Evolution of Virulence


Next tuesday, we are going to discuss the evolution of virulence, using the mosquito borne dengue fever as an example. (Handout and readings are at the end of this post)

The dengue virus causes dengue fever, aka breakbone fever. As the name suggests, sufferers of breakbone fever are typically laid up in bed.  Immobility is usually associated with high virulence (pathogen replication within its host). Immobility is also a trait that benefits the transmission of vector-borne diseases. An immobile victim cannot effectively swat a blood-sucking insect. Caretakers might also be at risk from bites from vectors because they have to stay in close quarters with the victim.

The virus that causes the common cold generally is transmitted better when its victims are not immobile. This is because it is transmitted by direct contact, not vectors. Pathogens that are transmitted by direct contact are generally less virulent than those that are vector-borne. If the cold virus made its victims unable to move and leave the house, then it would be less likely to spread like wildfire through offices and schools and universities!

Three main modes of transmission of pathogens include, direct, vectors, and environmental.

1) Direct contact diseases (like colds) are transmitted best when the victim is active. These are generally the least virulent.

2) Vector-borne diseases usually require the victim to be alive but are better transmitted if the victim is not moving. These are usually more virulent.

3) Environmentally transmitted diseases can be the most virulent. These do not require the victim to be active. Some are like vector borne diseases (cholera), in which the attendants who wash the soiled bedclothes in the river promote transmission of the bacteria. Some do just fine even after they kill the victim (anthrax), when the corpse releases infectious spores! In general, environmentally transmitted diseases might have very high virulence.

Hospital-acquired infections are like vector borne diseases. Doctors and nurses act like mosquitos. This is not because we are always drawing blood (although we are), but because our stethoscopes and hands are in contact with an immobile victim and then we often touch another patient.  Pathogens that are quite virulent and serious may attract even more attention from doctors and nurses (buzzing around the sicker patients like mosquitos!). So for the hospital acquired pneumonia, there is no downside to becoming more virulent and making the patient more immobile. Transmission of the hospital acquired pathogen depends on interaction with vectors: hospital workers who do not always wash their hands! This is in contrast to the community acquired pneumonia. Community acquired pneumonia might require direct contact (like the cold virus) for its transmission between patients. For this reason, a hospital acquired pneumonia is generally much more serious than a community acquired pneumonia.

Key points that we will cover in lecture:

1) Pathogens do not always evolve towards commensalism.

2) Pathogens that exclusively use vertical transmission – mother to baby – will evolve to a more benign state than those transmitted horizontally.

3) The mode of transmission matters: directly transmitted pathogens are less virulent than vector borne pathogens, and environmentally (or waterborne) illnesses tend to be the most severe.

4) Opportunities for transmission affect pathogen evolution – increased transmission selects for more virulent pathogens with a decreased duration of infectivity (and greater mortality). Decreased transmission selects for less virulent pathogens.

5) Competition between pathogens in a single host or single population can select for increased virulence.

6) Humans may be able to affect the evolution of pathogen virulence with some public health measures – but the definitive test awaits!

Readings for 10/12/10:

Ewald – Virulence

Brown – Evolution of Virulence

Kyle Harris – Dengue

Pay special attention to the Ewald and Brown readings for your writing project. Be sure to read ALL carefully prior to lecture.

Handout is here:

Vector Virulence

Writing Project:

Suppose the New Mexico Legislature is taking up the following bills for consideration. Bill #1 promotes the use of hand sanitizer before every handshake; it would be illegal for somebody to not use hand sanitizer before shaking hands with another person. Bill 2 promotes the use of condoms; it would be illegal under Bill 2 to engage in sexual intercourse without a condom, except at specially designated times reserved for procreation. Suppose that both bills would be accompanied by a massive public information campaign and incentive program, e.g free condoms and hand sanitizer. Explore the effects of these initiatives on the evolution of Rhinovirus, the virus that causes the common cold, and B) Neisseria gonorrhea, the bacteria that causes the sexually transmitted disease Gonorrhea. Assume that both bills, after enactment, result in a change in behavior of New Mexico citizens.  Could Bill #1 or Bill #2 change virulence, the duration of infectivity, transmission rates, and mortality of Rhinovirus or Neisseria gonorrhea in New Mexico?

Writing project is due, in hardcopy, in class at 5:30pm 10/12/10


Good Luck!

If you have questions, email Megan or me.


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