Zika Virus, Mosquitoes, and Microcephaly: A Worrying Infection
A Troubling Viral Infection
The Zika virus is transmitted to humans via a mosquito bite and causes disease. The symptoms of the disease are unpleasant but are generally mild. There may be no symptoms at all from the infection. The virus is worrying, however, because it has spread rapidly and because it may be linked to more serious disorders, including Guillain-Barré syndrome and microcephaly (abnormally small brain and head size in babies).
The virus is transmitted by a bite from a tropical mosquito belonging to the genus Aedes. This is not the same genus that causes malaria. Aedes does transmit diseases, though. It's responsible for dengue, yellow fever, and chikungunya as well as Zika virus disease. The genus lives in the United States. Researchers say that it's possible that the Zika virus will eventually reach the U.S. and be transmitted by bites from local mosquitoes.
Zika Virus Disease
The Zika virus is not new. It was discovered in 1947 in rhesus monkeys in Uganda. The virus is named after the Zika Forest in that country. It was found in humans in 1952, but for a long time was not common. The first episode that could be called an outbreak took place in 2007. In 2014, the first evidence of viral transmission from mother to fetus was discovered. Its significance wasn't appreciated at the time.
In recent times, the virus has spread rapidly and has attracted the attention of health authorities. Research suggests that a genetic change that is helpful during infection has spread through the viral population. This change has made the virus more successful in its attack on humans.
Possible Symptoms and Treatment
Symptoms of Zika virus disease include a fever, rash and headache, fatigue, muscle and joint pain, and conjunctivitis (red and inflamed eyes). The symptoms last for a few days to about a week. However, 80% of infected people show no symptoms at all. People generally recover from the infection without help. The infection may make a person feel miserable and may interfere with life for a while, but in general the illness isn't serious.
There is no specific treatment for Zika virus disease. Just as in any viral infection, rest, adequate fluids, and good nutrition will help the immune system to fight the virus and will aid recovery. Medications to reduce a fever and inflammation and to relieve discomfort can be bought in drugstores. If these don't help, a doctor's advice should be sought. Pregnant women and people with serious diseases should consult a doctor if they experience symptoms that suggest the presence of the Zika virus.
Secondary Effects of the Infection
The link between the Zika virus and other diseases is worrying. The potential relationship is being investigated by public health agencies. An increase in Guillain-Barré syndrome was noticed after one outbreak of the virus in French Polynesia and after another in Brazil. An increase in the number of babies born with microcephaly was noticed after an outbreak in Brazil. The latter situation is of special concern to pregnant women and to those who hope to become pregnant soon.
According to the CDC, there is now scientific consensus that the virus can cause microcephaly in newborn babies if the infection develops during pregnancy. The link between the virus and Guillain-Barré syndrome is strongly suspected but is not quite as certain as the link with microcephaly.
There is now scientific consensus that Zika virus infection during pregnancy is a cause of microcephaly. We do not know if a newborn who gets Zika virus infection around the time of birth will develop microcephaly after birth.— CDC (Centers for Disease Control and Prevention)
Viruses: More Complex Than They Appear
Unlike other living things, viruses aren't made of cells. They consist of nucleic acid—either DNA or RNA—surrounded by a protein coat. Some viruses have a lipid envelope surrounding the protein coat. Unlike cells, they can't reproduce on their own. They must enter the cell of another creature and then direct that cell to make new viruses.
For a long time viruses have been classified as non-living entities, but the idea that they are living things seems to be becoming more popular. Nucleic acids contain the genes that give living things many of their characteristics. Viruses have these genes, even though they can't make much use of them without the aid of a cell and its equipment. Nevertheless, researchers are showing that at least some viruses have surprisingly complex behaviours.
The video below describes how a virus invades a cell. Everyday terms are used for cell parts and processes instead of the correct scientific terms. Despite this simplification, I think the video does a good job of conveying the idea that the host cell is "tricked" into making new virus particles.
DNA stands for deoxyribonucleic acid while RNA stands for ribonucleic acid. Most viruses contain DNA, but some—including the Zika virus—contain RNA instead.
Zika Virus Infection
When foraging for food, a female mosquito pierces human skin and a blood vessel, injects an anticoagulant to stop blood from clotting, and then withdraws some blood. She needs substances in the blood in order to help her eggs to develop.
The mosquito's saliva may contain Zika virus particles that were obtained by withdrawing blood from an infected person. These particles enter the body of the mosquito's new victim during the bite. Researchers have found that the Aedes mosquito probes the skin before piercing a blood vessel and that during this process the Zika virus is deposited in the skin.
RNA from the Zika virus has been found in human saliva, urine, and semen. Although mosquito bites seem to be by far the most common route of infection, a few cases have suggested that the virus can be sexually transmitted. There may be additional methods of transmission.
Destruction of Human Cells
In a lab experiment, some French scientists observed the response of living patches of human skin cells to the presence of Zika virus virions. (A virion is an individual virus particle.) They discovered that none of the fibroblasts in the skin were able to block the entry of the virus and that all of them were infected within 72 hours. Other types of skin cells also allowed the virus to enter.
After a virion entered a skin cell, it took control of the cell and "forced" it to make new virions. Once the viral replication had finished, autophagy occurred. This is the process in which a cell destroys some of its contents. Autophagy was followed by the cell breaking up in a process known as apoptosis (self destruction) and by the spreading of virions to new cells.
The information discovered in the experiment is interesting, but there is still much that we don't understand about the Zika virus. Little attention has been paid to the virus until recently because the disease that it causes is so mild and because in the past the disease wasn't widespread. Now that the virus has been linked to microcephaly and is spreading fast, researchers are scrambling to learn more about it. If we understand its biology, we may be able to fight it.
At one time, mosquitoes in the genus Aedes were only found in tropical areas. Now they have spread to every continent except Antarctica. The mosquitoes have black and white markings on both their body and their legs. They are active during the day and may bite at any time during this time period. Most bites occur early in the morning and in the late afternoon and early evening.
Two species of Aedes are known to transmit the Zika virus. Aedes aegypti is the usual transmitter. It's sometimes known as the yellow fever mosquito. In the U.S., the species is most common in Hawaii and in the southern states that border the Gulf of Mexico. The insect has been found further north, though, especially when the weather is warm.
The Asian tiger mosquito, or Aedes albopictus, also transmits the Zika virus and is found in the warmer parts of the United States. It's unclear how significant this mosquito is in the spread of Zika virus disease.
Although a Zika virus infection is generally mild, the secondary effects that appear after the infection may be serious.
Microcephaly is a sad condition in which a baby is born with an unusually small head and brain. The disorder may also develop in the first few years of life. In this case the head and brain fail to enlarge sufficiently as the child grows. Occasionally a child with microcephaly has normal intelligence and abilities, but generally there is impairment, which ranges from mild to severe. Techniques such as speech and occupational therapy can be helpful for a person with microcephaly. The person may require special care throughout their life, however.
The New York Times has published some alarming statistics about microcephaly in Brazil.
- The state of Pernambuco in Brazil normally has around nine babies born with microcephaly in a year. In the year that ended in November, 2015, 646 children had been born with microcephaly.
- In a normal year, about 150 children are born with microcephaly in the whole of Brazil. In 2016, almost 4,000 cases were born with the disease.
It's no wonder that pregnant women in Brazil panicked during the disease outbreak, as reports suggest.
Guillain-Barré syndrome is a disorder in which the body's immune system attacks nerves. The immune system normally attacks invaders such as bacteria and viruses and protects us from disease. Something is very wrong when the system attacks normal structures that are an essential part of the body. The syndrome is rare but serious.
The first symptoms of Guillain-Barré syndrome are generally muscle weakness and tingling. The symptoms can escalate, resulting in paralysis. The paralysis may be a medical emergency. The disorder requires hospital treatment. Provided they get this treatment, most people recover from the disease. They may experience lingering symptoms, however. The cause of the disease is unknown, but it is known that it develops after a person has experienced a major infection of some kind.
Preventing a Zika Virus Infection
Researchers have started the process of creating a vaccine for the Zika virus. Unfortunately, this process will probably take a long time. The best thing that we can at the moment to prevent an infection is to avoid mosquito bites.
The CDC in the United States and health authorities in Canada are currently recommending that pregnant women avoid travelling to countries where the Zika virus is transmitted.
People should contact a health agency in their own country and do some careful research before they begin to travel. Recommended precautions to prevent mosquito bites are listed below. At the moment, these precautions are needed when travelling to a tropical country containing mosquitoes that transmit disease. Some of them may become necessary in other countries as mosquitoes spread.
- Wear insect repellent when outdoors during the day or early evening.
- Cover as much of the body as possible with clothing. Wear light-coloured clothing to reduce the chance of overheating.
- Consider treating clothing with insecticide.
- Make sure that all the windows in a building have secure screens that mosquitoes can't penetrate.
- Close windows and use air conditioning whenever possible.
- Keep doors closed as much as possible.
- Sleep under a mosquito net.
- Remove still and stagnant water around the home. Mosquitoes lay their eggs in still water. Even puddles or water in small items like buckets and flower pots may attract egg-laying females.
The Zika Virus in North America
WHO believes that the Zika virus will eventually appear in every country in North and South America where the Aedes mosquito lives. Only Canada and Chile are currently free of the mosquito.
The virus has already been found in Puerto Rico. There are fears that when the weather warms up in the spring and summer the virus will appear in the United States. People living in the more northerly and generally cooler states of the U.S. shouldn't ignore the situation. Aedes has been found as far north as Washington.
Every woman who becomes pregnant in the near future or who hopes to so should keep up to date with respect to Zika virus news. If the virus is discovered in her area—and perhaps even before this discovery—she should take steps to prevent mosquito bites. It's a good idea for other people to do this, too. Communities may take steps to reduce the spread of mosquitoes, but individuals need to protect themselves as well
The video below was produced in 2016. In 2018, there appears to be a lull in disease outbreaks. The disease still exists, however, and can still produce serious effects.
The Virus in 2018
As of 2018, no vaccine for a zika virus infection is available, although one potential candidate is undergoing a clinical trial and others are in development. New details about the structure of the virus have been discovered, however, which could be useful for the creation of a vaccine or for the production a medicine to fight the virus.
After the worrying situation in 2016, fewer cases of the disease were diagnosed in 2017. The trend is continuing in 2018. Researchers suspect that we have reached a stage of "herd immunity" in many places. This term is used when a large proportion of a population is immune to a disease. This greatly reduces the transmission of an infection from one person to another. It seems that our body develops immunity to the Zika virus after being infected.
Researchers warn that we shouldn't become complacent. They have observed a similar decline in infections caused by viruses related to the Zika virus, only to have the disease flare up again at a later date.
A Potential Problem
Some health experts in the United States are saying that most of us shouldn't be scared by the Zika virus. If we become infected, our symptoms will almost certainly be non-existent to mild. Guillain-Barré syndrome may be a secondary effect of the viral infection and can be very serious, but it's a rare disease and is usually treatable. The major problem linked to Zika virus is microcephaly in babies. This is the effect that is most worrying. Microcephaly is a lifelong condition.
Hopefully researchers will discover ways to protect us from a Zika virus infection. It's unknown when this protection will be available, however. Until it appears—assuming it ever does—we need to take steps of our own to prevent Zika virus disease.
- Zika virus information from the CDC
- Information about the virus and pregnancy from the CDC
- Virus facts from WHO
- Information about the virus from the New York Times
- Biology of the virus in human skin cells from the NCBI
- The link between Zika virus and other diseases from the ScienceDaily news service
- Zika virus disease in 2018 from PBS
Questions & Answers
© 2016 Linda Crampton