West Nile Virus


West Nile Virus
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Understanding West Nile Virus Transmission Methods

The dangerously infectious West Nile virus (WNV) is kept alive in a continuous cycle of mosquito-to-bird contacts and mosquito to offspring transference. A mosquito ingests the virus by feeding on the blood of a contaminated bird or an infected female mosquito transfers the virus to its eggs.  Typical scientific findings regarding the West Nile virus transmission have determined that female mosquitoes, once infected, can pass on the illness in the following manners:

  • West Nile Virus TransmissionHorizontally, to a person or mammal by taking a “blood meal” and thereafter transferring the virus vertically to her offspring during gestation.
  • Vertically, to her young during egg-laying, with no prior horizontal transference.
  • Horizontally, to a person or mammal, without further transferring the virus vertically.

About Horizontal Transmission
Within a few days of having fed off an infected bird, the West Nile virus is distributed throughout the female mosquito’s bloodstream and ultimately finds its way to the secretion glands. The infected mosquito then transmits the virus directly to a host (human or mammal) via its saliva as it bites the flesh.  This method of West Nile virus transmission is called horizontal.

Although horizontal transmission is of significant importance to sustaining the virus in nature during the year’s warm summer months and into the fall, a further process is required to preserve its existence during the cold winter months when mosquitoes become inactive. This other method of transmission is known as vertical.

About Vertical Transmission
The West Nile virus can be passed down from one generation to the next within the mosquito population by way of vertical transference (female adult to offspring), or in literal terms, transovarial transmission. This biological method of transfer occurs when the infected adult female mosquitoes infect their young as they develop inside the egg. Upon hatching, the larvae are already WNV-positive and able to transmit the virus when they grow into adulthood. Because the female mosquito is the only sex that bites, finding the virus in a male is only possible through vertical transmission.

Field and Facility Testing
Vertical West Nile virus transmission gives way to horizontal transmission, allowing the disease to endure the winter hibernation of the mosquito and launch a fresh wave of infection come springtime. This theory (and a subject under much debate for some years now) has been recently put to the test with natural and laboratory studies.

Reports from research conducted in nature have concluded that infected female mosquitos of the “Culex pipiens” genus can in fact vertically transmit the West Nile virus to its descendants, permitting the strain to remain effective while withstanding the northern winter climate. The virus quarantine has been collected from both adult males and nulliparous females (those who have not yet spawned) in the field over the summer. The same virus was also drawn from female mosquitoes resting in standing water the following winter. End results have indicated that vertical transmission is likely happening by course of nature.

Indoor lab studies have gone on to further prove the argument by manually injecting adult female mosquitoes with WNV. After a week of incubation, they were released for the next phase of testing. The procedure has essentially shown evidence of same: that even after a controlled period of hibernation, the vertically transmitted newborns of the contaminated female were still capable of infecting small test subjects with the West Nile virus.

When used side-by-side, the two West Nile virus transmission methods create a vicious loop. Together, they are responsible for the increase of infection cases over the summer months, which seem to spike especially in August, and the reintroduction of WNV the following spring.

The Culexbreed of mosquito (a.k.a. the common North American mosquito) is considered the primary carrier of the West Nile virus strain, and responsible for the widespread of the ever-present, potentially harmful disease.


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