Factors Affecting Disease Patterns PDF

# Factors Affecting Disease Patterns 1. **Pathogenicity and Virulence of the Mos:** * **Pathogenicity:** The ability of a disease-producing agent (DPA) to make a pathogenic effect, resulting in disease or morbidity. * **Virulence:** The severity of a specific strain of a disease-producing...

# Factors Affecting Disease Patterns 1. **Pathogenicity and Virulence of the Mos:** * **Pathogenicity:** The ability of a disease-producing agent (DPA) to make a pathogenic effect, resulting in disease or morbidity. * **Virulence:** The severity of a specific strain of a disease-producing agent. * **Pathogenicity and Virulence are Measured by:** * Ratio of clinical and subclinical cases * Case mortality 2. **Duration of Infectious State (Period of Communicability):** * Time during which the etiologic agent may be transferred directly or indirectly from infected to susceptible host. 3. **Ease of Communicability of the Disease:** * Droplets from infected host within a restricted population or herd can infect more hosts and are called **Secondary Attack Rate (SAR).** * **SAR** is calculated as: * No. of cases in outbreak-1ry (index) case (s) / Total No. of susceptible animals-1ry case (s) # Measures of Disease Occurrence Quantification of diseases can simply be done on the basis of counts of individuals infected, or dead. 1. **Prevalence "P":** * It is the amount of animals infected in a population at a specific time. It does not distinguish between old and new cases. * Therefore, it could be determined weekly, monthly, or annually. * Prevalence is usually expressed as a point prevalence, which means the amount of disease in a population at a particular point in time. * **P= **No. of individuals infected at a particular point in time / No. of all individuals in the population at risk 2. **Incidence "I":** * It is the No. of new cases infected in a population through a specific period of time. It measures the flow of the disease in the population. * **During the calculation of "I", two essential components are important:** * The No. of new cases. * The period of time through which the new cases occur. * **Incidence is calculated as a cumulative incidence,** which is the proportion of new cases infected through a specific period of time to the No. of healthy individuals at the beginning of that period. * **CI = ** No. of individuals that become diseased during a particular period of time / No. of healthy individuals in that population at the beginning of that period * **CI** can take values between 0 and 1. * **Example:** * In a herd of 100 horses, 20 animals get infected with strangles in the first week of November and 10 more cases were recorded during the second week. * **"I" during the first week:** 20/100 x 100 = 20% * **"I" during the second week:** 10/80 x 100= 12.5% * **CI after two weeks of recording:** 30/100 x 100 = 30% * The longer the period of observation, the greater the cumulative incidence. # Relationship Between Prevalence and Incidence * **Prevalence depends upon the duration of the disease "D" and the disease incidence "I".** * **So, P = I x D** * This means that a change in prevalence can be due to: * Change in Incidence * Change in disease duration * Change in both. * On the other hand, prevalence does not distinguish between old and new cases. It describes the probabilities of having the disease among a group of individuals at a point in time. * Anyway, both "I" and "P" can be used to make comparisons between risk factors like when comparing prevalence of disease in vaccinated and non-vaccinated animals. * **Calculation of one parameter from the other:** * Since **P = I x D**, any component of the equation could be determined if the other two components are available. * **In turn, I = P/D** * **Example:** * On a survey for leptospirosis among dairy farmers, 34% of them showed a serological reaction titer of 1/24. The persistence of antibodies at that titer was maintained for 10 years. * **So, P=34%** * **D = 10** * **I rate per year = 34/10 = 3.4%** # Other Measurement Factors * **Mortality Rate:** No. of dead cases at a given time / total No. of animals at risk * **Case Fatality Rate:** No. of dead animals at a given time / No. of sick animals at the same period * **Example:** * A veterinarian investigates a disease in a herd at July 1st, 1998 and the following data were recorded. * Total herd size was 600 * Clinically ill animals were 100 * Clinically ill animals between July '98 and July '99 were 200 * Total dead animals from July '98 and July '99 were 120 * **Calculate P, I MR, and case fatality rate.** # Disease Determinants (Causation) * **Infection:** invasion of a living organism to the host * **Infectivity:** the ability of DPA to establish itself in a host. * (ID50 = No. of DPA required to infect 50% of the susceptible host under experimental conditions) * **Pathogenicity:** the ability of DPA to produce the disease in the susceptible host * **Virulence:** the severity of a disease in the host. Commonly quantified using LD50 (No. of DPA required to kill 50% of exposed host under experimental conditions) * **Carrier State:** Infected host who is capable to disseminate the DPA without showing clinical evidence of the disease (true carrier state) * **Incubatory Carrier:** A host infected and disseminate DPA during incubation period (pre-clinical stage) * **Antigenic Variation:** DPA can evade host defenses by changing its antigenic characteristics * **Incubation Period:** Time between infection and the first appearance of the clinical signs * **Prepatent Period:** Time between infection and the first detection of the DPA * **Period of Communicability:** Time during which the infected host is capable to transmit the DPA # Etiology of Diseases "Determinants" * All factors that contribute to the production of the disease 1. **Some diseases require multiple factors "determinants".** 2. **Some diseases require only a single cause.** # Classification of Disease Determinants 1. **Primary Causes:** Specific factor and the disease doesn't occur without it 2. **Secondary Causes:** Non-specific factors and play a great role in the disease occurrence. It is called contributory or predisposing factor. Its importance vary widely from disease to another. ## Primary Causes 1. **Intrinsic factors:** * The causal agent is an integral part of the host itself. These include the following. * **Hereditary or gene transmitting diseases:** * Bull dog calves * Umbilical hernia in calves * Epistaxis in horses * Cryptorchidism in horses * Imperforated anus in dogs * Creeper in poultry (Short wings and long bones) * **Metabolic and hormonal diseases:** * Bloat in cattle * Hypocalcaemia in cattle * Transit tetany * Ketosis * **Behavioral disorders:** * Weaving in horses * Feather peaking in poultry * Cannibalism in poultry ## Extrinsic Factors * The causal agent is not an integral part of the host. These include the following. * **Non-living agents:** * **Physical agents:** * Trauma * Insect bites * Fracture * Horning * **Chemical agents:** * Organic and inorganic poisons * Toxins * Allergic agents etc. * **Living agents:** * Bacteria * Viruses * Parasites * Etc. ## Secondary Causes * These are predisposing factors, enabling or reinforcing factors. Classified to: 1. **Intrinsic factors:** * Inherited or acquired * Age * Sex * Species, breed and strain * Body confirmation * Metabolism and hormonal balance (cows more resistant than bulls?) * State of nutrition * Degree of function and fatigue * Physiological state 2. **Extrinsic factors (from 2ry):** * Environmental factors and not an integral part of the body * Climate and weather * Animal management (feeding, stable sanitation, handling, overcrowding, etc. ) # Host-Parasite Relationship * The fate of invading organisms depends on two factors: 1. The defense mechanisms of the body. 2. The pathogenicity or virulence of the invader. # Microbial Factors * **Pathogenicity** is the ability of the organism to cause the disease. * **Virulence** is the quantitative of an organism to cause the disease either by toxigenicity or invasiveness. * So virulence is measured in: * No. of MOs or micrograms of toxins required to kill a given host when administered by certain rout". * It is usually expressed as **LD50** i.e. No. of MOs or micrograms of toxins must be administered to kill 50% of animals under lab. conditions. # Virulence Factors 1. **Adherence factors:** Some DPAs have pili, which help their adhesion to the host cells. So they start disease processing like E.coli. 2. **Invasiveness:** is the ability of the DPA to invade the host tissues, multiply and spread rapidly. Capsulation of the DPA is an example of invasion as it protects it from phagocytosis like in pneumococci. 3. **Toxigenicity:** Toxins are bacterial products having a direct harmful action on the host tissues. There are two types of toxins: * **Exotoxins:** Are extracellular, diffuse freely into the surrounding medium as Cl. tetani, Cl. perfringens and Cl. Botulinum (G+ve bacteria) * **Endotoxins:** Usually produced by G-ve bacteria like E. coli, Salmonellae, shigella etc. These toxins are liberated only by disintegration of bacterial cells after death or by treatment with acids or alkalis. 4. **Production of certain enzymes:** * **Enzyme MW63:** Produced by all pathogenic staph. This enzyme protects the organism from phagocytosis by forming fibrin barriers around the organism and at the same time it inactivates the antibacterial substances in the blood. * **Fibrinolysin or kinase:** Secreted by Strep pyogenes (streptokinase). Promotes spreading of lesions by breaking down the fibrin (barriers) around the area of infection. * **Depolymerizing enzymes:** Such as mucinase, lipase, protease, nuclease which breakdown the tissues of the host. Collagenase produced by Cl. Perfringens play a role in gas gangrene. * **Hemolysins and leukocidins:** Dissolve RBC(hemolysin) and leukocytes (leukocidins). * **Hyaluronidase:** Spreading factor as it dissolves hyaluronic acid (cement like substances) that binds the cells together. So, it allows the pathogens to spread (Strep. Pyogenes). # Resistance Factors * Protective mechanisms of the animal body. There are two main classes of defenses: 1. **Non-specific protective mechanisms (1ry defenses) = innate immunity:** Operating non-specifically against most invading organisms and including: * **Mechanical Barriers:** Multiple mechanisms are present to prevent and destroy invaders. These physical barriers include: * **The skin:** Intact skin is unpenetrable by most of the DPA. After injury, a portal of entry can help invaders to enter host tissues, multiply and cause infection. * **Decontamination of the skin:** Important and occurs in three ways. * **Mechanically:** Desquamation of surface cells removes some superficial contaminants. * **Biologically:** Some resident flora living on the skin as Stapl. epidermidis, diphteroids, etc. produce antibiotic-like substances and compete with other organisms on the available nutrients. * **Chemically:** Sweat is normally acidic which is unsuitable for the growth of most pathogens. Moreover, this acidity of sweat is due to lactic acid. * **M.Ms:** Membranes are protected by mucous secretion which traps the invading pathogens. The body gets rid of the mucous with its content by many routes according to its location (via stools in the digestive tract and respiratory tract; via cough from the respiratory tract by the help of ciliated epithelium). * **GIT:** Saliva contains hydrolytic enzymes, stomach acidity, which may inactivate pathogens. In small intestines, proteolytic enzymes and active macrophages may attack invaders. * **Vagina:** Maintained by lactobacilli fermentation of glycogen from desquamated cells, making an acidic barrier (pH < 4.3), which prevents infection with yeasts, anaerobes and G-ve bacteria. As estrogen hormone is responsible for glycogen content of these cells, this acidic barrier is absent in childhood and old age. So, acute vulvovaginitis is therefore usually seen in these two age groups. * **Eyes:** Tears are unsuitable medium for invaders. Tears carry organisms to tear ducts and to the pharynx, then swallowed. Tears contain lysozymes which are a protective agent against most pathogens. * **Urinary tract:** M.Os are swept out with urine. Urine pH is unsuitable for mos. * **Phagocytosis:** All invaders that enter the lymphatic, lung or blood stream, are engulfed by any of a variety of phagocytic cells which include: * Polymorph nuclear leukocytes (microphages) and mononuclear cells: May be wandering in the tissues (histocyte) or in blood (monocytes) or fixed in tissues as Kupfer's cells in the liver. * Polymorph nuclear neutrophils granulocytes. * Eosinophils * Macrophages. * **Body secretions:** Contain many antimicrobial substances as: * Gastric secretions: Highly acidic * Vagina lactic acids * Nasal hairs * Saliva * Wave-like motion of tracheal cilia.