Livestock at pasture

Pastured livestock present a rather different problem from those being stall-fed in that they receive a diet that is less controlled and thus more difficult to assess. The risk of parasitic infestation and, in some cases, infectious disease is much greater in grazing animals. Inquiries should be made about the composition of the pasture, its probable nutritive value with particular reference to recent changes brought about by rain or drought, whether rotational grazing is practiced, the fertilizer program and whether or not minerals and trace elements are provided by top-dressing or mineral mixtures. The origin of mineral supplements, particularly phosphates, which may contain excess fluorine, and homemade mixtures, which may contain excessive quantities of other ingredients, should receive attention. Actual examination of the pasture area is usually more rewarding than a description of it.

Hand-fed/stall-fed animals

Hand-fed or stall-fed animals are subjected to a more or less controlled feed supply but, because of human error, they are frequently exposed to dietary mistakes. Types and amounts of feeds fed should be determined. Examples of disease caused by inadequate hand-fed diets include: osteodystrophia fibrosa in horses on diets containing excess grain; azoturia in the same species when heavy-carbohydrate diets are fed during periods of rest, and lactic acid indigestion in cattle introduced to high-level grain diets too rapidly. The sources of the dietary ingredients may also be of importance. Grains from some areas are often much heavier and contain a much greater proportion of starch to husk than grains from other areas so that when feed is measured, rather than weighed, overfeeding or underfeeding may occur.

Because the digestive enzyme capacity of newborn farm animals is most efficient in the digestion of whole milk, the use of non-milk sources of carbohydrates and proteins in the formulation of milk replacers may result in indigestion and nutritional diarrhea.

Exotic diseases may be imported in feed materials: anthrax, foot-and-mouth disease and hog cholera are well-known examples.

Variations in the preparation of ingredients of rations may produce variable diets. Overheating, as in pelleting or the cooking of feeds, can reduce their vitamin content; contamination with lubricating oil can result in poisoning by chlorinated naphthalene compounds; pressure extraction of linseed can leave considerable residues of hydrocyanic acid in the residual oil cake.

Feeding practices may in themselves contribute to the production of disease. Pigs fed in large numbers with inadequate trough space or calves fed from communal troughs are likely to be affected by overeating or inanition, depending on their size and vigor. High-level feeding and consequent rapid growth may create deficiency states by increasing the requirement for specific nutrients.

In both hand-fed and grazing animals changes in diet should be carefully noted. Movement of animals from one field to another, from pasture to cereal grazing, from unimproved to improved pasture may all precipitate the appearance of disease. Periods of sudden dietary deficiency can occur as a result of bad weather or transportation, or during change to unfamiliar feeds. Rapid changes are more important than gradual alterations, particularly in pregnant and lactating ruminants when metabolic diseases, including those caused by hypocalcemia, hypoglycemia and hypomagnesemia, are likely to occur.

The availability of drinking water must be determined: salt poisoning of swine occurs only when the supply of drinking water is inadequate.

Breeding history

The breeding history may be of importance with regard to inherited disease. The existence of a relationship between sires and dams should be noted. Hybrid vigor in crossbred animals should be considered when there is apparent variation in resistance to disease between groups maintained under similar environmental conditions. A general relationship between selection for high productivity and susceptibility to certain diseases is apparent in many breeds of animal and even in certain families. The possibility of genetotrophic disease, i.e. the inheritance of a greater requirement than normal of a specific nutrient, should be considered.

Reproductive history

The examination of the herd reproductive history involves comparing past and present reproductive performance with certain optimum objectives. The mean length of the interval between parturition and conception, the mean number of services per conception and the percentage of young animals weaned relative to the number of females that were originally exposed for breeding (calf or lamb crop, pigs weaned) are general measures of reproductive performance and efficiency.

Using cattle as an example, certain other observations may assist in determining the cause of failure to reach reproductive performance objectives. These are:

The percentage of females that need assistance at parturition and the percentage of calves that die at birth are also indices of reproductive performance that are indicative of the level of reproductive management provided.

Pasture and feed

On pastures the predominant plant types, both natural and introduced, should be observed as they are often associated with certain soil types and may be the cause of actual disease; the high estrogen content of some clovers, the occurrence of functional nervous diseases on pastures dominated by Phalaris aquatica (syn. P. tuberosa) and perennial rye grass and the presence of selective absorbing ‘converte’ plants on copper-rich and selenium-rich soils are all examples of the importance of the dominant vegetation. The presence of specific poisonous plants, evidence of overgrazing and the existence of a bone-chewing or bark-chewing habit can be determined by an examination of the environment.

Vital clues in the investigation of possible poisoning in a herd may be the existence of a garbage dump or ergotized grass or rye in the pasture, or the chewing of lead-based painted walls in the barn, or careless handling of poisons in the feed area. The possibility that the forage may have been contaminated by environmental pollution from nearby factories or highways should be examined. In some cases the physical nature of the pasture plants may be important; mature, bleached grass pasture can be seriously deficient in carotene, whereas lush young pasture can have rachitogenic potency because of its high carotene content or it may be capable of causing hypomagnesemia if it is dominated by grasses. Lush legume pasture or heavy concentrate feeding with insufficient roughage can cause a serious bloat problem.

The feed supplies for animals raised in confinement outdoors must be examined for evidence of moldy feed, contamination with feces and urine and excessive moisture due to lack of protection from rain and snow. Empty feed troughs may confirm a suspicion that the feeding system is faulty.

Water

The drinking water supply and its origin may be important in the production of disease. Water in ponds may be covered with algae containing neurotoxins or hepatotoxic agents and flowing streams may carry effluent from nearby industrial plants. In a feedlot, water may suddenly become unavailable because of frozen water lines or faulty water tank valves. This should not go unnoticed if one recognizes the anxiety of a group of cattle trying to obtain water from a dry tank.

Behavior

Separation of an animal from its group is often an indication of illness. The behavior is also a reflection of the animal’s health. If it responds normally to external stimuli, such as sound and movement, it is classified as bright. If the reactions are sluggish and the animal exhibits relative indifference to normal stimuli, it is said to be dull or apathetic. Cattle with carbohydrate engorgement are commonly reluctant to move unless coaxed. A pronounced state of indifference in which the animal remains standing and is able to move but does not respond at all to external stimuli is the ‘dummy’ syndrome. This occurs in subacute lead poisoning, listeriosis and some cases of acetonemia in cattle, and in encephalomyelitis and hepatic cirrhosis in horses. The terminal stage of apathy or depression is coma, in which the animal is unconscious and cannot be roused.

Respiratory rate

In normal animals under average conditions the rate should fall within the following limits:

An increased respiratory rate is designated as polypnea, decreased rate as oligopnea and complete cessation as apnea. The rate may be counted by observation of rib or nostril movements, by feeling the nasal air movements or by auscultation of the thorax or trachea. A significant rise in environmental temperature or humidity may double the normal respiratory rate. Animals that are acclimatized to cold outdoor temperatures are susceptible to heat stress when exposed suddenly to warmer temperatures. When brought indoors the respiratory rate may increase to six or eight times the normal, and panting open-mouth breathing may be evident within 2 hours.

Respiratory rhythm

The normal respiratory cycle consists of three phases of equal length: inspiration, expiration and pause; variation in the length of one or all phases constitutes an abnormality of rhythm. The breathing pattern of the neonatal foal is markedly different from that of the adult horse, and similar to that of other neonates. It has a higher respiratory rate, a higher airflow rate, and a higher minute ventilation on a body weight basis. In addition, in the standing neonatal foal, both the inspiratory and expiratory airflow patterns are essentially monophasic, whereas the adult horse typically has a biphasic inspiratory and expiratory airflow pattern. The transition from monophasic to biphasic flow patterns occurs within the first year of life.

Respiratory depth

The amplitude or depth of respiratory movements may be reduced in painful conditions of the chest or diaphragm and increased in any form of anoxia. Moderate increase in depth is referred to as hyperpnea and labored breathing as dyspnea. In dyspnea, the accessory respiratory movements become more prominent: there is extension of the head and neck, dilatation of the nostrils, abduction of the elbows and breathing through the mouth plus increased movement of the thoracic and abdominal walls. Loud respiratory sounds, especially grunting, may also be heard.

Type of respiration

In normal respiration there is movement of the thorax and abdomen. In painful conditions of the thorax, e.g. acute pleurisy, and in paralysis of the intercostal muscles, there is relative fixation of the thoracic wall and a marked increase in the movements of the abdominal wall; there also may be an associated pleuritic ridge caused by thoracic immobility with the thorax expanded. This syndrome is usually referred to as an abdominal-type respiration. The reverse situation is thoracic-type respiration, in which the movements are largely confined to the thoracic wall, as in peritonitis, particularly when there is diaphragmatic involvement.

Thorax symmetry

This can also be evaluated by inspection. Collapse or consolidation of one lung may lead to restriction of movements of the thoracic wall on the affected side. The ‘rachitic rosary’ of enlarged costochondral junctions is typical of rickets.

Respiratory noises or stridors

These include:

An important part of the clinical examination of a horse that produces an externally audible noise, usually a grunt, while working is to determine when the noise occurs in the respiratory cycle. This can be related to limb movements, expiration occurring as the leading foot hits the ground at the canter or gallop. Flexion of the head by the rider will exacerbate the noise.

Temperature

Normally the temperature is taken per rectum. When this is impossible the thermometer should be inserted into the vagina. Ensure that the mercury column is shaken down, moisten the bulb to facilitate entry and, if the anus is flaccid or the rectum full of hard feces, insert a finger also to ensure that the thermometer bulb is held against the mucosa. When the temperature is read immediately after defecation, or if the thermometer is stuck into a ball of feces or is left in the rectum for insufficient time, a false, low reading will result.

As a general rule the thermometer should be left in place for 2 minutes. If there is doubt as to the accuracy of the reading, the temperature should be taken again. The normal average temperature range for the various species at average environmental temperature is as shown in Table 1.1.

Table 1.1 Normal average temperatures with critical points

Temperature conversions are approximate.

The reference values in Table 1.1 indicate the average resting temperature for the species and the critical temperature above which hyperthermia can be said to be present. Normal physiological variations occur in body temperature and are not an indication of disease: a diurnal variation of up to 1°C (2°F) may occur, with the low point in the morning and the peak in the late afternoon. There may be a mild rise of about 0.6°C (1°F) in late pregnancy, but a precipitate although insignificant decline just before calving is not uncommon in cows and ewes and lower temperatures than normal occur just before estrus and at ovulation – the degree of change (about 0.3°C; 0.6°F) is unlikely to attract clinical attention.

In sows the body temperature is subnormal before farrowing and there is a significant rise in body temperature coinciding with parturition. This rise is commonly high enough to exceed the critical temperature of 40°C and may be considered erroneously as evidence of disease. The elevation of temperature that occurs in sows at the time of parturition, of the order of 1°C, is maintained through lactation and disappears at weaning.

High environmental humidity and temperature and exercise will cause elevation of the temperature; the deviation may be as much as 1.6°C (3°F) in the case of high environmental temperatures and as much as 2.5–3°C (4.5°F) after severe exercise; in horses, after racing, 2 hours may be required before the temperature returns to normal.

If animals that have been acclimatized to cold outside temperatures are brought indoors to a warmer temperature their body temperatures may exceed the critical temperature within 2–4 hours. Marked temperature variations are an indication of a pathological process:

Pulse

The pulse should be taken at the middle coccygeal or facial arteries in cattle, the facial artery in the horse and the femoral artery in sheep and goats. With careful palpation a number of characters may be determined, including rate, rhythm, amplitude, tone, maximum and minimum pulse pressures and the form of the arterial pulse. Some of these characters are more properly included in special examination of the circulatory system and are dealt with under that heading.

State of hydration

The state of hydration is assessed by inspection of the eyes for evidence of dehydration and evaluating the elasticity of the skin. Dehydration is characterized by sunken eyes of varying degrees, and the skin will ‘tent’ when lifted with the fingers and remain tented for varying lengths of time.

Cardiac area

Auscultation of the heart is aimed at determining the character of normal heart sounds and detecting the presence of abnormal sounds. Optimum auscultation sites are the fourth and fifth intercostal spaces and, because of the heavy shoulder muscles that cover the anterior border of the heart, the use of a flat phonendoscope chest piece pushed under the triceps muscles is necessary. Extension of the forelimb may facilitate auscultation if the animal is quiet. Areas where the various sounds are heard with maximum intensity are not directly over the anatomical sites of the cardiac orifices, because conduction of the sound through the fluid in the chamber gives optimum auscultation at the point where the fluid is closest to the chest wall.

The first (systolic) sound is heard best over the cardiac apex, the tricuspid closure being most audible over the right apex and mitral closure over the left apex. The second (diastolic) sound is heard best over the base of the heart, the aortic semilunar closure posteriorly and the pulmonary semilunar anteriorly, both on the left side.

In auscultation of the heart, the points to be noted are the rate, rhythm, intensity and quality of sounds and whether abnormal sounds are present. Comparison of the heart and pulse rates will determine whether there is a pulse deficit due to weak heart contractions failing to cause palpable pulse waves; this is most likely to occur in irregular hearts. Normally the rhythm is in three time and can be described as

LUBB — DUPP — pause,

the first sound being dull, deep, long and loud and the second sound sharper and shorter. As the heart rate increases the cycle becomes shortened, mainly at the expense of diastole and the rhythm assumes a two-time quality. More than two sounds per cycle is classified as a ‘gallop’ rhythm and may be due to reduplication of either the first or second sounds. Reduplication of the first sound is common in normal cattle and its significance in other species is discussed under diseases of the circulatory system.

The rhythm between successive cycles should be regular except in the normal sinus arrhythmia associated with respiration. With irregularity, there is usually variation in the time intervals between cycles and in the intensity of the sounds – louder sounds coming directly after prolonged pauses and being softer than normal sounds after shortened intervals, as in extrasystolic contractions. The intensity of the heart sounds may vary in two ways, absolutely or relatively: absolutely when the two sounds are louder than normal, and relatively when one sound is increased compared to the other in the cycle. For example, there is increased absolute intensity in anemia and in cardiac hypertrophy.

The intensity of the first sound depends on the force of ventricular contraction and is thus increased in ventricular hypertrophy and decreased in myocardial asthenia. The intensity of the second sound depends upon the semilunar closure, i.e. on the arterial blood pressure, and is therefore increased when the blood pressure is high and decreased when the pressure is low.

Abnormal sounds may replace one or both of the normal sounds or may accompany them. The heart sounds are muffled when the pericardial sac is distended with fluid. Sounds that are related to events in the cardiac cycle are murmurs or bruits and are caused mainly by endocardial lesions such as valvular vegetations or adhesions, by insufficiency of closure of valves and by abnormal orifices such as a patent interventricular septum or ductus arteriosus. Interference with normal blood flow causes the development of turbulence with resultant eddying and the creation of murmurs. In attempting to determine the site and type of the lesion it is necessary to identify its time of occurrence in the cardiac cycle: it may be presystolic, systolic or diastolic and it is usually necessary to palpate the arterial pulse and auscultate the heart simultaneously to determine accurately the time of occurrence. The site of maximum audibility may indicate the probable site of the lesion, but other observations, including abnormalities of the arterial pulse wave, should be taken into account. In many cases of advanced debility, anemia and toxemia, soft murmurs can be heard that wax and wane with respiration (hemic murmurs) and are probably due to myocardial asthenia. In cases of local pressure on the heart by other organs, for example in diaphragmatic hernia in cattle, loud systolic murmurs may be heard, probably due to distortion of the valvular orifices.

Abnormal sounds not related to the cardiac cycle include pericardial friction rubs, which occur with each heart cycle but are not specifically related to either systolic or diastolic sounds. They are more superficial, more distinctly heard than murmurs and have a to-and-fro character. Local pleuritic friction rubs may be confused with pericardial sounds, especially if respiratory and cardiac rates are equal.

Palpation of the heart beat has real value: the size of the cardiac impulses can be assessed and palpable thrills may on occasion be of more value than auscultation of murmurs. It is best carried out with the palm of the hand and should be performed on both sides. An increased cardiac impulse, the movements of the heart against the chest wall during systole, may be easily seen on close inspection of the left precordium and can be felt on both sides. It may be due to cardiac hypertrophy or dilatation associated with cardiac insufficiency or anemia or to distension of the pericardial sac with edema or inflammatory fluid. Care should be taken not to confuse a readily palpable cardiac impulse due to cardiac enlargement with one due to contraction of lung tissue and increased exposure of the heart to the chest wall.

Normally, the heart movements can be felt as distinct systolic and diastolic thumps. These thumps are replaced by thrills when valvular insufficiencies or stenoses or congenital defects are present. When the defects are large the murmur heard on auscultation may not be very loud but the thrill is readily palpable. Early pericarditis may also produce a friction thrill. The cardiac impulse should be much stronger on the left than the right side and reversal of this situation indicates displacement of the heart to the right side. Caudal or anterior displacement can also occur.

Percussion to determine the boundaries of the heart is of little value in large animal work because of the relatively large size of the heart and lungs and the depth of tissue involved. The area of cardiac dullness is increased in cardiac hypertrophy and dilatation and decreased when the heart is covered by more than the usual amount of lung, as in pulmonary emphysema. More detailed examination of the heart by electrocardiography, radiographic examination, test puncture and blood pressure are described under diseases of the heart.

Lung area

Auscultation, percussion and palpation are the major methods used for examination of the lungs.

The lung area available for satisfactory auscultation is slightly larger than that available for percussion. The normal breath sounds are heard over most of the lungs, particularly in the middle third anteriorly over the base of the lung, and consist of a soft, sipping VEE–EFF, the latter, softer sound occurring at expiration. The sounds are heard with variable ease depending on the thickness of the chest wall and the amplitude of the respiratory excursion. In well-fleshed horses and fat beef cattle the sounds may not be discernible at rest. The amplitude or loudness of the breath sounds is increased in dyspnea and in early pulmonary congestion and inflammation. The amplitude of the breath sounds is decreased or totally inaudible when there is pleural effusion, and in space-occupying lesions in the lung or pleural cavity. Abnormal lung sounds include crackles, wheezes and pleuritic friction rubs. They are the result of interference with the free movement of air in and out of the lungs, and of the presence of lesions that interfere with the normal movement of the lung and thus create additional respiratory sounds, which are an indication of disease. The descriptions and interpretations of the normal and abnormal lung sounds, and other respiratory noises are described in Chapter 10.

The intensity of abnormal lung sounds may be increased and their clarity improved by measuring the rate and depth of respirations with forced mild exercise such as walking for a few minutes followed by immediate auscultation. If exercise is undesirable the occlusion of both nostrils for 30–45 seconds will be followed by some deep inspirations and accentuation of abnormal lungs. An alternative maneuver which is effective in both horses and cattle is to pull a plastic bag over the muzzle and lower face. When respiratory movements become exaggerated the bag is removed and the lungs auscultated immediately.

Sounds of peristalsis are normally heard over the lung area on the left side in cattle and in horses. In cattle, these sounds are due to reticular movement and in horses to movements of the colon. Their presence is not of much significance in these species unless there are other signs. In cattle, too, sounds of swallowing, eructation and regurgitation may be confused with peristaltic sounds; ruminal movements and the esophagus should be observed for the passage of gas or a bolus to identify these sounds. Other techniques for examination of the thorax are described under diseases of the respiratory system (Ch. 10).

Palpation of the thoracic wall may reveal the presence of a pleuritic thrill, bulging of the intercostal spaces when fluid is present in the thoracic cavity, or narrowed intercostal spaces and decreased rib movement over areas of collapsed lung.

Percussion may be by the usual direct means, or indirectly by tracheal percussion when the trachea is tapped gently and the sound is listened for over the lung area. By direct percussion within the intercostal spaces the area of normal lung resonance can be defined and abnormal dullness or resonance detected. Increased dullness may indicate the presence of a space-occupying mass, consolidated lung, edematous lung or an accumulation of fluid. In a pleural effusion the upper limit of the area of dullness can be determined by percussion and the fluid line can be delineated and identified and used to assess the progress of therapy.

An overloud normal percussion note is obtained over tissue containing more air than usual, e.g. emphysematous lung. A definite tympanitic note can be elicited over pneumothorax or a gas-filled viscus penetrating through a diaphragmatic hernia. For percussion to be a satisfactory diagnostic aid, affected areas need to be large with maximum abnormality, and the chest wall must be thin.

Auscultation

Auscultation of the abdomen is an essential part of the clinical examination of cattle, horses and sheep. It is of limited value in pigs. The intestinal or stomach sounds will indicate the nature of the intraluminal contents and the frequency and amplitude of gastrointestinal movements, which are valuable aids in clinical diagnosis. The intensity, duration and frequency of the sounds should be noted. All these characteristics will be increased in animals that have just eaten or immediately following excitement.

Palpation and percussion through the abdominal wall

Because of the thickness and weight of the abdominal wall in mature cattle and horses, deep palpation of viscera and organs through the abdominal wall has limited value in these species compared to its usefulness in small animals. No viscera or organ, with the exception of the fetus, can be palpated with certainty through the abdominal wall in the horse. In cattle, the rumen and its contents can usually be palpated in the left paralumbar fossa. Ruminal distension is usually obvious while an inability to palpate the rumen may be due to a small, relatively empty rumen or to medial displacement, as in left-side displacement of the abomasum.

Nasogastric intubation

An important part of the examination of the abdomen and gastrointestinal tract of large animals, especially cattle and horses, is the passage of the nasogastric tube into the rumen of cattle and into the stomach of horses. Gastric reflux occurs commonly in the horse with colic and it is important to determine if the stomach is distended with fluid and to relieve it as necessary. This topic is presented in detail in the chapter dealing with equine colic. In cattle, when disease of the rumen is suspected, the nasogastric tube is passed into the rumen to relieve any distension and to obtain a sample of rumen juice for determination of rumen pH and the presence or absence of rumen protozoa.

Eyes

Any discharge from the eyes should be noted: it may be watery in obstruction of the lacrimal duct, serous in the early stages of inflammation and purulent in the later stages. Whether the discharge is unilateral or bilateral is of considerable importance; a unilateral discharge may be due to local inflammation, a bilateral discharge may denote a systemic disease. Abnormalities of the eyelids include abnormal movement, position and thickness. Movement may be excessive in painful eye conditions or in cases of nervous irritability including hypomagnesemia, lead poisoning and encephalitis. The lids may be kept permanently closed when there is pain in the eye or when the eyelids are swollen, as for instance in local edema due to photosensitization or allergy. The membrana nictitans may be carried across the eye when there is pain in the orbit or in tetanus or encephalitis. There may be tumors on the eyelids.

Nostrils

Particular attention should be paid to the odor of the nasal breath. There may be a sweet sickly smell of ketosis in cattle or a fetid odor, which may originate from any of a number of sources including gangrenous pneumonia, necrosis in the nasal cavities or the accumulation of nasal exudate. Odors originating in the respiratory tract are usually constant with each breath and may be unilateral. The sour smell of alimentary tract disturbance is detectable only periodically, coinciding with eructation. Odors originating in the mouth from bad teeth or from necrotic ulcers associated with Fusobacterium necrophorum in calves may be smelled on the nasal breath but are stronger on the oral breath.

In certain circumstances it may be important to note the volume of the breath expelled through the nostrils: it may be the only way of determining whether the animal is breathing and, in some cases, of counting the respiratory rate. Variation in volume between nostrils, as felt on the hands, may indicate obstruction or stenosis of one nasal cavity. This can be examined further by closing off the nostrils one at a time; if obstruction is present in one nostril, closure of the other causes severe respiratory embarrassment.

Any nasal discharge that is present should receive special attention and its examination should be carried out at the same time as an inspection of the nasal mucosa. Discharges may be restricted to one nostril in a local infection or be bilateral in systemic infection. The color and consistency of the exudate will indicate its source. In the early stages of inflammation the discharge will be a clear, colorless fluid, which later turns to a white to yellow exudate as leukocytes accumulate in it. In Channel Island cattle the color may be a deep orange, especially in allergic rhinitis. A rust or prune juice color indicates blood originating from the lower respiratory tract, as in pneumonia and in equine infectious anemia in the horse. Blood clots derived from the upper respiratory tract or pharynx may be present in large quantities, or appear as small flecks. In general, blood from the upper respiratory tract is unevenly mixed with any discharge, whereas that from the lower tract comes through as an even color.

The consistency of the nasal discharge will vary from watery in the early stages of inflammation, through thick, to cheesy in longstanding cases. Bubbles or foam may be present. When the bubbles are coarse it signifies that the discharge originates in the pharynx or nasal cavities; fine bubbles originate in the lower respiratory tract. In all species, vomiting or regurgitation caused by pharyngitis or esophageal obstruction may be accompanied by the discharge of food material from the nose or the presence of food particles in the nostrils. In some cases the volume of nasal discharge varies from time to time, often increasing when the animal is feeding from the ground, leading to infection of cranial sinuses.

Inflammation of the nasal mucosa varies from simple hyperemia, as in allergic rhinitis, to diffuse necrosis, as in bovine malignant catarrh and mucosal disease, to deep ulceration as in glanders. In hemorrhagic diseases variations in mucosal color can be observed and petechial hemorrhages may be present.

Mouth

Excessive salivation, with ropes of saliva hanging from the mouth and usually accompanied by chewing movements, occurs when a foreign body is present in the mouth and also in many forms of inflammation of the oral mucosa or of the tongue. Actinobacillosis of the tongue, foot-and-mouth disease and mucosal disease are typical examples. Excessive salivation may also occur in diseases of the central nervous system, as in acute lead poisoning in young cattle. Hypersalivation is a characteristic sign in epidermic hyperthermia associated with the mycotoxins of Acremonium coenophialum and Claviceps purpurea and by the fungus Rhizoctonia leguminicola sometimes found on red clover. Dryness of the mouth occurs in dehydration and poisoning with belladonna alkaloids, or when high levels of urea are fed.

Abnormalities of the buccal mucosa include local lesions, hemorrhages in purpuric diseases, the discolorations of jaundice and cyanosis and the pallor of anemia. Care must be taken to define the exact nature of lesions in the mouth, especially in cattle; differentiation between vesicles, erosive and ulcerative lesions is of diagnostic significance in the mucosal diseases of this species.

Submaxillary region

Abnormalities of the submaxillary region that should be noted include enlargement of lymph nodes due to local foci of infection, subcutaneous edema as part of a general edema, local cellulitis with swelling and pain, enlargement of salivary glands or guttural pouch distension in the horse. Thyroid gland enlargement is often missed or mistaken for other lesions, but its site, pulsation and surrounding edema are characteristic.

Neck

The most important part of the examination of the neck of cattle and horses is to determine the state of the jugular veins. Bilateral engorgement of the jugular veins may be due to obstruction of the veins by compression or constriction, or to right-side congestive heart failure. A jugular pulse of small magnitude moving up the jugular vein about one-third of the way up the neck is normal in most animals but it must be differentiated from a transmitted carotid pulse, which is not obliterated by compression of the jugular vein at a lower level. Variations in size of the vein may occur synchronously with deep respiratory movements but bear no relation to the cardiac cycles. When the jugular pulse is associated with each cardiac movement it should be determined whether it is physiological or pathological. The physiological pulse is presystolic and due to atrial systole, and is normal. The pathological pulse is systolic and occurs simultaneously with the arterial pulse and the first heart sound; it is characteristic of an insufficient tricuspid valve.

Local or general enlargement of the esophagus associated with vomiting or dysphagia occurs in esophageal diverticulum, stenosis and paralysis, and in cardial obstructions. Passage of a stomach tube or probang can assist in the examination of esophageal abnormalities.

Tracheal auscultation is a useful diagnostic aid. Normally, the sounds that are audible are louder and more distinct than breath sounds audible over the lung. In upper respiratory tract disease such as laryngitis and tracheitis, the sounds are louder and harsher and may be whistling in the presence of stenosis. Very loud stenotic tracheal sounds are characteristic of calves with tracheal collapse. Abnormal tracheal sounds, regardless of their cause, are usually transferred down the major bronchi and are audible on auscultation over the thorax, primarily during inspiration. They are commonly confused with abnormal lung sounds due to pneumonia, but in pneumonia the abnormal sounds are usually present both on inspiration and on expiration.

Rectal examination

Rectal exploration of the abdomen is a vital part of the complete examination of the abdomen of large animals, especially cattle and horses. Abnormalities that are completely unexpected may be present and may be the cause of illness in animals in which no other significant clinical abnormalities were detected on clinical examination. Special care is necessary to avoid injuring the patient and causing it to strain. Suitable lubrication and avoidance of force are the two most important factors. Rectal examination enables observations to be made on the alimentary, urinary and genital tracts and on the vessels, peritoneum and pelvic structures. The amount and nature of the feces in the rectum should be determined.

Palpable abnormalities of the digestive tract include paralysis and ballooning of the rectum, distension of the loops of the intestine with fluid or gas, the presence of hard masses of ingesta as in cecal and colonic impactions in the horse, and intestinal obstruction due to volvulus, intussusception or strangulation. The detection of tight bands of mesentery leading to displacement segments may be a valuable guide. In cattle, the caudal sacs of the rumen are readily palpable. When the rumen is distended as in bloat or vagus indigestion they may push well into the pelvis or be only just within reach when the rumen is empty. A distended abomasum may be felt in the right half of the abdomen in cases of abomasal torsion and occasionally in vagus indigestion. In healthy animals there is little to feel because of the space occupied by normal intestines. Palpable objects should be carefully examined.

The left kidney in the cow can be felt in the midline and distinct lobulations are evident. In the horse, the caudal pole of the left kidney is easily palpable, but the right kidney is not. There may be abnormalities of size in pyelonephritis, hydronephrosis and amyloidosis, and pain on pressure in pyelonephritis. The ureters are not normally palpable nor is the empty bladder. A distended bladder or chronic cystitis with thickening of the wall can be felt in the midline at the anterior end of the pelvic cavity. Abnormalities of the bladder and ureters in cattle are also palpable through the ventral aspects of the vagina. Large calculi have a stone-like hardness and are occasionally observed in horses in the same position. Pain with spasmodic jerking of the penis on palpation of the urethra occurs in urinary obstruction due to small calculi, cystitis and urethritis. Enlarged, thickened ureters such as occur in pyelonephritis can be felt between the kidney and the bladder.

On the peritoneum and mesentery one may feel the small, grape-like lesions of tuberculosis, the large, irregular, hard masses of fat necrosis and the enlarged lymph nodes of lymphomatosis. The abdominal aorta is palpable, and in horses the anterior mesenteric artery and some of its branches can be felt. This may be an important examination if a verminous aneurysm is suspected, in which case the vessels are thickened but still pulsate, have an uneven rough surface and may be painful. In horses the caudal edge of the spleen is usually palpable in the left abdomen. During a rectal examination in a horse it is advantageous in some cases to palpate the inguinal ring from inside the abdomen and, by pushing the other hand between the horse’s thighs, to palpate the external ring simultaneously. It is then easier to decide whether any abnormal structures are passing through the ring.

Paracentesis of the abdomen

Paracentesis of the abdomen includes obtaining a sample of peritoneal fluid when peritonitis or inflammation of the serosae of the intestines or other viscera of the abdomen is suspected. Aspiration of fluid from a distended abdominal viscus is also possible and may aid in the diagnosis.

Determination of the abnormality of function present

Disease is abnormality of function which is harmful to the animal. The first step is to decide what abnormality of function is present. There may of course be more than one and some clinically insignificant abnormalities may be present, e.g. a physiological cardiac murmur in a newborn foal. Definition of the abnormality is usually in general terms such as paralysis, state of the alimentary tract, hypoxia, respiratory insufficiency, nervous shock and so on. These terms are largely clinical, referring to abnormalities of normal physiological function, and their use requires a foreknowledge of normal physiology. It is at this point that the preclinical study of physiology merges into the clinical study of medicine.

The necessary familiarity with the normal, combined with observation of the case in hand, makes it possible to determine the physiological abnormality that may be, e.g. hypoxia. The next step is to determine the body system or body as a whole or organ involved in the production of the hypoxia.

Determination of the system or body as a whole or organ affected

Having made a careful physical examination and noted any abnormalities, it is then possible to consider which body system or organ is the cause of the abnormality. In some cases the body as a whole may be involved. This may not be difficult with some systems: for example, hypoxia may be due to failure of the respiratory or circulatory systems and examination of these is not difficult. However, special problems arise when attempting to examine the nervous system, the liver, kidney, endocrine glands, spleen and hemopoietic systems. Here, routine physical examination by palpation, auscultation and percussion is not very rewarding: special ancillary examination techniques with the aid of a laboratory are usually necessary. These are described under special examination methods for the various systems. As a guiding principle, all functions of the organ under examination should be observed and any abnormalities noted. For example, if the integrity of the central nervous system is to be examined, the clinician would look for abnormalities of mental state, gait, posture, muscle and sphincter tone and involuntary movements, abnormal posture and paralysis. Knowing the normal physiological functions of systems, one looks for aberrations of them.

When only simple physical examination is available it may be extremely difficult to choose between two or more systems as the possible location of the abnormality. For example, in an animal that is unable to rise from the recumbent position it may be difficult to decide whether the nervous system or the musculoskeletal system or generalized weakness from a systemic illness is the origin of the clinical recumbency. If special diagnostic techniques and laboratory evaluations are inconclusive or not available, it may be necessary to resort to probability as a guide. For example, paresis due to diseases of the muscles is most common in young calves, lambs and foals and generally uncommon in mature farm animals, with the exception of the myopathy associated with the downer cow syndrome in dairy cattle. However, paresis is common in mature cows affected with parturient hypocalcemia, peracute coliform mastitis and acute diffuse peritonitis.

Determination of the location of the lesion within the system or organ affected

The location of the lesion within the body system involved is not always obvious and may require special physical and laboratory examination techniques. For example, a detailed neurological examination may be necessary to localize the lesion in an animal with manifestation of disease of the nervous system. This may be combined with radiographic techniques such as myelography. An exploratory laparotomy with or without biopsy techniques may be necessary to determine the location of an intestinal lesion thought to be the cause of chronic diarrhea. Endoscopy is rapidly becoming standard practice for the localization of lesions of the respiratory tract of the horse. Radiography is often necessary to localize lesions of the musculoskeletal system and diseases of the feet of horses and cattle.

Determination of the type of lesion

The abnormality observed may be produced by lesions of different types. In general, lesions can be divided into anatomical or physical lesions and functional disturbances. The physical lesions can be further subdivided into inflammatory, degenerative or space-occupying. These classifications are not mutually exclusive, as a lesion may be both inflammatory and space-occupying: abscesses in the spinal cord or lung are typical examples. In these circumstances it is necessary to modify the diagnosis and say that such and such a lesion is space-occupying and may or may not be inflammatory.

The differentiation between functional disturbances and physical lesions is often extremely difficult because the abnormalities produced may be identical. For example, in a case of hypomagnesemia in a cow there is no physical lesion but differentiation from the encephalitis of furious rabies may be impossible. As a rule, functional disturbances are transient, often recurrent or fluctuating and are readily reversible by treatment, whereas structural lesions cause changes that are relatively static or at least change slowly and are affected only gradually by treatment. This is by no means a regular rule: the acute abdominal pain of intestinal obstruction usually fluctuates but the lesion is a physical one, whereas the paralysis of parturient paresis in cattle is static but the disturbance is functional only.

Differentiation between inflammatory, degenerative and space-occupying lesions is usually simpler. The latter produce signs characteristic of pressure on surrounding organs and can often be detected by physical means. Inflammatory lesions are characterized by heat, pain, swelling and a local or general leukocytosis and, in severe cases, a systemic toxemia. A total white blood cell count and differential is a sensitive but nonspecific test for the presence of an infection. A leukopenia, neutropenia and a degenerative left shift suggests a severe infection. A neutrophilia and regenerative shift suggests an active chronic infection. The most common infections of cattle, which are often not readily obvious, are in the thoracic and abdominal cavities (pleuritis, pulmonary abscesses, pericarditis and peritonitis). Degenerative lesions produce the same loss or abnormality of function as lesions of the other types but are not usually accompanied by evidence of inflammation unless they are extensive. If the lesion is accessible, biopsy should be considered as a means of determining its nature.

Determination of the specific cause of the lesion

If in the system involved, the nature of the abnormality and the type of lesion can be satisfactorily determined, it then remains to decide on the specific causative agent. If, for example, it could be said that a particular case of paralysis in a calf was caused by a degenerative lesion of the musculature, only a few specific etiological agents would have to be considered to make a final diagnosis. In many, if not most cases it is impossible to go beyond this stage without additional techniques of examination, particularly laboratory examinations, and it is a general practice to make a diagnosis without this confirmatory evidence because of limitations of time or facilities.

It is at this stage that a careful history-taking and examination of the environment show their real value. It is only by a detailed knowledge of specific disease entities, the conditions under which they occur, the epidemiology and the clinical characteristics of each disease that an informed judgment can be made with any degree of accuracy. If the diagnostic possibilities can be reduced to a small number, confirmation of the diagnosis by laboratory methods becomes so much easier because there are fewer examinations to be made and confirmation by response to treatment is easier to assess. If it is necessary to treat with a great many drugs serially or in combination to achieve a cure, the expense is greater and the satisfaction of both the client and the veterinarian is diluted in proportion to the range of treatments. Accuracy in diagnosis means increased efficiency, and this is the final criterion of veterinary practice.

Utility

To be useful, a diagnostic test must be accurate. An accurate test reliably differentiates between normal and diseased animals, thereby contributing to effective management of the animal or its disease. Inaccurate diagnostic tests provide unreliable data, which in the best scenario are useless and in the worst scenario cause mismanagement of the animal or its disease. The diagnostic accuracy of a test should be known before it is used extensively and a test of unknown diagnostic accuracy should be assumed to be inaccurate until proven otherwise.

The usefulness of a test to a veterinarian depends on a number of factors. Firstly, the test must be accurate, as discussed above. Secondly, it should be technically feasible and reliable, i.e. the test must be readily performed and its characteristics (listed above) must be known. A test that cannot be readily performed has minimal usefulness and unreliable tests are inaccurate. For testing of analytes, such as serum biochemical analysis or serology, it is important that the analysis yields results that are accurate and precise. Laboratory tests that are accurate yield results that are the same (or very close to) the true value of the variable being measured. Precise tests yield results that have very little variability around the expected value. Note that a test can be precise without being accurate, i.e. it has little variability but yields a value that is different from the actual value. Tests that are inaccurate or are highly variable (have poor precision) are not useful because the results are unreliable.

Thirdly, the test must have diagnostic utility in that the results of the test should enable the veterinarian to make a decision that will affect the subsequent management of the animal or its disease. If the results of the test will not alter the animal’s management or treatment of its disease nor improve its production or prognosis, then the test has no diagnostic utility and should not be performed. The diagnostic utility depends on the characteristics of the test in the population of animals being tested. The important characteristics, which should be known before the test is widely used, are the sensitivity and specificity of the test and the likelihood ratios associated with the possible results, in the population in which it will be used. That a test has sensitivity and specificity implies that there is a range of values expected in normal animals, the so-called ‘reference range’.

Reference range (Interval)

An important aspect of evaluating laboratory data is to decide whether or not the result of a test is consistent with the animal being healthy or diseased. Healthy animals are assumed to have values within a certain range, whereas diseased animals may have values that differ from that expected in a healthy animal. The range of values in healthy animals is often referred to as being the ‘normal range’ although, because of the statistical connotation of this term, ‘reference range’ or ‘reference interval’ is preferred.

The reference range represents the range of values of a test that are expected in a group of healthy animals. Animals with values outside the reference range are at increased risk of having the disease, compared to animals with values within the reference range. The actual increase in risk of being diseased depends on the way in which the reference range was determined, the sensitivity and specificity of the test and the prevalence of the disease in the population from which the animal was selected. Calculation of likelihood ratios, both positive and negative, is a useful means of quantitatively assessing the results of a test.

The reference range for a particular test is usually developed by collecting values from a large number of healthy or ‘normal’ animals and performing a statistical analysis of the values. For variables that have a range of possible values (e.g. serum urea nitrogen concentration), as opposed to being either present or absent (e.g. seropositive or seronegative for antibodies to a disease), the range of values in normal animals will have a characteristic spread. For the range of values of the variable in normal animals, an upper and a lower value are chosen that represent the upper and lower limits of the reference range. These values are usually chosen to include 95% of the values from normal animals, calculated as the mean value for the population of normal animals plus or minus 2 standard deviations, or as the 2.5–97.5 percentile range.

Sensitivity and specificity

The sensitivity of a test is a measure of the test’s ability to detect animals that are diseased and its numerical value represents the proportion of animals with the disease that are detected by the test (Table 1.3). A test with high sensitivity will detect most diseased animals within a population.

Table 1.3 Method for determining sensitivity, specificity, likelihood ratio for positive and negative tests, positive predictive value and negative predictive value of a test

Sensitivity = (TP/[TP + FN]) ×100

Specificity = (TN/[FP + TN]) × 100

Likelihood ratio positive test = Sensitivity/(1 – Specificity)

Likelihood ratio negative test = Specificity/(1 – Sensitivity)

Positive predictive value = TP/(TP + FP)

Negative predictive value = TN/(TN + FN)

The specificity of a test is a measure of the test’s ability to detect animals that are not diseased and its numerical value represents the proportion of normal animals detected by the test. A highly specific test will rule out the disease in most normal animals. Stated another way, a negative result for a test with high sensitivity effectively rules out the disease being tested for, whereas a positive test result for a test of high specificity effectively rules in the disease for which the animal is being tested.

Sensitivity and specificity are intrinsic properties of the test and their values are not influenced by the likelihood before the animal is tested that it has the disease for which it is being tested. The ability of a test to detect whether an animal has a particular disease depends on the likelihood that the animal has the disease at the time it is tested (the prevalence of disease in the population from which the animal being tested is drawn) as well as on the sensitivity and specificity of the test. The sensitivity and specificity can be combined to produce a single number, the likelihood ratio.

Positive and negative predictive value

The combined effects on the ability of the test to correctly detect diseased or healthy animals of (a) the prevalence of the disease and (b) the sensitivity and specificity of the test can be calculated and are called the positive predictive value (PPV) and negative predictive value (NPV) respectively. These are important values because they determine the usefulness of the test in detecting diseased, or normal, animals. The positive predictive value is the likelihood that a positive test is from an animal with the disease. The negative predictive value is the likelihood that a negative test is from an animal that does not have the disease.

Both the PPV and NPV are inextricably linked to the prevalence of the disease in the population being tested. Reports of the PPV and NPV are therefore only useful for populations of animals similar to those in which the values of these variables was determined, especially with regard to the prevalence of the disease in the population. The prevalence of the disease can also be viewed as the probability that an animal selected at random from the population has the disease – it is the pretest probability of disease in the animal. For a test of given sensitivity and specificity, the likelihood that a positive test correctly predicts the presence of disease (the PPV) increases as the proportion of diseased animals in the population increases (the disease has higher prevalence). Conversely, the NPV increases as the prevalence of the disease decreases.

The effect of changes in prevalence on the PPV and NPV of two tests with differing sensitivities and specificities is illustrated in Table 1.4. The probability that either test will detect the presence of disease in an animal with a high pretest likelihood of having the disease is very high. Similarly, the probability that a negative result is indicative of the absence of disease in an animal from a population with very low prevalence of disease is also very high. Importantly, the ability of a very good test (sensitivity and specificity both 95%) to correctly predict the presence of disease in an animal with a positive test from a population with a low prevalence (1% of animals affected) of the disease is very poor. Applied to an individual animal, this means that even a very good test is likely to yield an incorrect result in an animal that is unlikely to have the disease.

Table 1.4 Effect of changes in prevalence (pretest probability of disease) on the positive predictive value (PPV) and negative predictive value (NPV) of tests with 95% sensitivity and specificity (Test A) and 60% sensitivity and specificity (Test B)

Conversely, although the test result is very unlikely to be incorrect, a positive result in an animal with a very high pretest probability of having the disease yields little further information. The test result does not increase the likelihood of the animal having the disease by very much. The diagnostic test has its greatest utility when the pretest probability of disease is approximately 50% and the increase in PPV and NPV is much greater for a test with higher sensitivity and specificity.

The pre-test probability of disease, and thus the positive predictive value of the test, can be increased by selecting animals to be tested through careful physical examination and collection of an appropriate history. The PPV of a test in an animal that has signs of the disease being tested for is much higher than the PPV of a test in an animal without signs of the disease. Testing clinically normal animals is more likely to yield false-positive than true-positive results and such indiscriminate testing is not wise.

Step 1: Defining the abnormality

It is essential first to define the abnormality in either clinical or subclinical terms. This definition must be accurate, as this step of the examination determines the focus of the examination and the types of cases that will be included in the examination and analytical procedures. A case is defined as an animal or a group of animals that have the characteristics of the disease or a defined deviation from targets of production. With some investigations the problem will have obvious clinical manifestations and the primary definition of cases will be made by clinical examination of affected individuals. With others the primary complaint may be lowered production in the absence of clinical disease. An apparent problem in production efficiency can be focused by the examination of records. In many herds this will prove to be an immediate major limitation to the investigation because of a lack of sufficient records on reproduction, production and associated management to define the complaint. In these circumstances the criteria of the production inefficiency that will be considered in the examination will need to be determined and some form of measurement established.

Step 2: Defining the pattern of occurrence and risk factors

This step of the examination is often conducted in conjunction with step 1 above. It has the purpose of defining the characteristics of the animals that are affected in the disease problem and that have been established as cases, and of determining differences between them, as individuals or as a group, and the nonaffected animals within the herd. These differences may be attributes of the animals themselves or of environmental influences that affect them.

The initial examination is usually directed towards the determination of the characteristics of the animals involved and the temporal (when) and spatial (where) patterns of the disease. In general, the information that allows these examinations is collected at the same time and consists of such factors as:

In order to compute risk group analysis the number of animals present in both sick and well groupings must be recorded, as must be any similarities and differences in their management and environment. After the identity of the abnormality has been established, all the available clinical, production and laboratory data are examined according to the affected subgroups in the herd and according to time occurrence, management differences, nutritional and environmental influences and factors such as vaccination history.

In most herd examinations the analysis of these data is restricted to a cross-sectional study. Prevalence rates within the various groups are calculated and the population at risk can be determined. Animals or groups can be examined as those with and without disease and those with and without hypothesized risk factors, using a 2 × 2 contingency table generated for each variable. Relative risk, odds ratios or rate ratios can be calculated as a measure of association of the variable with chi-square and Mantel–Haenszel procedures used for evaluation of the significance of the risk. This attempts to determine if any associations exist between certain groups of animals and those factors that can influence the behavior of disease.

In some herds, where there has been extensive historical recording, it may be possible to examine the nature of the problem on the basis of a case-control study. However, in most herds this will not be possible because of the paucity of recording of factors of importance to the definition of the disease problem. With problems that are of obvious continuing importance to the economic viability of the herd it may be necessary to establish recording systems that allow a prospective examination of the problem.

Step 3: Defining the etiological group

Following characterization of the abnormality according to groups within the herd, and having made comparisons of the prevalence rates between groups, it may be possible to discern to which etiological category the abnormality most logically belongs. In many instances considerable difficulty may be encountered in deciding in which of the general areas of etiology the major determinant is located. In so many cases herd problems are not the result of a single error but are multifactorial, with several determinants contributing to a greater or lesser degree, and the problem may fall in several categories.

An example might be a problem of mortality in calves where examinations have determined that population mortality rates are highest in the winter period, that most mortality occurs between 4 days and 1 month of age, that calves that die early in this period have septicemia, or have scours associated with rotavirus and cryptosporidial infections, that the body condition scores of the calves fall during the third and fourth weeks of life and that calves that die later in the time period appear to die of starvation. Probable causes include improper feeding of colostrum, a poor environment leading to a high infection pressure and possibly also to excess cold exposure, malnutrition resulting either from the residual effects of enteric disease on intestinal absorption of nutrients or from an inadequate caloric intake or both. This complex could be placed in the categories of infectious disease, nutritional disease and also in the category of management error; further definition is the next step.

The use of path models that summarize current knowledge of the causality of the disease under consideration can help in this aspect of the herd examination. Path models specific to the problem at hand can be constructed and can show the interrelationships between various risk factors and give some indication of the dependence of any one factor on the occurrence of another. This information can be used to estimate the relative contributions of the various etiological categories and to give guidance as to the area where intervention is most likely to be effective.

Step 4: Defining the specific etiology

The final step is to select the probable most important determinant or combination of determinants from within one or more of the general areas and to make corrective interventions based on this diagnosis. In many instances the primary cause may be clear and the correction, be it alterations in nutrition, alterations in management, vaccination, etc., can be made at this stage. In other cases further prospective examinations may be conducted for a better definition before an intervention is attempted. In the example above, failure of passive transfer of colostral immunoglobulins and inadequate caloric intake would have been suspect or even identified as underlying determinants of the problem. However, with most farm recording systems there is likely to be no available data that would help delineate the specific reasons, and the specific management deficiencies that require correction, and so a prospective study to establish these would need to be established.

It can be very difficult to obtain a clearly defined diagnosis of cause of disease in a herd, because of its complexity, but the known important relationships are given for the individual diseases in the special medicine section. Methods for practical clinical quantitative assessment of the level of management expertise or, more importantly, the intensity with which it is applied, are not available. Consequently this must be assessed qualitatively for most management practices. Surrogates such as the percentage of cows presented for pregnancy diagnosis but not pregnant, bulk tank somatic count, rates of failure of passive transfer of colostral immunoglobulins, etc., can give some indication.

Numerical assessment of performance

Productivity indexes can be used as indicators of health; they can also be used to measure response to treatment or control measures. More and more they are being used as guides to husbandry and management questions to meet the present-day farmer’s concerns with costs and returns. If recording systems are present on the farm they can be invaluable data sources in the investigation of herd problems with disease. Monitors of production efficiency are used extensively in performance or production management veterinary practice and are detailed in texts on that subject in the reference literature section.

Intervention strategies and response trials

As the result of a herd examination, a clinician formulates a hypothesis concerning the disease. This may include hypotheses on the population of animals at risk, the determinants of the disease, the source of the problem and its methods of transmission or propagation. There may be sufficient confidence in these hypotheses that they may result in intervention strategies to correct the problem without further analysis. In other outbreaks the hypotheses may be less secure and may require further examination of response trials.

Response trials are often used in an approach to herd disease problems and problems of production inefficiency. They have several purposes: they may be used to establish or confirm a diagnosis, and when used for this purpose it is usually because of the difficulty in confirming the diagnosis by other methods. This may result from the lack of a suitable laboratory test or because the result of the test is supportive for the diagnosis but not confirmatory. Response trials can also be used to determine the degree of intervention that is required and the efficacy of the level of intervention that has been used.

There are many limitations to conducting response trials in clinical situations in private herds and their structure may not always meet the strict requirements of those conducted in research. It is not always possible to establish a controlled response trial in clinical practice but the efficacy of intervention strategies should still be monitored. The ultimate interest is in whether the disease or production problem is corrected; however, the efficacy of the individual strategies should be specifically monitored where possible. In the earlier example of calf mortality a decision might have been made to change the method of feeding colostrum and to improve the caloric intake of the calves. There can be various ways that either of these changes could be achieved. The overall efficacy of these changes will be determined by improved survival of the calves. However, the efficacy of the colostrum management change in improving passive transfer should be determined specifically by measurements of serum immunoglobulin concentrations in the serum of a proportion of calves and the efficacy of caloric improvement by weight measurements. Should calf mortality drop, these latter measures are of limited value but if it does not, then there are measures of whether the failure was due to misdiagnosis of the problem or due to poor efficacy of the suggested corrective strategies in correcting their respective target areas.

A diagnosis made on the basis of a response trial is often presumptive and it has become customary to couch the diagnosis in terms of response to a treatment, for instance, ‘selenium-responsive infertility’ in sheep. This is not a diagnosis in terms of satisfying the original concepts of Koch’s postulates, although it does satisfy the subsequent modifications of these postulates that are now generally accepted and have been based on a broader interpretation of disease causation. In populations of animals, diseases are largely the result of a number of interacting factors of different genres, including management, nutrition and environmental factors, interacting with traditional agent-causes of disease, including microbiological and toxic agents. The answer for the practical problem may be most economically derived by finding the cure rather than the cause. This is especially desirable if that course is cost-effective and finding the cause is more expensive than the wastage caused by the disease.

A simple example would be mortality in a group of cattle that followed a change of feed to a more concentrated ration. An epidemiological examination, including a temporal examination of cause or determinants, might closely link the mortality to the change in ration. This should be sufficient to indicate that the ration should be withdrawn or its method of feeding modified. The alternative approach would be to defer any decision for correction of the problem until the exact problem with the ration was established. This could involve a ration analysis and an examination for unknown toxic components. These examinations would take considerable time, would involve considerable costs, and could well give no additional information that would modify the immediate initial intervention strategy.

The role of the planned animal health and production program

Properly conducted herd health programs and planned animal health and production programs maintain accurate records on all matters of production and health. These are maintained against a background of epidemiological data, including number of animals in the herd, numbers of animals in the reproductive cycle segment group or age group that are therefore at risk. In many instances all the data required to effectively diagnose a disease or monitor its prevalence are already at hand in the records of these herds. It does put the veterinarian and the farmer in the position of almost being able to do a herd examination simply by consulting the records. This approach is detailed in texts on herd health and production medicine.