Why is species jumping a rare occurrence?
Dan Szyprowski writes: In terms of the flu, the strains of viruses that tend to “species-jump” are influenza type A viruses, which undergoes two kinds of changes. One is a series of mutations that occur over a period of time and cause a gradual evolution of the virus. This is called antigenic "drift." The other kind of change is an abrupt change in the hem agglutinin and/or the neuraminidase proteins. This is called antigenic "shift." In this case, a new subtype of the virus suddenly emerges. Type A viruses undergo both kinds of changes whereas influenza type B viruses change only by the more gradual process of antigenic drift.
In order for a virus to be able to infect a new species, it must be mutated to a certain degree. Although certain pathogens (like the influenza virus) have a very high rate of mutation, one must keep in mind that the genes of potential hosts are also mutating (although at a much lower rate.) Thus, not only does the virus have to accumulate enough mutations to become relatively suited to infect a new type of organism; it must also acquire specific mutations that will allow them to adapt to whatever the organism's genome is at that time. The probability of this occurring is relatively low, which is why species jumping is such a rare occurrence.
Christy Sylvester and Ruxy Pascanu write: Species jumping is a rare occurrence because of the differences in DNA and RNA between different species. In order for an infectious agent to transmit the illness to a different species, the agent will have to mutate with the other species DNA or RNA to cause harm. Species jumping can occur through direct or indirect contact between the infected animal(s) and humans. Virologists have learned to watch for corpses when a virus “jumps” from one species to another. Viruses can often coexist with a “host” species without harming them because they have built up an immunity to that virus, but if there is species to species contact, the virus can become transmitted to the non-host and infect that species. This relationship becomes parasitic because the new species does not have the built up immunity.
How do viruses spread around the world?
Dan Szyprowski writes: The increased volume of international travel that occurs in contemporary society has aided the spread of viruses. Recycled air used in airplane cabins also makes it especially easy for air-borne pathogens to be transferred between passengers. In addition, migratory animals can spread viruses (for example, birds and the avian flu.) These diseases may eventually species-jump so they can easily infect humans.
Christy Sylvester and Ruxy Pascanu write: Viruses exist for one purpose only: to reproduce. To do that they have to take over the reproductive machinery of suitable host cells. Many viruses can be transmitted by direct (person to person) or indirect (air) contact. This is when the virus can be easily transmitted and cause a pandemic.
What is meant by the term reservoir?
Katelyn O'Rourke writes: A reservoir is another name for a fluid containing sac or cavity. In terms of the influenza virus, a reservoir serves as a portal agent for transmitting the Avian flu.
Dan Szyprowski writes: When it comes to diseases, a “reservoir” is a long-term host of the pathogen of an infectious disease. Usually, these hosts do not get the disease that the pathogen carries, or they have an asymptomatic or non-lethal form of the disease. They are essential to the control and containment of contagious diseases (because reservoirs are indispensable in the pathogen's life cycle.) Well-known reservoirs include asymptomatic humans for typhoid fever, raccoons, skunks, foxes, and bats for rabies, marmots, black rats, prairie dogs, chipmunks, and squirrels for the bubonic plague, and, of course, fowl (geese and ducks) for the avian flu.
Christy Sylvester and Ruxy Pascanu write: A reservoir is an organism in which a parasite that is pathogenic for some other species lives and multiplies without damaging its host. There are also reservoir hosts. These are vertebrates that host an infection while it isn't infecting humans and is used by the disease as a source of maintenance. A single reservoir host maybe reinfected several times.
What are they for flu?
Katelyn O'Rourke writes: For the Avian flu, the reservoirs could be living organisms, such as birds, soils, as well as the chickens that raised on poultry farms as a means of food production. Also, once a human being becomes afflicted with this potentially live threatening illness, they themselves become a reservoir for the disease, which could lead to a pandemic.
Dan Szyprowski writes: In general, birds are natural reservoirs for influenza viruses. In fact, fifteen subtypes of influenza A virus are known to be circulating within bird populations. Currently, the most infamous strain of this virus is the avian influenza virus (AIV), which is naturally carried by shorebirds (like seagulls), ducks and geese. However, due to genetic mutations that have occurred, the virus has crossed species, and can now be carried by chickens and turkeys. Recently, the avian flu was also found in other animals, like civet cats in Vietnam.
Christy Sylvester and Ruxy Pascanu write: Humans are the only known reservoir of influenza types B and C. Influenza A may infect both humans and animals. Wild waterfowl are considered the natural reservoir of all influenza A viruses. They have probably carried influenza viruses, with no apparent harm, for centuries.