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CONCEPT OF ANIMAL DISEASES
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Disease is the result of complex interactions (some would say imbalance) between the triad of the agent (toxic or infectious), the host and the environment. The components of this interaction differ depending upon the specific circumstances of each group of affected animals. Particularly for agricultural animals, this triad is strongly influenced by husbandry and management factors, which are often the most important. For vector-borne diseases, vector factors are also linked to the other factors.
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Recognizing the different components of this triad is important because they are the source of opportunities to reduce disease at multiple points in the transmission cycle. A common mistake is to focus on only one aspect of the triad for disease control or prevention and to overlook the others.
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Examination of the past historical and contemporary writings on disease suggested that disease concepts were viewed as causal networks that represent relations among the symptoms, causes, and treatment of a disease. Conceptual change concerning disease is primarily driven by changes in causal theories about diseases.
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The most famous thories on diseases include
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The Humoral Theory
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The Contagion Theory &
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Germ Theory
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All of which were now superseded by the current medical advances.
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Ancient Greek viewed of diseases, whose concepts are closely connected to the humoral theory of the causes of disease. The same view dominated european medical thought until the development of the germ theory of disease, which was first hinted at in the sixteenth century but not developed and generally accepted until the nineteenth.
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Fracastoro, an Italian physician, wrote the first important work on contagion in 1546, but the modern germ theory of disease developed with the research of Pasteur, Lister, Koch, and others in the 1860s and 1870s.
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Transition from the humoral to the germ theory of disease required a major conceptual revolution, involving many kinds of conceptual change including a fundamental shift in how diseases are classified. Less radical conceptual changes occurred in the twentieth century with the discovery of genetic, nutritional, and immunological causes of disease.
Agent Factor Examples
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Dose
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Environmental hardiness
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Virulence (microbial)
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Infectivity (microbial)
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Toxicity (poisons)
Host Factor Examples
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Innate resistance (e.g. gastric barrier, mucocilliary transport mechanism)
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Previous exposure
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Passive immune status (neonates)
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Vaccination status and response
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Age
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Gender
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Behavior (e.g. mutual grooming, dominance, pica)
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Production status (e.g., lactating vs. non-lactating)
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Reproductive status (e.g., pregnant vs. non-pregnant, sterile vs. intact)
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Genetics
Intrinsic (non-changeable in the individual)
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Age is very important because the risk of many diseases change widely over the animals life time due to underlying physiological changes that are associated with age. Neonates are very susceptible to many enteric and respiratory infections but resistance increases as the animals mature. As immune function declines with advanced age, susceptibility begins increasing again.
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Clinical disease due to ubiquitous agents, such as the viral scour agents, can be reduced by delaying the neonate's exposure to the agent (innate resistance increases with age) and reducing the infectious dose by changing the environmental factors.
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Due to genetics different breeds have different risks for diseases, such as hip dysplasia in German Shepherds. Within breeds, some infectious diseases occur due to underlying genetic defects (e.g., Holstein BLAD, Arab CID, Quarter Horse HPP).
Extrinsic (changeable in the individual)
Environmental Factor Examples
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Animal stocking density
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Animal movement between groups
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Housing (e.g. ventilation, sanitation)
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Environmental conditions (e.g. temperature, humidity, wind velocity, precipitation)
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Nutrition (protein, energy and macromineral and micromineral adequacy)
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Many infectious agents are susceptible to the ultraviolet (UV) in direct sunlight and to desiccation. Many infectious agents survive for long periods in damp environments.
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Strangles (Strep. equi) in horses appears to occur more frequently during damp cold weather. This is likely because the agent is able to survive longer in damp environments.
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Salmonellosis in all animals including humans occurs more frequently during summer than during other times of the year. This is likely because the agent is able to replicate to infectious doses in moist feedstuffs at summer temperatures.
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Bluetongue virus grows more rapidly in Cuilicoides variipennis at higher temperatures. A strong association has been shown between bluetongue infection in cattle and both temperature and rainfall.
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These factors interact in complex ways that are often under the control of man.
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Eg: Increased animal density may lead to increased microbial load in the environment, a roof may prevent exposure of microbe to killing UV, low ventilation may increase humidity from animal respiration which in turn increases environmental survival of the organism which in turn increases exposure dose and infects more animals.
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It has been said that:
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"Bovine mastitis is a disease of man with signs in the cow."
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"Bad management will overwhelm the best immunology."
The "Iceberg" Concept
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In outbreaks of most disease in animal groups, both clinical cases (the tip of the iceberg) and subclinical cases (unobserved beneath the ocean surface) are present in the group.
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For many infectious agents, particularly those that are endemic, more of the infections in a group are subclinical (silent) than are clinical. For some exceptions, such as rabies, few if any subclinical infections occur and almost all if not all clinical infections end in death. This iceberg concept of severity distribution also holds for most induced, non-infectious diseases affecting a group, such as hypomagnesemia, ketosis and hypocalcemia. Disease in an individual is often evidence of a group phenomena because the factors that caused the disease in that individual are usually affecting others adversely as well.
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For most groups, the response to the host-agent-environment interaction that results in disease is usually not an either / or, black or white phenomenon. Instead, it is usually a continuum, with different individuals expressing different degrees of severity at different times as determined by the unique combinations of agent – host – environment risk factors that they experience. For each problem outbreak, the "shape" of this iceberg (the proportion affected, the proportion of the affected that become clinical and the proportion of these that die) at any point in time depends on the specific combination of agent, host, environment, vector (if one is involved), and human husbandry / management factors acting in that specific situation.
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Because these factors change over time (e.g., animal immune responses eliminate the infection, humans change their management practices, the environment changes both seasonally, day-to-day and year-to-year), this "shape" changes over time. This does make outbreak investigation and problem solving both challenging and rewarding for the clinician.
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