Outbreaks of S. pyogenes have recently been identified as among the most serious infectious diseases, with significant economic consequences in both cultured and wild populations of tilapia farms. Acute Streptococcosis infection can cause more than 50% mortality (Saleh et al., 2019). Streptococcus spp. are common in aquatic environments (Nasr-eldahan et al., 2021). Bacterial pathogenicity, like that of other opportunistic bacteria, is associated with a sharp increase in water physicochemical parameters (ammonia, salinity, and temperature), low dissolved oxygen, and stress factors (Younes et al., 2016).
The current study was an attempt to verify the pathogeny dose of streptococcus pyogenes into Nile tilapia infection. In addition to examining the appropriate dose for infection, estimated the LD50 of streptococcus pyogenes, in which fish reinjected at four different concentrations (1×106, 1×107, 1×108 and 1×109CFU), and found that 1×107 CFU showed a mortality rate of 60% compared to 1×106 CFU which show mortality rate by 10% and that on contract 1×108 and 1×109CFU which showing 100% mortality and this dose in contract with El-Gheit, (2005) which show that 1×107 CFU caused 100% mortality and 1×105 CFU was LD50. Furthermore, in day of infection 1×107 CFU as LD50 dose cause mortality from day 2 until day 7 and this agreement with El-Gheit, (2005) who found that S. pyogenes causes death and become virulence from day 2 and become normal in death after day 7 to day 10.
The clinical examination of S. pyogenes shows the same clinical sign of other Streptococcus species as in external examination show skin lesion, erratic swimming, eye optic, hemorrhagic in the body surface. Furthermore, internally show that the liver is enlarged with pale color and spleen is enlarged the abdominal cavity and intestine has fluid executed, which agreement with previous reports (Amal et al., 2011 and Mishra et al., 2018).
Hematological characteristics are a valuable tool that can be used as an effective and sensitive indicator for tracking physiological and pathological changes in fish (Parrino et al., 2018). The next discussion show that 1×107 agrees with previous studies, on contract with other concentration. In this study, going to comparing hematological parameter among four different concentration of S. pyogenes. Where WBCs, LYM and Neutrophils (a type of GRAN) show high increase in (1×107 and 1×108) and very low decrease in other concentration compared to the control. This increase indicates a response of the body resistance to disease-causing antigens taking into account a natural reaction to the presence of the bacterial pathogen by induction of the non-specific defense system (Alsaid et al., 2014). The Increase in total leukocytes indicated an increase in the body’s immunity, as evidenced by increased activity of phagocyte cells, which act to perform phagocytosis against foreign objects entering the fish body (Alamanda et al., 2007). On the other hand, the increase in lymphocytes may be due to stress of bacteria infect fish and end up causing lymphocyte proliferation (increased cell count and formation of changes in T cells and B cells) (Cano et al., 2013). Furthermore, the increase in neutrophils could be due to the response to a bacterial infection, as neutrophils escape from the marginal group and join the infection area, and the thymus releases its source of reserve, resulting in increased granulopoiesis, with this increase due to the presence of many immature cells. Neutrophils entering the blood circulation, killing and digesting bacteria. And this agreement with (Lawrence et al., 2018 and Afiyantithe et al., 2018). On the other hand, decreasing in WBC, lymphocytes and Neutrophils in S. pyogenes group, may due to the bacteria becoming virulent, and the action of bone marrow is temporarily disrupted, with the bone marrow being the factor that produces WBCs, lymphocytes, neutrophils, RBCs, and platelets (Nombela-Arrieta et al., 2017 and Seo et al., 2019).
On the other hand, the decrease in RBCs, HGB, and HCT other hematological parameters count in the different concentration except (1×108 in HGB and 1×106 in RBCs) could be due to the deterioration of the hematopoietic organs located in the spleen and pronephros, where the bacteria may cause pathologies in the hemopoietin organs, particularly in the kidneys, spleen, and liver of fish, which leads to a decrease in the production of RBCs, HGB, and HCT (Alsaid et al., 2014). And this agreement with Yu et al., (2010) who demonstrated that there was a significant decrease in RBCs in peripheral blood, HCT percentage, and HGB rate, all of which are indicators of anemia. Furthermore Alsaid et al., (2014) show same results when red hybrid tilapia injection with Streptococcus agalactiae. Our most recent discovery also noted that sharp decrease in these values that could have hampered oxygen and nutrient transport to tissues may be due to hypochromic microcytic anemia caused by bacteria (Harikrishnan et al., 2009) and that agree well with the results of Řehulka et al., (2007). The same results were obtained when fish infected with S. agalactiae bacteria (McNulty et al., 2003). MCHC
In most animals, the investigated hematological indices, MCHC (mean corpuscular hemoglobin concentration), MCH (mean corpuscular hemoglobin), and MPV (mean platelet volume), play a critical role in the diagnosis of anemia (Alsaid et al., 2014). In this study, the declines in these blood indictors (MCH and MCHC) may be linked to the declines in RBCs, HGB, and PCV caused by disruptions in hematopoietic organs of fish challenged with S. pyogenes. Similarly, to our findings Ranzani- Paiva et al., (2004) noticed that MCH and MCHC levels in blue tilapia infected with Corynebacterium sp. were found to be lower, due to lower RBCs. Furthermore, Haniffa and Mydeen, (2011) notice that there are decline in MCH and MCHC in infected catfish with Aeromonas hydrophila.
In elevated number of platelets may be due to innate immune cells, platelets contain PRRs (pattern recognition receptors), which recognize different components that are increased during infection in S. pyogenes group which show increasing in platelet count after infection. Bacteria release toxins called estreptolysin O, which is capable of activating platelets, increasing platelet production, and causing platelet aggregation (Portier et al., 2021). Also, during invasive infection, S. pyogenes bacteria may use a cloak of fibrinogen and activated platelets to evade the immune response and spread through the bloodstream. S. pyogenes bacteria have the ability to bind with fibrinogen to increase bacteria livelihood in the bloodstream (Carlsson et al., 2005), and this in agreement with fibrinogen (Svensson et al., 2014).