Zika Virus: Current Assessment

shutterstock_374400817James M Wilson V, MD

Director, Nevada State Infectious Disease Forecast Station

School of Community Health Science, University of Nevada-Reno


We have yet another exotic global public health threat on the radar – Zika virus – and there is no vaccine or anti-viral medication to stop it. While the vast majority of people infected experience minimal symptoms, many develop dengue-like symptoms of rash, fever, and joint pain. Worse, pregnant women who are infected are experiencing birth defects known as microencephaly (“small head”) as well as fetal death.  Microencephaly may lead to a child with neurological issues such as cerebral palsy. We have also noted paralysis (Guillain Barre Syndrome) in a limited group of 20-30 year old adults, where the case fatality in this group is reportedly as high as 50%.The literature suggests that the virus is in the blood for 10 days before symptoms develop, and remains in the blood for 12 days after initial onset of symptoms. The majority of patients are subclinical and therefore unlikely to seek care. There is also evidence of the potential for sexual and oral transmission of Zika Virus, however the primary transmission driver is mosquito-based.

While Aedes aegypti and albopictus mosquitoes are recognized as the  key vectors, the Brazilians are actively investigating a suspicion of Culex mosquito involvement. This is important because Culex species are involved in West Nile transmission among avian species. We have emphasized a priority research need to evaluate the potential for avian involvement. Should Zika transmit like West Nile, the threat assessment for the US will be different. It is important to note that avian involvement and West Nile-like transmission is not likely to be the case.

Zika has spread explosively in the tropical belt of the Americas, and now the expected media hyperbole has arrived. These kinds of public health emergencies are being recognized with increasing frequency and often cross the line from public health to national security implications.

It is believed that Zika virus is transmitted in a primate-mosquito cycle similar to dengue virus. However, we are not 100% sure of this, and we have called for studies to examine whether this virus is capable of infecting birds. Our threat assessment for the nation will be dramatically different if the virus spreads in a similar manner as West Nile versus dengue. West Nile is ecologically established, with locally acquired cases reported here in the United States every year. Dengue, on the other hand, does not transmit locally, but in occasional circumstances and in very focal areas of California, Texas, and Florida. Many cases are imported yearly.

The World Health Organization has drawn comparisons between the current Zika crisis and the Ebola disaster. The two situations could not be more dissimilar. Ebola was associated with an approximate 70% case fatality rate and 15,216 confirmed cases as of January 20th for all of West Africa. Recall there was great concern at the time of virus mutation leading to a more transmissible form of Ebola. It turns out this was not the case and CDC’s forecasts of over a million cases in West Africa were not realized. Zika virus infection estimates so far are as high as 2 million cases across 18 tropical American countries, with more than 3,500 birth defects documented. Fatalities have been rarely reported among patients with sickle cell disease. Zika is impacting drastically more people than the Ebola epidemic, however without the same degree of lethality. . So, to compare Ebola and Zika is truly not appropriate: these are two very different situations.

It should also be noted that Zika was not the only Africa-origin exotic virus to recently be introduced to tropical America: Chikungunya appeared two years ago and has spread rapidly as well. Chikungunya is also an African mosquito-transmitted virus. Brazil has, to-date, reported limited data regarding the emergence of Chikungunya in their country so we do not know how many people have been impacted. Brazil’s overall public health surveillance enterprise is not as robust as other countries and they have motivations to avoid reporting outbreaks such as the recent World Cup and upcoming Olympics.Both Chikungunya and Zika have been associated with birth defects so Chikungunya deserves our attention, too.

Many in the global public health community have embraced a public communications tactic of frightening politicians to action. It is unfortunate the community feels the need to take this path. However, fear mongering associated with public communication has badly eroded credibility of the entire global public health enterprise. Careful balancing of what is / is not known in context that avoids hyperbole is recommended. That said, WHO is under heavy fire right now due to the perception they are again slow to respond.

Our team at the Nevada State Infectious Disease Forecast Station at the School of Community Health Science, University of Nevada-Reno has emphasized toning down the language presented to the media regarding Zika, such as comparisons to Ebola. Further, we have emphasized clarity in regards to public messaging regarding sustained epidemic transmission potential. It is our assessment that sustained epidemic transmission of Zika in the United States is highly UNLIKELY except for focal, limited transmission potential in local areas in California, Texas, and Florida.  Importations are, of course, expected to occur to the relative same frequency as seen with Chikungunya. We propose that Chikungunya, because of a nearly identical transmission mechanism and the fact it is also transmitting among a population that has never been exposed to it before, is a useful analog to what we might see for Zika.

According to Ascel Bio, the forecasted peak of dengue / Chikungunya activity in Brazil is March / April.  Because this is a first wave transmission situation with Zika in the context of no herd immunity, we believe peak transmission will occur closer to March.  We emphasize the uncertainty in this aspect of the assessment. The implication is the peak of potential importation risk may correlate.

Neither a vaccine nor anti-viral agent is expected to be available until after herd immunity is established in tropical America. The experience in French Polynesia suggests there is, in fact, a limit to explosive transmission of Zika virus. There is evidence that herd immunity may be established quickly and will inhibit transmission beyond the first season.

Our team agrees with the core CDC guidance regarding a travel advisory for pregnant women, however the reality is more nuanced:

  • Many women do not realize or are proactive about determination of their pregnancy status. That’s likely why a recent global poll of more than 4,000 doctors on Sermo found that two thirds think the travel warning should be extended to all women of fertile ages.
  • Because the disease has the potential to be transmitted sexually (and possibly orally), the male half of a fertile couple should also be considered in a travel advisory.
  • It is not fully appreciated when the fetus is truly at-risk. There remain questions regarding risk in the 1st, 2nd, or 3rd trimester.
  • Many families are unable to comply with the travel advisory due to family, business, and other factors.
  • This then introduces uncertainty regarding testing and screening procedures.
  • Because of the uncertainty regarding the cause of microencephaly, we have encouraged clinical providers to consider Chikungunya and Zika testing together.

There remains uncertainty regarding protection of the blood supply. In French Polynesia they were compelled to abandon their national stockpile of blood and were forced to import blood from out of country. This raises questions about blood supply screening procedures here in the United States. Brazil has already reported suspected transmission of Zika virus via blood transfusion. The United States Red Cross says blood donors who have visited Zika outbreak zones should wait at least 28 days before giving blood.

In summary, we expect importations to continue in the United States, with near-zero probability of widespread sustained transmission.

  • We will need to keep an eye on focal areas of California, Texas, and Florida.
  • We also expect further confusion from clinical healthcare providers regarding patient counseling and testing procedures.
  • We recommend watching the discourse regarding review of the Brazil data very closely.

The time limit for this crisis is expected to be 2 seasons of transmission until herd immunity stabilizes the transmission dynamics south of the border.