Pathogens Can And Will Spread Quickly Biology Essay

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Public health is a major concern for health officials and members of the public because pathogens can and will spread quickly if introduced into the population, and the effect could be detrimental. According to the authors of Cross-Species Virus Transmission and the Emergence of New Epidemic Diseases, some of the most infectious diseases that have threatened our public health initiated from wildlife hosts. Diseases such as Ebola fever, SARS, and HIV, which all have zoonotic roots, not only infected their animal host, but gained the ability to also infect the human population. This type of behavior is discussed with emphasis on how some of these cross-species viruses have lead to epidemic diseases, and how different variables affected their ability to successfully do so.

Our lack of knowledge of viruses that infect wildlife has led to the emergence of high profile viruses such as Hantaviruses, Ebola, and HIV. All of these viruses are examples of viruses that were undiscovered before they appeared in humans. Once humans began to get infected due to the virus' host switching capabilities, they caused many fatalities despite an increased understanding of their behavior. HIV still infects an estimated 1.8 to 4.1 million people per year and the SARS virus infected thousands of people all over the world over a short period of time.

In order for a virus to switch hosts, there must be contact between an infected species and a new host. As such, the rate of occurrence of host-switching events can be lessened if there is a lack of contact between viruses and potential new hosts; HIV-1 and HIV-2 have been transmitted into the human population many times since the 1920's, but they failed to cause an epidemic due to the low level of virus-new host contact. Geographical distribution such as trading wildlife, behavioral separation such as bush meat hunting, and changes in human behavior such as an increase in travel, the increase and expansion of a population, sexual practices, and expansion in agriculture, all have the ability to aid in the emergence of new viruses. The size of a population also has a major effect on the emergence of a disease; generally, the higher the population density, the greater the chance that the disease will be able to spread and result in an epidemic.

A virus' ability to infect new hosts can be diminished at various levels of the infection process such as receptor binding, entry, genome replication, and innate antiviral responses. The most basic level of protection from an infection occurs at the skin's entry level or in the tissues. In order to be successful at host switching, viruses need to develop the ability to change their receptor binding to a site in the new host. FVP (feline panleukopenia virus) changed its host range from felines to dogs by binding to the orthologous receptor on their cells. Binding to a new receptor is not an easy task as it may require multiple mutations in the virus. If the virus gains this ability, then it may still be restricted to cause an infection at the intracellular level. Epidemic infections have occurred between hosts that are closely related, and hosts that are distantly related. HIV transferred between humans and chimpanzees (close relatives), while SARS transferred from bats to humans (distant relatives).

The kind of hosts a virus originally infects is a factor of its ability to switch hosts. Viruses that infect many hosts, called "generalist" viruses, are more likely to exhibit host switching behavior due to their ability to use the host cell mechanisms of multiple past hosts to replicate and infect new hosts. On the other hand, specialist viruses infect a small range of hosts and therefore are less likely to switch hosts.

Some viruses are required to "evolve" in order to emerge in new hosts. When a virus experiences a mutation that increases its efficiency at infecting new hosts its fitness in the donor host usually decreases. Viral evolutionary actions such as recombination and reassortment also aid in successful host switching. HIV and SARS both arose from recombinations and both are well known for their ability to switch hosts from chimpanzees to humans and from bats to humans respectively.

Whether or not viral intermediates with lower fitness are involved in host switching is a major question of this topic; the fact that FVP has transferred to dogs through a minimum of one lower fitness intermediate proves that they are indeed involved. "The first viruses that were collected from dogs were both less fit in cats that the FPV from which they were derived and less well adapted in dogs than the CPV variants that replaced them (464)."

Early detection of viruses that are unproductive at causing large infections will allow for their emergence to be controlled. When SARS first emerged, it failed to spread efficiently and early public health measures, like quarantines, disallowed the virus from becoming an established human virus. Figure 8 depicts how SARS became obsolete 5 months after a global issue was alerted. Although SARS did not become established in the human population, it gained host-adaptive changes among humans during its spread, which suggests that it may have gained the ability to fully adapt in the human population.

In order to spread efficiently, a virus may need to increase its ability to cause viral shedding in the sites of transmission such as feces, urine, and blood. These viruses may need to stimulate sneezing to accomplish respiratory shedding. Some individuals with higher levels of viral shedding may be "superspreading" individuals, in that they cause a large number of outbreaks in the early stages of the epidemic.

The information in this article suggests that if we could better understand the origins of emerging viruses, we would be able to identify and possibly control these emerging viruses in their original hosts. The data was well written and well formulated; it gave detailed explanations of how viruses that arose from animals have come to threaten our public health. The authors provided many examples of these infectious diseases including SARS and HIV. They also did a great job explaining how and why some viruses leave their original animal population and go on to successfully cause epidemics in the human population, and why some of them remain in the human population while others do not. In addition to understanding how and why viruses exhibit host-switching behavior, more research should be done on how to stop these infections in animals; if you stop the disease at its source then there will be no way for it to spread to other hosts and lead to an epidemic. Like the authors state, more measures should be taken in order to stop these viruses at the source such as vector control and vaccinating animals. If the world could cease to see the vaccination of animals as unethical, it would lead to an increase in the immunizations of animals and a huge decrease in emerging infectious diseases.

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