In my last two blog pieces, I began a blog series addressing the fact that we have seen a number of diseases that are not endemic to the U.S. pop up and cause outbreaks. Some of these are infections that we have not previously seen people acquire in the continental U.S., but rather were involving international travelers who acquired the infection elsewhere, then traveled to the U.S., became ill, and were diagnosed with the illness here.
In part I, I reviewed a short history of a number of these outbreaks focusing on those that have occurred since 2020 to illustrate that this is not just one or two diseases, nor ancient history.
In part II, I discussed the dengue virus that caused an outbreak in Florida in 2020 that was of note because we rarely see locally contracted infections in the continental U.S.
In this blog piece, I will discuss the Zika virus. Zika virus belongs to the same family of viruses as the dengue virus – flaviviruses. As I mentioned in my last blog piece, flaviviruses are vector-borne diseases, meaning that instead of human à human transmission or animal à human zoonotic transmission, these viruses are transmitted through the bites of mosquitoes (although there is some evidence that Zika can be transmitted through sexual contact, blood transfusions, and perhaps even tissue or organ donation, though these are all less common routes of transmission. Nevertheless, WHO recommends that men returning from an area with active Zika transmission use protection or abstain from sex for a period of 3 months after their return to protect their sexual partners and that women engage only in protected intercourse or abstain from sex for a period of two months following their return from an endemic area). The same type of mosquito that transmits dengue virus and the Chikungunya virus also transmits Zika virus, and thus we tend to see Zika infections in tropical and subtropical regions of the world.
Zika virus was first identified in Uganda in 1947 in a Rhesus macaque monkey followed by evidence of infection and disease in humans in other African countries in the 1950s. During the 1960s through the 1980s, Zika disease predominantly occurred in Asian and African countries. since 2007, outbreaks of Zika virus disease have been recorded in Africa, South and Central America, Asia and the Pacific rim countries that are in the southern hemisphere, where the Aedes mosquitos abound.
In 2015 and 2016, large outbreaks of Zika virus occurred in Central and South American countries, resulting in an increase in travel-associated cases detected in US states, and widespread transmission in Puerto Rico and the US Virgin Islands. In July of 2016, the first outbreak in the continental United States was identified in the Wynwood area of Miami-Dade County, Florida.
While like dengue fever, most cases of Zika virus disease are mild, in early outbreaks we observed an increased frequency of Guillain-Barre’ syndrome (GBS), a disorder that can range from a brief duration of mild weakness and abnormal sensation, to an ascending paralysis that can be severe and progressive enough to require a ventilator for respiratory support until the condition resolves, which it does in most cases. GBS can evolve rapidly over hours, or more slowly over days or even weeks.
When Zika emerged in the Americas with a large outbreak in Brazil in 2015, we noticed an alarming association between Zika virus infection in pregnant women and congenital microcephaly (an abnormally small head) of the infants delivered from these mothers. Infants with complications from their mothers’ Zika infection during pregnancy are said to have congenital Zika syndrome. This appears to occur in 5 – 15% of pregnancies during which the pregnant mom becomes infected (some reports are as high as one-third of infants born to mothers with Zika infection during pregnancy will develop microcephaly), even when the mother is completely asymptomatic and unaware that she was infected. Besides the microcephaly that is generally quite obvious and devastating on its own, this syndrome can include limb contractures (where the arms or legs are unable to move the full normal range of motion), abnormal muscle tone, eye abnormalities and hearing loss. Also tragically, some babies will become non-viable and the mother will experience fetal loss or the baby may be a stillbirth. Zika infection during pregnancy can also result in premature birth.
There is a study following the infants born with congenital Zika syndrome in Brazil following the large outbreak in 2015 that shows these infants have an 11-fold higher mortality risk during the first year of life.
Like with dengue fever, most persons who are infected with Zika virus do not develop symptoms or develop only mild symptoms. Those that do become symptomatic generally do so between days 3 and 14 following infection. Many of the symptoms are similar to those of dengue fever – headache, fever, muscle and joint pain and rash. The symptoms generally last 2 – 7 days.
Other complications of Zika virus disease tend to occur in older children and adults and include Guillain-Barre’ Syndrome (discussed above), peripheral neuropathies (loss of proper nerve functioning of one or more nerves to the extremities), or myelitis (an inflammation of the spinal cord).
Like dengue fever, there are no specific treatments or therapies for Zika virus disease; but unlike dengue fever, there is no vaccine against Zika.
Again, there is nothing in particular that Americans need to do in regard to Zika unless you are planning to travel to an endemic area, however, our public health planning needs to take into account these increasing occurrences of outbreaks of vector-borne diseases occurring in the southern U.S. that we have not previously dealt with in terms of locally acquired infections.
We have had outbreaks in 2016 (Zika), 2020 (dengue fever) and 2023 (malaria). We must consider these as warning signs. In my next blog post, we will discuss Chikungunya.
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