Like many of us in 2021, I miss the glee of traveling to new and exciting locations on vacation (and can't wait to get back these adventures post-pandemic). Yet, what I don't miss are the inevitable bugs I pick up along the way. Whether it be a cold or a stomach flu, many of us come down with some sort of infection when we are away from home. Many things could contribute to these illnesses, including travel-induced sleep deprivation, novel foods or the concentration of viruses on mass transit. But, could it be that our bodies are just not primed to defend us against the diseases in foreign locales? Agnes Piecyk from the Max Planck Institute for Evolutionary Biology, Germany, and colleagues from the USA and Germany wanted to investigate this theory to figure out whether hosts and parasites develop specialized relationships depending on their geographic origin.

So, they took advantage of a host–parasite system that is found throughout the northern hemisphere: the three-spined stickleback Gasterosteus aculeatus and its tapeworm Schistocephalus solidus. The stickleback picks up this nasty worm in a way commonly found on our own human travels – in its food. One of its favourite foods, the copepod, is commonly infected by this parasite. Once consumed, the parasite burrows through the fish's gut, invading its body and quickly growing to fill every nook and cranny of the fish's insides. The parasite steals much of its host's food, forcing the stickleback to feed constantly to keep up with its own energy needs and the demand of its unwanted guest. This voracious feeding makes it less likely to take shelter to avoid predators because of an overwhelming need for food, attracting the attention of its primary predators – and the next host in the parasite's life cycle – fish-eating birds. The parasite then gladly makes its way to the bird's gut, where it sexually reproduces, starting the process again when the bird deposits the parasite's eggs back into waterways containing the copepod (its first host) through its feces. This incredible chain of events has evolved through an evolutionary arms race among the parasite and its three host species.

Using sticklebacks and tapeworms from Europe and Alaska, the researchers set about testing the feasibility of inter-continental infections, exposing Alaskan sticklebacks to European tapeworms and vice versa. Surprisingly, while the Alaskan parasite readily infected the European fish host, the Alaskan stickleback put up a stronger fight against the European tapeworm, with hosts from certain Alaskan regions effectively preventing infection altogether. For those unlucky fish that succumbed to infection, their worms varied greatly in size depending on the host's origin. In addition, even when the Alaskan sticklebacks did pick up an infection, their immune systems succeeded in keeping the worm in check, with much smaller worms relative to their body size than European sticklebacks. While we would typically cheer the hosts that entirely prevented a parasite invasion, those uninfected sticklebacks were skinnier than otherwise healthy sticklebacks, indicating the hidden energetic cost of their unseen battle.

While the researchers’ work sheds light on the complex interactions between hosts and parasites on a local scale, we still have much to learn about how these geographic differences develop and whether there are epidemiological implications due to geographic differences in disease susceptibility. And hopefully this knowledge might one day help us to avoid the dreaded vacation bug (once we are finally back out there, galivanting in exciting new places).

Piecyk
,
A.
,
Hahn
,
M. A.
,
Roth
,
O.
,
Dheilly
,
N. M.
,
Heins
,
D. C.
,
Bell
,
M. A.
and
Kalbe
,
M.
(
2021
).
Cross-continental experimental infections reveal distinct defence mechanisms in populations of the three-spined stickleback Gasterosteus aculeatus
.
Proc. R. Soc. B.
288
,
20211758
. doi:10.1098/rspb.2021.1758