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Summary Veterinary Epidemiology and Economics
- Instelling
- Wageningen University (WUR)
The document is a summary of the theory explained during the lectures and of the recommended textbook.
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VETERINARY EPIDEMIOLOGY AND ECONOMICSCHAPTER 1 – THE DEVELOPMENT OF VETERINARY MEDICINE
About 60% of diseases in humans come from other vertebrate animals zoonoses. Humans get sick from
these diseases from food, direct contact with animals or from air and water. Examples are:
- Bird flu, found in birds and caused by a virus
- Q fever, found in domesticated animals and caused by a bacterium
- Rabies, found in cats, dogs, foxes, bats, cattle, goats and caused by the Rabies virus
- Swine fever, found in pigs and caused by a virus
- Tuberculosis, found in cattle, pigs, cats and caused by a bacterium
Definition of Health: health is a relative concept with many definitions. The World Health Organisation
defines it as a state of complete physical, mental and social well-being, and not only the absence of disease
of infirmity.
Health is about homeostasis = temporary or permanent anatomical, physiological or behavioural
change of the organism to reduce or overcome influences that can reduce health; this change can be
specific, e.g. respiration and vasodilation when it is too hot, or aspecific e.g. animals experience stress in
response to a stimulus. NB: it is not good to always avoid stressful situations in animals because they allow
them to learn how to live in a non-optimal condition, but of course the stressor stimulus should not last
long.
General Adaptation Syndrome: activated by the hypothalamic-pituitary-adrenal circuit when animals are
subject to a stressful situation; it has 3 phases:
1. Alarm reaction: first drop below the normal value of resistance to stress, then increase to above
normal
2. Resistance: phase in which the animal adapts to the stressful situation
3. Exhaustion: if the animal is stressed for a long time, its resistance decreases and it might cause
death.
NB: highly productive animals are more at risk of disease because their high production does not allow
them to adapt well to stressful situations, so they have lower resistance.
Example stressors: malnutrition, parasitism, dust, predators, overcrowding in the farm.
Types of diseases:
- “Simple” infectious diseases: caused by one infectious agent (e.g. salmonellosis) and still cause
problems in more developed countries.
- Complex infectious diseases: caused by multiple agents, e.g. enteric and respiratory diseases;
common in intensive farming systems.
- Subclinical diseases: they do not produce visible symptoms, but they often negatively affect
production (e.g. subclinical mastitis). Their identification often requires laboratory analyses.
- Non-infectious diseases: they are generally genetic (e.g. hip dysplasia in dogs), metabolic (e.g.
bovine ketosis) or neoplastic (tumors, cancer) diseases. Their cause is often associated with factors
such as sex, breed, age, diet and are not caused by a pathogen.
- Genetic diseases
In modern times, a multifactorial theory of disease has developed most diseases (both infectious and
non infectious) are influenced by many factors, e.g. social, geographical, economic factors. In multifactorial
diseases, there is usually a primary factor (e.g. pathogen) without which the disease does not occur, and
secondary factors e.g. host factors, or environmental factor (vedi capitolo 5).
, NB: abnormalities at birth do not necessarily have to be caused by hereditary defects, because the
environment already influences the embryo e.g. intrauterine infections, nutritional errors in the pregnant
mother.
The focus of veterinary medicine changed during the years: at the start of 1900s, veterinarians focussed on
the individual animal (treating diseases), then on the herd/flock (increasing herd health and productivity),
but now they focus on the food production chain to ensure good quality and safety of food.
Nowadays, society is concerned with animal welfare. Welfare in production systems is often evaluated with
the 5 freedoms: freedom from hunger and thirst, from discomfort, from pain/injury/disease, from fear and
distress, and to express normal behaviour.
CHAPTER 5 – DETERMINANTS OF DISEASE
Diseases are caused by multiple factors (multifactorial/multicausal diseases) that can have an effect on
health, called determinants of disease. Knowing these determinants helps to identify categories of animals
that are at risk of developing a certain disease (e.g. diet can be a determinant associated with a certain
disease; if an animal eats less, it can be an indicator of that disease).
Classification of determinants:
- Primary or secondary determinants: primary determinants are the main factors inducing a disease,
e.g. exposure to a virus/bacterium. Secondary determinants are factors that facilitate the
occurrence of the disease, e.g. sex is a secondary determinant of heart valve incompetence in dogs,
because male dogs are more likely to have this problem.
- Intrinsic and extrinsic determinants: intrinsic (or endogenous) determinants are factors internal in
the animal, e.g. genetic constitution (species, breed, sex), age, metabolism, behaviour. Extrinsic (or
exogenous) determinants are factors coming from the environment, e.g. physical trauma, presence
of stressors, management of the animal in the farm (e.g. type of housing, diet).
- Determinants associated with host, agent and environment: often called host-agent-environment
triad. More details below.
Host determinants
- Genotype: some diseases have a genetic cause. There are 3 types of genetic diseases: 1)
chromosomal aberrations (wrong numbers or structure of chromosomes), 2) Mendelian disorders
(simply inherited), 3) multifactorial disorders. The first 2 include diseases that are almost totally
genetic; the third category includes diseases that are caused by genetic problems, but are also
influenced by other factors, e.g. canine hip dysplasia is caused by weakness of muscles supporting
the hip joint due to genetic problems, but it is also influenced by conditions in early life that cause
osseous changes in the joint bone. Often, multifactorial diseases occur when the number and
combination of genes, in association with environmental factors, exceeds a threshold value beyond
which the disease occurs (see fig 5.3).
- Age: occurrence of many diseases is associated with age. E.g., fig 5.5 shows that canine neoplasia
(tumors) is more common in old dogs.
- Sex: sexual differences in disease occurrence can be attributed to:
o Hormonal determinants: effect of sex hormones can predispose animals to a disease.
, o Occupational determinants: influenced by how you “use” the animal, e.g. male dogs are
more likely to have heartworm infection because they are more used in hunting and are
more exposed to the mosquito that transmits the infection.
o Genetic determinants: genetic differences in disease occurrence can be 1) sex-linked,
when the genes causing the disease are on the X or Y sex chromosomes (e.g. dog
haemophilia is associated with the X chromosome and occurs mainly in males); 2) sex-
limited, when the genes causing the disease are only expressed in one sex; 3) sex-
influenced, when the threshold to have the disease (vedi sopra spiegazione, fig 5.3) is
lower in one sex, so the disease occurs more likely in that sex than in the other.
- Species and breed: some species/breeds are more susceptible to certain diseases, e.g. cattle are
more susceptible to respiratory diseases than pigs. Also, species/breeds that are more related from
an evolutionary perspective are more likely to be susceptible to the same diseases.
- Behaviour: some behaviours help the transmission of diseases, e.g. mutual grooming
(pulirsi/spulciarsi a vicenda) in meerkats helps the transmission of tuberculosis.
- Size and conformation: some diseases are more common in animals of a certain size, e.g. hip
dysplasia is more common in large dogs.
- Coat color: e.g. white cats develop deafness more often than cats with other colors.
Agent determinants
- Virulence and pathogenicity: virulence is the ability of a pathogen to cause disease in terms of
severity; pathogenicity refers to the quality of disease induction, e.g. a certain parasite can be
pathogenic only in warm water and not cold water. The genotype of infectious agents can change
frequently with different mechanisms:
o Mutation: sequence of DNA and RNA of the pathogen is changed. Can be point mutations
(change of one base), deletions (part of the genome is lost), insertion (additions) or
inversions (parts of genome change place). If mutations occur often in the pathogen,
animals are infected many times because they need to learn how to defend themselves
every time.
o Recombination: it occurs when two infectious agents exchange parts of their genome, or
when an infectious agent (usually virus) inserts its genome in the genome of the host.
o Conjugation: when bacteria transmit genetic material with each other in form of plasmids
(molecules in bacteria that can duplicate).
o Transduction: when a bacterial virus (phage) transfers a part of genome from a bacterium
to another.
o Transformation: when a bacterium releases DNA from its cell and it enters into another
bacterium of the same species.
- Gradient of infection: it refers to the possible responses of animals to an infectious agent:
o Silent or subclinical infection: animals do not show clinical signs (symptoms) so it can only
be detected with analyses in the lab. E.g. you can analyse faeces, urine, saliva (presence of
toxins, pH, blood in urine/faeces), make an autopsy to inspect organs after death,
serological tests (NB: serological tests are not always required).
Serological test: it is blood testing. You can determine the presence of antigens
(pathogens), antibodies (if they are present, it tells you if there is an infection now or
previously because the animal is producing them), or the blood composition (enzymes,
number of leukocytes, hormones). Advantage: fast and cheap; disadvantage: false-positive
, or false-negative results can be obtained. E.g. you obtain a false-positive if the animal is
vaccinated, because its blood contains antibodies but it is not infected, or you obtain a
false-negative e.g. when the animal has high tolerance so it does not produce antibodies
even though it is infected.
o Clinical infection: it can be mild or severe, and clinical signs are visible in the animal. When
the disease is very mild, it is sometimes impossible to make a diagnosis and it is called
abortive reaction; when the disease is severe, the signs are strong enough to allow a
diagnosis and it is called frank clinical reaction. Clinical health characteristics: abnormal
body temperature, no appetite, diarrhea, abnormal frequency of respiration, strange
behaviour, abnormalities on the skin, not shiny coat, decrease in production etc. These are
the symptoms that you have to notice to know if an animal is sick.
o Death
- Outcome of infection: animals can develop a chronic infection (and can infect others), recover or
die (dead animals are usually not infectious anymore). Recovered animals can develop sterile
immunity, so they cannot infect others anymore and do not get sick.
Animals can also be carriers of a disease, so they carry the pathogen but do not have symptoms.
Incubatory carriers excrete the pathogen during the incubation period; convalescent carriers
excrete the agent when they are recovering from the disease. Animals can also have a latent
infection, so the infection persists in the animal but they do not have symptoms, and can also
infect others.
- Microbial colonization of hosts: some pathogens are exogenous, so they are not usually present in
the host and cannot survive for long in the outside environment. Endogenous pathogens are
normally present in the host (often in GI or respiratory tract) and cause disease only when
influenced by other factors (e.g. E. coli is common in the GI tract of calves, and it cause infections
when for example calves do not drink colostrum). Other pathogens are opportunistic, so they
infect only when the host is already not well, e.g. they infect more easily animals that have wounds
after surgeries.
Environmental determinants
- Location: certain pathogens are more diffused in certain places. Also non-infectious diseases are
influenced by location, e.g. chronic pulmonary diseases in dogs are more common in urban areas
because of higher pollution.
- Climate: two types of climate: 1) macroclimate, includes the components of weather like rain,
temperature, solar radiation, wind, humidity, and they all affect health. E.g. pathogens develop
better in certain climatic conditions, or solar radiation can cause cancer because of UV radiations;
2) microclimate, includes the conditions in a limited space e.g. the farm so it includes the
temperature, humidity, ventilation etc. in the farm; e.g. poor ventilation is associated with
respiratory diseases.
- Husbandry: the bedding material and surfaces in farms can affect diseases (e.g. leg lesions are
common in animals raised on concrete floor); diet has effects because there can be energy, vitamin,
mineral or protein deficiencies that facilitate the occurrence of other diseases; having many
animals together and buying them from other farms increases chance of infection; the “use” you
make of the animal affects disease occurrence (e.g. racehorses have more leg injuries).
- Stress: it can be caused by weaning, transportation, excessive heat, changes in diet. The stimuli
that cause stress are called stressors.
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