Mechanisms of Decreased Growth and Decreased Weight Gain

Major pathogenic mechanisms that result in decreased growth and decreased weight gain include the following:

Protein-calorie malnutrition (PCM) is the most common clinical cause of decreased growth and decreased weight gain in young animals. It is characterized by smaller size and lower weight than the normal minimums for age, breed, and sex. Inadequate intake of digestible energy and protein (or essential fatty acids in the neonate primarily adapted to a milk diet) results in inadequate levels of amino acids, fats, and carbohydrates for normal metabolism and growth. Diets that lack any of the other essential nutrients (essential fatty acids, vitamins, macrominerals, or trace minerals) can also decrease growth. Deficiencies of calcium, phosphorus, and magnesium result in improper skeletal formation. Deficiencies in other macrominerals (e.g., sodium, chloride, potassium), trace minerals (e.g., copper, zinc, manganese, cobalt, iron), and vitamins (e.g., A, D, E, thiamin) cause biochemical dysfunctions that lead to inefficient metabolism and growth. Large animal patients that grow slowly as a result of inadequate diets often have normal or increased appetites until they are terminally ill. Physical findings and clinicopathologic data from animals with PCM are often within the normal range until the disease process is well advanced.

Infections or inflammatory processes are important causes of decreased growth and decreased weight gain in young horses and ruminants. The decrease in growth can be of short duration followed by recovery and compensatory gain (cryptosporidiosis) or can persist (chronic bronchopneumonia). Infections or inflammatory processes can also result in nutrient malabsorption (chronic salmonellosis, acute rotavirus diarrhea, equine proliferative enteropathy), anorexia (pharyngeal abscesses, systemic disease), increased nitrogen turnover, and direct protein losses (gastrointestinal disease). Both energy and protein requirements may be increased as a result of infection and inflammation.

Parasitism often affects young horses and ruminants and results in decreased growth and decreased weight gain by increasing nutrient requirements, increasing nutrient losses, and/or decreasing nutrient absorption. The animal’s metabolic rate and nutrient requirements may also increase as a result of inflammatory and immune reactions that arise secondary to parasitism.

Genetic diseases (α-mannosidosis, dwarfism) result in decreased growth through generalized errors in the genetic code or interference with strategic reactions in one or more metabolic pathways. Congenital cardiac malformations (tetralogy of Fallot, interventricular septal defect) create physiologic inefficiencies that require energy beyond the body’s ability to supply it. Congenital renal disease (agenesis, dysplasia, hypoplasia, polycystic kidney disease) affects homeostatic mechanisms that regulate electrolyte and acid-base balance, results in the production of uremic toxins, and often results in partial anorexia and PCM. Digestive tract malformations including cleft palate, megaesophagus, and brachygnathism can reduce nutrient ingestion and impair growth.

Toxicities, although rare in growing animals, result in decreased weight gain by interfering with metabolic pathways (e.g., ammonia toxicity, zinc-induced copper deficiency with abnormal skeletal development in foals), by causing loss of body reserves (e.g., thiamin deficiency in horses, bone marrow hypoplasia and associated bleeding diatheses in ruminants associated with bracken fern toxicity), by inducing anorexia, or by a combination of mechanisms. The pathogenic mechanisms of many toxins are not yet known.

Environmental factors including extreme heat or cold or high humidity result in decreased growth and decreased weight gain. Extremely cold conditions increase an animal’s daily energy requirements. During extremely hot weather feed intake often decreases, which may contribute to decreased growth. Often, environmental conditions influence the development of disease, resulting in a subsequent increase in nutrient requirements in a growing animal (e.g., calves with PCM housed in poorly ventilated or overly humid conditions become much more susceptible to infectious pneumonia).

In many cases a combination of these diverse factors may influence the growth and weight gain of young animals. A period of increased growth rate and weight gain, called compensatory gain, often occurs after a period of restricted growth once adequate dietary energy and protein are available. In growing foals, compensatory gain should be closely monitored to prevent rapid growth and abnormal skeletal development, which could lead to developmental orthopedic disease.