Pathogens worm their way in
The advantages and disadvantages to pathogens of being vectored by worms, compared with being directly transmitted.
Author(s): Sarah E. Perkins & Andy Fenton
Title: Helminths as vectors of pathogens in vertebrate hosts: A theoretical approach
E. coli bacteria labled with green fluorescent protein in the pharynx of a C. elegans nematode. Image courtesy of Sarah Perkins
Many insect and tick species act as vectors for pathogens — for example, mosquitoes transmit malaria and West Nile Virus. Similarly, some nematode worms carry certain plant and insect pathogens to new hosts. However, vectoring of pathogens by worms is less well studied in vertebrates.
Now, Sarah Perkins and her collaborator Andy Fenton (Liverpool, UK) have examined the role that helminth worms play in vectoring pathogens to vertebrate hosts. In a review of the scientific literature, they found 14 studies documenting transfer of bacteria and viruses by helminths, in hosts ranging from fish to sheep. In some cases, the vectoring was facultative: the pathogen could still infect the host in the absence of the vector. But in several cases the pathogen relied entirely on the vector for transmission.
To investigate when vectoring by helminths might evolve, Perkins and Fenton used mathematical models to examine under what circumstances:
- A vectored pathogen could invade and persist in a host population
- An obligately-vectored pathogen could supplant, or co-exist with a directly-transmitted progenitor
They found that vectoring by helminths can sometimes allow persistence of pathogens that would go extinct if direct transmission were the only way of reaching new hosts. However, they also found that vectored pathogens are at a selective disadvantage when:
- Helminths are highly aggregated within the host population (a few hosts contain many worms, most hosts contain few or none)
- Hosts are much more likely to die under dual infection (i.e. by pathogen as well as helminth)