Synopsis by Nina Wale
Within a population, individuals vary in their ability to transmit pathogens to other individuals. Hosts that are especially infectious, known as ‘super-spreaders,’ may have a disproportionate impact upon the transmission of infectious diseases – such individuals were implicated in the initial spread of both HIV and SARS. Given their potential importance for disease dynamics, finding out what makes a host a super-spreader is a major aim of disease ecologists. In a recent paper, a group of CIDD researchers utilized novel, non-invasive sampling methods to test the hypothesis that coinfection with multiple parasites causes certain hosts to become ‘super-shedders,’ which shed many more infectious parasite stages than the average host.
The authors compared the infectiousness of mice infected with both a respiratory bacterium (Bordetella bronchiseptica) and a helminth (Heligmosomoides polygyrus) with mice infected with just one of these pathogens. They found that the majority of coinfected mice could be considered super-shedders of the helminth. Throughout the experiment, coinfected mice shed significantly more helminth eggs, which transmit the parasite between individuals, than mice infected with helminths alone. Since adult helminths were found in greater numbers for longer periods in coinfected mice, the authors suggest that coinfection with bacteria might inhibit the clearance of helminths by the immune system, resulting in the super-shedder phenotype.
Coinfection was also found to impact the within-host success of the bacteria. By using a strain of Bordetella tagged with a bioluminescent marker, they were able to track its growth within the mouse body in time and space. This novel method allowed them to determine that coinfected animals had higher bacterial loads in the lungs than mice infected with Bordetella alone. How these higher loads translate into transmission of this pathogen is as yet unknown.
In addition to highlighting how bioluminescent-tagging might open a window into the within-host ecology of bacteria, this research experimentally demonstrates that coinfection may contribute to the existence of super-shedders in the field. The authors call for further research to assess the impact of super-shedders on the epidemiology of pathogens– do they drive the success of pathogens, or do the negative consequences of coinfection for host survival reduce the overall impact these individuals have on the transmission of parasites?
Author(s): Lass S, Hudson PJ, Thakar J, Saric J, Harvill E, Albert R, & Perkins SE
Title: Generating super-shedders: co-infection increases bacterial load and egg production of a gastrointestinal helminth