Reading: Evolution of Infectious Disease by Paul W Ewald, 1994

Guest: Dr John Ebersole Lecture: 10/09 Reading: Evolution of Infectious Disease by Paul W Ewald, 1994

Reading Notes: · Adaptive Severity Hypothesis o Diseases that are transmitted via host to host contact (or close proximity) benefit from a maintaining mobility of the host and concentrating viral reproduction in a body area most conducive for transmission o Diseases that are transmitted via a vector benefit from an immobile host and extensive systematic reproduction throughout the host as a means of increase probably of successful transmission o This trend towards virulence means that as cycling between vector and host increases, severity of the disease also increases; whereas the greater the cycling between host and host, the less severe the disease becomes . Demonstrated historically, diseases that have declined in virulence over time are typically non-vector borne; whereas vector-transmitted diseases have continued to cause severe disease . Virulence determined by mortality rates o Vertical vector transmission would be conducive to more benign infection, whereas horizontal vector transmission would be conducive to more virulent infections . Vertical transmission: Transmission of pathogen from parent to offspring. . Horizontal transmission: Transmission of pathogen between members of the same species, excluding parent-offspring transmission · Restricted Adaptation Hypothesis o Selective pressure is expected to trend pathogen/host relationship towards benign-ness; however, pathogens cannot evolve to be benign in both vector and host o Vector borne disease benefit most if they specialize in using their host as a resource for reproduction, and if they specialize in using their vector as a dispersal agents, thus the benign relationship occurs in the vector · Assumptions shape perceptions o Scientists often select evidence that supports their preconceived notions . Ex. Gambian sleeping sickness used to conclude that vector borne pathogens evolve toward benign-ness in vertebrate hosts, even though it is an anomaly in this regard . Typically, non-vector borne pathogens have declined in virulence . Post-natal studies of P. Falciparum fail to account for infant deaths when assessing virulence . Scientists subjectively select objects of study · F. falciparum v.s. other malarial strains . Nature always harbors exceptions · Insufficient Time Hypothesis o Belief that all pathogens evolve toward reduced virulence o Evidence omits non-vector borne pathogens o Difficulty determining arrival of disease . Ex. Disputes over when typhus first emerged, due to unclear records . Pathogens difficult to clearly identify within records . Absence of definitive documentation not evidence of absence of disease · Virulence Niche Hypothesis o A high volume/density of susceptible hosts along with a high volume/density of vectors results in continuously replenished resources and consistent opportunity to spread, making this the ideal environment for severe virulence o A low volume/density of susceptible hosts or a low volume/density of vectors results in inconsistent opportunities to spread, which increases the chance of severe virulence consuming available resources before they can be replenished, making this environment more ideal for benign infections o Pathogen virulence and host immunity in perpetual adaption dance, each trying to outsmart/out- compete the other · Under-reproduction v.s. Over reproduction o Pathogens that do not reproduce profligately may never achieve transmission from vector to vertebrate o Pathogens that do reproduce profligately may kill vertebrate host swiftly o This represents a a delicate balance of costs and benefits · Cultural Vector Hypothesis o Waterborne . Virulence of diarrheal pathogens is positively associated with tendencies for waterborne transmission, as the pathogen capitalizes on using water as a vector o Attendant-borne . Patients act as the host due to high susceptibility caused by prior illness or wounds, compromised immune systems (newborns, antibiotic and/or immunosuppressant treatment, etc.), and/or invasive procedures . Attendants act as vectors due to low susceptibility caused by better health, better hygiene practices (regular hand washing); even in the event that a disease does inadvertently infect an attendant, another attendant will step in as a replacement . Pathogens have increase propensity for virulence due to greater contact between attendant and severely ill, and because less ill patients are more likely to be discharged from care . Antibiotic resistance further promotes virulence · Antibiotic sensitive pathogens benefit from remaining benign to deter unwanted attention from attendants and antibiotic treatments; whereas antibiotic resistant pathogens have no concern towards an attendant’s antibiotic treatments . Agricultural diseases (those of farm animals and crops) can be partially explained by attendant-borne diseases, providing insight for solutions regarding food shortages caused by famine and increasing global population · Disease control o There is less opportunity of success if control is strictly focused on reducing the density of pathogens, rather than focusing on selective pressures regarding pathogen virulence . It’s important to note that the goal of manipulating virulence will not necessarily decrease the frequency of infections, but it does promote decreased virulence in the infections that do occur, thus leading to fewer complications, shorter hospital stays, and lower healthcare costs; similar to antibiotic resistance, this approach accepts that while pathogens can’t be completely eliminated, they can still be manipulated o Success with genetic or chemical control is difficult due to viruses’ high genetic variability . Particularly noticeable in antibiotic resistance . Resistance to antimalarial drugs has caused boom in more virulent strains o Control via exploitation of evolutionary principles provides greatest opportunity for success . Focus is on increasing the cost and decreasing the benefits of virulence, and on increasing the benefits and decreasing the costs of benign-ness . Reduction or elimination of vectors for transmission – disposable gloves and gowns, water purification, etc. . Increased competition with benign pathogens – mother-newborn contact and breast-feeding, reduction or elimination of preventative antibiotic use, etc. . Isolation of immobile hosts . Eradication v.s. Suppression: · Suppression of more virulent strains achieved by mosquito nets and more insular housing, capping opportunities for transmission from immobilized hosts · Suppression more attainable solution, due to P. Falciparum strain of malaria’s genetic variability o Similar to the problems faced in the smallpox debate and the HIV/AIDS epidemic, there is a certain finesse and rhetoric required to address the problems, i.e. hospitals do not want to take known for being responsible in creating and infecting their own patients with highly virulent diseases o Policy & disease . Ability to work and/or study from home would reduce transmission of pathogens . This may result in a reduction of virulence, as pathogen would no longer benefit from host’s immobilization · Similarity to “Plague, Typhus, Quarantine” reading o Appeal to logos o Multiple, conflicting interpretations of scientific evidence

Lecture Notes: · Darwin understood species to dynamic in space and time o Fossils resemble living species o Species composition varies with latitude, and with jumps in longitude, particular among niches that are geographically separated (bodies of water, mountain ranges, etc.) · Darwin linked known facts of biology to establish his theory evolution o It was known that populations of species have the potential to increase exponentially; however, populations generally maintain a steady state o It was known that resources are limited (known as carrying capacity) . Darwin inferred that this caused a struggle for existence among individuals of the same population o General observations demonstrates that there is variation among individuals and that individuals are unique o Jean-Baptiste Lamarck established the concept that features are heritable (i.e. offspring resemble parents) . Darwin inferred that differences in survival and reproduction among individuals occur due to these heritable traits . Darwin further inferred that heritable features that improve success in this struggle preponderate · This is microevolution (evolution within a lineage), and ultimately leads to macroevolution or speciation (branching of lineage) · Problems in Darwin’s theory o At the time, heritable traits were thought to blend (similar to the mixing of paints), which lead to the expectation that variation would gradual decline, particularly if unfit traits were killed off o Work by Gregor Mendel demonstrated that traits did not blend, but rather maintained individual function/identity