Amplification vs. Dilution

 

Dilution refers to the phenomenon whereby an introduction of a host in the system causes a reduction of the pathogen population in that system.  On the other hand, amplification refers to the phenomenon whereby the addition of a host to the system causes an increase in the pathogen population in that system. In other words, the dilution effect occurs when biodiversity and the risk of disease are inversely proportional while amplification is borne of a positive interaction between biodiversity and disease risk. Dilution and amplification scenarios are abundant in the human environment depending on the nature of the host-pathogen competition in a particular ecosystem.

In the dilution effect, once a host organism is introduced into a population of pathogens, it creates a diversity of species in the setting and causes competition between the diverse species. This competition, in turn, causes a reduction in the pathogenic efficiency of the more dominant of the two species.  Hence, this effect resulted in a decreased abundance of pathogens, a decreased disease risk and decreased the intraspecific transmission. In the amplification effect, the introduction of a host into an ecological setting enhances pathogenicity hence causes an increase in pathogen abundance, an increased disease risk, and interspecific transmission.

Of late, there has been a heated discussion as to whether the postulate that biodiversity dilutes disease holds ground. In some way, amplification and dilution mechanism have illustrated that there is a relationship between biodiversity and the outcome of the disease. Many believe that changes in biodiversity can cause different outcomes on the risk of diseases, such as amplification and dilution, and which can influence the disease effect in the community. Amplification effect is caused when there is a positive relationship between disease risk and biodiversity, whereas the dilution effect takes place when disease risk and biodiversity are inversely related (Ostfeld, Keesing & Eviner, 2008). When a variety of species are infected by the same pathogen, the target host species have different abilities in fighting against the pathogen, acquiring the pathogen, and in transmitting the pathogen to others. Therefore, alterations in biodiversity affect the composition of the host species resulting in a change in the community.

Amplification occurs in a scenario where adding species in a community that has lower diversity and contain unfavorable hosts increases the number of hosts that are highly competent. For instance, when intraspecific transmissions of the pathogen seem to drive the disease, the amplification will occur. In this case, the diversity of the species resulted in an increase in the disease.

Some disputants of the debate argue that dilution effects are more likely going to occur in the pathogens with frequency-dependent transmission, whereas amplification is common in pathogens with density–dependent transmission (Holt et al., 2003). In density-dependent transmission, the infection transferred from infected to uninfected depends on the total number of infecting organisms (population density). Therefore, the rate of disease transmission increases with the increase in population density. On the other hand, population density does not affect frequency-dependent transmissions; what matters in this case is the frequency of contact with the infected organism.

The Lyme disease system is a good instance of the dilution effect, and it is caused by bacteria that are factored by ticks. When larval ticks are born, they are uninfected, but become infected after consuming a blood meal from a host that is infected by the pathogen. Mice are, usually, highly competent hosts that transmit the bacteria to the ticks. After feeding on the blood, the nymph molts into a nymph that also has to take a blood meal to transform into an adult tick. The infected nymphs transmit the pathogen to the hosts as they take a blood meal. These nymphs feed on human beings and other mammalian hosts. In this disease, the pathogen infection is high when there is a large population of ticks that are infected, in an area.  The number of ticks that are infected can increase with high prevalence or high total densities of infected ticks. However, in Lyme disease, the vectors become infected after they bite the infected hosts; the pathogen is not transmitted from parent to offspring (Wood & Lafferty, 2013).  In other words, all hatched larval ticks are uninfected and become infected only after consuming a blood meal from a host that is infected.

I think that since that since the status quo is that the dilution effect has not been significantly implicated in disease reduction, more research is required to clarify this issue. However, I feel that the dilution effects hold great promise for disease management. According to the case study above, the population can be protected against diseases by altering the biodiversity to cause a dilution effect. Researchers and pathologists should work together towards the goal of preserving biodiversity while they reduce the risk of harmful and infectious disease. These experiments show that studying the community structure and biodiversity in relation to species variation, genetic variation and ecosystem variation may contribute to the study of disease dynamics. Understanding the variation in a particular community may enable researchers to control the effect of the disease to the population.

Columbia University