Walborg Thorsell was a Swedish parasitologist who specialised in mosquitoes and methods to repel them.

Walborg Thorsell was born in Stockholm in 1919. She was the daughter of the construction engineer Ernst Gottfrid Thorsell and his wife Alida Susanna Thorsell. During the 1940s Walborg Thorsell attended Stockholms högskola (college). In 1967 she gained her doctorate in the antiparasitical matter hexachlorophene (HCP) and its effects on the common liver fluke, which is a parasitical flatworm. In 1968 Walborg Thorsell was appointed docent in experimental parasitology.

Walborg Thorsell began studying mosquitoes in the early 1970s whilst an employee of Försvarets forskningsanstalt (FOA) (the Department of defence research institute). At this time both the Warsaw-pact countries and those in NATO had evidence indicating that enemy countries were undertaking research into the possibilities of spreading disease by using biting mosquitoes as a form of biological warfare. The Swedish Department of defence felt it was necessary to develop improved means of protection against mosquitoes for use by Swedish soldiers in times of war.

The anti-mosquito formulas which already existed were either too weak to effectively repel the insects or caused debilitating side-effects. The task which Walborg Thorsell and her research group worked on at the FOA was to develop a safer alternative to the current most-used mosquito-repellent, in which the active element was diethyltoluamide (DEET).

One element which revealed itself to have the desired effect on both malarial mosquitoes and ordinary Swedish forest mosquitoes was diethylamide. However, there was a drawback in that diethylamide is water soluble and is thus easily rinsed off by human sweat if applied to human skin. Walborg Thorsell got the idea of applying the same functional groups found in diethylamide to mandelic acid, namely mandelic acid diethylamide (DEM). The difference between DEM and DEET was that a smaller amount of the applied DEM was absorbed by the skin in comparison to DEET, which was an advantage in toxicological terms. The commercial mosquito-repellent known as DEMIDEX, containing the active element of DEM, was thus produced and sold for many years. However, the production costs were high and it was not long before the product lost out to its competitors. DEMIDEX was more effective against mosquitoes than its American predecessor DEET, albeit it is the latter which is most common active element in insect-repellents.

Partly due to her expertise but also due to her somewhat unusual research technique Walborg Thorsell appeared frequently in newspaper articles and on TV-programmes for many years. Out of respect for the suffering imposed on laboratory-animals Walborg Thorsell performed much of her testing on herself by allowing mosquitoes – in a controlled environment – to bite her own arm which was covered in various substances. Walborg Thorsell was not in favour of insecticides and believed that largescale eradication was risky because mosquitoes must serve an ecological purpose. Her goal instead was to get the mosquitoes not to suck blood from human prey and thereby to minimise the chance of disease transfer.

After her retirement from FOA Walborg Thorsell continued her research into mosquito repellents at the zoology department of Stockholm university and Uppsala university. Most of her later work was in plant extracts and natural products which repelled mosquitoes and ticks.

Walborg Thorsell’s contributions to research into mosquito repellents was recognised when her name was given to the Thorselliaceae bacterial family, which includes three strains of the Thorsellia family. These bacteria occur naturally within the intestinal flora of malarial mosquitoes and the most recent research had been focused on trying to eradicate the malaria parasite within the mosquitoes by using genetically modified Thorsellia bacteria. It is hoped that in this way the spread of malaria can be reduced without harming the mosquitoes, in accordance with Walborg Thorsell’s research ethic.

Walborg Thorsell died in Stockholm in 2016. She is buried at The Woodland Cemetery.