Veterinary · Clinical · also Herd Health Management, Production Animal Medicine
Medicine of the herd and flock rather than the individual: fertility, mastitis, calf and lameness control, biosecurity, performance records and economics across cattle, pigs, poultry and small ruminants.
In the exams: NAVLE bovine 13.3%, porcine 5% and ovine/caprine 3.3%; RCVS SME production animal (with VPH) 105 of 367; SAVC Paper 2; AVE Paper 2 cattle at least 13
Food-animal production medicine, also called herd health management, is the medicine of the group rather than the single animal. The patient is a dairy herd, a beef suckler herd, a feedlot pen, a sow unit, a broiler house or a sheep flock. The questions change accordingly: not "what is wrong with this cow?" but "why are one in five of our cows lame, what is it costing, and what change on this farm will bring it down?" The tools are records, key performance indicators (KPIs), targeted sampling, farm inspection, economics and a written plan agreed with the farmer, reviewed at regular visits.
The subject matters because most of the economic loss in livestock farming comes from subclinical and chronic disease, not from dramatic outbreaks. Poor fertility, subclinical mastitis, lameness, calf scour and pneumonia, parasitism and transition-cow disease quietly reduce output and raise costs in every production system, from a three-cow smallholding in Kenya or Punjab to a 3,000-cow dairy in California. Production medicine is also where the veterinarian protects public health: responsible antimicrobial use, residue avoidance and farm biosecurity sit here.
It is examined everywhere. The NAVLE blueprint gives bovine 13.3%, porcine 5% and ovine/caprine 3.3% of items, mostly as clinical vignettes that ask for the next step in a herd investigation. The RCVS Statutory Membership Examination devotes a large production-animal (with veterinary public health) section; the Australasian Veterinary Examination sets a minimum number of cattle items; the SAVC registration examination tests production animals in Paper 2; and Indian PG and PhD entrance papers ask about mastitis control, fertility indices and calf management within medicine, gynaecology and livestock production. This manual covers herd health programmes, fertility, mastitis, calf health, lameness, biosecurity, records and KPIs, economics, pig and poultry flock medicine and small-ruminant flock health. Numbers given as targets are widely used reference values; they differ by breed, system and country, and the manual says so where it matters.
Traditional farm-animal practice was reactive: the farmer calls when an animal is sick, the vet treats it, and the visit ends. A herd health programme replaces much of this with planned, regular visits at which the vet and farmer review the herd's performance against agreed targets, examine the animals that the records flag, and decide on changes. The idea took shape in the 1960s and 1970s in the United Kingdom, the Netherlands and North America, starting with dairy fertility and mastitis, and spread to pigs, poultry, beef and sheep.
A programme has a repeating cycle. First, set goals with the farmer: what does the business need, and which targets are realistic for this farm? Second, monitor: collect data (fertility, milk recording, treatment and death records, growth, feed use) and compare it with targets and with the farm's own past. Third, investigate: where a KPI falls short, find the cause by examining animals, the environment, feeding and management. Fourth, intervene: agree specific, written actions with named people and dates. Fifth, review: check at the next visit whether the actions happened and whether the KPI moved. This is the plan-do-check-act cycle of quality management applied to a farm.
Visit frequency depends on the enterprise. Large dairy herds are often visited weekly or fortnightly for routine fertility work; small herds monthly; beef and sheep flocks at key points of the production year (pre-mating, pregnancy diagnosis, pre-lambing or pre-calving, weaning).
A herd health plan is a written document, kept on the farm, that records the disease status of the herd, the agreed preventive measures and the targets. Typical contents are: the herd's known disease status (for example bovine viral diarrhoea, infectious bovine rhinotracheitis, leptospirosis, Johne's disease, bovine tuberculosis testing status); the vaccination programme; parasite control; biosecurity rules for purchased stock and visitors; the treatment protocols for common conditions, including which medicine, which route and the withdrawal period; the records to be kept; and the KPIs with their targets.
In many countries a health plan is required by milk buyers, retailers or national assurance schemes, and it is the document auditors read. A plan that sits in a drawer is useless; a good plan is short, specific to that farm, and updated at each review.
A herd problem investigation follows a structured path. Define the problem: what is the complaint, which animals, since when, and how big is it? Convert impressions into a rate with a denominator, for example "twelve cases of clinical mastitis in 180 cows in the last month" rather than "a lot of mastitis". Compare with the farm's history and with reference targets to decide whether there really is a problem. Then describe the pattern by animal (age, parity, stage of lactation, group), by place (pen, shed, paddock) and by time (season, a change of feed, a new batch of animals). The pattern points to the cause: cases concentrated in the first month after calving point to the dry period and transition; cases spread through lactation point to milking-time transmission.
Next, examine a sample of affected and unaffected animals, the environment and the feed, and collect targeted samples (milk cultures, blood metabolites, faecal egg counts, post-mortems). Form a hypothesis, test it where possible, and intervene. Finally, monitor to confirm that the intervention worked. Most herd problems are multifactorial: the pathogen, the animals' resistance and the environment all contribute, so the answer is usually several management changes rather than a single drug.
Fertility drives dairy economics because a cow gives most milk early in lactation and must calve regularly to start a new lactation. The calving interval (CI) is the time between successive calvings. It equals the days open (calving to conception interval) plus the gestation length, which is about 280 to 285 days in most dairy breeds. A 365-day calving interval, the traditional target for seasonal and many year-round herds, therefore needs conception by about 85 days after calving. In practice many year-round high-yielding Holstein herds run calving intervals of about 390 to 420 days, and the economic optimum differs between systems.
The calving interval is a lagging indicator: it is known only after the next calving and only for cows that conceived, so culled infertile cows disappear from it. Herd health visits therefore rely on leading indicators. The voluntary waiting period (VWP) is the deliberate interval after calving before cows are first served, often 40 to 60 days. The submission rate, or heat detection (insemination) rate, is the proportion of eligible cows served in a period, usually 21 days, the length of the bovine oestrous cycle. The conception rate is the proportion of services that result in pregnancy, also called pregnancies per AI. The 21-day pregnancy rate is the proportion of eligible, non-pregnant cows that become pregnant in each 21-day period; to a close approximation it equals submission rate multiplied by conception rate. A herd with a 60% submission rate and a 40% conception rate has a 21-day pregnancy rate of about 24%. Pregnancy rates above about 20% are generally regarded as good in year-round Holstein herds, but targets vary.
Other indices include calving to first service interval, first-service conception rate, services per conception, the proportion of cows pregnant by a fixed day (for example 100 or 150 days in milk), the percentage of cows culled for infertility, and in heifers the age at first calving, commonly targeted at about 22 to 24 months in Holsteins.
When fertility is poor, first ask which part is failing: are cows not being served (a submission problem), or are served cows not conceiving (a conception problem), or both? Poor submission comes from poor oestrus detection, anoestrus and long voluntary waiting periods. Oestrus detection fails through too few or too short observation periods, lameness (lame cows show fewer mounting behaviours), slippery floors, high yield (which shortens oestrus) and overcrowding. Aids include tail paint or chalk, heat-mount detectors, activity monitors and milk progesterone.
Anoestrus after calving is linked to negative energy balance, low body condition, dystocia, uterine disease and, in suckler cows, the suckling stimulus. Conception failure has many causes: timing of insemination, semen handling and technician skill, uterine infection (metritis and endometritis), energy deficit, heat stress, infectious disease (bovine viral diarrhoea, leptospirosis, campylobacteriosis and trichomoniasis in natural-service herds, and neosporosis as a cause of abortion), and trace-element deficiencies in some regions. Heat stress lowers conception markedly in hot climates and seasons.
Transition-cow disease is a major root cause: milk fever, retained fetal membranes, metritis, ketosis and displaced abomasum all delay return to cyclicity and reduce conception. Body condition score (BCS) is a practical tool; on the common five-point scale, loss of more than about one unit in early lactation is associated with poorer fertility.
Routine fertility visits examine cows at defined stages: post-calving checks for uterine disease in cows with risk factors, non-cycling cows past the voluntary waiting period, and pregnancy diagnosis by rectal palpation, transrectal ultrasound (commonly from about 28 to 35 days) or milk or blood pregnancy-associated glycoprotein tests. Early pregnancy diagnosis matters because it lets non-pregnant cows be re-served quickly.
Timed artificial insemination programmes use gonadotrophin-releasing hormone (GnRH) and prostaglandin F2-alpha to synchronise ovulation so that cows can be inseminated at a fixed time without detecting oestrus. The basic Ovsynch protocol is GnRH, prostaglandin seven days later, GnRH two days later, and insemination about 16 to 20 hours after the second GnRH; many variants exist, some with a progesterone insert. Such programmes raise submission rate but do not by themselves fix poor conception caused by disease or nutrition. Hormone use in food animals is regulated differently across countries, so check what is permitted and the withdrawal periods on the label.
In seasonal pasture herds (common in New Zealand, Ireland and parts of Australia), the key targets are compact calving: a high proportion of cows calving in the first three and six weeks of the calving season, a high six-week in-calf rate after mating starts, and a low empty rate at the end of the season. In beef suckler herds the aims are similar: a short breeding season, a high proportion of cows calving early in the season and a high weaned-calf percentage per cow exposed to the bull. Bull breeding soundness examination before the season, covering physical examination, scrotal circumference, semen evaluation and, where indicated, libido and serving capacity, is a core herd-health task.
The transition period is usually defined as three weeks before to three weeks after calving. In these weeks the cow's energy, protein and calcium demands rise steeply with fetal growth, colostrum synthesis and the onset of lactation, while dry matter intake falls in the days before calving and recovers only gradually afterwards. The result is negative energy balance: the cow mobilises body fat as non-esterified fatty acids (NEFA), which the liver oxidises, re-esterifies and stores as triglyceride (fatty liver) or converts to ketone bodies such as beta-hydroxybutyrate (BHB). Most of the metabolic and infectious disease of a dairy herd occurs in this window: milk fever (hypocalcaemia), ketosis, displaced abomasum, retained fetal membranes, metritis and a large share of clinical mastitis. These diseases cluster and increase the risk of one another, and they lower milk yield and fertility and increase early culling.
A herd-level view asks not "is this cow ketotic?" but "what proportion of fresh cows have subclinical ketosis, and why?" Subclinical ketosis is commonly defined as blood BHB of 1.2 mmol/L or more without clinical signs (some studies use 1.4 mmol/L). Elevated prepartum NEFA (commonly about 0.3 mmol/L or more in the last week before calving) and postpartum NEFA indicate excessive fat mobilisation. Herd testing samples a group of cows at the right stage (NEFA before calving, BHB in the first two weeks after calving) and compares the proportion above the cut point with an alarm level. Individual cow results are less useful than the proportion in the group.
Clinical milk fever is acute hypocalcaemia around calving, most common in older, high-yielding cows and in Jersey cattle. Subclinical hypocalcaemia is far more common than the clinical disease and is associated with other transition diseases. Prevention strategies include: feeding a low dietary cation-anion difference (DCAD) ration before calving, which induces a mild metabolic acidosis that increases calcium mobilisation from bone and absorption from the gut; restricting calcium intake in the dry period (hard to achieve with many forages) or using calcium binders; and oral calcium supplements at calving in high-risk cows.
DCAD is usually calculated in milliequivalents per kilogram of dry matter as (Na + K) − (Cl + S). Each mineral's percentage in the dry matter is converted to milliequivalents using its equivalent weight: sodium 23, potassium 39.1, chloride 35.45 and sulphur 16 (half its atomic weight, because it is divalent). Full anionic diets aim for a negative DCAD before calving, and urine pH is monitored to check the effect: targets for Holstein cows are often given as about 6.0 to 7.0 for a partially acidified diet and lower for a fully acidified diet, with values differing between sources. Potassium-rich forages, such as heavily fertilised grass, make low DCAD hard to achieve.
Ketosis prevention depends on avoiding over-condition at drying off (a body condition score above about 3.5 to 3.75 at calving on the five-point scale raises risk), maximising dry matter intake around calving through adequate feed-face space, comfortable housing, minimal regrouping and fresh palatable feed, and a well-balanced fresh-cow ration. Cows diagnosed with ketosis are commonly treated with oral propylene glycol, for example about 300 mL once daily for three to five days in an adult Holstein cow — confirm against the product label and your national formulary.
Fresh-cow monitoring is routine on many large dairies: daily checks of appetite, rectal temperature, rumen fill, uterine discharge, milk yield and urine or blood ketones for the first ten days or so after calving. Its purpose is early detection, but it also produces data. Recording every transition disease with a clear case definition lets the herd calculate incidence risks, for example the percentage of calvings followed by retained fetal membranes (commonly regarded as a problem above about 10%) or by displaced abomasum. Commonly quoted reference values vary between countries and systems, so compare the herd with its own past and with regional data.
Other monitoring tools include body condition scoring at drying off, calving and early lactation; rumen fill and manure scoring; feeding behaviour and feed-face space (often at least about 60 to 75 cm per cow for close-up cows); and milk components: a high fat-to-protein ratio in early lactation (for example above about 1.4 to 1.5 in Holsteins) suggests excessive fat mobilisation, while a very low milk fat percentage relative to protein suggests subacute ruminal acidosis. These thresholds depend on breed and should be read as flags for investigation, not diagnoses.
Mastitis is inflammation of the mammary gland, almost always caused by bacterial infection entering through the teat canal. Clinical mastitis shows visible changes in the milk (clots, flakes, watery secretion) with or without a swollen quarter or systemic illness. Subclinical mastitis has no visible signs but raises the somatic cell count (SCC) and lowers yield; it is much more common than clinical disease and causes most of the economic loss.
Pathogens are grouped by their reservoir. Contagious pathogens live in the udder and spread from cow to cow at milking, through milkers' hands, cloths and liners: Staphylococcus aureus, Streptococcus agalactiae and Mycoplasma species, with Corynebacterium bovis a minor contagious organism. Environmental pathogens live in bedding, manure, soil and water and infect teats between milkings: coliforms such as Escherichia coli and Klebsiella, Streptococcus uberis, and other environmental streptococci. Non-aureus staphylococci are common, usually cause mild increases in SCC, and have both skin and environmental reservoirs.
The pattern of a herd problem points to the reservoir. A high bulk milk SCC with few clinical cases and many chronically high-SCC cows suggests a contagious problem. A moderate bulk SCC with many clinical cases, especially in early lactation and in the dry period, suggests environmental infection. Culture of milk from clinical cases and high-SCC cows confirms which organisms dominate; bulk milk culture or PCR can detect Streptococcus agalactiae and Mycoplasma. Streptococcus agalactiae is an obligate udder pathogen that responds well to treatment and can be eradicated from a herd. Staphylococcus aureus responds poorly to lactation treatment in chronic cases because it forms abscesses and micro-abscesses, so segregation, milking order and culling of chronically infected cows are central to its control.
The five-point plan, developed at the National Institute for Research in Dairying in England in the 1960s, was the first herd mastitis programme. Its points are: disinfect all teats after every milking (post-milking teat disinfection); treat all quarters of all cows at drying off with a long-acting antimicrobial (blanket dry cow therapy); treat clinical cases promptly and record them; cull cows with chronic, repeated mastitis; and maintain the milking machine correctly. It greatly reduced contagious mastitis but did less against environmental mastitis.
Later programmes, such as the National Mastitis Council recommended programme in North America and the AHDB Dairy Mastitis Control Plan in the UK, add: setting goals and monitoring udder health records; proper milking routine (pre-milking teat preparation, forestripping, pre-dipping where environmental mastitis is a problem, clean dry teats, prompt unit attachment and avoiding over-milking); clean, dry, comfortable housing and bedding; dry-cow management including internal teat sealants; biosecurity for purchased cattle; and periodic review.
Blanket dry cow therapy is increasingly replaced by selective dry cow therapy, in which only cows likely to be infected at drying off receive an antimicrobial, while others receive an internal teat sealant alone. Selection is based on SCC history and clinical mastitis records, sometimes with culture. Selective therapy reduces antimicrobial use substantially without harming udder health in well-managed herds with low SCC. Several European countries restrict preventive antimicrobial use by law, and the EU veterinary medicines regulation limits prophylactic antimicrobial use to exceptional cases in individual animals.
Somatic cells in milk are mostly leucocytes. An uninfected quarter typically has an SCC below about 100,000 cells/mL, and 200,000 cells/mL is widely used as the cut-off above which a cow is regarded as probably infected. The linear score used in milk recording converts SCC to a scale where each unit doubles the count: linear score = log2(SCC/100,000) + 3. An SCC of 100,000 is linear score 3, 200,000 is 4 and 400,000 is 5.
The bulk milk SCC is the yield-weighted average of all cows contributing to the tank, so high-yielding infected cows raise it more than low-yielding ones. Legal and buyer limits differ: the European Union sets 400,000 cells/mL for cow's milk (as a rolling geometric mean), while the United States Pasteurized Milk Ordinance limit for Grade A cow milk is 750,000 cells/mL. Many buyers pay premiums at much lower counts.
Individual cow SCC from monthly milk recording yields herd indices. The proportion of cows above 200,000 shows the prevalence of infection. The new infection rate during lactation is the proportion of cows below 200,000 at one recording that are above it at the next; it measures how fast infections are spreading. The chronic rate is the proportion above the threshold at two consecutive recordings. Dry-period indices compare the last recording before drying off with the first after calving: the dry-period cure rate (high to low) and the dry-period new infection rate (low to high). A high dry-period new infection rate points to the dry-cow environment or teat sealing. Clinical mastitis is measured as an incidence rate, commonly expressed as cases per 100 cows per year; many advisers aim for fewer than about 25 to 35, with targets varying by country and herd type.
Treatment of clinical cases should follow the farm protocol: mild and moderate cases usually receive intramammary treatment; severe cases with systemic signs need a veterinary examination, supportive therapy including fluids and anti-inflammatories, and often systemic antimicrobials. On-farm culture lets some mild Gram-negative or no-growth cases go untreated, reducing antimicrobial use. Discarded milk during and after treatment, covering the label milk withdrawal period, is part of the cost of each case.
The milking machine can spread infection, damage teats and cause new infections. Its key components are the vacuum pump, the vacuum regulator, the pulsators, the clusters with liners, and the milk line. The pulsator alternates the liner between open (milking) and closed (massage) phases; the massage phase relieves congestion in the teat. Faults that promote mastitis include unstable or incorrect vacuum, faulty pulsation, worn or incorrect liners, liner slips (which drive milk droplets back against the teat end), and over-milking, which damages the teat end and produces hyperkeratosis. Machines should be tested dynamically (during milking) and statically by trained technicians at regular intervals, often yearly, and liners changed at the manufacturer's recommended number of milkings.
Teat-end condition scoring, teat skin scoring and observation of milking routine are part of every mastitis investigation.
Calves are born agammaglobulinaemic because the bovine placenta (epitheliochorial and cotyledonary) does not transfer immunoglobulins. They depend on absorbing immunoglobulin G (IgG) from colostrum across the small intestine, which is possible only for a short time after birth; absorption falls steadily over the first 24 hours as the gut closes. Failure of transfer of passive immunity raises the risk of diarrhoea, pneumonia and death and has long-term effects on growth and first-lactation yield.
Colostrum management is often summarised as quality, quantity, quickness, cleanliness and quantification. Quality: good colostrum is commonly defined as at least 50 g/L IgG, which corresponds to about 22% or more on a Brix refractometer. Quantity: feed about 10% to 12% of body weight at the first feeding, which is about 4 litres for a 40-kg Holstein calf. Quickness: within about two hours of birth where possible, and certainly within six. Cleanliness: collect and store colostrum hygienically, because bacteria in colostrum reduce IgG absorption. Quantification: monitor transfer by measuring serum total protein or serum Brix in calves aged about 1 to 7 days.
Apparent efficiency of absorption (AEA) is the proportion of IgG fed that appears in the circulation: AEA = serum IgG (g/L) × plasma volume (L) ÷ IgG fed (g), with plasma volume estimated as about 9% of body weight. Values of about 25% to 35% are typical. A 2020 international consensus proposed four categories of calf serum IgG: excellent at least 25 g/L, good 18 to 24.9, fair 10 to 17.9 and poor below 10 g/L (serum total protein about 6.2, 5.8 to 6.1, 5.1 to 5.7 and below 5.1 g/dL), with herd targets of more than 40% excellent and fewer than 10% poor.
Calf diarrhoea (scours) is the main cause of death in young dairy calves in many countries. The main infectious agents by age are enterotoxigenic Escherichia coli with the K99 (F5) adhesin in the first few days; rotavirus and coronavirus from a few days to about three weeks; Cryptosporidium parvum from about one to three weeks; and Salmonella at any age. Coccidiosis (Eimeria species) becomes important in older calves, usually after three weeks. Infection is usually mixed. The main clinical threats are dehydration, metabolic acidosis and hyperkalaemia; oral electrolyte solutions containing an alkalinising agent, continued milk feeding and, for severe cases, intravenous fluids are the basis of treatment. Herd control rests on colostrum, hygiene of the calving area and calf housing, all-in/all-out use of pens, and vaccination of dams in late pregnancy against rotavirus, coronavirus and K99 E. coli to raise colostral antibody.
Bovine respiratory disease (BRD) in calves and in weaned or transported cattle is multifactorial: viruses (bovine respiratory syncytial virus, parainfluenza-3, bovine herpesvirus 1, bovine viral diarrhoea virus, bovine coronavirus) and bacteria (Mannheimia haemolytica, Pasteurella multocida, Histophilus somni, Mycoplasma bovis) combine with stressors such as mixing, transport, weaning and poor ventilation. In housed calves ventilation, stocking density, avoiding draughts at calf level and good passive transfer are the main controls. In feedlots, arrival protocols, pre-conditioning of calves before sale, and metaphylaxis in high-risk groups are used; metaphylaxis is under increasing scrutiny because of antimicrobial use.
Heifer rearing is an investment that pays back only after the first lactation. Growth is measured as average daily gain (ADG), the weight gained divided by the number of days. For Holstein heifers, pre-weaning gains of about 0.7 to 0.9 kg/day or more are commonly targeted, and many programmes aim for calves to double their birth weight by about eight weeks. Doubling a 40-kg birth weight to 80 kg in 56 days is an ADG of about 0.71 kg/day.
Reaching first service at about 55% to 60% of mature body weight and first calving at about 85% of mature weight lets heifers calve at 22 to 24 months, which reduces rearing cost without harming performance in most studies; the right targets depend on breed and the farm's mature cow weight. Calf mortality is a key KPI: pre-weaning mortality below about 5% is a commonly quoted target in dairy calves, but reported rates vary widely between countries and systems.
Lameness is one of the most important welfare and economic problems of dairy cattle and is also important in sheep, pigs and broiler chickens. Most dairy lameness arises in the foot, especially the hind lateral claw. The main lesions are claw horn disruption lesions, namely sole ulcers, sole haemorrhage and white line disease, and infectious lesions, namely digital dermatitis, interdigital phlegmon (foul in the foot) and heel horn erosion. Claw horn lesions are linked to long standing times on hard floors, poor cow comfort, the calving period (when the suspensory apparatus of the pedal bone loosens and the digital cushion is thin), thin cows with a thin digital cushion, and overgrown or poorly trimmed claws. Digital dermatitis is an infectious, contagious disease associated with Treponema species and wet, slurry-covered floors.
Lameness prevalence is measured by locomotion or mobility scoring of every cow in the herd at regular intervals. Common systems are the four-point AHDB mobility score (0 good mobility, 1 imperfect, 2 impaired, 3 severely impaired) and the five-point Sprecher locomotion score. Farmers typically underestimate the prevalence of lameness by a large margin, so independent scoring is valuable. Early detection and prompt treatment of newly lame cows (for example within 48 hours of detection) improve cure rates.
Herd control combines several measures. Improve cow comfort so that cows lie down for long periods (often about 12 hours a day or more): well-designed cubicles or free stalls, deep soft bedding, and limited time standing in holding yards. Improve walking surfaces: non-slip, well-drained floors, regular scraping to reduce slurry exposure, and well-maintained cow tracks in pasture systems. Perform preventive (functional) foot trimming at regular intervals, commonly around drying off and in mid-lactation, using the Dutch five-step method or similar. Use footbathing to control digital dermatitis, with a solution, depth and bath length suitable for the purpose and with attention to local regulations on disposal of chemicals such as formalin and copper sulphate. Maintain body condition to preserve the digital cushion. Quarantine and inspect purchased cattle, because digital dermatitis is introduced with animals.
Treatment of sole ulcers and white line lesions involves corrective trimming to take weight off the affected claw, applying a block to the healthy claw, and pain relief with a non-steroidal anti-inflammatory drug, used at the dose and withdrawal period on the label of a product authorised for cattle in your country. Digital dermatitis lesions are treated with topical antimicrobials. In sheep, footrot (Dichelobacter nodosus with Fusobacterium necrophorum), scald and contagious ovine digital dermatitis are the main causes and are covered in the small-ruminant chapter.
Biosecurity is the set of measures that prevent infectious agents entering a herd (external biosecurity, or bioexclusion) and spreading within it (internal biosecurity, or biocontainment), and that prevent their spread from the herd to others. The main routes of entry are purchased and returning animals, semen and embryos, people, vehicles and equipment, feed and water, wildlife, birds, rodents and insects, neighbouring stock across fences, and airborne spread over short distances for some pathogens such as foot-and-mouth disease virus and porcine reproductive and respiratory syndrome virus.
Purchased animals are the highest risk for most cattle diseases. A closed herd that breeds its own replacements has the lowest risk. When animals must be bought, the rules are: buy from as few sources as possible, from herds of known and equal or better health status; obtain health information before purchase; test and vaccinate as the plan requires; and quarantine new arrivals for a period (commonly three to four weeks) in separate housing, without shared air space or drainage, while testing is completed. Diseases for which purchased cattle are typically checked include bovine viral diarrhoea (antigen testing to exclude persistently infected animals), infectious bovine rhinotracheitis, Johne's disease, leptospirosis, bovine tuberculosis according to national rules, and digital dermatitis on inspection. A persistently infected (PI) BVD animal, infected in utero before about 120 to 125 days of gestation, sheds virus for life and is the main source of infection; the fetus of a pregnant purchased cow can also be a PI, the so-called Trojan cow.
Internal biosecurity includes separating age groups, calving pens cleaned between uses, isolation of sick animals, all-in/all-out pen use, clean-to-dirty movement of people and equipment, and control of manure contamination of feed. Johne's disease control, for example, focuses on preventing calves ingesting adult faeces through the calving pen, colostrum and milk.
Pig and poultry units apply the strictest biosecurity. Typical measures are a perimeter fence with controlled entry; a shower-in or change-in system with a clean and dirty side; farm-specific clothing and boots; a minimum interval since contact with other pigs or poultry for visitors; loading bays so that lorries do not enter the clean area; feed delivered from outside the fence; bird-proof buildings; rodent control; and all-in/all-out management with cleaning, disinfection and a drying and empty period between batches. Semen and replacement gilts or day-old chicks come from sources of high, documented health status. Surveillance through slaughter checks, serology and post-mortems confirms status.
Transboundary animal diseases such as foot-and-mouth disease, African swine fever, classical swine fever, highly pathogenic avian influenza, peste des petits ruminants and lumpy skin disease are listed by the World Organisation for Animal Health (WOAH) and are notifiable in most countries. Herd biosecurity is the first line of defence against them; national rules on swill feeding, animal movement and standstill periods, identification and traceability form the next. Suspicion of a notifiable disease must be reported to the veterinary authority according to national law. The level of biosecurity that is realistic differs between a commercial broiler house and a village flock, and advice should be proportionate to the risk and the farm's resources.
Biosecurity is assessed with structured questionnaires and farm audits that score each risk pathway; scoring tools exist for pigs, poultry and cattle. The vet then prioritises the few changes that remove the most risk. Testing to show that a herd is free of a disease depends on the number of animals tested, the expected prevalence if the disease were present, and the sensitivity of the test. For a large herd and a perfect test, the number that must be tested to have a 95% chance of detecting at least one positive, if the true prevalence is p, is approximately n = ln(0.05) ÷ ln(1 − p). At a design prevalence of 10% this gives about 29 animals; at 5% about 59. An imperfect test needs a larger sample. Negative results support freedom only at the design prevalence used; they cannot prove complete absence.
Herd health depends on data. Minimum records for a dairy herd are animal identity, births, calvings, services and sires used, pregnancy diagnoses, disease events and treatments with medicine, dose, route and withdrawal period, milk recording results (yield, fat, protein, SCC), and removals with reason. Many countries require a medicine record by law for food-producing animals, listing the medicine, batch, quantity, date, animals treated and withdrawal period. Beef, sheep, pig and poultry units record their own equivalents: weights at fixed ages, feed delivered, mortality with cause, and output sold.
A KPI is useful only if it is well defined, calculated the same way each time, based on enough animals, and relevant to a decision. Small numbers make rates unstable: a conception rate based on ten services can swing widely by chance. Lag matters: some KPIs, such as calving interval, describe events months ago. Bias matters: indices that include only animals with a recorded event (conceived, sold) can mislead. Good practice is to track a small set of leading and lagging KPIs over time, often as a rolling figure or a chart, and investigate trends rather than single values.
The calving interval is days open plus gestation. Days open of 125 days with a 280-day gestation gives a calving interval of 405 days. The 21-day pregnancy rate is the number of cows becoming pregnant divided by the number of eligible, non-pregnant cows in the 21-day period; with 100 eligible cows, 55 inseminated and 22 pregnant, the insemination rate is 55%, the conception rate 40% and the pregnancy rate 22%.
Udder-health KPIs include the bulk milk SCC (a yield-weighted average), the proportion of cows above 200,000 cells/mL, the new infection rate, the chronic rate and the clinical mastitis incidence rate. With 45 clinical cases in a herd of 150 cows over a year, the incidence rate is 30 cases per 100 cows per year. Because bulk SCC is yield-weighted, a herd in which 80 cows yield 25 kg at 100,000 cells/mL, 15 cows yield 20 kg at 400,000 and 5 cows yield 15 kg at 1,500,000 has a bulk SCC of about 182,000, lower than the simple mean of 215,000, because the high-count cows give less milk.
The culling (removal) rate is the number of cows leaving the herd in a year divided by the average number of cows; 60 removals from an average of 200 cows is 30%. Replacement rate and the reasons for culling, especially involuntary culls for infertility, mastitis and lameness, show where disease is costing the herd. In beef systems the key output measures are the weaned calf percentage (calves weaned per cow exposed to the bull), calf weaning weight, and ADG; in feedlots, ADG, feed conversion and pull (morbidity) and death rates.
Average daily gain (ADG) is weight gain divided by days: a calf weighing 40 kg at birth and 96 kg at 56 days has an ADG of 1.0 kg/day. Feed conversion ratio (FCR) is the feed consumed divided by the weight gained, both in the same units; a pig that eats 250 kg of feed while gaining 95 kg has an FCR of about 2.63. A lower FCR is better. Its inverse, feed efficiency (gain ÷ feed), is 0.38 in the same example and is more common in cattle nutrition. FCR must be defined carefully: over which weight range, whether dead animals and their feed are included, and whether the feed is on an as-fed or dry-matter basis.
In broiler production the European Production Efficiency Factor (EPEF) combines growth, survival and feed efficiency into one number: EPEF = (liveability % × average live weight in kg) ÷ (age in days × FCR) × 100. A flock with 96% liveability, 2.4 kg average weight at 38 days and an FCR of 1.6 has an EPEF of about 379. In pig breeding herds the main output KPI is pigs weaned per sow per year, the product of litters per sow per year and pigs weaned per litter. With a 115-day gestation, 26-day lactation and a 6-day wean-to-service interval, a sow can produce at most about 365 ÷ 147 ≈ 2.48 litters a year; with 11.5 pigs weaned per litter this gives about 28.6 pigs weaned per sow per year. Non-productive days (days a sow is neither pregnant nor lactating) reduce litters per sow per year and are a key target in sow herds.
The cost of a disease has two parts: the output lost and the expenditure to deal with it. Losses include reduced milk yield, discarded milk, reduced growth, poorer feed efficiency, death, premature culling (the lost future value of the animal less its salvage value), reduced fertility, and penalties for product quality such as bulk SCC. Expenditures include treatment, veterinary time, labour, and prevention. A clinical mastitis case, for example, costs the drugs, the milk discarded during treatment and the withdrawal period, the lower yield for the rest of the lactation, the extra labour, and an increased risk of culling. Discarded milk alone for a cow giving 28 kg/day, treated for 3 days with a 4-day withdrawal after the last treatment, is 7 days × 28 kg = 196 kg; at a price of 0.40 per kg this is 78.4 currency units.
Published estimates of the cost of a clinical mastitis case, a lame cow, a displaced abomasum or an extra day open vary widely between countries, years and methods, and depend on milk and feed prices. Use local, current estimates rather than a single textbook figure. Most loss comes from subclinical disease, which farmers see less clearly than clinical cases.
A partial budget is the standard tool for assessing a change on a farm. It lists only what changes: extra income and reduced costs on one side, and reduced income and extra costs on the other. If the benefits exceed the costs, the change is worth making on that basis. For example, a vaccination programme costing 6,000 currency units a year that is expected to reduce losses by 18,000 has a net benefit of 12,000 and a benefit-cost ratio of 3.0. The benefit-cost ratio is total benefits divided by total costs; a ratio above 1 means the intervention pays.
Where costs and benefits occur in different years, as with heifer rearing, eradication programmes or building work, they are discounted to present values and compared as net present value or internal rate of return. Sensitivity analysis tests how the answer changes if prices or the disease reduction differ from expectations.
Residues are drugs or their metabolites remaining in food. Maximum residue limits (MRLs) are set for each substance, species and tissue by national authorities and, internationally, by the Codex Alimentarius Commission on the advice of the Joint FAO/WHO Expert Committee on Food Additives. The withdrawal period is the time after the last dose during which the animal must not be slaughtered for food, or its milk or eggs must not be used, so that residues fall below the MRL. It is stated on the label of each product for each authorised species and route. Some substances are prohibited in food animals in many jurisdictions, for example chloramphenicol and nitrofurans; lists differ by country.
When a medicine is used outside its label (the prescribing cascade in the UK and EU, extra-label drug use under AMDUCA in the United States), the vet must set a suitably extended withdrawal period. Under the EU veterinary medicines regulation, the statutory minimum for cascade use is generally the longest withdrawal period on the label for any species multiplied by 1.5, or 28 days for meat and 7 days for milk if the product is not authorised for food-producing species; national rules and later amendments differ, so check the current rule in your country. In the United States, the Food Animal Residue Avoidance Databank (FARAD) advises on extended withdrawal intervals.
Residue violations most often come from failure to observe withdrawal periods, poor identification of treated animals, extra-label use without an extended withdrawal, and treated cows being milked into the tank by mistake. Controls are clear marking of treated animals, a written treatment record, a single person responsible for releasing animals and milk, testing of milk from treated cows before return to the tank where screening tests are available, and not treating unnecessarily. Antimicrobial stewardship adds: diagnose before treating, use narrow-spectrum drugs where suitable, avoid critically important antimicrobials of highest priority where alternatives exist, reduce the need for treatment through prevention, and measure use (for example as mg per population correction unit or defined daily doses) so that it can be benchmarked and reduced.
Pig medicine is herd medicine: individual pigs are rarely treated for their own sake, and decisions rest on production records, slaughter checks, post-mortems and serology. The breeding herd is assessed by pigs weaned per sow per year, farrowing rate (the proportion of served sows that farrow), born alive per litter, pre-weaning mortality, wean-to-service interval and non-productive days. Pre-weaning mortality is the proportion of pigs born alive that die before weaning; with 12.5 born alive and 11 weaned per litter it is 12%. Crushing by the sow, starvation and low birth weight are major causes, influenced by farrowing accommodation, temperature and supervision at farrowing. The growing herd is assessed by ADG, FCR, mortality, and days to slaughter weight.
The major endemic diseases in many pig-producing regions include porcine reproductive and respiratory syndrome (PRRS), which causes reproductive failure in sows and respiratory disease in growing pigs; enzootic pneumonia caused by Mycoplasma hyopneumoniae; porcine circovirus type 2 (PCV2)-associated disease, which is largely controlled by vaccination; swine influenza; pleuropneumonia caused by Actinobacillus pleuropneumoniae; post-weaning diarrhoea from enterotoxigenic E. coli; proliferative enteropathy (Lawsonia intracellularis); swine dysentery (Brachyspira hyodysenteriae); and parasites such as Ascaris suum and sarcoptic mange. African swine fever has spread widely across Europe and Asia since 2007 and 2018 respectively, with no widely available vaccine in most countries, so biosecurity and national control measures are the defence.
Slaughter checks, which score lungs for enzootic pneumonia-like lesions and pleurisy and livers for Ascaris milk spots, give cheap herd-level data. All-in/all-out flow by room or site, multi-site production (breeding, nursery and finishing on separate sites), segregated early weaning, age-segregated flow and whole-herd depopulation and repopulation are strategies to reduce or eliminate disease. Herd programmes for PRRS include stabilising the sow herd (for example by exposure or vaccination with herd closure) and then producing negative weaned pigs.
Commercial poultry are managed as flocks of thousands placed at the same age, so health is measured by flock KPIs: daily and cumulative mortality, first-week mortality (often used as a check on hatchery and brooding quality), weight for age against the breed standard, FCR, uniformity, EPEF in broilers, and in layers hen-day egg production, egg weight, shell quality and mortality.
Disease control rests on biosecurity, all-in/all-out placement with cleaning and disinfection between flocks, good brooding (temperature, ventilation, litter, water and feed access in the first days), vaccination and monitoring. Key diseases include Newcastle disease, avian influenza, infectious bronchitis, infectious bursal disease (Gumboro), Marek's disease (vaccinated at hatch or in ovo), coccidiosis (controlled by anticoccidials or vaccination), necrotic enteritis, Mycoplasma gallisepticum and synoviae, and Salmonella, which many countries control under national programmes because of its public health importance. Vaccination programmes are region-specific and depend on local disease pressure, maternal antibody and regulation. Sampling for monitoring often uses dead birds collected daily, post-mortems on a sample of culls, serology at slaughter, and environmental samples such as boot swabs for Salmonella.
Sheep and goat flocks are managed by the calendar of the production year: preparation for mating (tupping), mating, pregnancy, lambing or kidding, lactation and weaning. Key flock KPIs include the lambing percentage, usually lambs born (or sometimes lambs alive at a set time) per ewe put to the ram, and the weaning percentage, lambs weaned per ewe put to the ram. With 200 ewes joined, 330 lambs born and 290 weaned, the lambing percentage is 165% and the weaning percentage 145%, meaning about 12% of lambs born were lost before weaning. Definitions vary between countries, so state which denominator is used. Other KPIs are barren (empty) ewe rate at scanning, lamb growth rates, ewe mortality and culling reasons.
The body condition score of ewes at mating affects ovulation rate and litter size; condition at lambing affects colostrum and lamb survival. Ram checks before mating examine teeth, feet, testes and body condition, since a ram with poor fertility or lameness can serve few ewes. Ultrasound pregnancy scanning allows ewes to be grouped and fed by the number of fetuses they carry, which reduces pregnancy toxaemia in multiple-bearing ewes and over-fatness in singles.
Pregnancy toxaemia (twin lamb disease) is a disorder of energy metabolism in late pregnancy in ewes and does carrying two or more fetuses, especially when feed quality falls or when stress or other disease reduces intake; flock control rests on scanning, condition scoring and adequate energy in the last six weeks of pregnancy. Hypocalcaemia also occurs around lambing.
Gastrointestinal nematodes are the main production-limiting disease of grazing sheep and goats in most climates. Haemonchus contortus, a blood-sucking abomasal worm dominant in warm, wet regions, causes anaemia and bottle jaw; Teladorsagia and Trichostrongylus cause diarrhoea and poor growth; Nematodirus battus causes acute disease in lambs after a mass spring hatch in temperate regions. Goats generally metabolise many anthelmintics faster than sheep and often need different doses, so use a product and dose authorised for goats where one exists — confirm against the product label and your national formulary.
Anthelmintic resistance is widespread worldwide in all the major broad-spectrum groups, and multiple resistance is common. Control now aims to keep anthelmintics working by maintaining refugia, the proportion of the worm population not exposed to treatment, so that susceptible genes persist. Tools include targeted selective treatment of only those animals that need it, using weight gain, body condition, dag score or, for Haemonchus, the FAMACHA system, which scores the colour of the lower eyelid conjunctiva on a five-point chart from red (1) to white (5) with treatment of animals scoring 4 and 5 and sometimes 3; faecal egg count monitoring; quarantine treatment and faecal egg testing of bought-in animals; correct dosing to the weight of the heaviest animals in a group; and grazing management.
The faecal egg count reduction test (FECRT) compares egg counts before and after treatment: percentage reduction = (pre-treatment count − post-treatment count) ÷ pre-treatment count × 100. A reduction from 800 to 120 eggs per gram is 85%. A reduction below 95% has traditionally been taken to indicate resistance, and current guidelines of the World Association for the Advancement of Veterinary Parasitology refine the interpretation using confidence intervals and minimum egg counts.
Many small-ruminant diseases are "iceberg diseases": the clinical cases seen are a small part of the infected flock. Examples are Johne's disease (paratuberculosis), maedi-visna in sheep and caprine arthritis-encephalitis in goats (both small ruminant lentiviruses), caseous lymphadenitis (Corynebacterium pseudotuberculosis) and ovine pulmonary adenocarcinoma. Control depends on buying from accredited or tested flocks, blood testing, culling and, for lentiviruses, preventing transmission through colostrum and milk.
Infectious abortion is a key flock problem: enzootic abortion of ewes (Chlamydia abortus), toxoplasmosis (Toxoplasma gondii, from cat faeces), campylobacteriosis, Q fever (Coxiella burnetii), listeriosis and, in some regions, brucellosis (Brucella melitensis) and border disease. Several are zoonoses, and pregnant women should avoid contact with lambing ewes. Investigation needs aborted fetuses and placentas submitted to a laboratory. Vaccines exist for some agents in some countries.
Lameness in sheep is dominated by footrot (Dichelobacter nodosus with Fusobacterium necrophorum), interdigital dermatitis (scald) and contagious ovine digital dermatitis. The UK five-point plan for sheep lameness is: cull persistently lame sheep, quarantine incoming stock, treat lame sheep promptly, avoid spreading infection in gathering areas and handling, and vaccinate. Prompt individual treatment of footrot with systemic antimicrobials is more effective than routine foot trimming, which can spread infection and damage the foot. Clostridial diseases (for example enterotoxaemia, pulpy kidney and tetanus) are prevented by vaccination of ewes before lambing and of lambs, and peste des petits ruminants, sheep and goat pox and foot-and-mouth disease are major transboundary threats in Africa and Asia.
Learn to turn a herd's records into rates with denominators and to compare them with targets. Be able to calculate calving interval, pregnancy rate, linear score, incidence rate, ADG, FCR, EPEF, lambing percentage and FECR by hand, and to explain what each tells you. Learn the contagious and environmental mastitis pathogens, the five-point plan, the colostrum rules and the basic biosecurity measures for purchased stock. On farm placements, sit in on a routine fertility visit, mobility-score a herd, and read a milk recording report; these are the skills that NAVLE, SME and AVE vignettes test.
A five-year farm vet runs routine visits for a list of herds: fertility examinations, reviewing monthly milk recording and fertility reports, writing and updating health plans, treatment protocols and medicine records, and advising on BVD, Johne's and lameness programmes. The day-to-day decisions are: which cows to examine, whether a KPI change is real or noise, which two or three actions to agree with the farmer this month, how to cut antimicrobial use (selective dry cow therapy, on-farm culture) without harming health, and what withdrawal to set for an off-label treatment.
With 25 years in practice the vet leads a herd health team, trains farm staff and younger vets, handles multi-factorial investigations (a herd with rising bulk SCC, a sow unit with PRRS, a flock with anthelmintic failure) and advises on building design, eradication programmes and business decisions using partial budgets. They work with nutritionists, hoof trimmers, milking machine technicians, processors and assurance auditors, and they represent the practice in regional disease control schemes and in discussions of national antimicrobial-use targets.
A researcher with long field experience designs the intervention trials and observational studies that set the targets used in practice: the cut points for BHB and NEFA, the effect of selective dry cow therapy, colostrum transfer standards, lameness risk factors and the economics of disease. They build herd-level models and decision tools, evaluate data from sensors and milk recording, define benchmarks for antimicrobial use, and advise governments and industry on disease eradication, surveillance design and the trade-offs among productivity, welfare, public health and the environment.
The manual does not give country-specific vaccination schedules, drug doses beyond a single example, or legal withdrawal periods for named products, because these depend on the label and national law. It gives only broad targets for KPIs, which vary by breed, system and country, and no monetary costs of disease, which change with prices. Camelid, buffalo-specific, deer and aquaculture herd health, detailed feedlot nutrition, precision-dairy sensor analytics, national eradication-scheme rules (for example bovine tuberculosis testing regimes) and detailed statistical methods for herd surveillance are not yet covered.
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