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On Pandemics: Deadly Diseases from Bubonic Plague to Coronavirus.

From the Publisher’s Website

Authored by a leading epidemiologist, this engrossing book answers our questions about animal diseases that jump to humans—called zoonoses—including what attracts them to humans, why they have become more common in recent history, and how we can keep them at bay.

Published originally in 2007 as The Chickens Fight Back: Pandemic Panics and Deadly Diseases that Jump from Animals to Humans, this book has been re-thought, revised and rewritten in light of the COVID-19 pandemic (as well as new and/or ongoing pandemics and epidemics of H1N1, Ebola, and a few other diseases). It is no longer just about people and animals. It’s about everything: naive globalization, climate crises, population explosions, habitat loss, reckless economic development, medicalization of health, blindered science, and thoughtless food systems.

Canadian Geographic interview and excerpt:

https://canadiangeographic.ca/article/epidemiologists-book-pandemics-takes-interdisciplinary-look-spread-diseases

Ottawa International Writers Festival Book Launch https://biblioottawalibrary.ca/en/virtual-book-launch-pandemics-david-waltner-toews

Northumberland Learning Connection https://www.connectnlc.ca/

Toronto Public Library – a joint presentation with science author Sonia Shah https://www.crowdcast.io/e/pandemicstpl 

Sensitivity and Specificity

Several readers from different parts of the world have pointed out to me that the numbers in Table 1 (p52) don’t add up. I hadn’t wanted to get into a lengthy discussion about sensitive and specificity in “On Pandemics”, but having messed up the numbers, I suppose I need to say a bit more than I did so that readers are not distracted from my general argument—which remains true—about the uncertainty of tests and the importance of clinical judgement for interpreting them.

This is a very oversimplified, traditional, binary, way to understand the sensitive and specificity of tests, and how that relates to how common the disease is (prevalence).  Let’s assume we have a test that is said to be 99% “sensitive” and 99% “specific.” Of those who actually have the disease, 99% get a positive test (below 990/ 1000). Of those that don’t have the disease, 99% test negative (98,010/99, 000).

Table 1.  Say one percent of people (1000/100,000) have the disease. Then only about half the people who test positive (990/ 990+ 990) actually have the disease. That’s a lot of false positives!  Of the 98020 who test negative, 98,010 don’t have the disease, which is about as good as you can get.

 Disease +Disease-TotalPredictive value
Test +9909901,980990/ 1980= 50%
Test –1098,01098,02098,010/ 98,020= 99.9%
Total100099,000100,000 

Table 2. BUT if we up the prevalence of the disease to 20 percent and keep the sensitivity (19,800/ 20,000=99%)  and specificity (79,200/ 80,000 = 99%) the same, we get the following:

 Disease +Disease-TotalPredictive value
Test +19 80080020,60019,800/20,600= 96.1%
Test –20079,20079,40079,200/79,400= 99.7%
Total20,00080,000100,000 

This means that if you get a positive test, you probably have the disease (95% positive predictive value). Of those that get a negative test, we still have close to 100% who don’t have it, but if the prevalence increases a lot more, the negative predictive value will really drop. So, the general, qualitative, rule is that when the disease is rare, false positive tests (i.e. the person does not have the disease, but the test is positive) are common, but when the disease is common, you see a rise in false negatives (the person has the disease, but the test is negative). These characteristics are important for interpreting tests when you want to manage an infectious disease. If you have more false positives, I’d say it’s better to control the disease, but more annoying for people whose activities are restricted based on tests. More false negatives mean that more infectious people are slipping through the quarantine nets and the disease is spreading “invisibly.”

In many medical situations, physicians will only test people for whom there are other reasons to believe they have the disease (clinical signs, history, contact with known cases); this group of people will probably have a higher prevalence than the general population. This increases the likelihood that someone who tests positive actually has the disease. It’s one of the reasons agencies and governments tend to focus testing programs on people who already have clinical signs.

Complicating all this further are the different kinds of tests available and how to decide on a positive/negative cut-off point. Are we looking at whole viable viruses or bits of nonviable viral protein? How many can we detect? Does it matter if you have 10 viruses versus 100? Are the tests done by different laboratories comparable? Are the testers equally skilled?

Next time you see discussions about how many people are testing positive in this pandemic (or another other), keep these things in mind. For myself, I consider the test results to be probably biased guidelines, and look at the longer trends without fussing over the exact day-to-day numbers.

The Chickens Fight Back: Pandemic Panics and Deadly Diseases that Jump from Animals to Humans (Greystone Books, 2007)

The Ecology and Culture of Diseases Other Animals Share with People

Emerging diseases like mad cow, SARS, and avian flu are — for the moment, at least — far more prevalent in animals than in humans. Still, the knowledge that measles, TB, and smallpox were at one time “emerging” diseases that eventually made a permanent, and quite deadly, jump to humans gives epidemiologists pause.

The Chickens Fight Back examines the various groups of animal diseases, explains what attracts them to the human population — from food to sex to living conditions — and offers suggestions for keeping them at bay. It also points out that diseases must be looked at from an ecological, cultural, and economic point of view as well as from a biological standpoint. Cooking meat till its well done and slathering on insect repellent for a hike in the woods are effective preventative measures, but as David Waltner-Toews notes, it’s more important to fundamentally rethink humankind’s place in the world.

Watch David’s interview on George Strombolopolous’ The Hour here.

Alexander Varty, Georgia Strait, June 13, 2007 – This is definitely one of those “Do as I say, not as I do” scenarios. If you have any interest at all in epidemiology, modern medicine, or the survival of the human race, do read Ontario veterinarian and University of Guelph professor David Waltner-Toews’s The Chickens Fight Back: Pandemic Panics and Deadly Diseases That Jump From Animals to Humans. But if you’re feeling feverish and nauseous, do not, as I did, bring it into bed for a little light reading.

“But whether the subject is sleeping sickness or Chagas disease or how televisions and air conditioning helped take the bite out of western equine encephalitis, Waltner-Toews makes truly entertaining reading.” — Globe and Mail

“Waltner-Toews tells these truly curious fables with a charming elegance and colourful precision that reminded me of the blessed clarity practised by Robert Desowitz, another great microbial story tracker. The Chickens Fight Back is exactly the kind of book medical and nursing students should be reading.” — Globe and Mail

“We need more like David Waltner-Toews: informed folks who not only care deeply about animals but can explain why humans have turned our dysfunctional yet collective fate into comedy or tragedy.” — Globe and Mail

“The latest book by Kitchener, Ontario, veterinarian and epidemiologist Dr. David Waltner-Toews, however, may just be a vaccination against fear and ignorance.” — Quill and Quire

“In plain (and occasionally saucy, funny, and contrarian) language, Waltner-Toews explains how humans throughout history have picked up diseases from animals ranging from fleas and ticks to cats, dogs, rats, pigs, mice, chickens, and cattle.” — Quill and Quire

“Though often funny and occasionally eccentric, Waltner-Toews is no armchair homilist. Sifting through dog poop in Kathmandu to discover the source of canine tapeworm has a way of humbling a person and opening one’s mind to the basic questions that fuel good science and help avoid bad public policy.” — Quill and Quire

“In the tradition of Silent Spring, the 1962 Rachel Carson book that acted as an environmental wake-up call, The Chickens Fight Back asks us to examine the societal set-up that makes these diseases possible, including such inequities as overcrowded cities, poverty, slums, and a lack of clean water. This book is a quiet little gem of understanding in a cacophony of panic and fear.” — Quill and Quire

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