Tag Archives: virulence

“Be careful of what you wish for…”

The sustained transmission of Ebola Virus within urban centres in Sierra Leone, Liberia and Guinea is unprecedented, and the consistent mortality rate of greater than 50% is frightening.

It would be great if that mortality rate fell to 10-20%, right?

From a purely scientific viewpoint, not necessarily so….

Ebola is certainly very transmissable, but not as transmissable as other viruses we encounter, such as measles. So far it has failed to cause a large outbreak in a country with a well developed and well financed Public Healthcare system. That is encouraging to some extent but we should not get complacent. I believe the virus in its current form is probably incapable of causing a true worldwide pandemic but….

This is the first time we have seen a period of sustained and intense transmission in the human population. You can be sure that the virus will be mutating and adapting to its new host (the human species) in order that it can attempt to maintain its long term survival within that species. To do this the virus needs to facilitate its transmission within the human host, and decrease it’s virulence. By decreasing its virulence the virus would then have the potential to be transmissable to other potential contacts for a longer period.  Decreased virulence would also give rise to the possibility of sub-clinical cases thus facilitating cross border spread.

So from a humanitarian point of view, it would be great to see the mortality from Ebola virus decreasing. However from a scientific view this would raise the alarm bells that the virus has adapted to the human host to the extent that it is much more capable of causing a true pandemic. (There could of course be other reasons why the mortality rate might decrease, e.g. better treatment…)

The longer the outbreak continues, the longer the Ebola virus has to mutate and adapt and cause exactly this sort of scenario. This is why the governments of developed countries should be pouring resources into controlling the Ebola epidemic in West Africa, because a few months down the line we may be dealing with a quite different virus than the one we have at the moment….


Keep up to date with what is happening with Ebola on an international scale by reading the WHO Ebola situation updates. Click here for the link.

“MRSA, MSSA and resistance v virulence”

There are three sound reasons why mortality in sepsisStaph aureus culture due to Methicillin Resistant Staphylococcus aureus (MRSA) is higher than that of Methicillin Susceptible Staphylococcus aureus (MSSA).


  • If the patient is put on initial treatment that just covers MSSA and not MRSA, then effective therapy may be delayed.
  • The antibiotic most commonly used for treating sepsis due to MSSA (flucloxacillin, cloxacillin) is a more effective antibiotic than that usually used for MRSA sepsis (vancomycin)
  • The cohort of patients with MRSA sepsis tends to be a “sicker” cohort (ie more co-morbidities, more prior antibiotics, more hospital admissions etc) than that of patients with sepsis due to MSSA. (This is particularly true for “hospital type” MRSA strains.)

Some people/experts suggest a fourth reason, that MRSA may be a more virulent bacterium per se than MSSA. Some research trials have actually claimed this. However such trials are fraught with difficulty. It is hard enough to attribute mortality to a particular micro-organism. Throw in the confounding variables as described above and you are on a hiding to nothing.

However there are theoretical reasons why MRSA should not be more virulent than MSSA. Generally a bacterium utilises energy in order to become resistant. Sometimes, but not always, this energy is the energy used for making virulence factors within the bacterium.

It doesn’t make any logical or evolutionary sense that MRSA should be both more resistant and more virulent per se than MSSA.

On the contrary it may well be that the converse is true, that MRSA is a slightly less virulent bacterium than MSSA.

Trying to convince the “experts” of this is another matter…..


The link between bacterial resistance and virulence is very interesting. Unfortunately the more you read about it, the more complex it gets. More on this topic in later articles….