating virus in the blood.4 The secondary case developed symptomatic Zika infection, but subsequently recovered. It is possible that hormonal treatment for prostate cancer somehow accelerated viral replication in the index case, investigators concluded.
First 2017 Case
Though it probably should include an asterisk, Florida health officials recently announced what is apparently the first locally transmitted U.S. Zika case in 2017 after a blood donation tested positive in Miami. The individual reported no symptoms, but screening conducted after blood donation in January showed evidence of a past infection.
“This individual had multiple exposures in Miami-Dade County and likely contracted Zika in 2016,” the Florida health department concluded. “Because the individual was asymptomatic, it is difficult to determine when infection occurred. Since the first positive sample was collected in January, this is considered our first locally reported case of Zika in 2017. Florida does not have any identified areas with ongoing, active Zika transmission.”
With the rest travel-related infection, the total number of Zika cases reported in Florida for 2017 is 18. In CDC surveillance including last year and up to March 1, 2017, there have been 5,074 cases of Zika in U.S. states. While 4,779 cases were in travelers returning from Zika-affected areas, 221 cases of local mosquito-borne transmission occurred in Florida (215) and Texas (6), the CDC reports. In additional U.S. cases breakdowns, 74 acquired the virus through non-mosquito routes, including 45 via sexual transmission, 27 via congenital infection, one laboratory transmission via needlestick, and the aforementioned case of person-to-person spread through an unknown route.
“One of things we are concerned about this summer [2017] is a Zika outbreak on the U.S.-Mexico border and Southern Texas,” Petersen said. “It was introduced there toward the end of the season. They have had a mild winter, and transmission still appears to be going on the Mexican side of the border.”
In U.S. territories, including hard-hit Puerto Rico, there were 38,306 Zika virus disease cases reported for the period. Of those, only 145 cases were travelers returning from affected areas. A staggering 38,161 cases were acquired through presumed local mosquito-borne transmission, though sexual transmission may have occurred in some cases. The CDC is not listing birth defects in the U.S. territories currently, but has previously projected that up to 270 afflicted infants could be born to mothers infected with Zika.
Underscoring the threat of Zika virus transmission via the blood supply, the FDA last year called for all states to begin screening donations. In addition, the CDC continues to recommend that pregnant women not travel to areas with Zika. If a pregnant woman must travel to or lives in an area with Zika, she should talk with her healthcare provider and strictly follow steps to prevent mosquito bites and sexual transmission of Zika virus. Pregnant women with possible exposure to Zika virus should be tested for Zika infection even if they do not have symptoms, the CDC advises.
Vector-control Woes
As the CDC braced for the return of Zika, the sobering consensus at the summit was that the current state of vector control programs in the United States will be hard pressed to control the resourceful, resilient Aedes aegypti mosquito. On a positive note, similar Aedes albopictus mosquitoes — which have a broad range well up the East Coast — are not efficient transmitters of Zika, and have only occasionally spread infections, Petersen said.
“I think Zika virus emergence brought to light all of the problems with the existing systems and controls [for vector-borne disease] and strategies we have,” he said. “Clearly, some new paradigm is needed. Existing tools have [not been] adequately tested in the field for efficacy in reducing human disease. We have quite a lot of entomological data, but there several examples where it looked like the intervention was going to work but it really didn’t help reduce human disease.”
In addition, vector-control programs vary widely by state in terms of resources and expertise. “We need to figure out how to raise the bar so they can implement better existing approaches as well as new ones that may come on line,” he said. “We have talked at length about public distrust and lack of knowledge. I think this remains a real problem in this day and age.”
Indeed, since massive mosquito abatement programs have been rarely needed in recent years, the public could be concerned about pesticide exposures and novel approaches like genetically modified mosquitoes. Vector control efforts are also challenged by pesticide resistance, which is roughly analogous to antibiotic resistance in terms of misuse and the need for new alternatives.
“There is an urgent need to try and overcome this problem either through better training of vector control units how to reduce the problem and better apply pesticides, but also looking at non-pesticide alternatives,” Peterson said.
New pesticides and repellents may come on line in the next few years, but must pass muster with environmental health regulations. “We are dealing with pesticides that were developed 50 years ago, hence the big resistance [problem],” he said.
REFERENCES
- CDC. Baseline Prevalence of Birth Defects Associated with Congenital Zika Virus Infection — Massachusetts, North Carolina, and Atlanta, Georgia, 2013–2014. MMWR 2017;66(8):219–222.
- CDC. Outcomes of Pregnancies with Laboratory Evidence of Possible Zika Virus Infection in the United States. Feb. 21, 2017: http://bit.ly/28QH7dM
- Boggil AK, Geduld J, Libman M, et al. Surveillance report of Zika virus among Canadian travelers returning from the Americas. CMAJ March 6, 2017;189:9: doi:10.1503/cmaj.161241
- Swaminathan S, Schlaberg R, Lewis J, et al. Correspondence: Fatal Zika Virus Infection with Secondary Nonsexual Transmission. New Engl Jrl Med 2016;375(19):1907-1909.
Any hopes that Zika virus would strike and disappear like SARS and other episodic infectious disease outbreaks have been dashed by the CDC. Zika is fully expected be a mosquito-borne infection threat in the U.S. as the warmer months arrive, primarily in the form of a horrific panoply of birth defects to infants born to infected mothers.
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