(a) Classification of zoonoses based on etiological agents (10 marks) (b) Significance of the cohort, case-control and…

(a) Classification of Zoonoses Based on Etiological Agents

Zoonoses are diseases naturally transmissible between vertebrate animals and humans. Based on the causative etiological agent, they are classified into the following groups (they can also be classified by maintenance cycle into direct zoonoses, cyclozoonoses, metazoonoses and saprozoonoses, or by the direction of transmission):

Bacterial Zoonoses: Caused by bacteria, these represent major public health threats. Prominent Indian examples include Anthrax (Bacillus anthracis), Brucellosis (Brucella abortus and B. melitensis), Bovine Tuberculosis (Mycobacterium bovis), and Leptospirosis (Leptospira interrogans).

Viral Zoonoses: Caused by viruses capable of cross-species spillover. Key examples in India include Rabies (Lyssavirus), Kyasanur Forest Disease (Flavivirus), Japanese Encephalitis, and Avian Influenza (H5N1).

Parasitic Zoonoses: Divided into protozoal and helminthic infections. Major Indian examples include Toxoplasmosis (Toxoplasma gondii), Cryptosporidiosis (Cryptosporidium parvum), Hydatidosis/Echinococcosis (Echinococcus granulosus), Cysticercosis (Taenia solium, from pigs) and taeniasis (T. saginata, from cattle), and scabies from animals (Sarcoptes scabiei).

Fungal Zoonoses: Caused by mycotic agents, commonly presenting as cutaneous or systemic mycoses. Examples include Dermatophytosis or Ringworm (Microsporum canis, Trichophyton verrucosum) and Sporotrichosis (Sporothrix schenckii).

Rickettsial and Chlamydial Zoonoses: Caused by obligate intracellular pathogens. Notable examples include Q Fever (Coxiella burnetii), Scrub Typhus (Orientia tsutsugamushi), and Psittacosis/Avian Chlamydiosis (Chlamydia psittaci).

Prion Zoonoses: Caused by misfolded proteins without nucleic acid. Bovine spongiform encephalopathy (BSE) caused variant Creutzfeldt–Jakob disease in humans who ate infected beef products; BSE has never been reported in India.

(b) Significance of Epidemiological Studies in Animals

Observational epidemiological study designs provide critical evidence for disease surveillance, etiology, and herd health management:

Cohort Studies: Longitudinal and prospective in nature, these follow exposed and unexposed animal cohorts over time to determine disease incidence. They allow direct calculation of Relative Risk (RR) and Attributable Risk, proving vital for establishing causal relationships in multi-factorial production diseases, such as the association between transition cow negative energy balance and subsequent clinical mastitis or metritis.

Case-Control Studies: Retrospective designs that compare animals with a specific disease (cases) to disease-free animals (controls) regarding past exposure. They yield an Odds Ratio (OR) as an estimate of relative risk. These studies are indispensable for investigating rare conditions, outbreaks with long incubation periods (such as Bovine Spongiform Encephalopathy or Scrapie), and emergent herd outbreaks where quick, cost-effective source tracing is essential.

Cross-Sectional Studies: Point-in-time surveys assessing exposure and outcome simultaneously across a population. They determine point or period prevalence, providing baseline data for disease mapping, herd health monitoring, and evaluating national control programmes like the National Animal Disease Control Programme (NADCP).

(c) Procedures for the Pasteurization of Market Milk

Pasteurization is the heating of every particle of milk to a set temperature for a set time, followed by prompt cooling, to destroy all vegetative pathogenic microorganisms with minimal change in composition, flavour and nutritive value. The time-temperature combinations are based on Coxiella burnetii, the most heat-resistant non-spore-forming pathogen in milk (earlier Mycobacterium tuberculosis was the index organism):

Low-Temperature Long-Time (LTLT) or Holder Method: Milk is heated to 63°C (145°F) and held for at least 30 minutes in an insulated batch vat, followed by rapid cooling to below 4°C. It is typically utilized in small-scale processing units.

High-Temperature Short-Time (HTST) Method: A continuous flow process using plate heat exchangers where milk is heated to 72°C (161°F) for at least 15 seconds, followed by prompt cooling to 4°C. It represents the commercial standard for market milk. The plate heat exchanger has a regeneration section, where incoming cold milk is warmed by outgoing hot milk (saving up to about 90% of heat), a heating section, a holding tube that gives the 15-second hold, a flow diversion valve that automatically returns under-heated milk, and cooling sections with chilled water.

Flash and higher-heat methods: flash pasteurization (about 85–90°C for 1–2 seconds) and higher-heat shorter-time processes (89–100°C for a second or less) are used for some products, and extended-shelf-life (ESL) milk combines higher heat with very clean filling.

Ultra-High Temperature (UHT) Processing: Milk is heated to 135°C–150°C for 1 to 4 seconds using direct steam injection or indirect heating, followed by aseptic packaging, rendering the product commercially sterile with extended shelf life without refrigeration. Strictly, UHT is a sterilization process rather than pasteurization, because it also destroys spores.

In accordance with Food Safety and Standards Authority of India (FSSAI) standards, the efficacy of pasteurization is routinely verified using the Alkaline Phosphatase (ALP) test, where complete enzyme inactivation confirms adequate thermal treatment (it cannot detect post-pasteurization contamination). Post-pasteurization contamination is checked by coliform counts (coliforms should be absent in 1 mL of pasteurized milk), and keeping quality by the methylene blue reduction test and standard plate count. Pasteurized milk must be stored and distributed below about 5°C.

(d) Mechanism of Rigor Mortis in the Goat Carcass

Rigor mortis is the post-mortem physiological stiffening of skeletal muscle resulting from the irreversible formation of actomyosin cross-bridges:

Following slaughter and exsanguination, oxygen supply ceases, switching muscle metabolism to anaerobic glycogenolysis. This pathway generates limited Adenosine Triphosphate (ATP) and accumulates lactic acid, causing the muscle pH to drop from 7.0–7.2 to an ultimate pH of 5.5–5.8. As glycogen stores deplete, ATP resynthesis fails.

ATP is essential for sequestering calcium ions into the sarcoplasmic reticulum and breaking the bond between actin and myosin filaments. When intracellular ATP concentrations fall below a critical threshold, actin and myosin bind permanently to form rigid actomyosin cross-bridges, resulting in muscle inextensibility.

In goat carcasses, the onset of rigor mortis typically occurs within 1 to 6 hours post-slaughter, reaching full rigor around 12 hours, and resolving after 24 to 48 hours via endogenous proteolytic enzyme action (calpains and cathepsins). Pre-slaughter stress, exhaustive exercise, or starvation depletes muscle glycogen, precipitating rapid rigor and resulting in Dark, Firm, and Dry (DFD) meat, whereas rapid chilling before rigor onset can induce cold shortening and severe toughening.

(e) Preslaughter Handling and Transportation of Poultry

Preslaughter handling and transportation directly affect broiler welfare, carcass yield, and meat quality:

Feed and Water Withdrawal: Feed must be withdrawn 8 to 12 hours prior to slaughter. This optimal window ensures full gastrointestinal evacuation, preventing carcass fecal contamination during evisceration, while avoiding excessive live-weight shrink, dehydration, and watery meat associated with prolonged fasting (>14 hours). Water is normally withdrawn only 1-2 hours before slaughter, since early water withdrawal worsens dehydration and live-weight shrink.

Catching and Crating: Birds should be caught in dim or blue light, held gently by both legs or around the body upright to minimize wing flapping, bruising, and bone dislocations. Crating densities must adhere to standards (approximately 20–25 kg live weight per crate), ensuring adequate headroom and ventilation.

Transport and Lairage: Transport should occur during cooler hours to prevent thermal stress. Overcrowding, rough transit, and extreme temperatures elevate Dead-on-Arrival (DOA) rates and lead to Pale, Soft, Exudative (PSE)-like meat. Upon arrival at the abattoir, birds require a 1 to 2 hour lairage resting period under fans and misting systems to normalize physiological parameters.

Adherence to the Prevention of Cruelty to Animals Act, 1960 and the PCA (Transport of Animals) Rules is essential to ensure humane handling, lower mortality, and optimize product quality.