(a) What is ELISA? Explain the principle, types, procedure and applications of ELISA. (20 marks) (b) Describe the symptoms…

ELISA. ELISA is an enzyme-linked immunosorbent assay that detects antigen or antibody by immobilising one on a solid phase and using an enzyme-conjugated reagent to convert substrate into measurable colour. The principle is specific antigen–antibody binding: if target is present, enzyme is retained after washing and substrate produces a chromogenic signal proportional to analyte. In direct ELISA, antigen is coated and enzyme-labelled primary antibody detects it; it is simple but less sensitive. In indirect ELISA, antigen is coated, unlabelled primary antibody binds, then enzyme-labelled anti-immunoglobulin amplifies signal; it detects antibodies to pathogens. In sandwich ELISA, capture antibody coats the well, antigen is bound, then detection antibody and enzyme conjugate are added; it is highly specific for antigens with two epitopes. In competitive ELISA, limited labelled antigen competes with sample antigen for antibody, so colour inversely indicates concentration; it suits small molecules and hormones. Procedure: coat plate with antigen/antibody, block with BSA or casein, incubate sample, wash, add labelled reagent, wash, add substrate such as TMB, stop and read absorbance. Applications include HIV, hepatitis B/C, COVID-19 antibody/antigen testing, and veterinary serology for FMD and brucellosis.

Livestock diseases. FMD causes fever, vesicles on tongue, teats and interdigital skin, lameness and reduced milk yield in cattle, buffalo, sheep and goats; diagnosis combines clinical signs, RT-PCR, virus isolation and serology. Treatment is supportive, with antivirals only where available, analgesics, wound care and antibiotics for secondary infection; scheduled FMD vaccination, e.g. FMD-Raksha, under NADCP is central. Anthrax, caused by Bacillus anthracis, causes sudden death, bloating, dark blood from orifices and terminal haemorrhage; diagnosis uses post-mortem blood smear, PCR and culture, but carcasses must not be opened carelessly. Early cases receive antibiotics such as penicillin or tetracycline; prevention relies on live attenuated spore vaccines, e.g. Sterne strain, plus burial of carcasses and pasture management. Black quarter, a Clostridial myonecrosis, causes painful swelling, crepitus and rapid death in cattle; diagnosis is clinical plus post-mortem gas-filled muscle and culture. Treatment is penicillin, supportive care and surgical drainage where possible; prevention uses alum-precipitated toxoid vaccine and wound care. Mastitis in cattle and buffalo presents swollen, hot udder, clotted or watery milk, fever and reduced yield; diagnosis uses CMT, somatic cell count, culture and sensitivity. Treatment is intramammary/systemic antibiotics, anti-inflammatories and dry-cow therapy; prevention uses milking hygiene, footbaths, dry-cow protocols and herd health monitoring. Differential diagnosis therefore integrates clinical signs, laboratory tests and post-mortem findings. In India, NADCP covers FMD and brucellosis, while anthrax and black quarter are managed through state vaccination, IVRI support, quarantine, biosecurity and herd health programmes.

Crypsis. Crypsis is concealment that reduces detection by matching an animal’s appearance to its visual environment. It works because predators and prey detect objects by contrast: edges, colour, brightness and motion are processed against background noise, so lowering signal-to-noise ratio makes an animal harder to find. Background matching, as in leaf insects (Phyllium), uses shape, colour and texture to resemble leaves or twigs; masquerade mimics a harmless object, reducing recognition even when seen. Disruptive coloration breaks the body outline with stripes or blotches, as in tiger stripes in Ranthambore grasslands, where bands interrupt the silhouette against dappled vegetation. Countershading, darker above and lighter below, compensates for overhead light, making a body appear flatter, as in many fish. Chameleons alter skin pigment cells and posture to match substrates, though change is not instantaneous camouflage. These mechanisms are maintained by an evolutionary arms race: predators improve visual acuity, motion detection and pattern recognition, while prey refine concealment. Frequency-dependent selection maintains cryptic polymorphisms because rare morphs are less learned by predators, as in cryptic insects and moths. Thus crypsis is a sensory-ecological strategy shaped by detection thresholds, habitat structure and coevolution.