Virulence Factors

Viruses are pathogens that are able to cause disease. The ability of this pathogenicity is termed as virulence. Meaning, the more the pathogen is able to cause a disease, the more virulent it is.

Factors that enable pathogens to be more virulent mainly consists of the pathogen adhering and colonization in the host, evading the host’s immune system, entering and exiting host cells, and obtaining nutrition from the host. With these factors, many of the host’s functions are inhibited, and usually, the host will die.

Below is a comparison between Bacterial Virulence and Viral Virulence.

Click to enlarge!

I’m focusing on viral virulence, but if you want to know more about bacterial virulence, such as what adhesion, extracellular enzymes, toxins, anti-phagocytic factors are, and what they do to infect and replicate in the host, check out this page 2 of this pdf.

Adhesion
Viruses have to attach to surface proteins of cells before infecting them.

Host Evasion
Some viruses are able to “hide” within host cells in order to avoid the host’s immune system.

Latency
Viral latency refers to when viruses lie dormant in the cell. This can be seen by the lysogenic part of a virus life cycle. The virus’s genetic material may combine with the host cell’s DNA and remain there indefinitely, not replicating or infecting other cells. This allows them to reactivate and begin producing large amounts of viral progeny (replicated virus) without the host being infected by new outside virus.

High Mutability
Some viruses are able to mutate to evade the host cell’s immune system. The immune system recognizes foreign particles(in this case, the virus) due to antigens on the surface of the virus. Before the immune system can act on the virus with antibodies, the virus change their surface antigens to avoid recognition from the body. This is called antigenic variation.

So you’ve seen the virulence factors, now, you must be a little worried about getting infected by the many different kinds of deadly viruses you’ve heard of, such as HIV, mumps virus, and Ebola virus. Well, fret not, as different viruses have different intensities of virulence.

The diagram below shows an example of the relative virulence of some viruses.

As you can see, the influenza virus is one of the most virulent viruses among humans. Influenza virus easily infects and cause diseases in the body, known commonly as the flu.

There are five stages of infectious disease, which are the order in which diseases often occur in. They are:

1. Incubation
2. Prodormal
3. Illness
4. Decline
5. Convalescence

Image from here

Incubation
During the incubation period, the pathogen infects and replicates rapidly in the host. There are no signs or symptoms during this period. However, the host is able to spread the disease to another host. Some incubation periods can range from less than a day for Salmonella food-borne infection, to up to 8 years for HIV!


Prodormal
The prodormal period signals to the host that there is an impending illness. Some symptoms and signs appear. (Not all diseases have the prodormal stage)


Illness
Also called the invasive phase, this is when the most severe symptoms and signs pop up and are very clear. The acme is the peak of the disease symptoms. If the host’s immune system is unsuccessful in overcoming the pathogens during the invasive/illness phase, the host will die. Otherwise, it will go on to the next stage.


Decline
During the decline phase, symptoms and signs decrease, as the person gets better and the infection is brought under control.


Convalescence
There are no signs or symptoms in the convalescence period. Cells in the host repair damage caused by the infection, and everything goes back to normal.


Something else related to the stages of infectious disease is the iceberg concept of infection. What is it? You’ve heard of the expression tip of the iceberg, well, the iceberg concept of infection is just like that, look at the image below.

I’m not that good at drawing on the computer, or drawing at all, so pardon the bad graphics and animation. The white mountain-shaped thing is the iceberg, and the blue moving things is the sea. Like the phrase “the tip of the iceberg” implies, when people realize that they have been infected, it is usually when they have severe symptoms. They do not realize the initial stages of infection before that, in the iceberg analogy, below the surface of the sea. The exposure, infection and mild symptoms usually go unnoticed.


Viruses are scary aren’t they? What frightens me the most is that they aren’t even living organisms; they behave and act like microorganisms such as parasitic bacteria, but do not have cells to divide. Virus infection also cannot be easily defeated by antibiotics like bacteria. The infection must be fought off by the body’s immune system or other drugs. There are so many different types of viruses and with the high ability to constantly mutate, it is pretty impossible to come up with new vaccines all the time. A widely known example is the human immunodeficiency virus (HIV); it has caused many deaths and long term suffering through AIDS, and despite many years of finding a cure for it, scientists have failed. More than 25 million people have died from AIDS since 1981, and it was estimated that there were 33.4 million more living with it at the end of 2008 (Avert.org).


What comforts me a little, is that viruses cannot survive for long when they leave the human body, as they are acellular particles, unable to carry out even the most basic life processes. They are not made up of cells, they cannot reproduce on their own, and they have no metabolism. So how do viruses spread from person to person, from plant to plant, from animal to animal?
Well, in order to maintain life outside of the host (whom they infect and replicate), they need to be maintained in sites called reservoirs of infection.


The three types of reservoirs are… (See animation below)

Note that the virus does not replicate in the reservoirs, but merely use them as a means (vector) to infect hosts, where they replicate.

Animal Reservoirs

Zoonoses (also called zoonotic diseases) are diseases that spread from animal hosts to humans. Humans are susceptible to infection from the virus when:

- They have direct contact with animals
- They have direct contact with faeces
- They eat the animals
- Vectors such as mosquitoes, fleas and other insects transfer fluids from animal to human.

Humans are usually the dead-end host to zoonotic pathogens.

Human Reservoirs

Some human virus reservoirs may appear to be healthy and asymtomatic, but these carriers actually are prone to infect others. The virus simply lies dormant in them, it may be that they do not have the protein receptor for the virus to attach to their cells, and thus preventing the virus from infecting them, or, that the human reservoir’s immune system is able to fight off the illness. Transfer of virus from human to human may be through contact or exchange of fluids.

Non-living Reservoirs

Soil, water and food are some examples of non-living reservoirs. Usually, the presence of viruses in these places is due to contamination with faeces and urine.

Ah, there you go. Now you know about virus virulent factors, stages of infectious disease, the iceberg concept of infection, and the reservoirs that viruses reside in before they infect. I hope that you have now understood, and learned more about viruses!


- Kiat Yi