in upland game birds, i)r()ducing coccidiosis ; Toxo- 

 plasma becomes encysted in tbe brain of rodents ; 

 Lcucocytozoon is a blood parasite common among 

 waterfowl and game birds. 



3. Bacteria cause a variety of diseases, notably 

 tularemia, paratyplioid, and tuberculosis among birds 

 and mammals, as well as otber diseases in lower types 

 of organisms. 



4. Viruses are .so suhmicroscopic in size that 

 many kinds pass through the finest filters. They are 

 the potent agents of hoof and mouth disea.se in deer. 

 Slotted fever in rodents, encephalitis and distemper 

 in fo.xes and dogs. 



5. Fungus spores of Asfrrgilliis that occur in 

 moldy pine litter may be drawn into the lungs of 

 ground-feeding birds where they germinate and grow, 

 causing aspergillosis. Fungus may also develop on 

 the external surface of animals. 



6. External parasites such as ticks, fleas, lice, 

 mites, and flies do not commonly produce serious 

 mortality by themselves, but they are often vectors 

 transmitting protozoa, bacteria, and viruses from one 

 animal to another. Heavy infestations of external 

 parasites may, however, lower the vitality or vigor of 

 an animal and cause diseases of fur (mange) or 

 feathers. 



7. Nutritional deficiencies in vitamins or min- 

 erals, or improper balance between carbohydrates, 

 proteins, and fats, may produce malformations, lack 

 of vigor, even death. Variations in amount, compo- 

 sition, and intensity of solar radiation may affect the 

 vitamin content of the food an animal consumes. 

 Long restriction to emergency foods of low energy 

 content and outright starvation often cause consider- 

 able loss of life during periods of climatic stress. 



8. Food poisoning, botulism, occurs when cer- 

 tain foods become contaminated with the toxms re- 

 leased by the bacterium Clostridium botulinum. 

 Many waterfowl are stricken in some localities. 

 Waterfowl also often pick up and swallow^ gun-shot 

 from marshes in which there has been much hunting, 

 and get lead poisoning. 



9. Physiological stress (Selye 1953) is a term 

 that has come to be applied to changes produced in 

 the body non-specifically by many different agencies 

 which may accompany any disease. Effects of stress 

 include loss of appetite and vigor, aches and pains, 

 and loss of weight. Internally, the stress syndrome is 

 characterized by acute involution of the lymphatic 



organs, diminution of the blood eosinophiks, en- 

 largement and increased secretory activity of the ad- 

 renal cortex, and a variety of changes in the chemical 

 constitution of the blood and tissues. 



Stress gives rise to abnormal conditions, but it 

 simultaneously elicits from the body defense mech- 

 anisms against those abnormal conditions. It is pres- 

 ently believed that the anterior ])ituitary gland and 

 the adrenal cortex are chiefly responsible for integrat- 

 ing the defense mechanisms. Three stages are in- 

 volved: the alarm reaction, in which adaptation has 

 not yet been acquired : the stage oj resistance, in 

 which the body's adaptation is optimum ; and the 

 stage oj exhaustion, in which the acquired adaptation 

 is lost. Characteristic of the exhaustion phase are, 

 among others, hypoglycemia, adrenal cortical hyper- 

 trophy, decreased li\er glycogen, and negative nitro- 

 gen balance. 



10. Accidents must be included as an important 

 cause of mortality. 



Organisms that produce disease generally fall into 

 one or two categories. They are either present in the 

 body at all times but not normally virulent, or they 

 are normally absent but are virulent from the moment 

 the host is infected by them. Even the healthiest ani- 

 mals chronically entrain many parasites and noxious 

 organisms in the body, but these organisms wreak 

 overt harm only when they become unusually abun- 

 dant, when virulent mutant strains develop, or if, for 

 one or another reason, the host's vitality and resist- 

 ance decline to the point where the host is no longer 

 able to withstand the effects of their presence. Any 

 animal suffering an unusually heavy infestation of 

 parasites will show the tax thus put upon its vitality 

 as a loss of vigor and weight, decreased growth rate, 

 and low resistance to vicissitudes of its natural en- 

 vironment. Normally, a more or less mutual toler- 

 ance exists between host and parasite such that the 

 demands of the parasite are in equilibrium w^ith the 

 host's capacity to meet them. Host-tolerant parasites 

 liave been naturally selected for ; mutant strains that 

 are exceptionally virulent quickly die out because 

 they kill the host, without which they cannot survive. 



A single attack, even a mild one, of some diseases 

 often confers a partial or complete immunity from 

 further attacks of the same disease, even though the 

 agent of the disease may still be carried in the body 

 of the recovered victim. Immunity is an acquired 

 physiological adaptation by which the immune is able 

 to withstand the presence of an otherwise noxious or- 

 ganism, suffering little or no deleterious consequence 

 of that presence. The fact of immunity is demon- 

 strated when parasites not conspicuously harmful to 

 their normal hosts are introduced into a species to 

 which they are normally exotic. The novel host has 

 had no prior occasion or opportunity to adapt im- 



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