I'm interested in literature from both animal and plant systems examining how multiple vectors can influence both 1) a successful pathogen invasion (qualitative) and 2) the severity of an infection (quantitative).
I'm wondering if multiple points of entry increases the probability or severity of infection. For instance, if a mouse gets bit by 5 ticks carrying Lyme disease, is the probability of successful infection higher than if it gets bit by only 1 (assuming all vectors are equally infective and 100% competent)? If a wheat plant is infected with Barley Yellow Dwarf virus, will it become sicker if it is fed upon by additional BYDV-infective vectors? Or does it not matter?
I'm more interested in the quantitative side of that question (how do multiple vectors influence disease severity), but would appreciate citations addressing either aspect.
Hi Paul,
I'm assuming that you mean multiple individuals of the same vector species; if you mean multiple species then many aspects of my answer will change. Also, I don't really have citations for you, but maybe some suggestions on how to think about the problem and look for citations.
So for the first case of how probability of infection changes with the number of vectors, assuming that the vectors are independent of each other, each with a probability, p, of infecting the host, then we expect:
P(infection | k vectors) = 1-(1-p)k
That is, 1 minus the probability of not being infected given k vectors. I cannot think of a study off hand that has explicitly tested this relationship, but I do know of some that use it successfully.
The second case of how the intensity or severity of infection changes with the number of vectors will likely depend a great deal on disease system. For instance, with vector-borne diseases of animals (e.g., tick-borne infections, malaria) we usually assume that the hosts are either infected or not infected; if they have a low-level infection it will quickly grow to a typical-level infection pretty quickly, or be cleared. This of course masks dose-effects and there are many dose-response studies showing that the severity of infections increases with increasing dose of inoculum for viruses, bacteria, etc. So our assumption about the binary status of infections is not perfect, but it works well enough for most purposes. Note that in these dose-response experiments, the effect generally increases with ten-fold increases in doses. What this means is that if each vector is contributing v virus particles, you might notice a change in the effect or severity between 1 and 10 vectors feeding or 10 and 100, but probably not between 1 and 2 or 3.
One caveat may be with nonsystemic infections in plants, where only a part of the plant is experiencing the infection and disease. In this case one might expect that two vectors feeding on and infecting different parts of the plant would essentially double the impact on that plant.
I'll be interested in seeing if anyone has any good citations for you. If you find them, please post there here.
Cheers,
Jesse
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