Effects of water contamination with heavy metals on pigmented macrophages in spleen of Vardar chub (Squalius vardarensis Karaman) from three rivers in the North-Eastern region of North Macedonia

Background and purpose: Pigmented macrophage aggregates (PMAs) are a part of the immune system of fish and are considered as valuable biomarkers of the impact of water pollution on fish health.Materials and methods: Immunomodulatory effect of water contaminated with high levels of metals on spleen and splenic PMAs of Vardar chub (Squalius vardarensis) (n=129) was examined in two seasons, spring, and autumn 2012. For this purpose, three rivers were chosen: the Zletovska River, heavily contaminated with metals, the Kriva River, moderately contaminated with metals combined with high level of organic matter, and the Bregalnica River, without heavy metal contamination. Spleen masses and splenosomatic indices (SSI) were used for determining the general impact of water contamination on the immune system of Vardar chub. Moreover, volumes and numbers of PMAs within spleen tissue were determined stereologically.Results: The isolated effect of high exposure to metals caused the reduction of spleen mass and/or SSI in the fish from the Zletovska River. The combined effect of exposure to metals and high levels of nutrients in the fish from the Kriva River referred to stimulation of PMAs response, namely increased volumes of splenic PMAs. The existence of seasonally dependent differences was noted since the observed variability in PMAs between rivers occurred only in the autumn.Conclusions: Splenic PMAs in the Vardar chub can be used as an additional biomarker of metal pollution, but with caution, since enhancement of the volumes and numbers of MACs depends on the exposure level of heavy metals, on the presence of the other contaminants, as well as on the physiological condition of the fish.