Introduction to Health, Disease, and Etiological Factors

Core Concepts of Health and Disease

Health is defined as a state of the organism where all organs function in harmony. In this state, all vital life processes—such as metabolism, movement, and excretion—proceed without disturbance. A descriptive term used for health is the "silence of organs," signifying that they function so flawlessly that the organism is entirely unaware of their operation. Conversely, disease is a collection of an organism's responses to a specific stimulus, which leads to changes in one or more organs and their respective functions. When diseased, organs no longer function flawlessly. An example of this is a broken leg caused by the application of force; the fractured bone in the limb results in the inability to walk. Disease involves various disturbances in biological processes and changes in bodily tissues. In short, disease is any deviation from the physiological state. The term "physiological" refers to something that is natural or normal, characterizing the normal functioning of organs and organ systems. The term "pathological" is the opposite of physiological. In professional terminology, the word "pathological" is used more frequently than the Slovenian equivalent "bolezensko."

The World Health Organization and Global Perspectives

The World Health Organization, known by the acronym WHO, is the leading international body for health issues on a global scale. Its purpose is to assist in achieving the best possible health for people worldwide. The WHO maintains disease statistics, fights against hunger and malnutrition, manages vaccinations, conducts research on various diseases, and remains active in many other fields. According to the WHO definition, health is a state of complete physical, mental, and social well-being, and not merely the absence of disease or infirmity. Disease is characterized as a collective name for organic, functional, and mental changes in the organism that exceed physiological fluctuations. While this definition does not perfectly fit the animal world, it is considered more than acceptable in the field of veterinary medicine.

Characteristics and Developmental Periods of Disease

Every disease possesses three fundamental characteristics: a cause that triggers the disease, the capacity of the organism to respond to a stimulus, and a progression involving a beginning, development, and end. The course of a disease is generally divided into four specific periods. The first is the latent period, which is the duration from the initial action of the cause until the moment the first clinical signs appear. The term "latency" implies a period of inactivity or concealment; thus, the organism appears healthy on the outside while an internal battle occurs as the body attempts to destroy or remove the harmful factor. The second stage is the prodromal period, which occurs if the harmful factor wins the initial battle with the organism. This period is marked by general malaise without distinct signs of disease. In infectious diseases, for example, one might notice a fever and slight pain during this phase. The third stage is the manifest period, which is when typical disease signs fully develop. The final stage is the end of the disease, which can result in three outcomes: complete recovery, where damage is fully repaired and the organism returns to its pre-disease state; incomplete recovery, where certain permanent changes remain; or death, which occurs when at least one vital organ, such as the lungs, is irreparably damaged. Not all diseases follow every stage; for instance, a bone fracture only displays the manifest period and the end of the disease.

Etiology and the Classification of Disease Factors

Etiology is the science dedicated to studying the causes of disease. A factor that triggers a disease is called a "noxa" (from the Latin noxa, meaning harm), though the term "agens" is also frequently used. Disease development depends not only on harmful factors but also on the responsiveness or reactivity of the organism. Therefore, the onset of disease involves external (exogenous) causal factors and internal (endogenous) factors. Disease processes result from external influences, internal influences, or a combination of both.

Internal (Endogenous) Factors and Organism Reactivity

Internal factors are circumstances within the organism that contribute to disease, such as genetic factors or other internal conditions. The organism's reactivity describes how it responds to a disease factor. Several properties are associated with this: constitution, which refers to the innate physical traits of an animal including skeleton, muscle development, organ function, and temperament; condition, which refers to the animal's physical fitness and capacity for effort, depending on training and nutrition; and disposition (or predisposition), which is a tendency toward a certain disease. Disposition can be innate or acquired. Innate dispositions include species disposition (e.g., foot-and-mouth disease affects cloven-hoofed animals), breed disposition (e.g., certain dog breeds are more susceptible to hip dysplasia), age disposition (e.g., intrauterine diseases, newborn diseases, or diseases of the elderly), sex disposition (diseases appearing more frequently in one sex than the other), organ disposition (e.g., the mammary gland is prone to inflammation, the liver to fattiness, or a horse's intestine to torsion), and individual disposition, which is uniquely tied to a specific animal's constitution.

Physical External Factors and Trauma

External factors act on the organism from the outside and include physical, nutritional, toxic, hygienic, and biological factors. Physical or inanimate factors cause injuries, also known as trauma, leading to tissue damage and subsequent functional disturbances in surrounding areas. Mechanical factors—such as impact, pressure, tension, cutting, friction, or stretching—cause mechanical injuries. These are divided into closed injuries, where the skin remains intact, and open injuries (wounds), where the skin is broken. Thermal factors involve both heat and cold. Heat can act locally to cause burns of varying degrees based on size and depth. High temperatures can also affect the entire organism, leading to overheating when the body cannot maintain normal temperature. This results in heatstroke, also called heat exhaustion. Sunstroke is specifically caused by direct solar radiation on the head. Cold also acts locally, causing frostbite of different levels. General exposure to cold leads to hypothermia and the common cold. Hypothermia occurs when the body temperature falls below $35\,^\circ\text{C}$. When the temperature drops to $27-20\,^\circ\text{C}$, death occurs because metabolic processes slow down or stop entirely. The heartbeat slows so much that the cells are no longer sufficiently supplied with oxygen and nutrients via the blood. The common cold is a viral disease of the upper respiratory tract that occurs due to overall or local cooling, which lowers the organism's resistance and allows viruses to invade.

Radiation, Electricity, and Climate Factors

Radiation also impacts the organism, including solar rays (visible, ultraviolet, and infrared), X-rays, and gamma rays. The harm depends on the type of rays, their intensity, and the duration of exposure. Radiation can cause burns, act carcinogenically, and damage hereditary material in cells. However, certain rays have positive effects, such as destroying microorganisms, and sunlight is essential for life. Electrical factors can be technical (household) or atmospheric (lightning). If current flows through the organism, it causes electric shock. Household current causes burns and, if it passes through the heart or brain, death. Atmospheric electricity in the form of lightning causes death through paralysis of the nervous system or heart failure. Interestingly, lightning striking a pregnant cow can kill the fetus while the cow survives. Climate factors include temperature, air pressure, precipitation, light, humidity, and winds. Animals adapt differently to climate, and changes in weather affect them as they do humans. Microclimate—the conditions in the specific living environment of the animal like a stable or pasture—often has a greater impact than the general climate. Poor microclimates (e.g., too warm, too humid, or high concentrations of $NH_3$ and $CO_2$) act as stress factors, increasing disease risk and reducing milk/meat production or work capacity. Microclimate is closely linked to hygiene conditions.

Nutritional and Alimentary Factors

Proper nutrition and high-quality food are prerequisites for health. Nutritional factors are also called alimentary or nutritional factors. Diseases linked to improper nutrition are called nutritional diseases. Quantitative disturbances relate to the amount of food consumed. Chronic excessive intake leads to obesity, which causes circulatory disturbances, reproductive issues (e.g., birth difficulties), movement problems, and a higher risk of sudden heart failure. If an animal consumes too much food at once, it can suffer from laminitis (inflammation of the hoof laminae) in horses, gastric torsion in dogs, or acute gastric dilation in horses, which can cause the stomach to burst. Cattle can suffer from alimentary indigestion, which corresponds to digestive problems in the forestomachs. Deficient nutrition constitutes either insufficient food intake or starvation (no food at all). In both cases, the organism consumes its reserves, starting with carbohydrates, then fats, and finally proteins. This leads to weight loss and eventually cachexia (a final stage of wasting), which lowers resistance to infections and parasites. Survival durations without food (with water) are: humans ($40-70$ days), dogs ($40-50$ days), horses ($30$ days), and laboratory animals ($1-3$ weeks). Qualitative disturbances involve food quality; even if food is plentiful, it may lack essential nutrients, leading to deficiency diseases (minerals, vitamins, amino acids). These can be caused by food shortages, increased needs during pregnancy, lactation, or growth, or poor utilization/absorption.

Water Balance and Dehydration

Water deficiency causes dehydration. The body constantly loses water, typically losing $2\,\%$ of its body weight daily. A loss of $10\,\%$ causes severe disturbances, and a loss of $20\,\%$ leads to death. Rehydration is the process of replacing lost fluids. Causes for decreased water in the body include insufficient intake or excessive loss (sweating, diarrhea, vomiting, burns, blood loss, or increased urination). Consequences of water deficiency include tissue desiccation, increased blood density (slower blood flow), decreased production of saliva, sweat, and hormones, and the accumulation of toxic metabolic products. Death results from damage caused by these waste products that cannot be excreted. Dehydrated animals do not eat; their skin becomes excessive, wrinkled, and dry, and the coat loses its shine. A skin fold test on a dehydrated animal will show that the fold does not flatten immediately due to reduced skin elasticity.

Toxic Substances and Intoxication

A poison is any substance harmful to the organism that can cause death. The disease is called poisoning or intoxication, and the degree of harm is toxicity. Toxicity depends primarily on the quantity of the substance. Poisons can enter the body perorally (mouth), via inhalation (respiratory), percutaneously (skin), or parenterally (injection/wound). The organism attempts to excrete poisons via sweat, milk, urine, feces, vomit, or exhaled air. Exogenous poisons come from the outside and can be inorganic (Arsenic, Mercury, acids, bases), organic (toxins from microorganisms, plants, or animals, leading to toxicosis), or synthetic (medications, pesticides). Food poisoning is termed alimentary intoxication. Endogenous poisons are produced within the body, and the resulting condition is autointoxication. Examples include substances produced during digestion that aren't excreted, urea returning to the blood (uremia), excessive hormones, or decay fragments from dead tissue (e.g., from burns or crush injuries). The liver is the primary detoxifier in the body.

Hygiene and Biological Factors in Disease

Inadequate zoo-hygiene conditions facilitate the rapid spread of infectious diseases by promoting the growth of microorganisms, especially in overcrowded settings. Poor housing can cause respiratory diseases, distal limb (hoof) diseases, or poisoning if cleaning agents are ingested. Biological factors include bacteria, viruses, prions, fungi, and parasites. To cause disease, these factors must invade the organism and overcome its defense mechanisms. Microorganisms (bacteria, viruses, fungi) cause infectious diseases. Zoonoses are specific infectious diseases transmitted between animals and humans. Infection (or infectiousness) refers to the invasion, settlement, and reproduction of microorganisms in the body. It can be local (one site) or general/generalized (spreading to the environment, lymph, and blood). The presence of bacteria in the blood is sepsis or septicemia, while viruses in the blood is viremia. For parasites, the term used is invasion or infestation, as "infection" is reserved for microorganisms. Parasitic diseases are parazitoses. Parasites include single-celled protozoa and multicellular organisms. They can be external (ectoparasites) or internal (endoparasites), and temporary or permanent. Parasites harm the host by taking food or blood, secreting toxins, acting mechanically (clogging intestines or damaging mucosa), transmitting other microorganisms (e.g., Borreliosis), or weakening the organism for further infections.