The Zika Virus is an example of an invasive species. Originally from Africa, it has now spread around the globe to every continent but Antarctica and hundreds of remote islands. The Zika Virus depends on a mosquito, Aedes aegypti,
for most of its transmission.
Aedes aegypti is also an invasive species. A native of Sub-Saharan Africa, Aedes aegypti began a long association with humans when humans first moved into the African forests. Aedes aegypti was able to find suitable hosts (humans) and nearby habitat in the water collected by people. Aedes aegypti adapted to the urban environment and has lived in close association with humans since then. As trade from Europe increased, Aedes aegypti spread as larvae swimming in the casks of water brought on board the ships. Aedes aegypti had colonized much of the world by the 17th century.
Successful invasive species can pave the way for other species that depend on them. Without the spread of the mosquito, viruses such as Zika and yellow fever could not themselves become invasive.
Francesco Redi was a poet and a physician, a learned man in 17th century Florence, Italy. He is considered by some to be the author of the “experimental control” and the author of parasitology. He had a particular interest in flies and studied the life cycle of the blow fly. Redi conducted experiments that showed that maggots were produced not from rotting meat, but by flies that laid eggs on the meat. This observation was a refutation of spontaneous generation and part of the emerging paradigm of learning through observation and experimentation rather than sole reliance on authorities such as Aristotle.
Based on his observations of blow flies, Redi hypothesized that plant galls and the gall flies that emerged from galls, originated from fly eggs. However, Redi was not able to observe gall flies laying eggs or adequately exclude them in his study of galls. This led him to conclude (erroneously) that galls were produced by plants by the same process that caused plants to produce flowers and fruits. Redi’s error was soon corrected by Vallisneri, who observed the eggs laid in the plants.
Today, Redi’s error is obvious, but in the 17th century, genes, DNA and plant hormones were all unknown and no good model of flower production had been proposed. Even today, the mechanisms behind gall creation (secretions of the fly induce gall formation) are vaguely understood and are the subject of intense investigation.
Silkworm, Bombyx mori
The silk produced by larvae of the silkworm, Bombyx mori, is a long thread composed of shorter interlocking strands of silk protein. Silk thread can be disassociated into its protein units, silk nanofibrils. These nanofibrils can be dissolved in solution and when appropriately filtered, will form a membrane of interlocking silk nanofibrils.
The membranes are useful for filtration applications They are free standing, do not dissolve in water or solvent and can be cut to a desired shape. The membranes contain nanopores from 4 to 20 nm. The thickness of the membrane can be varied by altering the conditions used to produce the membrane. Increasing the membrane thickness from 40 nm to 120 nm decreases the pore size. The membranes have water permeabilities similar to other filtration materials. Insect biomaterials such as silk are increasingly important in nanoscale materials applications.
Shengjie Ling, Kai Jin, David L. Kaplan, and Markus J. Bueller. Ultrathin Free-Standing Bombyx mori Silk Nanofibril Membranes. Nano Letters 2016 16 (6), 3795-3800
Left: Silkworm caterpillar, sham injected
Right: Silkworm caterpillar injected with bacteria
Photo: Ishii & Colleagues*
Caterpillars suffering from bacterial infections often stop moving and become lethargic. The paralytic proteins (cytokines) present in caterpillar hemolymph are involved. The same contractile paralysis that is initiated by paralytic proteins can be induced by injecting bacteria into the hemocoel of caterpillars. (Note the significant contraction in the photo) This has been demonstrated* by comparing injection of bacteria to injection of the paralytic protein. The bacteria trigger the activation of the paralytic protein which affects numerous organs and stimulates nitrous oxide release. Is the paralysis adaptive? The answer is unknown. Immobilization may slow the movement of pathogens and confine their location which may make them easier for the caterpillar immune system to eliminate them. Some pathogens are present on food and paralysis will stop a caterpillar from ingesting addition food that may be contaminated.
*Ishii, K., Hamamoto, H. and Sekimizu, K. (2015), PARALYTIC PEPTIDE: AN INSECT CYTOKINE THAT MEDIATES INNATE IMMUNITY. Arch. Insect Biochem. Physiol., 88: 18–30.
Silkworm, Bombyx mori
Caterpillars of silkworms and other moths are known to produce peptides (cytokines) that can cause larval paralysis. It is speculated that these peptides may serve as a partial defense against parasitoids. The levels of paralytic peptide that can be isolated from the hemolymph of a caterpillar are many times that needed to induce paralysis. How can the caterpillar avoid the paralytic effects of its own peptides?
The peptide circulates in the hemolymph in an inactive form. It can be converted to a modified “paralytic” form by serine proteases that respond to damage to the caterpillar cuticle and loss of hemolymph. The activated protein has a number of biological effects such as paralysis accompanying muscle contraction and changing the shape of hemocytes as part of the immune defensive response of the caterpillar.
Caterpillars are important models for the study of immune response. Knowing how caterpillars fight infections could lead to new methods for fighting human infections.
Soon-Duck Ha, Shinji Nagata, Akinori Suzuki, Hiroshi Karaoke. Isolation and structure determination of a paralytic peptide from the hemolymph of the silkworm, Bombyx mori. Peptides. Volume 20, Issue 5, June 1999, Pages 561-568.
Allergic Reaction To Mosquito Bite
First Chikungunya and now Zika have “gone viral (literally) in the Caribbean and South America. It is unknown why these viruses went from relative obscurity in the African rain forests to become widespread epidemics.
They are not the only viruses being transmitted in the Caribbean by mosquitoes. Mayaro virus, which is related to the Chikungunya virus is also transmitted by mosquitos. Reported symptoms include (but are not limited to) high fever at 40 °C, joint pain, myalgia, headache, asthenia and some diarrhea.
The primary transmission zones are in Brazil and French Guiana. Tourists from France have been diagnosed with Mayaro* on their return from French Guiana. Mayaro virus has also been detected in some of the Caribbean Islands. The similarity of symptoms produced by mosquito vectored viruses is challenging medical authorities with identification. A viral infection may be misreported based on symptoms alone. Genetic diagnoses are more reliable, but are underutilized because of the cost. The focus on Zika has led to more genetic testing for viruses and is detecting less common viruses such as Mayaro. Research on these diseases can potentially lead to better identification, management programs and health outcomes.
*Marion Llagonne-Barets & colleagues. A case of Mayaro virus infection imported from French Guiana. Journal of Clinical Virology. Volume 77, April 2016, Pages 66–68
Florida is making some progress with its aggressive spraying of insecticides to control mosquitos that vector Zika Virus. The Wynwood area north of Miami, the first site of local Zika transmission, has not had a new transmission case in 45 days. This suggests that Zika is no longer present in the local Wynwood mosquito population.
Meanwhile Miami Beach has reported several new cases of local transmission and the area at risk for local transmission of Zika is tripled. The mosquito sprays are receiving criticism from environmentalists and local residents concerned about the environmental and health effects of the insecticides. However, local authorities and Zika experts agree that the risks from NOT controlling mosquitos and Zika are far greater than the risks from the insecticide for the population as a whole. Unfortunately, there are no good options for mosquito control that do not involve insecticides. The authorities are acting to minimize the severe and consequential risk of a larger Zika outbreak against a much lower but uncertain risk from spraying the insecticide naled.