Efficacy and safety of boric acid as a preventive treatment against Saprolegnia infection in Nile tilapia (Oreochromis niloticus).

Saprolegniosis is a worldwide fungal-like infection affecting freshwater fishes and their eggs. Reports show high mortalities and subsequent economic losses annually from Saprolegnia infections. Most therapeutants against Saprolegnia spp. infections are inefficient and some have negative impact on the environment. In this study, we have investigated the ability of boric acid (BA) to prevent Saprolegnia infection in Nile tilapia (Oreochromis niloticus). BA inhibited radial growth of Saprolegnia hyphae in vitro. Complete in vitro growth inhibition was found at a concentration of ≥0.6 g/L. Inhibitory effects were also observed in vivo when Nile tilapia were experimentally challenged with Saprolegnia spores and followed over 10 days post challenge and under continuous exposure to different BA concentrations. No signs of saprolegniosis were observed in fish treated with BA at concentrations of 0.4 g/L and above. Comet assay revealed that BA has low toxicity in tilapia continuously exposed to concentrations of 0.2-0.6 g/L for 96 h. Additionally, no significant histomorphological changes were observed in BA-treated fish compared to non-treated controls. Alanine Aminotransferase (ALT) and Aspartate Aminotransferase (AST) enzyme levels indicated reduction in systemic tissue damage associated with Saprolegnia infection. This study demonstrates the potential of BA as a prophylactic measure against Saprolegnia infection in tilapia, and we recommend additional studies on environmental impact.

Effects of aflatoxin B1 on reproductive performance of farmed Nile tilapia.

This study evaluated the effect of dietary aflatoxin B1 (AFB1) on growth, milt and egg quality in matured Nile tilapia (Oreochromis niloticus). Triplicate groups of Nile tilapia (initial body weight 24.1 ± 2.6 g) were fed with either of four diets (Diets 1 to 4) designed to contain 0, 20, 200 and 2000 µg AFB1 kg-1 diets for 24 weeks. After 24 weeks of AFB1 exposure, growth was significantly (P <0.05) different between the control and the AFB1 exposed treatments in both sexes. No significant differences were observed in 17ß-oestradiol, absolute fecundity, oocytes volume and diameters between AFB1 exposure groups and the control group. However, we observed a significant reduction in relative fecundity and gonad somatic index (GSI) in females fed 2000 µg AFB1 kg-1 diet. On the other hand, we observed significant differences (P <0.05) in gonadosomatic index (GSI), testosterone, milt count and motility between males in the control group and AFB1 treatments. We conclude that rearing Nile tilapia with aflatoxin-contaminated diets for a prolonged period affects milt quality, fecundity (at higher doses) and growth performance. This implies that for optimal seed production, provision of aflatoxin free diets should be part of the management practices in Nile tilapia hatcheries.

Dietary administration of probiotics modulates non-specific immunity and gut microbiota of Nile tilapia (Oreochromis niloticus) cultured in low input ponds.

Poor culture conditions in low input ponds make fish highly susceptible to infectious pathogens which lead to diseases and mortalities yet the effects of probiotics on immunity, gut microbiota and microbiological quality of fish in low input ponds are unknown. Nile tilapia, Oreochromis niloticus fingerlings (40 g) were randomly stocked at 50 fish m-3 in 1.25 m3 cages in low input ponds. The fish were fed on diets supplemented with either Saccharomyces cerevisiae (1 × 1010 CFU g-1) or Bacillus subtilis (1 × 109 CFU g-1) at six levels: Diet 0 (No probiotic); S. cerevisiae at 2 g kg-1 (Diet 1); 4 g kg-1 (Diet 2) and 6 g kg-1 (Diet 3) and B. subtilis at 5 g kg-1 (Diet 4); 10 g kg-1 (Diet 5) and 15 g kg-1 (Diet 6) for 180 days. Results indicate that hemato-immunological parameters (hemoglobin (Hb), red blood cells (RBC), white blood cells (WBC) serum protein, albumin, globulin and lysozyme activity) were significantly higher in fish fed on probiotic treated diets compared to the control (P < 0.05). On the contrary, fish fed on Diet 6 presented significantly similar Hb and globulin values compared to the control (P > 0.05). Additionally, fish fed on probiotic treated diets retained the probiotics in their guts and lower microbial load was realized in their muscle (P < 0.05). In conclusion, B. subtilis and S. cerevisiae supplementation in diets of Nile tilapia reared in low input ponds improves immunity, manipulates gut microbiota and enhances fish flesh quality.

A review of aquaculture production and health management practices of farmed fish in Kenya.

Warm water aquaculture is widely practiced in Kenya and is dominated by the culture of Nile tilapia (Oreochromis niloticus) (75% of total production) followed by African catfish (Clarias gariepinus) at 18%. Aquaculture started in Kenya in 1920's and has been on upward trend until 2014 when it peaked at 24,096 MT. However, production reduced drastically in the past 3 years, with 14,952 metric tonnes (MT) reported in 2016. Most farmers practice earthen pond based semi-intensive culture system. Commercial intensive culture of Nile tilapia (O. niloticus) in cages in Lake Victoria has grown significantly in the last five years with a production of 12 million kg of fish every cycle (about 8 months). Recirculation aquaculture system (RAS) is also gaining popularity mainly in intensive hatcheries. The freshwater cages have been marred by increasing frequencies of fish kills with obvious financial and environmental implications. Although limited information exists on fish disease outbreaks across the country, certain well known diseases in farmed fish have been reported. These include; fungal, mainly saprolegniasis, bacterial, mainly hemorrhagic disease and pop-eye diseases. Parasites have also been documented in farmed O. niloticus and C. gariepinus. Although prophylactic treatments are used in some hatcheries in order to prevent infections, limited biosecurity measures are in place to prevent diseases in farmed fish. This is because of inadequate knowledge of the economics of fish diseases, poor infrastructure and inadequate human resource specialized in fish diseases. This review describes the aquaculture production and health mangement practices of farmed fish in Kenya in order to document actions required for effective monitoring and regulation of future fish health problems across the country.

The morphology and systematics of Rhabdochona paski Baylis, 1928 (Nematoda: Rhabdochonidae), a widespread parasite of freshwater fishes in Africa.

Nematodes of the genus Rhabdochona Railliet, 1916, identified as R. paski Baylis, 1928, were collected from the intestine of Hydrocynus forskahlii (Cuvier) (Characiformes: Alestidae), Oreochromis niloticus (Linnaeus) and Tilapia zillii (Gervais) (both Perciformes: Cichlidae) from Lake Turkana, Kenya during 2007-2009. Their morphology was studied in detail using light and scanning electron microscopy. Paratypes of R. paski and museum specimens of R. congolensis Campana-Rouget, 1961 from six other host species were examined for comparison. Based on these studies and the available literature data, Rhabdochona congolensis, R. aegyptiaca El-Nafar & Saoud, 1974 (emend.) and R. vesterae Boomker & Petter, 1993 are considered to be junior synonyms of R. paski. The occurrence of this widely distributed African nematode in many fish species belonging to different families and orders suggests that most of them are probably not definitive hosts of this parasite, but only serve as paratenic, paradefinitive or postcyclic hosts (sensu Odening, 1976). True definitive hosts of R. paski appear to be characiform species belonging to some genera (e.g., Alestes, Brycinus, Hydrocynus) of the family Alestidae.

Mexiconema africanum sp. n. (Nematoda: Daniconematidae) from the catfish Auchenoglanis occidentalis from Lake Turkana, Kenya.

A new species of dracunculoid nematode, Mexiconema africanum sp. n. (Daniconematidae), is described from the abdominal cavity and the intestine (rarely also the gall bladder) of the catfish Auchenoglanis occidentalis (Valenciennes) (Claroteidae, Siluriformes) from Lake Turkana, Kenya. The new species differs from two other congeners mainly in the absence of two large cell nuclei in the glandular oesophagus, presence of well-developed lateral cephalic elevations, more numerous (14) cephalic papillae and a much longer body of the gravid female (18-22 mm); from Mexiconema cichlasomae Moravec, Vidal and Salgado Maldonado, 1992 also in less numerous (two) caudal processes and a different arrangement of genital papillae in the male. M. africanum is the first representative of the dracunculoid family Daniconematidae described from Africa.

Philometrids (Nematoda: Philometridae) from fishes of Lake Turkana, Kenya, including two new species of Philometra and erection of Afrophilometra gen. n.

The following four species (only females available) of the Philometridae (Nematoda: Dracunculoidea) were recorded from freshwater fishes of Lake Turkana, northwestern Kenya in 2007-2008: Philometra bagri (Khalil, 1965) from the subcutaneous tissue around the mouth, on gill covers and the fin base of the bayad Bagrus bajad (Forsskål) (Bagridae: Siluriformes), Philometra lati sp. n. from the abdominal cavity of the Nile perch Lates niloticus (Linnaeus) (Latidae: Perciformes), Philometra spiriformis sp. n. from capsules on the inner surface of gill covers of L. niloticus and Afrophilometra hydrocyoni (Fahmy, Mandour et El-Nafar, 1976) comb. n. from the fins of Hydrocynus forskahlii (Cuvier) (Alestidae: Characiformes). The new species P. lati is characterized mainly by the presence of distinct oesophageal teeth, absence of large cephalic lobes and caudal projections, and by a combination of other features. Philometra spiriformis differs from all congeners principally in the spirally coiled body and from individual species by a combination of other morphological features. The already known species P. bagri and A. hydrocyoni are redescribed based on light and scanning electron microscopy; findings of both these species in Kenya represent new geographical records.