Environmental trace metal status and their effects on sharp-tooth catfish (Clarias gariepinus) immune cell population and functioning in Tanzania

Trace metals are a group of naturally occurring elements whose presence in the environment at low concentrations may affect biota. They are non-biodegradable and have the property of long persistence in the environment and bioaccumulating in the food chain. They have become a global concern as they have many effects on biota, including the immune system.

The immune status of catfish was assessed using blood cells and microflora parameters as they play important immune roles. Catfish blood and gill swab samples were collected from the upper Ruvu River, the lower Ruvu River, and a pond in Java-Saadani in Tanzania. The levels of white blood cells (WBC), red blood cells (RBC), haemoglobin (Hb) level, and bacterial load were measured using MS4 haematology analyzer and bacteria isolation was done by culture, respectively.

Trace metals can deter the production and functioning of blood cells and the abundance of microflora in the catfish gills. This study measured the concentration of eleven trace metals (Al, Cd, Co, Cu, Cr, Fe, Mn, Ni, Pb, V, and Zn) and arsenic in sediments and catfish collected from Rivers Matandu, Rufiji, Upper and lower Ruvu and a pond at Java near Saadani National Park, using induction-coupled plasma mass spectroscopy (ICP-MS). Cadmium was experimentally assessed for its potential effects on the population and functioning of immune cells using Cd concentrations of 0.03 and 0.24 mg/l. The results for trace metal concentrations indicate a low to moderate pollution level at all the study sites of Rivers Matandu, Rufiji, upper and lower Ruvu and a pond at Java-Saadani. The geo-accumulation index indicated low contamination with Ni showing the lowest ecological risk index suggests a low risk to biota. Concentrations of trace metals in catfish tissue did not exceed the WHO/FAO or the European Union limits. The Target Hazard Quotient (THQ) and the Hazard Index (HI) were < 1, and the carcinogenic target risk (TR) was < 0.0001, indicating a low risk.

The results for in-situ values of blood cells of catfish showed a significant difference (p ≤ 0.05) in RBC among sites, with lower Ruvu having the highest volumes, while WBC and Hb parameters did not vary statistically. The overall in-situ levels of blood cells mean results were 37.32 x103 cells/µl for WBC, 1.4 x106 cells/µl for RBC, and 6.04 g/dl for Hb. Lower Ruvu had the highest mean blood values, followed by Java-Saadani then upper Ruvu with the lowest blood parameters. The mean colony forming units (CFU) results for microflora were 47, 46, and 405, x108 bacteria cells /ml for Saadan, upper Ruvu, and lower Ruvu respectively.
The microflora cells also varied significantly among sites (P ≤ 0.05), with the highest level obtained from catfish from lower Ruvu.

The results for experimental Cd stress of catfish on immune cells showed that Cd reduced WBC and RBC cell count, increased cell fragility, and impaired phagocytosis function. The results for white blood cells in the control tanks were stable within a narrow range (28.8 to 32.0) x103 cells / µl, while treatment results indicated a progressive decrease in the white blood cells ranging from 18.7 to 32.0 x103 cells / µl in week 1, 13.5 to 29.8 x103 cells / µl in
week 2 and 9.6 to 28.8 x103 cells / µl for week 3. The decrease of red blood cells was noted each week from 1.9 x106 cells / µl in week 1 to 1.2 x106 cells / µl in week 3, while the average Hb declined from 9.3 g/dl in week 1 to 5.0 g/dl in week 3. It is recommended that continued monitoring of trace metals should be done to detect early signs of pollution.