The nematode experiment earned Fibiger the 1926 Nobel Prize in Physiology or Medicine, but under controversial circumstances.

Erling Norrby, who had served as the Permanent Secretary of the Royal Swedish Academy of Sciences and Professor and Chairman of Virology at the Karolinska Institute, declared Fibiger's Nobel Prize as "one of the biggest blunders made by the Karolinska Institute.

"[2] A Danish physician, Johannes Fibiger, while working as Director of the Institute of Pathological Anatomy at the University of Copenhagen, dissected some wild rats collected from Dorpat (now, Tartu, in Estonia) in 1907.

He published his discovery in a series of three papers in 1913, and also presented them at the Académie Royale des Sciences et des Lettres de Danemark (Royal Danish Academy of Sciences and Letters), and Troisième Conférence Internationale pour l’Étude du Cancer (Third International Conference for Researches in Cancer) at Brussels the same year.

The adult roundworms are present in the epithelium of the anterior portion of the digestive tract, including the mouth, tongue, oesophagus and fundus.

The tail end contains cloaca and precloacal sucker on the ventral side, which are surrounded by sensory structures called caudal papillae.

The male reproductive system consists of a single testis, vas deferens, seminal vesicles, ejaculatory duct, two spicules, gubernaculum and bursa.

Yamagiwa and Ichikawa showed that it was possible to induce cancer (carcinoma) in rabbits, and that the simplest method was by painting coal-tar on the inner surface of the ear.

They even showed that different mechanical or chemical irritation, especially the painting of coal-tar upon the inner surface of the ear was the most effective in inducing carcinoma in rabbits.

But the two assessors appointed by the Nobel Committee, Folke Henschen and Hilding Bergstrand could not come to a mutual agreement.

Bergstand conclusion was that there was not "much support for the possibility that the work of Fibiger and Yamagiwa will have great importance in the solving of the riddle of cancer.

There were two other nominees: Otto Heinrich Warburg, for his works of cancer metabolism and respiratory enzymes, and Julius Wagner-Jauregg, for the discovery of malariotherapy.

Fibiger became the sole winner of the 1926 Nobel Prize in Physiology or Medicine,[4][15] "for his discovery of the Spiroptera carcinoma".

They repeated Fibiger's experiments using advanced techniques, and concluded that the tumours induced by the nematode in rats were metaplasia, not cancer.

[23] Systematic reanalysis of Fibiger's data also gave the same conclusion that G. neoplasticum can cause tumour, but is not carcinogenic by itself.