Slugs on Anti-depressants are More Suicidal
By: Science Explained
Tags: Biology, Slug, Science, Parasites, Parasitology

A study in 2018 by Morris et al named “Zombie Slugs on Drugs”, discovered that the parasite Phasmarhabditis hermaphrodita uses serotonin to manipulate the behaviour of infected slugs, leading them to move towards P.hermaphrodita instead of away. To do this they gave slugs anti-depressants and serotonin inhibitors to determine their behaviour.

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Mind control is a commonly seen tactic for a variety of parasites, with famous examples including the funky disco snail and the fungal parasite that controls carpenter ants. And another unfortunate victim to the brainwashing is commonly found UK slug and snail species, who are the target of the lethal nematode Phasmarhabditis hermaphrodita.

P.hermaphrodita is found in the soil, where it reproduces on rotting matter. But the nematode can’t produce the optimal amount of offspring on the leaf litter and decides to enter passing slugs through its mantle, bring the slug back to its buddies, and kills it to produce a nice location to hold a parasite orgy.

The slugs, on the other hand, they don’t really like being used in this way, and so they detect where P.hermaphrodita are living, and avoid it. To show this avoidance, the scientists placed 5 slugs in the middle of a box with nematodes on one side and water on the other, the side the slugs was on was then noted for four days, and the slugs were returned to the middle after each measurement.

Most of the slug species avoided the side with the nematodes on, excluding M.sowerbyi which spent more time on the nematode side, and L.valentiana which spent equal amounts of time on each side. All of the species with the exception of M.sowerbyi spent more time on the nematode side when infected by P.hermaphrodita, showing that infection did cause the slugs to move towards the nematodes.

When uninfected slugs of the D.panormitanum species were fed the SSRI drug Fluoxetine (commonly known as Prozac), they were found to spend more time on the nematode side than when they weren’t drugged. There was a slight decrease in the number of infected D.panormitanum found on the nematode side when they were dosed compared to without.

To determine whether it was Seratonin that caused this behavioural change, both infected and uninfected D.panormitanum were given the serotonin suppressor cyproheptadine. The results of this showed that there was no significant difference between dosed and undoped uninfected slugs, however infected slugs that were dosed with cyproheptadine spent equal time on both sides of the box, instead of mainly on the nematode side like their undrugged predecessors.

To show that this was due to the effects of the drugs, and not because the drugs were killing the parasites, they exposed P.hermaphrodita to either 10µM fluoxetine, cyproheptadine, or water as a control. There was no effect on their behaviour, and after 4 days the majority of the nematodes were still alive.

Another experiment by Cutler et al in 2019 showed that other serotonin increasing drugs such as sertraline and apomorphine had the same effect on the slugs, making them move towards the nematode side, while haloperidol made uninfected slugs prefer the control side while having no significant effect on the infected slugs, further showing that serotonin is involved in the mind control mechanisms of P.hermaphrodita.

Conclusion

Although not really related to the infectious diseases of humans, it’s interesting to see how parasites evolve to survive in the optimal conditions, and the powers of mind control they seem to have. Maybe depressed people with slug problems should start grinding up some of their anti-depressants to add with their nematode-based pesticides.