Kamika and Vanessa
Reading Summary:
How are doctors and nurses like mosquitos, machetes and moving water?
In chapter 6 of Evolution of Infectious Diseases, Paul Ewald discusses how virulence in certain diseases has evolved in different settings such as hospitals. The reasoning behind the change in virulence was due to many factors. The biggest factor affecting virulence was the change in sanitation practices in hospitals. Proper and routine sanitation was replaced with an increased use of antibiotics.
Another contributing factor for the change in virulence was the sheer amount of immunosuppressed patients in the facilities. Newborns who had little to no immune system development were at increased risk for transmission of Escherichia coli from their caretakers. This was known as attendant-borne transmission.
“At first glance one might speculate that the association between lethality and duration of outbreak was a nonevolutionary consequence of poor hygiene.” But it was noticed that when the facility was shut down there were still outbreaks of virulent strains of E-coli. After the facilities were thoroughly cleaned with disinfectants would the outbreaks of E-coli go down in number. Other stringent guidelines for handwashing and glove use also brought the number of cases down. When proper sanitation and stringent guidelines were not followed an increase in outbreaks would occur again.
“Another indicator for the severity of infection is whether the infecting organism has entered the bloodstream. Once this invasion has occurred, death commonly occurs in one-third to one-half of patients.” The reason for the ability of these organisms to infect a person’s bloodstream so easily is due to the vast number of patients in hospitals with open wounds from trauma or surgery. Again people within the community, outside of hospitals, had a much lower rate of death because the same disease would not be given a chance to become as virulent with a lower number of patients in close proximity to each other and little to no attendant-borne transmission.
One recent observation from the 1980’s showed that infants born vaginally and breastfed were less likely to acquire diarrheal diseases than babies born via c-section and having less chance to breast feed. “Because this study was not carried out from an evolutionary point of view, the contribution to the evolutionary changes in pathogen virulence was not addressed.”
Homes for the elderly also showed increases for attendant borne-transmission. Serious bloodstream infections were also prevalent. This was due to the vast number of immunosuppressed patients in close proximity and a decline in rigorous handwashing and glove use. The major contributing factor was their increased vulnerability to their environment and caretakers.
Attendant-borne transmission does not just stop in hospitals. Kennels, puppy mills and veterinary hospitals also had high numbers of highly virulent attendant-borne infections. “These infections are highly resistant to heat, disinfectants, and environmental exposure.” The pathogens are spread from cage-to-cage, through attendant’s footwear and clothing.
Lecture Summary:
He discusses the 4 horsemen (war, death, famine, conquest/pestilence) from Revelations and points out how pestilence is not the real name of the 4th horseman but shows how powerful the conception of epidemic is that it has become culturally accepted as more terrifying than conquest.
For many years and through when Ebersole was an undergrad, it was thought that pathogens evolved towards being benign. It was believed that it doesn’t serve a pathogen to be too virulent. However, growing antibiotic resistance challenged this theory. In addition, it is important to note that cholera, plague, and many other diseases have not evolved that direction over time despite having had hundreds or thousands of years of time to do so. The modern conception of the evolution of virulence,the severity and harmfulness of a disease,
required several mental leaps and breakthroughs that changed the prevailing master narrative.
These breakthroughs included Darwin and the idea that organisms can adapt over time. This gets to the nature of pathogen virulence and the question of what quality of a pathogen gives it the ability to do harm. One major contributor to this ability to do harm is that a pathogen must be able to replicate fast. There is a connection between replication and virulence.
Another mental breakthrough came from Louis Pasteur whose germ theory allowed epidemics and pestilence to become something we can fight. The idea of organisms we can fight and who adapt came forcefully upon the medical community with the advent of antibiotics. These were so revolutionary that many antiseptic practices after Pasteur weren’t practiced which resulted in antibiotic resistant pathogens through natural selection as hospitals were no longer being sanitized.
There is a cost/benefit relationship between virulence and fitness. There is an optimal level of virulence before costs outweigh benefits. This optimal point is called adaptive virulence and natural selection should be working towards this point. For example, if the common cold infects nasal passages and can rapidly spread, there is no benefit to further replicating in other parts of the body or in being more virulent because the costs would outweigh the benefit. However, vector diseases do not need to worry about killing their hosts as much so they can afford to be more virulent (Lyme disease, typhus, yellow fever, dengue, etc..) It should be noted that many collective group behaviors , called cultural vectors, can cause a contact spread disease to become rapidly more virulent. Cultural vectors are practices such as burials, war, famine, medical training, care of the sick, and sewer systems that can spread contact diseases through methods that simulate vector borne diseases. This in part explains why Cholera and other enteric (intestinal) diseases can be so virulent because they do not have to abide by the usual cost/benefit curve for contact diseases.
Thoughts/Questions:
Would increasing our understanding of cultural vectors help alleviate overuse of antibiotics and result in less virulence? What other ways are there to reduce virulence? How did the change in the conception of germ theory change the way we view our responses to controlling disease and therefore virulence? How will control of vectors such as mosquitos influence the level of virulence in the future?