"So, naturalists observe, a flea has smaller fleas that on him prey; and these have smaller still to bite ’em; and so proceed ad infinitum."
- Jonathan Swift

September 22, 2020

Parapulex chephrenis

Here's the second student guest posts from the third year Evolutionary Parasitology unit (ZOOL329) class of 2020. This post was written by Patra Petrohilos and it is about the social life of Egyptian Spiny Mouse and how that relates to their fleas. (you can also read a previous post about how a muscle-dwelling worm survives under a cover of snow here).

It doesn’t require a particularly vivid imagination to appreciate that being eaten by fleas is not exactly the most stress-free experience for an animal. Neither (to the surprise of introverts nowhere) is being bullied into submission by the resident bossy boots in your social group. Surely, then, it would logically follow that being bullied by your peers AND preyed upon by parasites at the same time would be the most stressful option of all? That’s certainly what some researchers thought – and were stunned to discover that the answer was not quite what they expected.

Photo of spiny mouse from here, photo of Parapulex flea from here

Before we get any further, you may be wondering how exactly one measures the stress levels of an animal. I’m so glad you asked. Turns out, when we get stressed our bodies produce this stuff called glucocorticoids – which is such a long clunky word that I’ll just refer to it from here on in as GC. In the short term (let’s say we see a predator across the street) this is a good thing – a short burst of GC takes the energy that we’d usually spend on boring things like digesting food and diverts it to more useful activities – like running away from predators. But in the long term (let’s say we are trapped in a cage with that predator for a year) it is a very bad thing. Too much GC can do all kinds of awful things, wreaking havoc on our immune system and our fertility. Scientists can measure how much GC an animal is producing (and therefore how stressed out it is) by analysing its poo. It’s all pretty glamorous.

These particular scientists were interested in how two different negative experiences (parasitism and social interaction) interact to affect an animal’s stress levels. They decided to investigate this by studying the Egyptian spiny mouse (Acomys cahirinus) – an incredibly social little fella that is found living in groups of one male and multiple females. Within this little society, one of those females usually stakes a claim to “Queen Bee” of the group. Bizarrely, they are also especially attractive to one particular species of flea (Parapulex chephrenis), who for some reason steer clear of all other mouse species in favour of this one.

Once they had gathered their mice, the scientists split the females into two groups. The first consisted of pairs of mice, two to a cage. As tends to happen in these situations, one of the pair invariably emerged as the bossier one. This two-mouse hierarchy was well and truly established after a week, by which time the submissive one knew her place well enough to not even attempt to rock the social boat. The second group was divided into single ladies. Each mouse in this group got an entire cage to herself (and peace from any potential bickering over petty things like food).

They then divided the groups further. Half of the paired mice and half of the single ladies were infected with P. chephrenis fleas, while the other half were left flea-free. For a brief period, a male was also added to each cage (just long enough to do the kinds of things that male mice like to do with female mice) and then mouse poo was collected at various points so the scientists could gauge each mouse’s stress levels.

To their amazement, the single mice were more stressed than their paired up counterparts – even the ones being dominated by the bossy boots cagemates. Apparently company is so important to such a social species that being alone is more traumatic than being at the bottom of the pecking order. But even more astoundingly, it was the mice who were not only solitary but also flea-free that were more stressed out than anyone!

It’s possible that flea infestation made these already-anxious solitary mice more likely to indulge in a bit of grooming (a behaviour that tends to soothe rodents), but regardless – it’s fascinating that the results were the exact opposite of expected. Rather than one stressful thing exacerbating the other (like adding Carolina Reaper chili peppers to an already hot sauce would) they almost seemed to cancel each other out (like adding yogurt to a vindaloo curry).

So what’s the moral of the story? If you’re an Egyptian spiny mouse, even having awful, flea infested friends that bully you is better than having no friends at all. And for those poor waifs who don’t have friends - any distraction is preferable to the loneliness of a solitary life. Even when that distraction is being eaten by fleas.


This post was written by Patra Petrohilos

September 15, 2020

Trichinella britovi

It's time for some student guest posts! One of the assessments I set for students in my ZOOL329 Evolutionary Parasitology class is for them to summarise and write about a paper that they have read in the manner of a blog post. The best blog posts from the class are selected for re-posting (with their permission) here on the Parasite of the Day blog. For the class of 2020, two students' posts were selected. So to kick things off, here's a post written by Anna Clemann, and it's all about how a muscle-infecting nematode survives under cover through winter. 

Photo of Trichinella britovi from this paper
We’ve all heard the stories of people lost in the snow building ‘snow caves’ to survive the cold temperatures. Turns out the nematode Trichinella britovi, a small parasitic worm which have larvae that are found in the striated muscles of carnivorous animals, also survives better in ‘snow cave’ type conditions. 

Trichinella britovi can be found in a number of different hosts, with many scavenger species acting as carriers or reservoir hosts that themselves do not experience much ill effects from the parasite, but can be a source of infection for other host species. Trichinella britovi larvae are transmitted when the striated muscle (where the larval worm resides) of an infected host is consumed by another animal. This parasite has adapted to surviving in the decaying muscle of hosts via engaging in anaerobic metabolism, so they can survive in tissue that has little oxygen for long periods of time. 

A recent study has found that temperature and humidity also play a major role in the chances of survival for T. britovi. If a carcass infested with T. britovi is frozen, they can survive for up to several months in the muscles, which increase their chances of being ingested by another host. Researchers from Italy and Latvia decided to test whether the chance of survival for T. britovi was better if the infested carcass was buried under snow or above the snow. 

The researchers conducted their study on two carnivore scavengers, fox and raccoon dog. First, they placed the animal carcasses in a scavenger-proof netted mesh box that was surrounded by snow. They then divided the box into two sections and placed one set of the carcasses on each side. One side was filled almost to the top with snow while the other side was left exposed (see image below). 

A picture depicting the experimental set-up, taken from Fig. 1 of the paper.

Over the course of the study (112 days) the researchers collected muscle samples from all the carcasses and recorded the temperature and humidity for both environments over key periods of time. The muscle from all carcasses were fed to lab mice and researchers then looked at the prevalence of T britovi larvae surviving and reproducing within the mice. Through that, they found that T. britovi survived better if they were buried in the snow! 

Researchers found little difference in the reproduction capacity of T. britovi in the mice from the carcasses which are beneath and above the snow in the first two months of the experiment. However, during the last 42 days of the study, mice that were fed muscle from exposed carcasses above the snow (which were subjected to more temperature and humidity variation) showed a 100% reduction in T. britovi infection, meaning the worms they were fed with were not infecting them at all. While mice fed with muscles from carcasses that were buried also had a reduction in larvae reproduction, but at least some were successful in establishing infection in those final weeks up to the end of the experiment. 

The researchers found that the difference in temperature and humidity above snow and below snow were enough to provide a better environment for T. britovi over a longer period. They also noted that below the snow, the variation in temperature was 5.5 times lower than above the snow, producing a more stable and warmer environment. This was further confirmed when the researchers found that the extent of rotting in the muscle (which was more in the buried carcasses than the unburied) was not detrimental to T. britovi reproductive capacity. 

 So, life is better if you’re buried alive, at least if you’re a Trichinella britovi larvae. 


This post was written by Anna Clemann