Fish Diseases and Management

3.1.2.Non specific humoral factors

Growth inhibitors

1. Transferrin: Transferrin has very high capacity for iron, which is an essential growth element for all the organisms. Due to this ability, transferrin has bacteriostatic and fungistatic effect. To circumvent this, many microorganisms produce their own siderophores, which compete with transferrin for iron.

2. Interferon: These are substances produced by animal cells following viral infection. The rate of production is temperature dependent. Interferons have substantial non-specific antiviral activity.

Enzyme inhibitors These include antiproteases, which act against the enzymes produced by the microorganisms. The antiproteases 2-macroglobulin protects salmonids against Aeromonas salmonicida

Lysins : These compounds either themselves or in combination may cause lysis of the pathogen’s cells. Two important lysins are complement and lysozyme.

i. Complement: Complement is an enzyme cascade system present in serum and tissue fluids and has 12 protein components. This cascade system is activated in 2 major pathways viz. a) classical pathway, which require the target cell to be coated with antibody which activates and orientate complement to damage target cell membrane and b) alternative pathway, which is initiated by certain agents such as bacte ria l endotoxin, solid polysaccharides zymosan and insulin. These compounds first activate properdin, which in turn activate the rest of the complement cascade.

ii. Lysozyme: Lysozyme acts on peptidoglycan component of the cell wall of microorganisms. In most cases, the outer membrane in bacterial cell wall needs to be disrupted by complement to allow lysozyme to act. Lysozyme is abundantly present in phagocytic cells, serum and mucus.

Precipitins and agglutinins

i. CRP (C-reactive protein): This binds to phosphoryl – ester groups present on many microbial cell walls. CRP is detected in normal serum and eggs of many teleost species. Action needs presence of Ca²⁺ ions and is solubilized by c hela ting agents. In mammals, CRP activates complement. In fish the role of  is not fully documented.

ii. Natural antibodies: These compounds are reported to have agglutinating or precipitating activity to a wide variety of erythrocytes, bacteria and polysaccharides. They belong to immunoglobulins or lectins. Lectins are proteins, which bind specifically to simple sugars and are important in invertebrate defence. In fish these are thought to act as opsonin s.