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Biological transmission occurs when the pathogen reproduces within a biological vector that transmits the pathogen from one host to another ( [link] ). Arthropods are the main vectors responsible for biological transmission ( [link] ). Most arthropod vectors transmit the pathogen by biting the host, creating a wound that serves as a portal of entry. The pathogen may go through part of its reproductive cycle in the gut or salivary glands of the arthropod to facilitate its transmission through the bite. For example, hemipterans (called “kissing bugs” or “assassin bugs”) transmit Chagas disease to humans by defecating when they bite, after which the human scratches or rubs the infected feces into a mucous membrane or break in the skin.

Biological insect vectors include mosquitoes , which transmit malaria and other diseases, and lice , which transmit typhus . Other arthropod vectors can include arachnids, primarily ticks , which transmit Lyme disease and other diseases, and mites , which transmit scrub typhus and rickettsial pox . Biological transmission, because it involves survival and reproduction within a parasitized vector, complicates the biology of the pathogen and its transmission. There are also important non-arthropod vectors of disease, including mammals and birds. Various species of mammals can transmit rabies to humans, usually by means of a bite that transmits the rabies virus. Chickens and other domestic poultry can transmit avian influenza to humans through direct or indirect contact with avian influenza virus A shed in the birds’ saliva, mucous, and feces.

a) Step 1: fly picks up pathogen from fecal matter and carries it on its body. 2: Fly transfers pathogen to food. 3: Person eats contaminated food and gets sick. B) Step 1: Infected mosquito bites uninfected person. 2: Infections spreads through body and into red blood cells. 3: Second mosquito bites infected person. Mosquito may now transmit infection to another person.
(a) A mechanical vector carries a pathogen on its body from one host to another, not as an infection. (b) A biological vector carries a pathogen from one host to another after becoming infected itself.
Table titled common arthropod vectors and selected pathogens. Columns: Vector; species, pathogen; disease. Black fly; Simulium spp.; Onchocerca volvulus; Onchocerciasis (river blindness). Flea (has 2). Ctenocephalides felis; Bartonella henselae; cat scratch disease. Xenopsylla cheopis (has 2). Rickettsia typhi; murine typhus. Yersinia pestis; plague. Kissing bug, Triatoma spp.; Trypanosoma cruzi; Chagas disease. Louse; Pediculus humanus humanus (has 3) Bartonella quintana; trench fever. Borrelia recurrentis; relapsing fever. Rickettsia prowazekii; typhus. Mite/chigger (has 2).  Leptotrombidium spp.; Orientia tsutsugamushi; scrub typhus. Rickettsia akari; rickettsialpox. Moquito (has 3). Aedes spp and Haemogogus spp.; yellow fever virus; yellow fever. Anopheles spp.; Plasmodium falciparum; malaria. Cutex pipiens; west nile virus, west nile disease. Sand fly; Phlebotomus spp.; Leishmania spp.; Leishmaniasis. Tick (has 2): Ixodes spp; Borrelia spp.; Lyme disease. Dermacentor spp. And others; Rickettsi rickettsia; rocky mountain spotted fever. Tsetse fly; Glossina spp. Trypanosoma brucei Trypanosomiasis (sleepting sickness).
(credit “Black fly”, “Tick”, “Tsetse fly”: modification of work by USDA; credit: “Flea”: modification of work by Centers for Disease Control and Prevention; credit: “Louse”, “Mosquito”, “Sand fly”: modification of work by James Gathany, Centers for Disease Control and Prevention; credit “Kissing bug”: modification of work by Glenn Seplak; credit “Mite”: modification of work by Michael Wunderli)
  • Describe how diseases can be transmitted through the air.
  • Explain the difference between a mechanical vector and a biological vector.

Using gmos to stop the spread of zika

In 2016, an epidemic of the Zika virus was linked to a high incidence of birth defects in South America and Central America. As winter turned to spring in the northern hemisphere, health officials correctly predicted the virus would spread to North America, coinciding with the breeding season of its major vector, the Aedes aegypti mosquito.

The range of the A. aegypti mosquito extends well into the southern United States ( [link] ). Because these same mosquitoes serve as vectors for other problematic diseases ( dengue fever , yellow fever , and others), various methods of mosquito control have been proposed as solutions. Chemical pesticides have been used effectively in the past, and are likely to be used again; but because chemical pesticides can have negative impacts on the environment, some scientists have proposed an alternative that involves genetically engineering A. aegypti so that it cannot reproduce. This method, however, has been the subject of some controversy.

One method that has worked in the past to control pests, with little apparent downside, has been sterile male introductions. This method controlled the screw-worm fly pest in the southwest United States and fruit fly pests of fruit crops. In this method, males of the target species are reared in the lab, sterilized with radiation, and released into the environment where they mate with wild females, who subsequently bear no live offspring. Repeated releases shrink the pest population.

A similar method, taking advantage of recombinant DNA technology, Blandine Massonnet-Bruneel, Nicole Corre-Catelin, Renaud Lacroix, Rosemary S. Lees, Kim Phuc Hoang, Derric Nimmo, Luke Alphey, and Paul Reiter. “Fitness of Transgenic Mosquito Aedes aegypti Males Carrying a Dominant Lethal Genetic System.” PLOS ONE 8, no. 5 (2013): e62711. introduces a dominant lethal allele into male mosquitoes that is suppressed in the presence of tetracycline (an antibiotic) during laboratory rearing. The males are released into the environment and mate with female mosquitoes. Unlike the sterile male method, these matings produce offspring, but they die as larvae from the lethal gene in the absence of tetracycline in the environment. As of 2016, this method has yet to be implemented in the United States, but a UK company tested the method in Piracicaba, Brazil, and found an 82% reduction in wild A. aegypti larvae and a 91% reduction in dengue cases in the treated area. Richard Levine. “Cases of Dengue Drop 91 Percent Due to Genetically Modified Mosquitoes.” Entomology Today. https://entomologytoday.org/2016/07/14/cases-of-dengue-drop-91-due-to-genetically-modified-mosquitoes. In August 2016, amid news of Zika infections in several Florida communities, the FDA gave the UK company permission to test this same mosquito control method in Key West, Florida, pending compliance with local and state regulations and a referendum in the affected communities.

The use of genetically modified organisms (GMOs) to control a disease vector has its advocates as well as its opponents. In theory, the system could be used to drive the A. aegypti mosquito extinct—a noble goal according to some, given the damage they do to human populations. Olivia Judson. “A Bug’s Death.” The New York Times , September 25, 2003. http://www.nytimes.com/2003/09/25/opinion/a-bug-s-death.html. But opponents of the idea are concerned that the gene could escape the species boundary of A. aegypti and cause problems in other species, leading to unforeseen ecological consequences. Opponents are also wary of the program because it is being administered by a for-profit corporation, creating the potential for conflicts of interest that would have to be tightly regulated; and it is not clear how any unintended consequences of the program could be reversed.

There are other epidemiological considerations as well. Aedes aegypti is apparently not the only vector for the Zika virus. Aedes albopictus , the Asian tiger mosquito, is also a vector for the Zika virus. Gilda Grard, Mélanie Caron, Illich Manfred Mombo, Dieudonné Nkoghe, Statiana Mboui Ondo, Davy Jiolle, Didier Fontenille, Christophe Paupy, and Eric Maurice Leroy. “Zika Virus in Gabon (Central Africa)–2007: A New Threat from Aedes albopictus ?” PLOS Neglected Tropical Diseases 8, no. 2 (2014): e2681. A. albopictus is now widespread around the planet including much of the United States ( [link] ). Many other mosquitoes have been found to harbor Zika virus, though their capacity to act as vectors is unknown. Constância F.J. Ayres. “Identification of Zika Virus Vectors and Implications for Control.” The Lancet Infectious Diseases 16, no. 3 (2016): 278–279. Genetically modified strains of A. aegypti will not control the other species of vectors. Finally, the Zika virus can apparently be transmitted sexually between human hosts, from mother to child, and possibly through blood transfusion. All of these factors must be considered in any approach to controlling the spread of the virus.

Clearly there are risks and unknowns involved in conducting an open-environment experiment of an as-yet poorly understood technology. But allowing the Zika virus to spread unchecked is also risky. Does the threat of a Zika epidemic justify the ecological risk of genetically engineering mosquitos? Are current methods of mosquito control sufficiently ineffective or harmful that we need to try untested alternatives? These are the questions being put to public health officials now.

Micrograph of brown dots of about 50 nm inside cells; dots re labeled Zika virus. Photo of mosquito labeled Aedes aegypti. Map of where mosquitoes are found in the US. Aedes aegypti and Aedes albopictus are both found in the lower half of the US, reaching up to Connecticut, Missouri, and California. Aedes albopictus reaches further north in the eastern part o the country; through Minnosota. Aedes aegypti reaches a bit further into Utah and is in Puerto Rico.
The Zika virus is an enveloped virus transmitted by mosquitoes, especially Aedes aegypti. The range of this mosquito includes much of the United States, from the Southwest and Southeast to as far north as the Mid-Atlantic. The range of A. albopictus , another vector, extends even farther north to New England and parts of the Midwest. (credit micrograph: modification of work by Cynthia Goldsmith, Centers for Disease Control and Prevention; credit photo: modification of work by James Gathany, Centers for Disease Control and Prevention; credit map: modification of work by Centers for Disease Control and Prevention)

Questions & Answers

what a tropism in host
Khaliil Reply
HPV vaccine given to school children
Jayani Reply
Differentiation between electron, proton and neutron
Zainab Reply
proton .possitive charge electron . negative charge neutron . having no charge
Kifayat
proton positive charge. electron negative charge. And no charge of the neutron.
Saman
the nucleus is composed of electrons (-) charge and they turn around the Nucleon the Nucleon = neutron(no charge) + proton (+) a neutron can turn to a proton and vice versa (cuz they have the same mass=1)
BENNINI
what are the roles of microorganisms in human being
Buhari Reply
some causes disease, others are not disease causing
Clark
they're necessary in our digestive system+the skin,everywhere actually the number of them in the human body alone is higher(by millions)than the number of humans cells,they're indispensable in the food industry,others are fundamental to make medicines and more,what exactly are you asking about ?
BENNINI
What are the natural occurring elements found in organisms on earth?
Otu Reply
some of the naturally occurring elements found in organisms are carbon, nitrogen, hydrogen, oxygen
Yashi
thanks
Mariam
some of them are carbon, oxygen, nitrogen, hydrogen, phosphorus and sulfur
Jamal
what is Zika virus?
Somali Reply
Zika virus is a mosquito-borne flavivirus that was first identified in Uganda in 1947 in monkeys. 
Cumar
in pregnancy it can cause the unborn child's head to underdevelop so that it does not grow along with the rest of the body
Bethany
intracellular vesicles are found in
Akshay Reply
how do i report widal slide results
Isaac Reply
how food can be used as a substrate for the growth of microorganisms
Ashi Reply
what is the importance of understanding chemistry in the field of microbiology and food technology?
Charmaine Reply
What is different between eukaryotes and prokcaryotic
Abia Reply
main difference is that eukaryotic cells possess membrane bound organelles
Iqra
prokaryotes are primitive organisms that doesn't contain membrane bound nucleus or any orgenelle while in eukaryotes membrane bound orgenelles and nucleus is present
Yashi
what s anatomy
jane Reply
its mean body structure, function, and systems
gazi
study of internal structure of living things
Falere
anatomy is the study structure of the made human
isir
anotomy is the actual study of body internally and externally Which include how itis made. for what and what is need of this
Iqra
la science anatomie c'est la science qui nous aide à étudier l'homme
Baraka
okay
Baraka
Ancestor are they real
Rapheal Reply
yes of course
David
yes please
Clark
50 50, depending on the accuracy of the clan records.
Vincent
okay
Baraka
Why protist is not a kingdom of Linnaeus 'S taxonomy?
Neha Reply
I dont know
Jeewraj
Linnaeus used Aristotle's criteria for dividing living organisms into kingdoms. Plants are immobile and insensitive, while animals are mobile and sensitive. the creatures he saw through the microscope were mobile, so he attributed them to animals.
Lad
name the different types of media use in lab to detect the micro organisms
Shehzadi Reply
The different types of media used in the lab to detect the microorganisms is known as cell culture..
Tean
solid media or broth is used to grow and detect microorganisms
Pohor
what is microbiology
Baraka
the study of large living organisms
Pratibha
the study of organisms which are micro in range
Vency
what is autoclaving?
Yashi
process for sterilization
Vency
is a machine used on the process of sterilisation
Clark
what is difference between hot air oven and autoclave as they both are used for sterilization ?
Vency
autoclave basically do moist heat sterilization while hot air oven do sterilization by dry heat.....
Yashi
what is the microbe
HUSSAIN
which method out of these two is best?
Yashi
why human have microbe
HUSSAIN
Hi
Sadam
hi to all
Sadam
depends on what to sterilize
Vency
microbes
Md
what is knowledge
HUSSAIN
well idea
Daniel

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Source:  OpenStax, Microbiology. OpenStax CNX. Nov 01, 2016 Download for free at http://cnx.org/content/col12087/1.4
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