BY Dr ARCHANA SARAF

M.V.Sc Scholar Indian Veterinary Research Institute-Bareilly

DISEASE : Any condition that impairs the normal functioning of the body .

THE CONCEPT OF DISEASE : includes the causes or the factors associated that directly or indirectly cause disease.

Eg Anthrax

Eg Mastitis

Outbreaks or disease in a population often involves several factor and entities.

Epidemiologists and many scientists have created models to help explain the cause of disease causation.

INTRODUCTION ……

A TRADITIONAL MODEL OF INFECTIOUS DISEASE CAUSATION

This model comprises of ; Susceptible host (the person at risk for the disease), A disease agent (the proximate cause), and An environmental for the interaction between host and agent.

Vector, an organism which transmits infection by conveying the pathogen from one host to another without causing disease itself, may be part of the infectious process.

Contd…………..

Agents associated with disease can be categorized into two broad groups ;

Living agents Non-living agents

.

AGENT1.

Bacteria Viruses Protozoa Helminths Fungal Arthropods

Nutritional : proteins, fats, carbohydrates, vitamins, minerals- any excess or deficiency leads to disease.

Chemical agents Exogenous – allergens, metals, fumes, dust, gas,

insecticide Endogenous – bilirubin in jaundice, Ca++ in Kidney

stones, uraemia in kidney failure

Physical agents: exposure to heat, cold, humidity, Pressure, radiation, electricity, sound, etc.

Absence insufficiency or excess of factors necessary for health ,like Hormones – insulin (Diabetes) ACTH – Cushing's syndrome, Thyroxin – myxodema, thyrotoxicosis .

CONTD……….

Dose Infectivity Virulence Toxicity Pathogenicity

Dose is the quantity or the amount of agent to be exposed.

Infectivity is a measure of the ability of a disease agent to establish itself in the host. (Quantified- ID50. )

Virulence can be defined as a measure of the severity of a disease caused by a specified agent.(Quantified - LD50 )

Pathogenicity the ability of a particular disease agent of known virulence to produce disease in a hosts.

Toxicity how much that agent is toxic for that host.

CONTD …….

Intrinsic (non-changeable in the individual): Age Genetic make up Immune status Colour of skin Sex

Extrinsic (changeable in the individual): Eg. Intact bitches are at risk of pyometra and mammary gland tumors than

spayed (excluding stump pyometras) are not.

Species susceptibilities eg. FMD virus doesn’t infect equines

Breed susceptibilitieso eg In Africa, there are certain breeds of cattle ( N’dama and muturu ) ,

horses, sheep and goats are more tolerant of trypanosomiasis than others.

o Dalmation more prone to deafness and hyperuricemia

Age susceptibilitieso eg. Pups are more prone to parvo viral infection

Sex susceptibilities Eg Baldness common in males.

Genotype

Habits of animalso e.g. Greedy habit of cattle make them susceptible to choke .

Reproductive status eg. pregnant & non pregnant , sterile & intact.

Production status eg. lactating or non lactating .

Behaviour e.g. cannibalism, dominance, pica.

Innate resistance e.g. gastric barrier, mucocilliary transport mechanism.

Previous exposure.

Macro environment

Microenvironment

Temperature heat, cold, rainfall, wind, humidity etc

Weather can also affect the ability of a disease agent, or its intermediate host or vector.

Actual climatic conditions prevailing in the specific, restricted environment where the host, agent, vector or intermediate host actually live

eg Animal stocking density Housing (e.g. Ventilation ,sanitation) Environmental condition of that area. Feed

PASTEURELLOSIS : “SHIPPING FEVER” Pasteurella sp normal inhabitant of respiratory tract

STRESS •Recently weaning of calves •Transportation •Mixing groups of cattle from different sources •Confinement of cattle •Ineffective housing and ventilation

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.

Salmonellosis in all animals including humans occurs more frequently during summer. This is because the agent is able to replicate to infectious doses in moist feedstuffs at summer temperatures.

Epidemiological Triad Applied to Smoking-related Disease

Peer pressure,stress Thus, the traditional model of disease transmission can be useful to identify areas to reduce disease prevalence, whether infectious or non-infectious.

Cigarette, other filters

(Mausner & Kramer,1985)

The Wheel of Causation de-emphasizes the agent as the sole cause of disease.

It emphasizes the interplay of physical, biological and social environments.

It also brings genetics into the mix.

The wheel of causation model has a characteristic core which is the host The other aspects that surround the core is biological, social and physical determinants of disease. This model emphasizes the unity of gene and host within an interactive environmental envelope

CONTD……….

One sufficient wheel might include: contact with a carrier, low immunity, and crowding.

Another wheel might include contact with a carrier, having AIDS, poor hygiene. 

Both wheels are sufficient to produce disease, but factors differ. 

Thus for TB, both wheels must have the TB organism, as a necessary factor for TB to occur.

CONTD……….

Wheel of causation illustrates how disease may occur through the interplay of several causal factors without any being necessary in itself.

CONTD …….

A causal web is the linkages between a set of agent, environment and host factors that have been shown to result in disease.

Disease usually develops because of a chain of causes, rather than a single cause, that have acted one after another or together, often in complex ways.

This model is best used in the study of chronic diseases, where the disease agent is often not known.

Effects never depend on single isolated cause but as a result of chains of causation, interlinked.

The complex nature of relationship between factors and diseases is due to several factors ;

Factors may in combination with other. Some factors may be necessary for disease and some may act as supporting. Several factors together work to cause a disease but no one factor can cause a disease.

CONTD………

The purpose of developing a causal web is to provide "the big picture" a framework for thinking about the relationships between these causes and for developing strategies for controlling and preventing the condition in a group of animals.

CONTD…….

Anderson, RM, RM May (1982). Population Biology of Infectious Diseases. Springer-Verlag.

Chapter 17: Medical Ecology in: Schwabe, CW (1985). Veterinary Medicine and Human Health, 3rd ed. Williams and Wilkins. ISBN 0-683-07594-2.

Numerous examples of the causal webs for different types of diseases. Ewald, PW (1996). Evolution of Infectious Disease, Oxford Univ

Press; ISBN: 0195111397 Mims, C, N Dimmock, A Nash, J Stephen (2000). Mims' Pathogenesis

of Infectious Disease, 5th ed. Academic Press. Amazon General description of the natural history of infectious disease

processes, emphasizing the interaction between host and agent factors.

Thrusfield, MV (2005). Veterinary Epidemiology, 3rd ed., Blackwell Publishing. Amazon

"Epidemiology of Pneumonia in Feedlot Cattle" (S. Wikse) http://www.colorado-serum.com/vets/vol_3/vol3_6.htm