(a) What are zero disease concept and herd immunity? Discuss in detail about chemoprophylaxis. (20 marks) (b) Discuss the…

(a) Zero Disease Concept, Herd Immunity and Chemoprophylaxis The zero disease concept aims at bringing the incidence of a particular disease to zero, in a herd, a region or worldwide (eradication), after which no control measures are needed. It is achieved by vaccination, testing and removal of infected animals, movement control and surveillance, for example rinderpest (declared eradicated worldwide in 2011), and the national programmes for FMD, brucellosis and PPR.

Herd immunity is the indirect protection of susceptible animals when a sufficient proportion of the herd is immune, so chains of transmission break. The threshold is 1 − 1/R₀: a disease with R₀ of 4 needs about 75% immune animals. Vaccination coverage therefore must exceed the threshold, and more transmissible diseases need higher coverage.

Chemoprophylaxis is the use of drugs to prevent infection or disease in healthy or exposed animals, not to treat clinical cases. Examples:

• Isometamidium chloride against trypanosomiasis.
• Tetracyclines against anaplasmosis, and imidocarb against babesiosis.
• Ionophores such as monensin and other coccidiostats (decoquinate, amprolium) against coccidiosis.
• Strategic anthelmintic dosing against helminths.
• Preventive antibiotics at high-risk periods such as dry-cow therapy for mastitis, or surgery.

Merits: protection when no vaccine exists, short-term and quick, useful in outbreaks. Demerits: antimicrobial resistance, drug residues in milk, meat and eggs (withdrawal periods must be observed; FSSAI sets residue limits), cost, disturbance of gut flora, and masked infection without lasting immunity.

(b) Methods of Ghee Manufacture, FSSAI and Agmark Standards Methods:

• Indigenous (desi) butter method: milk is soured to curd, churned for makkhan, and the butter is heated. Good flavour, but labour-intensive and the yield is lower.
• Direct cream method: cream is separated and heated directly to ghee. Quick and cheap, with a higher yield, but a less characteristic flavour.
• Creamery butter method: cream is churned to butter, melted and clarified. It is efficient, and the butter can be stored, but it needs extra steps and equipment.
• Pre-stratification method: melted butter is held at about 80°C, the lower serum layer is drained, and then the fat is clarified. It gives low acidity and better keeping quality, but needs care in temperature control.
• Continuous methods: butter oil is made by centrifugation and vacuum drying, at industrial scale, with consistent quality, but capital cost is high.

Standards: FSSAI (Food Products Standards regulations): milk fat at least 99.5%, moisture at most 0.5%, free fatty acids at most 3.0% as oleic acid, BR reading 40–43 at 40°C, Reichert Meissl value at least 28, negative Baudouin test, and no added colour or preservative. Agmark: moisture at most 0.3%, FFA at most 1.4% (Special), 2.5% (General) or 3.0% (Standard), the same BR reading and RM value, and negative Baudouin and phytosterol acetate tests.

(c) Food-Borne Diseases: Epidemiology, Investigation and Control Epidemiological principles: the agent, host and environment triad; the chain of infection (source, route, susceptible host); multiple causation; the iceberg concept (reported cases are a fraction of all cases); the incubation period and infective dose; the time-place-person description; and the role of carriers and food handlers. Common agents include Salmonella, Campylobacter, E. coli, Staphylococcus aureus toxin, Clostridium, Listeria, Brucella, Taenia, and toxins such as aflatoxin.

Investigation: confirm the outbreak and diagnosis; define cases; line list and food histories with attack rate calculation by food item; case-control or cohort analysis to identify the vehicle; laboratory testing of patients, food and water; environmental inspection and traceback to the source.

Control: control at source (healthy animals, vaccination of food animals, safe feed); hygiene in slaughter, milking and processing; pasteurisation and thorough cooking; cold chain; HACCP and FSSAI regulation with licensing and inspection; education of food handlers; surveillance (IDSP) and prompt reporting. Listeria can multiply at refrigeration temperatures, so hygiene and shelf-life control are required.