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

(a) ELISA: principle, types, procedure and applications

ELISA (enzyme-linked immunosorbent assay) is a plate-based immunoassay that detects an antigen or antibody through an enzyme attached to an antibody, which turns a colourless substrate into a coloured product. Principle: an antigen or antibody is fixed to the plastic surface of a microtitre well, and specific binding of its partner retains an enzyme-labelled reagent after washing. The enzyme, such as horseradish peroxidase or alkaline phosphatase, changes the substrate, and the colour intensity measures the amount of analyte. Types: Direct ELISA: antigen is coated and an enzyme-labelled antibody detects it. It is quick but less sensitive. Indirect ELISA: antigen is coated, an unlabelled primary antibody binds, and an enzyme-labelled anti-immunoglobulin gives amplified colour. It detects antibodies in serum. Sandwich ELISA: a capture antibody is coated, the sample antigen binds it, and an enzyme-labelled second antibody detects it. It is specific and sensitive for antigens with two epitopes. Competitive ELISA: sample antigen competes with labelled antigen for limited antibody, so less colour means more antigen. It is suited to small molecules. Procedure: Step 1: Coat the wells with antigen or capture antibody. Step 2: Block the remaining sites with bovine serum albumin or casein, and wash. Step 3: Add the sample, incubate, and wash. Step 4: Add the enzyme-labelled antibody, incubate and wash. Step 5: Add the substrate (for example TMB), stop the reaction with acid, and read absorbance in a plate reader. Applications: detection of HIV and hepatitis B and C infection, dengue and COVID-19 antibodies, screening of blood donations, tests for hormones and drugs, food allergen testing, and veterinary tests for brucellosis and foot-and-mouth disease.

(b) Livestock diseases: symptoms, diagnosis, treatment and prevention

Foot-and-mouth disease (virus): Symptoms: fever, blisters on the tongue, gums, teats and between the hooves, drooling, lameness and fall in milk. Diagnosis: clinical signs, with virus detection by PCR and antibody tests. Treatment: supportive, with antiseptic washing of lesions and antibiotics for secondary infection. Prevention: regular vaccination under the National Animal Disease Control Programme, quarantine and movement control. Anthrax (Bacillus anthracis): Symptoms: sudden death, high fever, bloating and dark blood oozing from body openings. Diagnosis: blood smear of a carcass showing rods, culture and PCR. A carcass should not be opened. Treatment: penicillin or oxytetracycline in early cases. Prevention: annual live spore vaccine (Sterne strain), and deep burial or burning of carcasses. Black quarter (Clostridium chauvoei): Symptoms: fever, hot painful swelling over a thigh or shoulder with gas that crackles, and lameness. Diagnosis: clinical signs and demonstration of the bacterium in muscle. Treatment: high doses of penicillin early. Prevention: vaccination before the monsoon. Haemorrhagic septicaemia (Pasteurella multocida): Symptoms: fever, throat swelling and difficult breathing in buffalo and cattle. Treatment: sulphonamides or oxytetracycline early. Prevention: pre-monsoon vaccination.

(c) Crypsis: definition, types and mechanism

Definition: crypsis is the ability of an animal to avoid detection by blending with its background, so that a predator or prey fails to see it. Mechanism: the animal reduces the contrast of its outline, colour and shadow against the background, and thus the signal the observer's visual system can pick out. Selection by predators preserves the best matched individuals. Types: Background matching: colour and pattern resemble the surroundings. Examples: the stick insect and the leaf insect, and the flounder on sand. Disruptive colouration: bold stripes or patches break up the body outline. Example: the stripes of a tiger in tall grass. Countershading: the back is dark and the belly is light, which cancels the shadow from overhead light and makes the body appear flat. Example: many fishes and deer. Masquerade: resemblance to a harmless object such as a twig or bird dropping, as in geometrid caterpillars. Colour change: some animals alter their colour, as in the chameleon and cuttlefish. Selection example: in the peppered moth, dark forms increased on soot-darkened trees in industrial areas because they were better hidden from birds.