In a recent study published in Plus BiologyAnd the Researchers have shown that life-history traits of pathogens help determine the age of humans when they have the highest likelihood of contracting an infectious disease.
Ecological succession theories predict a change of direction in the composition of society. The leading species are generated from the regional species pool based on life history traits, which can modify the specialized space in the community to accelerate the establishment of secondary successive species with different characteristics. As such, grouping of existing species facilitates the introduction/growth of some species while hindering others.
Facilitation can occur through habitat enhancement or mutual interactions, while resistance occurs through direct competition for resources or degradation of habitat suitability. Species turnover occurs when the growth of an existing species with similar life history traits declines to the point that a new group of species with different life history traits replaces it. The concept of species rotation has been successfully applied to explore and understand how an individual’s microbiome affects certain medical conditions.
The immune system and previous infections affect the habitat quality of new pathogens. The invasion and suitability of the host environment for the newly introduced pathogens is due to 1) the immune function of the individual host shaped by previous exposures to diseases and 2) the population of hosts that have suffered from various diseases but whose immune functions are affected in similar ways.
Therefore, a group of hosts acts as an assembly of individuals interacting with distinct immune memories, cumulatively forming a habitat patch. The host population’s microbiome can hinder the success of new infections by competition for resources within the host or by habitat degradation due to previous exposure to pathogens leading to cross-protection. Conversely, succession of pathogens can occur through 1) opportunistic secondary infection and 2) immune amnesia, a disturbance of previously acquired immune memory.
Study and results
In this study, the researchers hypothesized that diseases that share life history traits may most likely occur at the same demographic life stages of the hosts in a population (that is, the age of greatest prevalence). The authors identified six candidate characteristics that influence how successfully humans become infected over time.
The six traits were 1) the length of the incubation period, 2) the length of the contagious period, 3) the survivability of the pathogen outside its host, 4) the physical distance of disease transmission between hosts, 5) the number of times the disease is opportunistic, and 6) the mutagenic disease. The analysis of succession theory has been limited to disease burdens in developing countries.
The authors performed a literature search on Google Scholar and PubMed for etiology and each of the 30 infectious pathogens to find estimates of the six characteristics. Pathogens were explicitly selected to include epidemiological diversity in pathogen classifications and transmission mechanisms.
A qualitative succession score was presented for each pathogen to summarize the effects of these (six) traits and to test the ability to predict the mean age of the greatest prevalence of a given disease. Linear regression was performed on pairwise values to test whether successive score yielded a valid association model for the age of greatest prevalence for each infectious disease.
The authors found a significant association between consecutive scores for 30 pathogens and specific demographic categories during the lifetime of greatest prevalence for each disease. Moreover, categorizing the age groups based on the successive score only was meaningful. This was consistent with the hypothesis of some successive progression in the ecology of the disease in the host human habitat.
Five of the 10 diseases with the largest regression regressions (in absolute terms) were sexually transmitted diseases (STIs). Ebola showed the largest residual; The age estimate for the greatest Ebola prevalence was driven by the small sample size and initial dynamics after onset rather than a long-term sequential pattern.
Repeated regression analysis without STDs and Ebola resulted in a much stronger association, suggesting that the concept of succession theory could inform discussions about disease ecology. Alternatively, applying the reproduction number for each pathogen as a correlation of the greatest prevalence age did not lead to a significant prediction.
In summary, the results suggest that community dynamics of infection should be viewed more broadly in discussions of human health and disease. Although the cascading result is not perfect, it did provide important insights into the era of greater diffusion. An important future step is to replicate existing analyzes in modern healthcare settings, specifically developed countries, and to investigate whether the same causative traits yield similar sequential traits.