----start epidem.lec.01.05.98----
epidemiology
1/5/98 11-12
gary smith


garys@vet.upenn.edu

Principles of Epidemiology #7021
handout was given. 
make sure you have one.

He's talking about the Ashanti kingdom which was 
wellplanned, with wide streets and carefully planted trees. In 1817 each 
house had a lavatory. Roast sheep, turkey, plum pudding, nuts, ale, and 
wine were nicely served. But at the base of the society, was a valuation 
of power. it was an aggressive society. If power is for sale, sell your 
mother to buy it because you can always buy her back, they said. So these 
people seemed murderous, queer, and alarming to british people. Their 
presence had an effect on prices of exported goods. The ashanti and 
british threatened each other over control of the Gold Coast. 
eventually 
there was a war. The british shipped 4000 soldiers to the Gold Coast, 
with respirators, veils, cholera belts, quinine, etc. They brought three 
hospital ships, lots of artillery, etc. As they advanced on the capitol, 
they built a bunch of bridges, and stuff, and all this was watched and 
recorded by the media. Finally, the british destroyed the Ashanti city - 
they found 120,000 bodies. Why did the Ashanti think they could win? They 
fought, almost completely ignorant of what they were up against. they 
were proud and nationalistic, and had never met an enemy they couldn't 
beat. They thought they were protected by some kind of spiritual chain 
mail, and they lost because they were misled in their beliefs. We are the 
Ashanti. Epidemiology is the British. You must pay attention or it will 
catch up to you. 

phew. that was weird.

note re: note service - he will 
correct errors, but will not add things into the note service that the 
transcriber failed to put down on the paper.

exam questions are based 
entirely on lecture questions. you do not have to do additional reading. 
if you would like to, there are three suggested texts you may choose to 
consult. 

both exams are closed book, unlike the previous exams. there 
was abuse of the old open book system, so now it's closed book. 

these 
handouts are new. there are bound to be typos in them. if you see one 
please tell him so he can fix them for next year. that would be a service 
to him. also note that at the bottom  of the schedule are many contact 
numbers, office locations, home number, email, etc, for the instructor.. 
if there is a problem, contact him - before the exam. just don't call 
during masterpiece theatre or buffy the vampire slayer.

epidemiology: 
the study of the occurrence of disease in animal populations. it provides 
data on which to base a rational decision for the control and prevention 
of dz. it's the study of the frequency, distribution, and determinants of 
health and disease in animal populations.

an epidemiologist doesn't care 
why one cow had a calving difficulty causing her death and the death of 
the calf, but would care why dystocia is more common in this herd than in 
the herd at a neighboring farm. epidemiology is concerned with 
populations. the unit of study is a group, not an individual. this is 
different from much of what we study. don't misunderstand - it is 
relevant to the individual, despite the fact that it is the study of 
groups.

consider the human literature...most of us would consider that 
diabetes, untreated, is always associated with higher than normal blood 
sugar levels. but this isn't so. epidemiologists would argue based on 
these charts that only when one understands on the basis of the 
population can you make clinical decisions - eg, clinical data mean 
nothing unless interpreted within expected values for the population.


another example of how study of groups benefits the individual:

epidemiology is used to find cause of disease, and the study of groups 
led to the conclusion that smokers get more lung cancer than nonsmokers. 
so now individuals can choose to lower their risk by quitting. 
epidemiology also identifies factors that protect from disease. in the 
1940s, it was seen that communities with different levels of natural 
fluoride in the drinking water had different levels of dental caries. a 
high level of fluoride correlated with a low level of caries. so please 
do not disregard this discipline because it deals with groups - it 
informs what you do as a clinician for the individual.

we will use fewer 
human examples as we go on, because this is a course on veterinary 
epidemiology, and because veterinary epidemiology pays much more 
attention to health related parameters like productivity. it's easy to 
raise a prizewinning animal if you keep it confined to a pen and feed it 
specially formulated rations. it is more difficult to maintain the health 
of groups of animals kept in remote pastures or feedlots. you have to 
step back and think of dz as a phenomenon of the group vs individual. 
health in humans is defined as a state of physical, mental and spiritual 
well being, but in vet med, productivity is also used as a surrogate 
measure of health, in that the epidemiologist uses productivity to 
measure health indirectly. 

consider one sick bird in a flock of 25,000 
chickens - not a problem. but 10,000 sick birds is a big problem. it's in 
this context that although disease may limit productivity, it may not be 
the most important limiting factor. we also shift our attention to other 
things affecting productivity - so epidemiologists may look at links b/w 
productivity and housing, or ventilation, or flooring, or whatever. so in 
vet med, epidemiologists consider both health and productivity.

now. 
epidemiology is used instead of epizootology. epidemiology is technically 
the study of disease among human populations because epi=among, 
demos=people, logos=doctrine. But, demos can refer to animal populations 
as well, and epizootology is meaningless pedantry.

purpose of 
epidemiology: to provide data upon which a rational decision for the 
prevention and control of disease in animal populations can be based. 


the discipline provides:
guidelines for collection of data
methods of 
presenting and analyzing data
methods of making decisions based on the 
data.

can use epidemiology to answer:
why did an outbreak occur?
what 
was the cause of disease?
what are the expected clinical data of the 
population?
how to plan disease control programs?
also to assess economic 
impacts of disease and control plans.

activities that comprise 
epidemiology fall into distinct subcategories:

descriptive - what we're 
talking about for the rest of the day. involves sampling, data gathering, 
surveillance - monitoring disease occurence. many ways to do this. also, 
it's that branch of epidemiology that gives you the case series - the 
name given to that collection of data which describes the range of signs 
seen in the diseased population and lets you compare to the normal 
population. description of many cases of disease.

ecological - involves 
study of those factors which affect disease transmission for infectious 
diseases. dynamics of infectious disease. maintenance of causative 
organisms in environment. nowadays is dominated by mathematical 
modelling. 

analytical - classically what you read about in most 
epidemiology books. concerned with statistical techniques that find 
associations b/w disease occurence and putative causative factors.  often 
the first step in documenting a factor as a cause of disease. 

clinical 
- most experimental of all the subdisciplines. evaluates relative value 
of various interventions or preventative approaches. very concerned with 
clinical trials. also with ways in which we can improve ability to 
diagnose. 

as you move from descriptive to clinical epidemiology you 
move from an activity useful for framing hypotheses to an activity useful 
for testing hypotheses.

aims of the course:

-to acquaint us with basic 
tenets of epidemiology
-to show us how they are useful to practicing vets

-to introduce us to one of the "ways of knowing" that contributes to the 
volume of clinical information we have to know by the end of our time 
here. 

descriptive epidemiology:

some examples of veterinary 
epidemiological studies - most of which are descriptive - that part of 
the discipline involving observing and recording diseases and possible 
causative factors. first part of the investigation. the power of this is 
that once you have this info, you can hypothesize about cause of disease. 
if you find the cause, you can do something about it. all epidemiologists 
believe that disease is multicausal. don't be naive and think ok, the 
cause of fascioliasis is the common liver fluke. sure, the fluke is part 
of the causal chain. but if the animal is on a high protein diet it won't 
have clinical disease - so low protein is also a cause. lots of different 
factors come together before disease is manifest. if you can control even 
one factor, you can help to control disease. so this is really important. 


historical examples: note how useful it was to study the disease at the 
population level, and note that you can use descriptive epidemiology to 
control disease even when little/nothing is known of the disease.

years 
ago, after civil war, cattle range expanded very widely. for 20-30 yrs, 
there were cattle drives. finished cattle were driven across a number of 
trails with weird names to cowtowns like Abilene, Kansas City, etc...now, 
along these cattle drives, through the dust and heat and rattlesnakes, 
the cattle left a weird trail of disease - cattle deaths - financial 
ruin. one was lung fever, and one was texas cattle fever. both were 
eradicated. lung fever in 1892, texas cattle fever by 1949. the critical 
observations leading to eradication of lung fever were that it was 
imported cattle that led to the disease in almost all of the cases. now 
remember at this time, people still considered spontaneous generation as 
a cause of disease. so this was a big leap. but the epidemiologists found 
infected animals and killed them. infected premises were disinfected and 
imported animals were quarantined. the disease was eradicated 6 years 
before the mycoplasma that caused it was identified. texas cattle fever 
was tougher. in 1884 it was found that to be endemic in south/southwest 
over an area known to be the range of cattle ticks. this resulted in a 
flurry of studies - they found boophilus to be a vector. so they 
eradicated the ticks, limiting them to a small area around the rio 
grande. dipping the animals was very expensive, and farmers resisted. 
some farmers shot at the drivers of the tick wagons, and one was actually 
killed (one epidemiologist, not one farmer).
last case was in 1949. so 
careful study led in both cases to a hypothesis of causation and a means 
of dealing with the problem. 

it was also epidemiological studies in 
britain that suggested that cattle developed BSE after eating feed 
containing ruminant derived protein. now feeding ruminant derived protein 
to animals is illegal. this halted the epidemic before the prion was 
found. 

note that we know we can prevent grass fever in horses by 
stabling them in spring and summer and preventing them from grazing, even 
though we don't know what causes it.

there are many veterinary recording 
systems to chart progress of various diseases - some french named group 
formed to monitor rinderpest. NAHMS is the system in the US, it monitors 
livestock disease. The VMDB on the web has a lot of information on 
disease outbreaks. also individual hospital records. Penn has been 
recording incidences of diseases over many years. note that prevalence of 
tapeworms in dogs is unchanged over the past ten years, but is increasing 
in cats. why? 

hookworm prevalence in dogs is decreasing, but not 
changing at all in cats. why?
if we knew, maybe we could speed up the 
decline.

prevalence of heartworm in dogs is clearly decreasing - due to 
introduction of heartgard and similar products. this may also explain why 
hookworm is decreasing in dogs.

two ways to monitor - can monitor the 
same population at different times, or different populations at the same 
time.

the same population over time:

usual reason for slaughterhouse 
condemnation of pig livers is cirrhosis due to migration of larval 
ascaris suum. it used to be believed that ascaris suum was totally under 
control but a study showed a rise in the % of pig liver condemnation over 
time, not a drop.

also - there was a sharp rise in the % of sheep lungs 
being condemned for pleurisy and pneumonia in the early spring - 
correlating with lots of rain and wind chill two months prior. the 
animals were getting sick in the winter. this suggested a change in 
management as a solution - could make a barn for them to go into so they 
weren't out in the wet and cold.

crocodile hatching mortality-  
crocodile farming is very important in queensland australia, but there is 
high mortality in young crocs, and production is not yet optimized fully. 
a recent study of seven farms found that hatchling mortality in unheated 
facilities was high in winter, and that heating the facility reduces 
mortality by about 50%. one explanation is that since crocodiles are 
poikilothermic- body temp depends on ambient temp and that the dropped 
temp causes reduced immune function. 



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