Thyroid hyperplasia in wild brook trout from Lake Florentien, Abitibi-Témiscamingue (QC).

A small lake among many others in Abitibi-Témiscamingue (Quebec), Lake Florentien is distinguished by its project to restore a natural population of brook trout (Salvelinus fontinalis) carried out in 2012. During the summers 2016 and 2017, goiters (increases in the size of the thyroid gland) were observed in several specimens of brook trout in this lake, on approximately 1% of the fish caught. The lesions were characterized by the presence of one or more masses located under the head at the junction of the two opercula, or visible at the base of the gills (see pictures).

Goiter (increase in size of the thyroid gland) observed on several brook trout from Lake Florentien in 2016 and 2017. An external ventral mass between the two opercula is evocative of a goiter (photo: Jean-Pierre Hamel, MFFP).

photo: Jean-Pierre Hamel, MFFP.

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Thyroid follicular hyperplasia has been confirmed by histopathology by the CWHC-Quebec. In order to better characterize the lesions and better understand their origin, a project was carried out in collaboration with the Ministère des Forêts, de la Faune et des Parcs (MFFP; Quebec Ministry of Forests, Wildlife and Parks) and the Quebec node of CWHC (Centre québécois sur la santé des animaux sauvages) during the summer 2019. Fish were sampled from two lakes for gross and microscopic examination: Lake Florentien and a neighboring control lake for comparison. Thyroid hyperplasia was diagnosed by histology on several fish caught in Lake Florentien. However, the lesions were slight and did not appear to affect the general condition of the fish.

Thyroid hyperplasia is a non-tumor proliferative lesion secondary to excessive stimulation of the thyroid gland for the production of thyroid hormones. This condition is poorly documented in wild fish. The only documentation of thyroid hyperplasia has been reported in wild salmon from the Great Lakes in the 1970s. Various hypotheses have been put forward to explain the development of goiter in fish: low iodine in water, a diet deficient in iodine or the presence of goitrogenic substances in the environment or in the diet. A nutritional cause is strongly suspected in the brook trout from Lake Florentien due to the high density of fish (intraspecific competition possibly leading to malnutrition). Iodine analyzes carried out during this field project showed a potentially low iodine content in the trout diet. Consumption of these fish is therefore harmless to humans.

Ackowledgements:

The authors would like to thank the MFFP staff, Jean-Pierre Hamel and Isabelle Dumais; all of the volunteer fishermen for the logistics, for catching the fish and for their precious help in examining the fish; Sarah Bosisio, summer student of CQSAS in 2019, for her help during the field project, the sample management and the histological preparation.

REFERENCES 

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Marion Jalenques and Stéphane Lair – CWHC-Quebec