(a) Twinning during embryonic development in animals (10 marks) (b) Classification of antimicrobial agents based on the…

(a) Twinning during Embryonic Development Twinning arises through two distinct mechanisms: dizygotic (fraternal) and monozygotic (identical). Dizygotic twinning results from the simultaneous ovulation and fertilization of two ova, producing dichorionic-diamniotic twins. This is the predominant form in cattle, where twins occur in about 1–4% of births (more in high-yielding multiparous dairy cows) and double ovulation is linked to high IGF-1 and FSH levels, breed and parity. In contrast, monozygotic twinning involves the splitting of a single zygote; the timing of this split determines membrane patterns: splitting at the 2-cell to morula stage (days 0–3) gives separate chorions and amnions, splitting of the inner cell mass of the blastocyst (about days 4–8) gives monochorionic-diamniotic twins, later splitting gives monoamniotic twins, and incomplete late splitting gives conjoined twins. The nine-banded armadillo regularly produces identical quadruplets by this process (polyembryony), and embryo splitting is used to produce identical calves. In sheep and goats, prolificacy is often genetically determined. For instance, the FecB gene in sheep breeds like Garole and Booroola Merino significantly enhances ovulation rates. Goat breeds such as Black Bengal and Barbari also commonly bear twins and triplets. In cattle the chorions of dizygotic twins usually fuse, so the twins share blood (vascular anastomoses) and become blood chimeras; over 90% of heifers born co-twin to a bull are sterile freemartins, because the male's hormones and cells masculinise the female's reproductive tract. Twin pregnancies in cattle also raise the risks of abortion, dystocia, retained placenta and metritis, so twinning is undesirable in cattle but desirable in sheep, goats and pigs.

(b) Classification of Antimicrobial Agents by Mechanism of Action Antimicrobial agents are classified by their mechanism of action, targeting specific bacterial structures or processes. Cell wall synthesis inhibitors, such as β-lactams (penicillins, cephalosporins), prevent peptidoglycan cross-linking, leading to osmotic lysis. Protein synthesis inhibitors interfere with ribosomal function; aminoglycosides bind to the 30S subunit causing misreading, while tetracyclines block aminoacyl-tRNA attachment at the 30S site. Macrolides (tylosin, erythromycin), lincosamides, florfenicol and chloramphenicol act on the 50S subunit; chloramphenicol is banned in food animals. Nucleic acid synthesis inhibitors include fluoroquinolones (enrofloxacin, ciprofloxacin), which inhibit DNA gyrase and topoisomerase IV, halting replication; rifampicin, which blocks DNA-dependent RNA polymerase; and metronidazole, whose reduced products break DNA in anaerobes. Antimetabolites (folate antagonists): sulfonamides compete with para-aminobenzoic acid (PABA) for dihydropteroate synthase, and trimethoprim inhibits dihydrofolate reductase; the combination is synergistic and bactericidal. Additionally, agents that disrupt cell membrane function, like polymyxins (colistin), bind to lipopolysaccharides in Gram-negative bacteria, increasing permeability. Nitrofurans are reduced inside bacteria to reactive intermediates that damage DNA and ribosomes; they are banned in food-producing animals because their residues are carcinogenic. Knowing the mechanism helps in choosing bactericidal or bacteriostatic drugs, combining drugs sensibly, and understanding resistance (for example β-lactamase production against penicillins).

(c) Specific Requirements of a Brooder House for Chicken A brooder house must provide a controlled environment for chicks from day one to six weeks. Temperature management is critical, starting at 95°F (35°C) under the brooder and reducing by approximately 5°F (3°C) weekly until about 21–24°C is reached, with relative humidity of about 60–70%. Heat comes from electric hovers, infrared bulbs or gas brooders, one hover of about 1.5 m diameter serving about 250–300 chicks; a chick guard (a 45 cm high ring of hardboard about 1 m from the hover) keeps chicks near heat, feed and water for the first week. Chick behaviour is the best thermometer: crowding under the heat means too cold, chicks panting at the edges means too hot, and even spreading means comfortable. Space requirements are stringent, with about 0.5 sq ft per chick during brooding, rising to about 1 sq ft for broilers by market age, to prevent overcrowding and cannibalism; about 2.5 cm of feeder space and 1.5 cm of waterer space per chick are needed at first. Litter management is vital for foot health; a 2-4 inch depth of dry, absorbent material like rice husk or wood shavings is recommended to maintain low humidity and prevent coccidiosis. Ventilation must be adequate to remove ammonia and excess moisture without creating drafts, often achieved through adjustable side curtains and exhaust fans. Lighting is typically maintained at 23 hours initially to stimulate feeding and drinking, gradually reducing to natural photoperiods. Biosecurity is paramount, involving strict zoning, footbaths, and exclusion of wild birds to prevent pathogen introduction, ensuring a hygienic start for the flock.

(d) Clinical Signs of Vitamin E Deficiency in Poultry Vitamin E deficiency in poultry manifests primarily through neurological and muscular disorders, often exacerbated by selenium deficiency. The most distinct sign is Encephalomalacia, or "crazy chick disease," characterized by ataxia, torticollis, opisthotonus, head retraction, falling on the side and paddling of the legs, usually at 2–4 weeks of age, due to softening, haemorrhage and necrosis of the cerebellum. Exudative Diathesis, due to combined vitamin E and selenium deficiency, presents with greenish-blue subcutaneous edema under the breast and abdomen, which makes chicks stand with legs wide apart, with hemorrhages and fluid accumulation, resulting from capillary damage. Muscular Dystrophy, or "white muscle disease," involves degeneration of skeletal and cardiac muscles, visible as white striations in breast and leg muscles, leading to weakness and paralysis; it appears when vitamin E and sulphur amino acids are both deficient, and gizzard myopathy occurs in turkeys. In breeders, deficiency causes poor hatchability with early embryonic death (about day 4 of incubation) and testicular degeneration and infertility in cocks. These signs are often confused with Newcastle Disease or Botulism, but the absence of acute respiratory signs and the specific muscular pathology help in differential diagnosis. The condition is particularly prevalent in fast-growing broilers due to high metabolic demands. Supplementation with Vitamin E and Selenium in feed or via injection rapidly reverses symptoms, highlighting the synergistic antioxidant role of these nutrients in maintaining cellular integrity.

(e) Clinical Symptoms and Treatment of Impaction in Bovines Impaction in bovines, particularly ruminal impaction, is a common digestive disorder. It is classified as primary, caused by ingestion of indigestible feed like dry hay or straw, or secondary, resulting from conditions like vagal indigestion or traumatic reticuloperitonitis. Clinical signs include anorexia, reduced rumination, and a doughy, firm consistency of the rumen upon transabdominal palpation and ballottement. Dehydration is a key feature, often accompanied by metabolic acidosis or mixed acid-base disturbances, rather than the systemic hypochloremic alkalosis seen in abomasal impaction. Rectal examination reveals the firm, doughy, distended dorsal sac of the rumen and complements transabdominal palpation. Treatment involves aggressive fluid therapy to correct dehydration and electrolyte imbalances, withholding the offending feed, oral laxatives such as magnesium sulphate (about 250–500 g in 4–5 litres of water for an adult cow) or liquid paraffin (1–2 litres), ruminotorics or prokinetics like neostigmine or bethanechol to stimulate motility, rumen transfaunation (4–8 litres of rumen liquor from a healthy animal), and dietary correction to ensure soft, palatable feed. Omasal impaction (drought feeding on dry, fibrous feed) and abomasal impaction (in buffaloes and cattle fed chopped straw, often with vagal indigestion) respond poorly and may need abomasotomy. In severe cases of complete impaction, rumenotomy may be required to remove the impacted material. Early intervention is critical to prevent progression to rumenitis or peritonitis.