Fleas seem to have it made. Not only do they get to live right on their source of food, but if their host is a burrowing animal, they get to spend their days in a warm, cozy hole in the ground. Biologist Cynthia Downs knows that this good life isn’t all it’s cracked up to be, though. In a new study published in the Journal of Experimental Biology she shows that burrows aren’t safe shelters for fleas, as the carbon dioxide (CO2) that builds up in them can actually turn them into death traps for the insects. 

A few years ago, Downs was working in Israel studying the jird, a rodent closely related to the gerbil. Jirds can build sprawling, complex burrows with multiple entrances, nest and food chambers, and long tunnels. While these burrows provide shelter and a stable microclimate, the air in them can get pretty stale, and some burrows have CO2 levels that are 50 times higher than the air above ground. Downs was investigating how the layouts of the burrows affect those CO2 concentrations and what impact that had on the animals. After learning that high CO2 levels can affect the jirds’ immune systems, she started to wonder how CO2 might also affect their parasites.

To find out, Downs collected 18 Sundevall’s jirds (pictured above) from a colony kept in her lab and put each of them in an airtight plastic cage attached to an air pump. Half of the cages were supplied with regular air from the room, while the other half were fed a mix of room air and CO2 designed to mimic conditions in the jirds’ burrows. After the jirds settled in, Downs supplied each of them with some house guests—150 Xenopsylla ramesis fleas, the same number that jirds typically carry in the wild. 

These fleas normally don’t spend a whole lot of time on a jird—just a few days to fill up on blood and reproduce before moving on. To mimic the fleas’ transient ways, Downs combed the bugs from her jirds and collected them from the cages’ sandy floors every few days and then transferred in a new batch of fleas. As each group of fleas was removed, Downs placed them in an incubator so she and her colleagues could count how many had survived and how many eggs they had laid, and track how many eggs hatched.

Downs figured that because of their long shared evolutionary history with their hosts, the fleas would have evolved adaptations for living in the jirds’ burrows and could deal with the high CO2 levels. To her surprise, however, 27 percent more fleas died per day in the burrow-like cages than in the cages filled with room air, and the burrow fleas also laid 25 percent fewer eggs. In a second experiment where fleas kept in the same “burrow” or room air conditions, but without the jirds, the fleas in the simulated burrow air again had higher mortality rates and were also less mobile. 

Fleas don’t do well in stale burrow air, contrary to what Downs expected. But maybe they don’t need to, she now thinks. Fleas are fairly cosmopolitan and can infest a variety of different mammal species. With many hosts to choose from, they might not have had to adapt to underground living and the high CO2 levels in burrows.

As for why the fleas couldn’t cope with the carbon dioxide, Downs think that the gas forces them to increase their respiration to get the oxygen they need. The longer they hold their spiracles, or breathing tubes, open, the faster they dry out and die. As the fleas struggle to breathe, they also become less active, leaving them more vulnerable to being dislodged and killed when a jird scratches or grooms itself. They also spend less time feeding and drinking, which means they have fewer of the bodily resources they need to produce their eggs. It’s also possible that burrows are bad for fleas not only because of the direct effects on them, but also because of how the air conditions affect the jirds. The rodents are adapted to breathe the stale air, but the CO2 can still alter their body chemistry and immune functions, which might make their blood less nutritious for the fleas and contribute to their demise. 

Jirds and other burrowing animals have ways of limiting the CO2 levels in their homes down by keeping the entrances unsealed or adding vents. Yet, not all of them do that, and Downs says that her results may help explain why. If high CO2 levels kill fleas off and help with jirds’ parasite problems, that could be one reason they design their burrows the way they do.