VET 120: Applied Lab Animal Nutrition

Unit 2: Introduction to Animal Biology and Medicine

II Applied Nutrition and Reproduction

Reading Recommendations: Follow the in-text links

Applied Nutrition

Animal care must be carefully planned before animals arrive at a research facility so that the animals will be maintained in the best of health. A written protocol (the Standard Operating Procedures) must be developed that includes feeding and other husbandry activities.

To develop a protocol that provides the proper kinds and amounts of nutrients, it is essential to understand nutrition--the study of nutrients and how they are used in the body. Without this understanding, errors may occur in feeding that can adversely affect animal health and experimental results.

Nutrient Requirements
Substances used for metabolism and growth of an animal, such as water, vitamin C and protein, are called nutrients. Essential nutrients can only be obtained from the diet. Vitamin C is an essential nutrient for primates (including humans) and guinea pigs, because they cannot manufacture vitamin C and must obtain it from the diet. Other animals, including rats, rabbits and dogs, produce their own vitamin C so it is not an essential nutrient for them.

Nutrients are used by all animals for growth, maintenance, repair and reproduction. Each species of animal has similar nutrient needs: all animals need water, protein, carbohydrates and fats. Each species has unique nutritional needs that must be known and addressed in the selection of a diet.

An individual animal's nutrient requirements are affected by many factors. Young animals generally need increased amounts of many nutrients. Reproduction places many demands on female animals, and nutrient requirements are very high in gestating and lactating animals. Environmental temperature and humidity may also affect food intake and nutrient needs.

Nutrients

Water is the most important and the most common nutrient: up to 80% of an animal's weight is due to water.

Water must be supplied to animals in sufficient quantity and in a way that an animal can use. A dog that has previously been given water in a bowl, for example, may need to be taught how to obtain water from a bottle.

When open containers, like bowls, are used for watering animals they must be placed to minimize urine and fecal contamination. Fresh water should be supplied at least once per day and the watering receptacles should be washed at least weekly. Most laboratory animals are given water in bottles or with an automatic watering system.

Water is the solvent for all of the biochemical processes involved in metabolism and it is the primary transport medium of the body, forming the basis of blood and urine. Through circulation, these water-based fluids move nutrients and waste materials throughout and out of the body. In addition, water can play an important role in temperature regulation, with excess heat lost in sweat or by panting.

Proteins are complex organic molecules composed of carbon, hydrogen, oxygen and nitrogen. The building blocks of proteins are amino acids. Proteins have many functions in the animal's body, including the structure of cells, tissues and organs, formation of bioactive molecules, such as enzymes, hormones and antibodies, it is an energy source, and composes skin, hair, fur and nails.

Fats (lipids) yield the greatest amount of energy per gram when metabolized. They are also important components of cell membranes and help insulate and protect the body. Lipids provide storage for the fat-soluble vitamins. Sources of fats in the diet include plant oils, dairy products and animal fat.

There are several different types of carbohydrates, but they all contain carbon, hydrogen and oxygen. Carbohydrates are the primary energy source for an animal, and simple sugars, such as glucose, are ready for direct use by the body. Starches are complex carbohydrates composed simple sugar subunits; they can be digested by enzymes in the small intestine. Fibers, such as cellulose and lignin, are impervious to enzymatic action and are digested by microbial fermentation in rodents, rabbits, horses and ruminants.

Vitamins are organic molecules that are required in small amounts. Their role is to assist in the regulation of metabolism and cellular function. The fat-soluble vitamins (A, D, E and K) are stored in lipids in the body and can be ingested in excess, resulting in hypervitaminosis and disease. The water-soluble vitamins (C and the Bs) are not stored in the body and must be replaced.

Minerals are also required in small amounts. They are inorganic molecules needed for structure (bones and teeth), enzymes, muscle contraction and nerve impulse transmission. There are many different essential minerals, including calcium, copper, fluorine, potassium, iron, iodine and selenium.

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Digestive Physiology

Prehension is the grasping of food in the mouth by the teeth and tongue. Food is then masticated (chewed) to break into smaller pieces and lubricate it prior to swallowing.

The esophagus connects the mouth to the monogastric stomach or rumen complex. Most animals, including horses, dogs and cats, rodents and rabbits have a simple monogastric stomach consisting of a single chamber. It is the site where food is mixed and chemical breakdown begins. Digestion of carbohydrates is limited to simple sugars and starches.

The ruminant "stomach" is actually a complex of four organs: the rumen, reticulum, omasum and abomasum (which is equivalent to the monogastric stomach). Symbiotic bacteria and protozoa in the rumen complex ferment cellulose, producing carbohydrates and vitamins for the ruminant to use.

The small intestine consists of the duodenum, jejunum and ileum. The duodenum is the primary site for enzymatic digestion, using enzymes produced by the pancreas. Most nutrient absorption, and all sugar and amino acid absorption, occurs in the jejunum.

The cecum, colon, rectum and anus make up the large intestine. The cecum is the primary site for "hindgut" fermentation, the method by which horses, rabbits and rodents digest cellulose. The process is similar to rumen (or "foregut") fermentation, utilizing micro-organisms to break enzymatically resistant fibers into usable nutrients. The colon absorbs water and other substances, such as sodium and chloride.

Foregut fermentation (rumination) is more efficient than hindgut fermentation, because most nutrients are absorbed in the small intestine. Nutrients made available by digestion in the rumen are readily absorbed on their normal transit through the gastrointestinal tract. Hindgut fermentation, however, occurs after the small intestine, so less nutrients are available. Rodents and rabbits use "cecotrophy" to acquire these nutrients that might otherwise be lost--they eat nutrient-filled soft feces called cecotrophs.

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Feeding Research Animals

Standards for feeding animals in research are part of the Animal Welfare Act and are published in the Guide for the Care and Use of Laboratory Animals.

There are a number of requirements that must be assessed when determining what to feed a laboratory animal (or any other animal), such as:

Is it a herbivore, carnivore or omnivore?
Is it a monogastric, a foregut fermenter (ruminant) or a hindgut fermenter?
Does it require certain nutrients in its diet, such as vitamin C?
What is the animal's function (growth, reproduction, etc.)?
What are the requirements of the experiment?

You must always buy the best quality, most standardized diet, make sure that it is clean, palatable and fresh and that it is sufficient for the animal's needs. This will minimize nonexperimental variables and ensure the well-being of the animals in your care.

The food selected must be a balanced diet, providing all required nutrients. Feeds come in a wide variety of forms. Most feeds are processed so that the animal cannot select out a preferred food. This food selection can result in an unbalanced diet and may lead to osteoporosis in rodents.

Most rabbit and rodent food is pelleted--large hard pieces must be gnawed, helping to wear down their open-rooted teeth. Rodents often hoard food, hiding it under bedding, making it difficult to assess how much food is being eaten.

Herbivores need sufficient roughage (fiber) for proper digestive function.

Diets can be classified by purity. A nonpurified diet is the standard diet used for pets and animals not on nutrient-critical experiments. The ingredients of the diet vary somewhat, based on price and availability of raw materials during manufacture. Purified diets are formulated with specific ingredients- you know that the feed always contains (for example) soybean meal. The exact composition of chemically-defined feeds is known- you know that the feed always contains (for example) 14% soybean meal.

Most feeds contain supplemental vitamins and minerals. Some diets have antibiotics or other drugs added during their manufacture.

Food should be stored in a room dedicated for that use. It should be cool (4° C) and well-ventilated. Feed should be located off the floor, on pallets or shelves. Check the milling dates on the bags and use the oldest food first, rotating stock as feed is delivered. Most food should be used within six months of milling (not delivery). If the food contains supplemental vitamin C, which is unstable, it should be used within three months of milling.

Animal diets should be changed gradually to avoid gastrointestinal disturbances. Begin by adding a few pieces of the new diet to the old, and slowly increase the amount of new food and decrease the amount of old diet. Anorexia (loss of appetite) or diarrhea may occur in response to a sudden change in diet. Some animals are neophobic and refuse to eat food that they have never seen before.

It is important to be able to recognize signs of nutrient deficiency. Animals that are dehydrated may have:

Dull dry hair coat
Decreased skin pliability (the skin "tents" and the tent persists when lifted)
Dry mucous membranes
Anorexia
Decreased urine and feces output

Animals that are starving can be recognized by weight loss and a prominent skeleton. Nutrient deficiencies can produce a wide variety of clinical signs, from deformed bones to bleeding gums.

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Genetics and Applied Reproduction

Introduction

Genetics is the study of inheritance (heredity), how certain characteristics are passed from parent to offspring. It is essential to understand the principles of genetics and how these principles affect the development and maintenance of different strains of animals.

Kittens nursing

Genes are the functional units of heredity. They are composed of deoxyribonucleic acid (DNA) and are located on chromosomes within the nuclei of cells. The order of DNA components codes for genetic information.

Chromosomes consist of long rows of genes and occur in pairs in body cells. This paired arrangement is called diploid (2N). Paired chromosomes separate randomly during reproduction, resulting in haploid (1N) sperm and ova (gametes). These gametes then combine to form a diploid zygote at fertilization, with one set of chromosomes (1N) inherited from each parent.

Forms of a gene that occur at the same site and code for the same trait on matching chromosomes are called alleles. The alleles may be different, resulting in different expressions of that trait. A dominant allele is expressed, masking other genes that may be present. The recessive allele is masked unless both chromosomes contain the same gene. Dominant genes are indicated by upper case letters (ex. A for agouti) and recessive genes are identified by lower case letters (ex. a for nonagouti). If only one type of allele is present, the animal is homozygous (ex. AA) for that trait. A heterozygous animal has more than one type of each allele (ex. aa).

The genotype of an animal is its genetic make up and includes the dominant and recessive genes. The phenotype is the physical appearance--how the genotype is expressed. The phenotype of this gerbil, for example, is agouti. Because agouti is a dominant trait, its genotype may be AA or Aa (it may be either homozygous or heterozygous agouti).

This black mouse, however, has the genotype aa, because any color other than agouti is recessive. Additional alleles determine the actual color of the mouse. B, for example, is the gene for black fur and b is the gene for brown fur. This mouse, therefore, would either have the genotype aaBB or the genotype aaBb.

The gene for albino is c, which is recessive. If an animal has homozygous (cc), no color will be present. If, however, the mouse has a genotype of aaCC or aaCc, the color will be determined by additional alleles, such as a for agouti.

Albino mouse

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3. Genetic Modifications

Modification of an animal genetic make-up (genome) can be used to develop better animal models.

Selective breeding has been used for thousands of year to create larger or smaller animals, animals with more fat or less fat, guard dogs or guide dogs, etc.

Genetic engineering (physically adding or subtracting genes) is frequently used to improve the usefulness of animals in the lab. Transgenic animals have had genes added to the genome. Knock out animals have had genes removed.

4. Reproduction Revisited

Young animals are incapable of reproduction until they reach puberty, the time of sexual maturity. Once puberty is attained, the female animal develops an estrous cycle, a pattern of physiologic and behavior changes that makes the female receptive to and capable of breeding. The estrous cycle consists of a number of stages. The stage of sexual receptivity is called estrus and the female is considered "in heat" or "in season." The female is influenced by the hormone estrogen and will mate with a male.

The duration of the estrous cycle varies between species. The length of the estrous cycle of the rat, for example, is 5 - 6 days long, while that of the horse and cow is 21 days and the human estrous cycle is approximately 28 days long.

Some animals, such as red foxes, skunks, bears and some dogs, are monestrous: they have one estrous cycle per year. Polyestrous animals have several estrous cycles per year.
Humans, rats and cattle are non-seasonal breeders; they cycle throughout the entire year. Seasonally polyestrous animals, such as horses, sheep and hamsters, respond to photoperiod (length of daylight), cycling so that their offspring are born when the weather is warmer.

Haploid (1N) gametes are produced in sexually mature animals. Eggs released from the ovary at ovulation are fertilized in the oviduct. The resulting zygote is implanted in the uterus or uterine horns and a placenta forms to nourish and protect the developing embryo.

Most animals ovulate spontaneously , releasing ova after natural cyclic hormone secretion during estrus. Some animals, however, are induced ovulators. Ovulation in cats, ferrets and rabbits must be induced by copulation, which causes a build up of hormones.

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5. Breeding Management

It is important to know and understand the species of animal with which you are working. By using the animal's natural physiology and behavior, you can optimize the breeding program at your facility.

There are two basic types of mating systems:
      Monogamous, where one male and one female are put together
      Polygamous, where one male is mated with more than one female

There are several advantages to using a monogamous mating system. Some animals, such as gerbils and hamsters, are monogamous in the wild. Gerbils form very strong pair bonds and may fight if you put a strange male and female together. Record-keeping is simplified because you will know the sire and dam of all the offspring. However, the monogamous system is less cost efficient, because one male must be kept for every female and more cages are required. More cages mean more cages to clean...and that means more work!

The polygamous mating system uses one male for several females. In the colony system, the male and females are housed together continuously throughout the reproductive cycle. In the harem system, the female is removed from the group cage prior to parturition, so she will have her babies in a separate cage. Colony and harem mating systems are commonly used with rats and mice. These systems are the most economical and efficient. The harem system requires additional cages when the female is separated, but it is easy to track the genetics and birth dates of the offspring. Female rats will share nursing duties, so babies from two or three litters may be nursing on the same dam; with the harem system, there will be no baby-sharing. The colony system is the most efficient, but record-keeping is more difficult, because you may not be sure of the dam of a particular rat or mouse pup or when it was born.

Pheromones may interfere with breeding management, especially in mice. The estrous cycles of females housed together will coincide with each other--this is called the Whitten effect. It is useful when large numbers of offspring of the same age are desired. If, however, you want to produce one litter every two weeks (for example), it is important to house the females in separate cages. If no male is present in a group of females, the Lee-Boot effect may occur and all of the females may enter anestrus.

If a mouse bred within the previous twenty-four hours is exposed to a strange male, the existing pregnancy may be aborted and the female will mate with and have the offspring of the new male. This is the Bruce effect.

Breeding females must be monitored to ensure that no difficulties occur. Sometimes animals in all-female groups may undergo a pseudopregnancy (pseudocyesis), where a female may appear to be pregnant, may produce milk and have nesting behavior, but she is not actually pregnant. Any animal, especially guinea pigs, may have difficulty during the birthing process. This is called dystocia and may be life-threatening for both the mom and her offspring. Animals approaching parturition should be monitored closely to ensure that no problems occur.

Mammals tend their offspring, supplying nutrition via the female's milk. The level of care, however, varies widely with the species of animal. Rats, ferrets and hamsters have altricial offspring. These babies are born helpless. Their eyes and ears are closed, they have no fur and very poor thermoregulatory (heating) systems. They are totally dependent upon their mother for the first several weeks of life. Animals such as guinea pigs, fawns and calves are precocial: they are born fully furred and are able to move about within hours of their birth. It is important to know whether offspring are altricial or precocial so the appropriate husbandry can be given them.

Breeding Strategies:

If related animals are bred for several generations (such as brother to sister or parent to offspring), the resulting animals are called inbred. The animals have very similar genotypes (and usually phenotypes).

Outbred animals are created when unrelated parents are bred. These offspring have much more genetic diversity and are similar to a natural population.

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