Mechanism on transmission of viral diseases:
Skimming through the history of Zoonotic diseases, most of them are epidemic in nature sweeping away all the beings in its way. Though the causative organisms are mostly bacteria, viruses and parasites which usually spread from animals, the host cannot be infected unless it is most susceptible. The spread of infection depends on the strength of immune system of the host.
At a Maryland country fair in 2017 the prize pigs were not looking the best. Farmers reported feverish hogs with inflamed eyes and running snouts. But while fair officials worried about the pigs, the Maryland Department of health was concerned about the sick fair goers.
Some had pet the pigs while others had nearly been near the barns but soon 40 of these attendees would be diagnosed with swine flu.
More often than not sick animals don’t infect humans but when they do, these cross infections where viral host jumps have the potential to produce deadly epidemics so how can pathogens from one species infect another and what makes host jumps so dangerous?
Viruses are a type of organic parasite infecting nearly all forms of life. Just revive and reproduce, they must go through three stages: contact with a susceptible host, infection and replication and transmission to other individuals.
As an example, let’s look at human influenza: first the flu virus encounters a new host and makes its way into their respiratory tracts. This isn’t so difficult but to survive in this new body, a virus must mount a successful infection before it’s caught and broken down by an immune response.
To accomplish this task, viruses have evolved a specific interaction with their host species. Human flu viruses are covered in proteins adapted to bind with their matching receptors on human respiratory cells.
Once inside a cell, the virus employs additional adaptation to hijack the host cell’s reproductive machinery and replicate its own genetic material. Now the virus only needs to suppress or evade the host’s immune system long enough to replicate to sufficient levels and infect more cells.
At this point, the flu can be passed on to its next victim via any transmission of infected bottle-y fluid. However this simple sneeze also brings the virus in contact with pets, plants or even your lunch.
Viruses are constantly encountering new species and attempting to infect them, more often than not this ends in failure. In most cases, the genetic dissimilarities between the two hosts are too great.
For a virus adapted to infect humans, a lettuce cell would be a foreign and inhospitable landscape. But there are a staggering number of viruses circulating in the environment all with the potential to encounter new hosts and because viruses rapidly replicate by the millions, they can quickly develop random mutations.
Most mutations will have no effect or even prove detrimental to virus but a small proportion may enable a pathogen to better infect a new species. The odds of winning this destructive genetic lottery increase over time or if the new species is closely related to the virus’s usual host.
For a virus adapted to another mammal infecting to human might just take a few lucky mutations and a virus adapted to chimpanzees, one of our closest genetic relatives, might barely require any changes at all.
It takes more than time and genetic similarity for a host jump to be successful. Some viruses come equipped to easily infect new host’s cells but are then unable to evade immune response.
Others might have a difficult time transmitting to new host. For example, they might make the host’s blood contagious but not their saliva. However, once a host jump reaches the transmission stage, a virus become much more dangerous.
Now gestating within two host, the pathogen have twice the odds of mutating into a more successful virus and each new host increases the potential for a full blown epidemic.
Virologists are constantly looking for mutations that might make viruses such as influenza more likely to jump however predicting the next potential epidemic is a major challenge.
There is a huge diversity of viruses that were only just beginning to uncover. Researchers are tirelessly studying the biology of these pathogens and by monitoring populations to quickly identify new outbreaks. They can develop vaccines to stop these deadly diseases.