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system

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the assembly of factors connected with each other in some form of coherent relationship; all working towards the same or common goal

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An epidemiologic study must begin with

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1. the various states of health (or disease) 2. the transition from one state to the other (dynamism) 3. the determinants that affect the system under study 4. the relationships amongst the variables as these interact to affect the outcome of the specific condition under study

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determinant

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any factor, which when altered can affect the frequency with which the disease can occur in a population or any characteristic which affect the health of the population

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Determinants of disease can be divided into:

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1. primary determinants 2. secondary determinants 3. intrinsic determinants 4. extrinsic determinants

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Primary determinants

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factors whose variations exert a major effect in inducing disease; frequently are necessary causes ex: distemper virus, rinderpest virus

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Secondary determinants

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correspond to predisposing, enabling and reinforcing factors

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Intrinsic determinants

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aka. endogenous genetically governed factors ex: host, agent, vectors

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Extrinsic determinants

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aka. exogenous environmental influences, which alter the frequency of disease occurrence

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A critical premise of epidemiology:

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disease and other health events do not occur randomly in a population, but are more likely to occur in some members of the population than others because of risk factors that may not be distributed randomly in the population

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Important use of epidemiology

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to identify the factors that place some members at greater risk than others

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Determinants of disease include:

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1. Both the etiologic agents directly responsible for disease and other factors that facilitate exposure, multiplication, and spread in the population =agent, host, and environmental factors

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Epidemiologic triad or triangle

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traditional model for infectious disease; consists of an external agent, a susceptible host, and an environment that brings the host and agent together -disease results from the interaction b/w the agent and the susceptible host in an environment that supports transmission of the agent from a source to that host -Agent, host and environmental factors interrelate in a variety of complex ways to produce disease -proven inadequate for cardiovascular disease, cancer, and other diseases that appear to have multiple contributing causes without a single necessary one

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Agent

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an infectious microorganism or pathogen: a virus, bacterium, parasite, or other microbe -must be present for disese to occur; however, presence of that agent alone is not always sufficient to cause disease -can also include chemical and physical causes of disease /injury

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Pathogenicity

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ability to cause disease

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Host

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the humans/animals who can get the disese -risk (intrinsic) factors can influence an individual's exposure, susceptibility, or response to a causative agent -exposure is often influenced by behaviors such as sexual practices, hygiene, and other personal choices choices as well as by age and sex -susceptibility and response to an agent are influenced by factors such as genetic composition, nutritional and immunologic status, anatomic structure, presence of disease or medications, and psychological makeup

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Environment

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extrinsic factors that affect the agent and the opportunity for exposure

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Environmental Factors

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1. physical factors = geology and climate 2. biologic factors = insects that transmit the agent 3. socioeconomic factors = crowding, sanitation, and the availability of health services

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Component causes & causal pies

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-an individual factor that contributes to cause disease is shown as a piece of a pie -after all the pieces of a pie fall into place, the pie is complete and disease occurs -a disease may have more than one sufficient cause, with each sufficient cause being composed of several component causes that may or may not overlap

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Component causes

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individual factors; may include intrinsic host factors as well as the agent and the environmental factors of the agent-host-environment triad -a single component cause is rarely a sufficient cause by itself

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Sufficient cause

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the complete pie, which might be considered a causal pathway

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Necessary cause

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a component that appears in every pie or pathway, because without it, disease does not occur

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Note to self

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an agent that is usually harmless in healthy persons may cause devastating disease under different conditions

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Note to self

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a particular disease may result from a variety of different sufficient causes or pathways

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Note to self

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public health action does not depend on the identification of every component cause

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Note to self

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disease prevention can be accomplished by blocking any single component of a sufficient cause, at least through that pathway

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Host Factors

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- the type of events that an agent can trigger in a host including manifested lesions and clinical analysis of data by "host" may use inherent characterisiitcs of the host, biologic characteristics (immune status), activities (occupation, leisure activities, use of meds/tobacco/drugs) most disease -most diseases can be associated with identified segment of a population or the population at risk

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INTRINSIC HOST FACTORS

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genetically governed factors

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Species

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certain diseases involve a wide range of species, but the outcome is different -the variation in susceptibility and response to disease agents is due to efficacy of immune mechanism, which can be general or specific, and availability of receptor cells

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Ethnic and racial groups

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differences in racial, ethnic, or other group variables may reflect differences in susceptibility or exposure, or differences in other factors that influence the risk of disease, such as socioeconomicstatus and access to health care

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Age

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the occurrence of many disease shows a distinct association with age: bacterial and viral disease are more fatal in young than in adults, either because of an absence of acquired immunity or because of low non-immunological host resistance; rickettsial and protozoan diseases induce milder responses in young compared to adults -if immunity is long-lasting and all ages are susceptible, a disease often occurs in ages of first exposure; endemic diseases generally affect the young part of the population at risk -normal age factors also affect the occurrence of the disease

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Sex

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males have higher rates of illness and death than do females for many diseases - may be due to genetic, hormonal, anatomic, or other inherent differences b/w the sexes -these differences affect susceptibility or physiologic responses -ex: mastitis, milk fever, caliculi

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Hormonal

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-bitches are more likely to develop diabetes -premenopausal women have a lower risk of heart disease than men of the same age

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Occupational or different levels of exposure

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-D. immitis (mosquito-borne) is more common in male dogs (go hunting) -differences in lung cancer rates among men and women

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EXTRINSIC FACTORS

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the husbandry system can influence the host, which in turn alter the pattern of disease occurrence in the population ex: fatigue, overwork, exhaustion deplete defense mechanism of the host and alter the nature of disease occurrence

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Socioeconomic status

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made up of many variables such as occupation, family income, educational achievement or census track, living conditions, and social standing -these variables do measure socioeconomic status precisely -the frequency of many adverse health conditions increases with decreasing socioeconomic status -these patterns may reflect more harmful exposures, lower resistance, and less access to health care -a few adverse health conditions occur more frequently among persons of higher socioeconomic status

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ENVIRONMENTAL FACTORS

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-can have effect on agents, hosts, and their interaction -generally extrinsic determinants of disease occurrence -can be physical (climate, shelter, soil, and geography) or biological (man, vector, plants, and free-living animals)

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Climate

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Macroclimate - comprises the normal components of weather to which animals are exposed-rainfall, temperature, solar radiation, humidity, and wind -may also affect the stability of the agent, since the development of the agent is governed by the environment -may affect the transmission of diseases

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Temperature

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may be a primary determinant of hypothermia in newborns, may be a stressor, predisposing to disease causing agents

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Microclimate

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a climate that occurs in a small defined space where the host, agent and vector are surviving -it is part and parcel of the macroclimate

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Shelter

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a climatological sheath that would buffer the effect of macroclimate on the host -it can either have a positive or negative influence on disease occurrence -it can prevent sunstroke, wind and rainfall -unhygienic condition and increased density will favor the occurrence of disease -it can limit the distribution of disease (photosensitivity) -negative influences are often the result of intensification (mastitis)

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Soil

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-it can affect the vegetation in which the animals live -it can be a cause of disease indirectly (nutritional deficiency) -it can contravene the survivability of the agent (oocytes, spores, anthrax spores, eggs) -organic constituents like feces of birds and animals increase the risk of fungal infection in appropriate environment

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Biological Factors

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-vectors, wild animals (reservoir of infection) -man may create means by which disease causing agents are spread, perpetuate or survive -intensification (contact transmitted diseases are favored) -new diseases can be introduced by visitors/vehicles (also zoonotics like TB and salmonellosis) -can reduce incidence of disease through chemotherapy, chemoprophylaxis, quarantine, movement control and vaccination

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Note to self

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-analyzing data by place can identify communities at increased risk of disease - even if the data cannot reveal why these people have an increased risk, it can help generate hypotheses to test with additional studies

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Causes of disease can be

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1. internal (certain disturbances within the host 2. external (introduced from outside)

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External causes

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agents introduced from outside can be of living or non-living nature

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Non-living agents

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1. chemical 2. physical 3. nutritional 4. physiological 5. genetic 6. psychic 7. social -these are often not contagious, but simultaneous exposure to the suspected cause, may at times, cause mass problems

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Living agents

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-range from virus to bacteria to protozoa and to arthropods - with the infectious diseases, entry of the agent is a must, although the mere presence of the infectious agents does not necessarily lead to the disease state: if there is a disease, there should be an agent

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EPIDEMIOLOGIC ROLE OF INFECTIOUS AGENTS IN THE DEVELOPMENT OF DISEASE STATES

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...

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Infectivity

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-a measure of an agent's ability to penetrate, multiply and produce some detectable changes in a host -can be expressed qualitatively or quantatively by ID50 -after infection, there may or may not be disease, which governed by host, agent, vector and their interaction

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Pathogenicity and Virulence

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-measure the ability to transfer infection to disease states

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Pathogenicity

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a measure of an agent's ability to produce clinical disease once it has infected a host

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Virulence

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-the disease producing power of an organism -a measure of the severity of a disease once the disease is produced -quantified by LD50 -can be manipulated in such a way that it is either attenuated or enhanced: passage of disease agents in sensitive animals enhances the virulence whereas passage in less sensitive animals attenuates the virulence -physical agents (heat) and chemical agents (formalin, phenol) all can attenuate the virulence

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Aggressiveness

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-measured by penetrability (ability of the agent to penetrate the host tissue) -this is governed by microbial enzymes (hyaluronidase in Streptococcus and Staphylococcus and collagenase in Clostridium perfringens) and reproducibility (ability of the agent to replicate in the host tissue) -reproducibility is governed by agent's inherent ability, physical, chemical and biological envrionment in the host (e.g. ascaris lays a number of eggs within limited time)

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Invasiveness

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-ability of the agent to fight against defense mechanism of the host -usually effected by the formation of capsule and production of aggressins -capsule is protective against phagocytic activity -aggressins inhibit phagocytosis

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Toxigenicity

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a measure of disease-causing agent's ability to release toxins (endotoxins or exotoxins) which can destroy tissue cells

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Immunogenicity

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ability of the agent to produce a detectable antibody response in a host

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Plurality

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shows antigenic differences of the same species of the aetiological agents (FMD, AHS) -one needs to typify agents prevalent in that particular locality

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Adaptability

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antibiotic resistance factors

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Transmissibility

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a measure of ability of the agent to be transferred from one host to the other -usually the means for the agents survival

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Strability

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aka. Tenacity, Vitality -the duration for which the organism can remain infective outside the host -certain agents are labile (Rinderpest virus) -others are resistant (FMD virus) -secretions and excretions of the body facilitate the survival of the agents by prohibiting direct exposure to external environment

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Tissue Tropism

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signifies the affinity of the agents to certain tissues: -Panthropic (eg.Rinderpest and canine distemper virus) = do not have fixed residing sites -Organothropic (eg. enterothropic, neurothropic, pneumthropic) = have certain tissues to which they have affinity -this is related to biological activity (nutrition) of the agent

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Natural History

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the progression of a disease process in an individual over time, in the absence of treatment

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NATURAL HISTORY OF DISEASE TIMELINE

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1. Stage of Susceptibility -APPROPRIATE EXPOSURE TO OR ACCUMULATION OF FACTORS SUFFICIENT FOR THE DISEASE PROCESS TO BEGIN IN A SUSCEPTIBLE HOST

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NATURAL HISTORY OF DISEASE TIMELINE

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2. Stage of Subclinical Disease -After the disease process has been triggered, pathological changes then occur without the individual being aware of them -extends from the time of exposure to onset of disease symptoms = incubation period = infectious diseases =latency period = chronic diseases -during this stage, disease is said to be asymptomatic (no symptoms) or inapparent

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NATURAL HISTORY OF DISEASE TIMELINE

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3. Stage of Clinical Disease -onset of symptoms -this is when most diagnoses are made

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NATURAL HISTORY OF DISEASE TIMELINE

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4. Stage of Recovery, Disability or Death -Ultimately the disease process ends in one of these 3

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spectrum of disease

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-in some people, the disease process may result in illness that ranges from mild to severe or fatal -many additional cases may be too early to diagnose or may never progress to the clinical stage -unfortunately, persons with inapparent or undiagnosed infections may nonetheless by able to transmit infection to others

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Carriers

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persons who are infectious but have subclinical disease -persons with incubating disease or inapparent infection -persons who appear to have recovered from their clinical illness but remain infectious (eg. chronic carriers of Hepatitis B virus, or persons who never exhibited symptoms) -more likely to unwittingly spread infection than are people with obvious illness

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AGENT-HOST INTERACTION

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Agents can either cause infection or cause disease -there are many factors involved: 1.host resistance 2. Pathogenicity or virulence of the agent 3. Predisposing/Stress factors

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Host resistance

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a physical or immunological state of the animal is a governing factor -prior infection with TB, brucellosis, listeriosis, makes the animal more susceptible to some other infections

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Pathogenicity or virulence of the agent

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as agents pass in sensitive animals, the pathogenicity increases

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Predisposing/Stress factors

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-an environmental impact -an exposed host will acquire per-acute diseases from highly explosive agents -a decisive factor for the agent's survival where there would be no transmission -the agent increases its survival by increasing its infectivity and decreasing pathogenicity as well as through shorter generation intervals -those agents causing a chronic disease in animals have high infectivity and lower pathogenicity -there are certain agents for which death of the host is determinant, where their transmission is enhanced by flesh eating animals in order to perpetuate their cycle

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Note to self

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Host agent relationship is not a natural phenomenon and hence disease process does not occur randomly in a population

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Note to self

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the agent always tries to overcome host defense mechanism

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Note to self

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Disease causing agents are continuously evolving because they have short generation intervals and try to overcome defense ability of the host by antigenic variation and creation of a carrier state

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Antigenic variation

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biological situations where an agent evades the host defense by changing its antigenic characteristics -usually as a result of mutation or genetic recombination of the outer proteins -natural selection procedure is an important factor in antigenic variation

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Carrier

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any animal that sheds an infectious agent without demonstrating frank clinical signs -an inapparently or sub-clinically infected animal may be a carrier, and may shed the agent either continuously or intermittenly

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Incubatory carriers

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excrete the agent during the disease's incubation period (Rabies)

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Convalescent carriers

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shed the agent when they are recovering (FMD)

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