Malnutrition and Starvation
The terms malnutrition and starvation are used interchangeably, when in reality, there are specific definitions for each. Malnutrition is the inadequate intake of any of the required nutrients. This can even occur in an animal receiving large amounts of food, but is not able to ingest, digest, absorb, or utilize this food. Causes for this inability are injuries, poor teeth, parasitism, disease, foreign bodies in the digestive tract, tumors, or an increased motility of the digestive tract. Malnutrition can also occur if the food is inadequate in one or more of the required nutrients. If an animal is not able to obtain food for an extended period of time either for the above reasons or due to an unavailability of food or insufficient energy intake, this is defined as starvation. Malnutrition and starvation can be caused by diseases, injuries, the range the animal lives on, or the environmental conditions it must live in. Starvation and malnutrition occur in several wildlife species and routinely eliminates the young, old, weak, and sick animals. Winter is when mortality usually occurs due to the negative energy balance brought about by the cold weather, deep snow, increased energy demands, snow covered food, and human and predator induced stress.
Historically, in Michigan the number of species diagnosed at the Laboratory as dying from malnutrition and starvation are second only to those dying from traumatic injuries. Numerous bird and mammal species annually (depending on the severity of the winter) die from insufficient nutrition. Currently we have 3 primary species that die from malnutrition or starvation: white-tailed deer, mute swan, and wild turkey. The majority of the animals have come from the upper peninsula and the northern half of the lower peninsula with mortality occurring almost exclusively during the winter when food availability is at its lowest.
Susceptibility to starvation and malnutrition usually occurs in the winter and early spring months for wildlife in Michigan. Animals cope with the severe weather and shortage of food in 1 of 3 ways: hibernate (amphibians, reptiles, and several mammals), migrate (most avian species), or remain active and attempt to survive. Juvenile, yearling, and old animals are the age groups most susceptible to starvation and malnutrition because they enter the winter with the smallest fat reserves, the highest nutritional demands, the greatest heat loss, and the lowest position in the social hierarchy. Of the winter starvation deaths observed, 60 to 70% may consist of animals less than 1 year of age.
Adult males and females and juveniles of both sexes of various species may have smaller reserves of fat due to breeding activities, rearing of the previous year's offspring, and their growth requirements, respectively. Most wild animals in colder climates undergo an annual fat cycle whereby fat is deposited and then utilized as the physical condition declines. During a severe winter, adult deer may lose as much as 25 to 30% of their body weight and still survive. The loss of weight occurs during the winter because of snow depth, ambient temperatures, and the quality and quantity of the available forage. Because of these factors, the physical condition of animals at the onset of winter is critical to their survival. The adequacy of summer and fall ranges is thereby very important. The duration and severity of the winter is critical to the animal's chances for survival, as it determines the length of time the animal must depend on its body fat reserves and on poorer forage for survival. Deep snow and cold temperatures, especially during the latter part of the winter can result in very high numbers of deaths attributed to malnutrition and starvation. The cover available to the animal is also of utmost importance as this allows the animal to escape from the low temperatures and the wind. Offspring of weakened animals that survive the winter are also susceptible to the effects of starvation as they may be absorbed or aborted as fetuses, or if born, may be improperly cared for.
Avian species are highly mobile and usually migrate, thereby lessening the chances of mass starvation. Extremes of weather (sudden snow or ice storms) may result in birds becoming trapped in inhospitable areas and not having food available. Some species experience mortality of the females due to nesting activities in the early spring when they are unable to leave the nest to feed. Deaths attributed to malnutrition and starvation are seen in young birds during the hatching season due to parental neglect, or once they are fledged from the nest, the inability to acquire their own food.
If an animal is forced into an inadequate plane of nutrition, there are many physiological changes as the animal attempts to satisfy its energy requirements. At the cellular level, catabolism (the breaking down in the body of complex chemical compounds into simpler ones) continues to supply the substances required for anabolism (the usage of nutritive matter and its conversion into living substance) and to continue vital functions. Reserve stores of nutrients contained in the individual are utilized to compensate for the lack of nutritional intake. Energy is generated from the utilization of carbohydrates, fats, and proteins. The most readily usable material, the carbohydrate glycogen, is utilized first. This is derived from glycogen stored in the liver and is exhausted within a few hours. This is followed by stored fat from the various subcutaneous deposits, around the kidney, and in the mesentery and omentum tissue. Fat deposits in the parenchymatous organs are utilized next. The last area of the body to lose its fat deposits is the marrow of the bones. The final source of energy available is the protein comprising the cytoplasm of the cells. It is at this time that ketosis and an increase in nitrogen excretion may occur. Ketosis (a condition in which ketone substances appear in the blood and urine) is commonly seen in malnourished animals. This is because it is necessary for the animal to derive its energy from the stored fat and protein. After all the fat reserves have been exhausted, nitrogen excretion rises due to the protein catabolism which occurs just prior to death. The animal will eventually reach a point where the cells of the body are unable to perform the functions necessary for life. Death results from lack of sufficient blood glucose to provide the energy needs of the brain and hypoglycemic shock occurs.
At the microbial level, inadequate food intake, especially in a ruminant species, results in a rapid decrease in the number of bacteria and protozoa present and in the volatile fatty acid concentration in the stomach. The ruminant obtains approximately 70% of its energy from these fatty acids, so a reduction in the level has a significant impact on the animal. The pH of the rumen becomes more alkaline because of the lack of these fatty acids. Decreasing the microbial populations probably diminishes the animal's ability to digest cellulose (fibrous material).
At the individual level, weight loss of 25 to 30% can occur and the animal may survive, but death is often the result. Wild ruminant physiology has developed to allow them to withstand dietary deficiencies and obtain energy from the consumption of poor-quality forage. Adult ruminants are able to store large amounts of nutrients and fat within their body tissues. They are also able to store minerals and nitrogen in their tissues for secretion into the rumen during the winter months. These fatter, older animals are thereby able to utilize more fat than protein, especially in the early stages of malnutrition and starvation. Young animals, which have smaller fat reserves because of their higher nutritional demands for growth, smaller body size, and position in the social hierarchy, utilize more protein than fat under starvation conditions.
Clinical Signs and Pathology
Clinically, mammals suffering from malnutrition or starvation are lethargic, unsteady, listless, and unafraid of humans. The skin may appear loose, the hair coat erect, dull, and rough and the body more angular. The animal may have a humped or sagged back, a swollen appearing face, sunken eyes, and a small tucked up abdomen. Due to atrophy (shrinkage) of the muscles, there is usually an increased prominence of the bones of the shoulders, ribs, vertebrate, and pelvis. The muscles appear more prominent, but usually do not appear full, and consequently a definite demarcation may be seen between the neck and shoulders and the upper forelegs and chest.
Clinical signs of an avian species dying from malnutrition or starvation are listlessness, unsteady locomotion, ruffed feathers, and a lack of fear of humans.
Pathological changes which occur in a starved animal are many and varied. The most striking gross change is a lack of fat in the subcutaneous, visceral, and bone marrow locations, and atrophic changes which occur in the musculature. Serous atrophy, a reddish gelatinous appearance to the fat tissue, is commonly seen in starving animals. The organs of the body decrease in size and weight. The digestive tract of most species is empty and/or shrunken with dark green bile staining of the lining and contents. The stomachs of ruminant species usually contain food, but the contents are often dry and of poor quality. The rumen lining may be ulcerated, have erosions present, and shrunken villi. The femur marrow, due to a lack of fat present, will be red or yellow in color, transparent, and gelatinous in a starved animal.
Severe weight loss (up to 50%) is a common occurrence in malnourished and starved avian species. Gross lesions seen are an absence of fat deposits and atrophy of the musculature, with breast muscle atrophy being the most noticeable. The digestive tract is shrunken and/or empty with dark green stained linings, and there is a marked increase in the size (possibly 2 to 3 times normal) of the gall bladder due to an accumulation of bile. In avian species, malnutrition may increase the susceptibility of the bird to parasitic infection (lice and other endoparasites are more common), and may result in the drawing of contaminants from the fat deposits being used, thereby resulting in the circulating and redistribution of these compounds. Diseases which could cause a malnourished condition, such as chronic infections of aspergillosis and lead poisoning must be ruled out when a definitive diagnosis is made.
Starvation can be diagnosed either by field techniques through gross examination, or by laboratory analyses. To grossly diagnose starvation, the overall physical condition of the animal must be determined by examining for the presence or lack of adipose tissue (fat deposits) in the various subcutaneous and visceral locations. In ruminants, the femoral or mandibular bone marrow fat can be examined, and the percentage of fat present estimated visually. Some care must be taken when examining the femur marrow as it is used for fat storage in adult animals but serves as a production area for red blood cells rather than fat storage in young animals. Laboratory methods that are used nationwide are varied. There is a femur marrow compression method, ether-extract method, kidney fat index, and wet weight-dry weight method. We have used the latter 3 methods for determining physical condition of the various mammals we examine. Blood parameters provide little information that can't be gained from gross examination of the carcass. Gross examination of birds dying from malnutrition or starvation is sufficient for a diagnosis providing other disease entities are investigated and ruled out.
Supplemental feeding of starving wildlife is an alternative to allowing wildlife species to die. This, however, involves a philosophical question of maintaining wildlife populations at a level above their normal carrying capacity, interfering with nature's checks and balances on populations and encouraging transmission of diseases (bovine tuberculosis). It may also be cost prohibitive. If a feeding program is to be used to maintain a high plane of nutrition it needs to be started early in the winter, continued throughout, and a surplus of food must be provided. If food is not provided (especially in ruminants) until malnutrition is in its advanced stages, the animal will probably die anyway. This is because once food is made available, the ruminant must be able to live in a negative energy balance for up to 2 weeks, before its digestive tract can adjust to the new diet and change to a positive energy balance. Generally, starved ruminants do not eat large quantities of food when sudden access to unlimited food occurs. However, due to an altered microbial population in the stomachs, it is possible to observe mortality in deer when shelled corn is overeaten. The reason for this is that lactic acid from the fermentation of starch accumulates to toxic levels. High quality palatable feed is essential in a feeding program: feed which contains readily available carbohydrates, roughage, minerals, and vitamins. Pelleted formulated feeds are the best ration that can be provided for ruminants. Elk can survive on high quality second or third cutting alfalfa, but deer have greater difficulty in obtaining adequate energy from roughages like this that are high in fiber. If baled hay is all that is provided for deer, it must be high quality alfalfa fed at a level where the deer do not have to consume anything but the leaves and small stems.
Supplemental feeding of birds is usually only done for songbirds but does occur with waterfowl species and turkeys under certain circumstances. The feeding of waterfowl during the winter may encourage alterations of normal migration patterns and possibly be of disease importance. Avian species respond faster to the providing of food once physical condition has been affected. Consequently, if the necessity arises, supplemental feeding can be started at anytime and probably be successful.
The plane of nutrition animals are on in the winter influences the severity of mortalities due to starvation, the reproductive success of the females, and the animal's resistance to disease, parasitism, and predation. Deaths attributable to starvation may have a great and persistent effect on a population, not only due to the loss of individuals, but also due to the disruption of fertility and reproductive success. In mammals, the effects of starvation on a pregnant female and her fetus(es) may be seen in utero, or after birth. Pregnant females catabolize their own fat and protein reserves during periods of food deprivation. The fetus is protected by these actions and continues its development. If a pregnant female is forced to withstand prolonged malnutrition, however, the fetus may die and be absorbed or aborted. A fetus that is not absorbed or aborted, but survives and continues its development to term, may be born small and have a reduced chance of surviving. This is especially true if malnutrition occurs during the third trimester of the pregnancy. These small offspring will probably have difficulty in suckling and the female may not permit them to do so, thus rejecting them. It is, however, possible that a malnourished pregnant ruminant will maintain its pregnancy until it dies from starvation.
The ability of the malnourished animal to resist bacterial and parasitic infection is markedly reduced when the body's immunological system has been compromised by the animal's malnourished condition.
In conclusion, malnutrition and starvation can be significant influences on a population of animals, but usually this effect is short term and the population returns to its normal level.
For questions about wildlife diseases, please contact the Michigan DNR Wildlife Disease Laboratory.