Dynamic Facts

 

Great fleas have lesser fleas to bite em; and so on, ad infinitum

According to the Centers for Disease Control, as of September 21, 2016, there are a total of 3,358 cases of Zika in the U.S.. 43 of these were acquired by being infected by local mosquitoes; 3,314 infections were acquired outside of the U.S.; 28 cases were acquired by sexual contact; with 1 having been acquired in a laboratory. There are also 8 cases of Guillian-Barré syndrome;  “a rare disorder in which your body’s immune system attacks your nerves. Weakness and tingling in your extremities are usually the first symptoms.”These sensations can quickly spread, eventually paralyzing your whole body. In its most severe form Guillain-Barre syndrome is a medical emergency. (Mayo Clinic)

The case of Zika provides insight into our natural world. The questions of why Zika, a disease that originated in a distant forest in central Africa and why it has become of particular health concern in the U.S. can be understood by looking at how natural phenomena appear to work.

Natural phenomena can be thought of as facts but are perhaps better understood as facts that are in dynamic relationship with one another: insects, viruses, humans, environments. The facts you think you know now are busily changing into new facts that.

Insects have a variety of ways of getting their nourishment: they can sip (like butterflies); they can chew (locusts); they can pierce and suck (mosquitoes).  The mosquito way of dining is different for males and females. The males are sippers; their mouthparts are adapted to collecting plant nectars.

Females can both sip nectar but in order to successfully lay eggs that will hatch, they need vertebrate blood and its protein and iron. To get this mother mosquitoes use their mouthparts to pierce and suck reliable sources of blood like humans.

Valerie Choumet, a scientist at the Pasteur Institute in Paris captured the process used by a female mosquito to get her blood meal.

As the female mosquito pushes its bundle of six mouth parts (two maxillae, two mandibles, a double tube (hypopharynx and labrum) into the skin, the labium or sheath folds back. The two mandibles and a pair of maxillae are used to saw through the skin and then to anchor the insect so that she can push a double tube (hypopharynx and labrum) to search for a blood vessel. While the tube appears to be stiff, it is actually highly flexible and is fully controllable by the mosquito, even bending it at right angles.

She spits saliva down the hypopharynx, this prevents blood from coagulating and the labrun is used to pierce a blood vessel and to suck up the blood.

You can view the video here.

The mosquito’s blood dinner will pay off when she lays her eggs in or near water. The eggs nourished by the proteins and iron from the blood will hatch out and eventually become mosquitoes. If the female mosquito is herself infected with Zika, the eggs will develop into mosquitoes that are also infected with Zika.(University of Texas at Galveston, 2016)

But the blood dinner has another another organism with a different purpose. There is a whole group of viruses that have adapted themselves to using the mosquito as a way to get into vertebrates where they can breed. These are known as arboviruses (from arthropod borne viruses).

When she sucks up the blood of a human who has the virus in his blood, the mosquito becomes infected. The virus does not grow in the mosquito but will remain in the mosquito until she spits, along with the spit the virus particles will hitch a ride, putting the virus into the blood stream of a vertebrate where it can fulfill its destiny.

The  mosquito and its  passenger virus are apparently well-adapted to one another.  New research has shown that the saliva of the mosquito “causes an inflammation that helps the virus particles multiply and quickly spread to other parts of your body.” (Kupferschmidt, 2016)

The Zika virus is little more than RNA covered with a protein coat shaped as a 20-sided polygon called icosahedron, resembling “sinister Christmas ornaments.”

The virus attaches itself to the surface of the host’s cells, penetrates, turns its RNA into DNA and “hijacks the internal machinery of the host’s cells to copy its own DNA and make new cells. Like commandos invading a town and converting its car factory into a bomb factory, the virus makes thousands of copies of itself. Eventually the cell explodes, and the viruses are released to attack other cells, spreading the illness.” (McNeil, 2016, p. 23) You can see a more detailed animated account of this process the HHIBioInteractive Website .

The Zika virus was first identified in 1947 by researchers in the Zika forest on the northern shore of Lake Victoria in central Africa. The infected animal was an Asian monkey. The first human case didn’t occur until 1952.

In the early cases the Zika virus caused a fever along with body aches and discomfort but cleared up with no apparent ill-effects.

The twentieth century has seen changes like  “a combination of warmer weather moving mosquitoes north, of cheaper, more frequent jet travel helping people reach new continents with viruses still fresh in their blood” meaning that even obscure diseases from remote parts of the world can quickly become notable in highly populated places. (McNeil, 2016, p. )

Zika appeared in the western Pacific on the island of Yap, then in Tahiti, in French Polynesia.

It made its appearance in the Western Hemisphere in 2014 with  grim discoveries in the maternity wards of hospitals in the northeastern Brazilian state of Pernambuco.

Doctors and nurses noticed that there were suddenly new borns with “this thing we had never seen…” “Children with normal faces up to the eyebrows, and then you have no foreheads…The doctors were saying, ‘Well, I saw four today,’ and ‘Oh, that’s strange, because I saw two.’” (McNeil, 2016, pp.13-14)

Later it was realized that what they were seeing was microcephaly, a birth defect that was the result of the fetal brain failing to develop. Further investigation revealed that the mothers whose children had microcephaly had also been infected by the Zika virus. It was also the case that apparently it didn’t matter at what stage in their pregnancy infection occurred.

Our habit of teaching science as sets of facts makes it easy to forget that facts are dynamic; the natural world is constant motion.  The genomic revolution has made it possible to investigate change at the level of the mechanisms of change.

In an research paper that focuses on dengue, a relative of Zika, Ciota & Kramer (2010) observe that in order to survive the virus cycles back and forth between vertebrates and blood-eating arthropods. This means that there is a premium on the virus being “plastic” in order to be able to take advantage of new environments. RNA viruses like Zika are subject to rapid replication as well as large error rates. These two factors may explain why such viruses are able to undertake “quick exploration of fitness landscapes and production of variance which may have an advantage in different host environments.” (Ciota & Kramer, 2010).

It is therefore not surprising that the Zika, in its travels around the world, would evolve new ways of spreading itself (crossing the placenta barrier; sexual transmission) and developing new targets for infection (nerve tissue).

Resources:

Ciota, Alexander T. & Laura D. Kramer (2010). Insights into Arbovirus Evolution and Adaptation from Experimental Studies. Viruses. 2010 Dec; 2(12): 2594-2617. doi:  10.3390/v2122594

You can watch a detailed account of how a flavivirus (Dengue) “hijacks” a cell at HHMI BioInteractive (2016. Dengue Transmission. http://www.hhmi.org/biointeractive/dengue-virus-life-cycle

Kupferschmidt, Kai (2016).  Mosquito spit helps viruses make us sick. Science, June 21, 2016.

http://www.sciencemag.org/news/2016/06/mosquito-spit-helps-viruses-make-us-sick

DOI: 10.1126/science.aaf5794

McNeil, Donald G. Jr. (2016). ZIKA: THE EMERGING EPIDEMIC. W.W. Norton & Company, New York.

Yong, Ed (2013). NOT EXACTLY ROCKET SCIENCE: Here’s What Happens Inside You When a Mosquito Bites. (August 8, 2013).

Wang, Lulan, Stephanie G. Valderramos, and others. From Mosquitoes to Humans: Genetic Evolution of Zika Virus. Cell Host & Microbe. 19(5), pp. 561-565. May 2016.

doi: http://dx.doi.org/10.1016/j.chom.2016.04.006

University of Texas Medical Branch at Galveston. (2016, August 30). Female mosquitoes can transmit Zika virus to their eggs, offspring: Killing only adult mosquitoes may not end Zika outbreaks. ScienceDaily. Retrieved September 22, 2016 from www.sciencedaily.com/releases/2016/08/160830091524.htm

 

Tags:

Zika, arbovirus, mosquitoes, transmission, adaptation, organisms, environments

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