Evolution of Virulence

Next tdengue-fever-on-the-feveruesday 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.

Joe Alcock MD

Readings for 9/29/09

Elliott – Vector

Galvani – Epidemiology

Ewald – Virulence

Brown – Evolution of Virulence

Kyle Harris – Dengue

Try to read all 5. But pay special attention to the Kyle Harris and Brown readings. Be sure to read those carefully prior to lecture.

Handout is here:

Vector Virulence

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