Weight Loss

The clinical problem of weight loss suggests that an individual large animal patient or a herd has lost weight over a known period of time. It may also suggest that the patient has reached a sub­normal adult weight and size (see section on decreased growth and decreased weight gain, earlier).

Weight loss in adult animals is most commonly associated with one or more of the following circumstances (other causes are listed in Boxes 9.6 and 9.7):

• Anorexia

• Increased nutrient demands

• PCM

• Micronutrient deficiencies

• Parasitism

Anorexia usually occurs secondary to a primary disease. A decrease in DMI occurs in dairy cows when temperature and humidity (Temperature Humidity Index, THI) exceed the coping mechanisms of the animals.¹² Increased nutrient require­ments are associated with normal physiologic conditions (e.g., pregnancy, lactation, exercise, cold weather) and pathologic processes (e.g., sepsis, trauma, parasitism, burns). Mild to severe PCM is often associated with inadequate feed quality or quantity but can also be caused by increased energy requirements resulting from adverse environmental conditions. Weight loss can also occur with a deficiency of essential micronutrients such as copper, cobalt (vitamin B₁₂), or vitamin A. Parasitism should always be on the differential list in an animal that has lost weight.

Mechanisms of Weight Loss

Anorexia is the loss of appetite or lack of desire for food; it may be complete or partial.

In domestic species, anorexia is usually associated with a primary disease condition and is regulated by cytokines including interleukin (IL-1) and tumor necrosis factor alpha (TNF-α), released during an inflammatory response.¹³ Resolution of the primary disease process usually results in a return to voluntary food consumption. Anorexia must be differentiated from dysphagia by observation. The distinction between the condi­tions that cause anorexia and those that control hunger and satiety is not clear; however, many diseases that cause anorexia also result in dehydration, electrolyte imbalances, and/or acid-base disorders.

In addition to causing anorexia, many disease processes cause an increase in the nutrient requirements for basal metabolism. Nutrient requirements for maintenance, growth, pregnancy, lactation, and exercise have been well defined for many large animal species. Nutrient requirements in disease have not been adequately evaluated in large animals, and most information is currently extrapolated from humans, laboratory animals, and small animal species. In human patients, published estimates indicate that requirements for energy and protein increase approximately 10% after elective surgery, 20% with fractures, 30% to 60% with severe infection or sepsis, 40% with peritonitis, and 50% to 110% with major burns.^14,15 In humans, the resting energy expenditure is estimated to increase by 14% for each degree Celsius increase in body temperature.¹⁶ Extrapolation of these data directly to equine and ruminant patients is probably not possible; however, the figures do indicate the degree of change in nutrient requirements as a result of disease.

In conditions such as burns, peritonitis, pleuritis, colitis, and granulomatous bowel disease, nutrients (particularly proteins) are lost. In many disease conditions, concurrent anorexia and increased nutrient requirements greatly increase the risk of PCM and weight loss. Certain conditions such as Johne's disease in ruminants and granulomatous enteritis in horses are also associated with a malabsorption or malassimila- tion syndrome. In these types of diseases, nutrients are not efficiently digested and absorbed; anorexia may be absent, and dietary intake may appear normal, but weight loss still occurs.

PCM continues to be a persistent problem in domestic animals. Inadequate ingestion of energy and protein obviously

FIG. 9.2 Body condition scoring system for dairy cows. (From Edmonson AJ, Lean IJ, Weaver LD, et al: A body condition scoring chart for Holstein dairy cows. J Dairy Science. 72(1)68-78, 1989.)

■ BOX 9.6

Causes of Weight Loss in Horses—cont'd

Pentachlorophenol

Phenylbutazone, flunιxιn meglumine, and other nonsteroidal antiinflammatory drugs

Selenium toxicity

Vitamin D calcinosis

Vitamin K₃ toxicity

Zinc toxicity

Plant Toxins

Crofton weed (Ageratina adenophora)

Plant calcinosis (Cestrum diurnum, Trisetum flavescens, others)

Pyrrolizidine alkaloid toxicosis (Senecio spp., Amsinckia spp., Crotolaria spp., others)

Red maple leaf (Acer rubrum)

Jimsonweed (Datura spp.)

White snakeroot (Eupatorium rugosum)

Yellow star thistle (Centaurea solstitialis), Russian knapweed (Centaurea repens)

Exotic Diseases

Besnoitiosis (Besnoitia bennetti)

Burkholderia pseudomallei Dourine (Trypanosoma equiperdum)

Equine dysautonomia (grass sickness) Glanders (Burkholderia mallei)

Mal de caderas (Trypanosoma equinum) Nagana (Trypanosoma spp.)

Stachybotryotoxicosis

Surra (Trypanosoma evansi) Trombiculiasis

Trypanosoma hippicum

Continued

B, Bovine; O, ovine.

results in weight loss, but PCM and associated weight loss can occur through several other mechanisms. The most direct cause is that the animals receive an inadequate volume of feed to meet their dietary requirements. This can occur as frank underfeeding of all animals or as a consequence of inadequate feeding facilities that create competition among animals for available feed. The latter circumstance occurs most dramatically when animals of varying ages are mixed; the younger animals with the highest requirements are often pushed away by older, dominant individuals.

The quality of the diet, particularly dietary forages such as hay and pasture, is an important factor in the development of PCM and total nutrient intake. Table 9.1 lists guidelines for estimating the maximum daily intake by cattle. It is evident that as forage quality (digestibility) decreases, maximum daily intake decreases because poor-quality feed must remain in the rumen for an increased period of time before it is sufficiently digested to allow passage through the reticuloomasal orifice. Maximum dry matter intake (DMI) as a percentage of body weight is somewhat higher in small ruminants than in cattle. However, the energy requirement per kilogram of body weight is higher in small ruminants than in cattle. Similar estimates for maximum DMI in horses related to forage quality are not available, yet horses will consume on average 2% of their body weight as DMI, with some studies reporting intakes as high as 4% of body weight. Horses do not have a pregastric fermenta­tion organ (rumen) and can ingest slightly more of the same quality forage than cattle. Low-quality forages are often the

cause of PCM, even when an unlimited quantity is available. The best way to determine the nutrient content of forage, grain, and pelleted feeds is to have the feed analyzed by a forage laboratory. Feed analysis instructions are provided in Box 9.3. Feed tag labels or forage databases can be referenced if an analysis is not performed.

Environmental factors can have a major influence on nutrient requirements and can increase the subsequent risk for PCM and weight loss. The most important environmental factor is the ambient temperature. Nutrient requirements for mainte­nance change with a decreasing ambient temperature as follows:

• Adult horses: Estimated increase in digestible energy (DE) requirements by 2.5% for every degree Celsius below the lower critical temperature (LCT). The LCT for adult horses ranges from 5° C to -15° C, depending on the horse's adaptation to the environment. In cold temperatures, when the hair coat is wet, the maintenance DE requirement may be increased by as much as 50%.⁷

• Beef cattle: 1% increase in maintenance energy requirements (total digestible nutrients [TDN], net energy of maintenance [NE_m], digestible energy [DE], and metabolizable energy [ME]) for each 1° C drop below 20° C (68° F).

• Dairy cattle (lactating): 25% increase in energy requirements (TDN, net energy of lactation [NE_l]) as ambient temperature drops from 20° C (68° F) to -10° C (14° F).

• Sheep with 10-cm wool: 1% increase in energy requirements (TDN, ME, DE) for each 1° C drop below lower critical temperature (approximately -10° C).

There are also additive effects of wind and rain that increase energy requirements in large animal species. As nutrient requirements increase, the dietary intake must also increase to prevent weight loss associated with PCM. Horses in inclem­ent weather may not be able to consume enough forage to meet their increased energy requirements, and for these animals, dietary fat and limited grain supplementation may be required.

Deficiencies of micronutrients (trace minerals, B vitamins) often result in inefficiencies in basic biochemical pathways. These inefficiencies, if marked, can be associated with weight loss. Genetic errors in metabolism can cause similar distur­bances, but these usually manifest as decreased growth and even death in young animals.

Parasitism is a common cause of weight loss in adult domestic animals. The mechanisms by which parasite infestation can result in weight loss include a loss of body fluid and tissues resulting in increased nutrient requirements, competition for nutrients in the gastrointestinal tract, malassimilation and malabsorption, inflammation resulting in increased nutrient requirements, micronutrient deficiencies, and organ or vascular damage from migrating parasite larvae. Anorexia may also develop in the advanced stages of severe parasitism.