Anthropogenic evolution, ���externalities, and public health

Carl T. BergstromDepartment of Biology

University of Washington

Department of Ecology and Evolution, UCLA���January 6, 2016

Mark Tanaka Ben Althouse Ted Bergstrom Marc LipsitchUNSW IDM USCB Harvard

Participants in the 2011 Heron

Island Summit on Evolutionary Challenges in

Food, Health, and the Environment

Overview

Anthropogenic evolution and externalities.

Carroll et al (2014) Science 346:313

Public choice theory for public health

Althouse et al (2010) PNAS 107:1696

Timing of antiviral use during a pandemic

Tanaka et al (2014) Evolution, Medicine, and Public Health 2014:150

http://octavia.zoology.washington.edu

Anthropogenic evolution and externalities.Carroll al (2014) Science 346:313

Herbicide resistance

Heap (2012) OnlineGlyphosate (Roundup) resistant weeds

Antibiotic resistance

Bergstrom and Dugatkin (2016) Evolution 2nd ed.

Fisheries Induced Evolution

Borrell (2013) Nature

Harvesting Induced Evolution

Decreasing size in harvested, but not non-harvested, species of snow lotus in the Himalayas.

Law and Salick (2005) PNAS

Conservation & captive breeding

Gene drive

Harvard Wyss Institute Gene Drive FAQ

In general, the problem with managing anthropogenic evolution is that

The species we ���want to evolve fast,evolve slow,

And the species we ���want to evolve slow,���evolve fast.

Images:  superbwallpapers.com,  free-­‐designer.net  

Evolutionary changes in wild populations generate externalities

What are externalities? ���������

Economic side-effects that are���not captured by market prices.���

Negative ���externalities

Images:  Wikimedia  

Many other situations with negative externalities involve���common-pool resources: they are non-excludable, ���

but rivalrous. The result in tragedies of the commons.

cc  by  flickr:bekhiann  

Positiveexternalities

cc  by  flickr:Brigit  Murphy  

Meanwhile, pure public goods are non-excludable ���and non-rivalrous. Creating these generates positive ���externalities.

Private markets tend to under-allocate to pure public ���goods. Governments need to intervene.

National Defense is the example acceptedmost broadly on bothsides of the political aisle.

cfr.org  

Evolutionary changes in wild populations generate externalities

Harmful changes tend to be common pool issues. •  Non-excludable but rivalrous•  Hard to monitor, hard to attribute cause / blame

Beneficial changes tend to be pure public goods•  Non-excludable, non-rivalrous

How can we deal with externalities?���������

Economists have a rich toolkit���(and extensive theory, of course!)

Command ���and control���

policies

wikimedia  

Sir Alexander Fleming,

Nobel Prize Lecture, 1945

wikimedia  

(We need) Antibiotic regulation

Bergstrom and Dugatkin (2016) Evolution 2nd ed.

Tort Law

wikimedia  

Aventis Starlink Corn

Images:  NCI,  wikimedia.  

Establish property rights (cap and trade)

cc  flickr:Thomas  Berg  

Catch shares for fisheries

wikimedia  

Pigovian taxes

In Sweden, spirits are taxed €50 euro / liter

Pigovian subsidiesPoster:  Wikimedia  

As evolutionary biologists we can explain how human actions lead

to anthropogenic evolution ���

We would do well to understand the the policy tools available for modulating these actions.

Public choice theory for public healthAlthouse et al (2010) PNAS 107:1696

When should the government���get involved in public health?

When it can improve the lives of the most needy members of society.

On efficiency grounds, when ���does government investment���in public health out-perform ���

private market solutions?

Public choice theory

SuscepEble� Infected � Removed  

Epidemiological modeling

Paul Samuelson 1955

Tetanus

Measles

Otitis media

Pandemic Influenza

Tetanus

Tetanus occurs when the soil microbe ���Clostridium tetani infects a deep wound.

It is not transmitted from human to humanVaccine targets the toxin, not the microbe.

My tetanus vaccine doesn’t affect you, positively or negatively.

It is a pure private good. cc flickr : Penn State

Measles

Image: Helen Bergstrom, then age 6.

Images: kinderhelper.com

My problem�

Your problem �

Images: kinderhelper.com

Private good�

Positive externality�

SuscepEble� Infected � Removed  

Vaccinate fraction p�

SuscepEble� Infected � Removed  

Vaccinate fraction p�

Infections decrease linearly with ���fraction of the population vaccinated.

OptimalSubsidy

€

v(q) −C'(qN) − kX(q) = 0

Direct value of ���treating one individual

Cost of treating ���one individual

Effect of treating one ���person on all others.

First-order condition for social efficiency

€

v(q) −C'(qN) − k q∂htreated (q)∂q

+ (1− q)∂huntreated (q)∂q

$

% &

'

( ) = 0

Direct value of ���treating one individual

Cost of treating ���one individual

Cost of one infection

Effect on other treated individuals

Effect on untreated individuals

€

s = −k q∂htreated (q)∂q

+ (1− q)∂huntreated (q)∂q

$

% &

'

( )

In our measles example, vaccinationaffects untreated but not treated individuals

A Pigouvian subsidy of this magnitudealigns private and public interest

Otitis mediawikimedia

Otitis media is caused Streptococcus pneumonia and Haemophilus influenzae. ���Half of preschool aged children carry ���them asymptomatically.

Image: Dennis Kunkel

Otitis media occurs when these bacteria make their way up the eustachian tube into the tympanic cavity - and thus is rarely transmissible from the infection site.

www.patienteducationcenter.org

American Academy of Pediatrics (2004)

Minimize use of antibiotics for uncomplicated otitis media.

Use amoxicillin rather than a late generation drug if antibiotics are necessary.

Patients vary in their ���need for the drug

Otherwise healthy children ���can easily clear the infection ���without treatment.

Children suffering from other complications mayneed drug treatment

After Bonhoeffer et al. 1996

Steady-state resistanceincreases with fractiontreated.

Marginal private benefit

Marginal public benefit

Marginal cost

Optimaltax

€

s = −k q∂htreated (q)∂q

+ (1− q)∂huntreated (q)∂q

$

% &

'

( )

In our Otitis media example, ���treatment affects other treated ���individuals but not those who are untreated

A Pigouvian tax of this magnitude aligns private and public interest

Pandemic Influenza

Images: http://www.coo.kz/

•  Social distancing

•  Vaccination

•  Antivirals

Antivirals

Reduce mortality & morbidity������Reduce transmission ������Select for resistance

  Private good

  Positive externality

  Negative externality

How should we distribute antivirals?

Flu Kits:���Families purchase in advance as insurance against pandemic

���Pharmacy distribution: Doses are distributed to individuals once infected

After Lipsitch et al 2007

Flu kit scenario

Pharmacy distribution scenario

Timing of antiviral use during a pandemicTanaka et al (2014) Evolution, Medicine, and Public Health 2014:150

Staphylococcus aureus

Pseudomonas aeruginosa

Enterococcus sp.

Klebsiella pneumoniae

Surgical  ICU  central  line  catheter  infecEon  rates    Denver  Health  Quality  Report  2009  

When should we deploy a limited stockpile of antimicrobials to maximize benefits over the course of an epidemic?

Direct benefits (private goods): reduction of mortality and morbidity in treated individuals.

Indirect benefits (externalities): changes in the epidemic trajectory due to antimicrobial use.

See  Handel,  Longini,  and  AnEa  (2007)  Proc  R.  Soc.  B  274:833-­‐837  

See  Handel,  Longini,  and  AnEa  (2007)  Proc  R.  Soc.  B  274:833-­‐837  

See  Handel,  Longini,  and  AnEa  (2007)  Proc  R.  Soc.  B  274:833-­‐837  

See  Handel,  Longini,  and  AnEa  (2007)  Proc  R.  Soc.  B  274:833-­‐837  

X  

YSU  YST  

YR  

Z  θ   ν/(1-­‐ε)  

ν  

ν  

1-­‐fξ  

fξ  

Treatment  start  Eme  (days)  

FracEo

n  of  stockpile  used  successfully  

Model results (≠policy suggestions)

Limited stockpile? Don’t use it right away.

Even with an unlimited stockpile, the total social welfare is greatest when one delays antimicrobial use some time into the epidemic.

Why ?

Indirect benefits come from reducing overshoot by reducing transmission at and beyond the epidemic threshold, i.e., late in the epidemic, to minimize overshoot.

Direct benefits depend on the number ���of treatment failures due to resistance.Here timing is everything.  

Direct benefits depend on the number ���of treatment failures due to resistance.Here timing is everything.  

To summarize:

Early use generates negative externalities by promoting the evolution of antiviral resistance.

Late use creates positive externalities, by reducing the overshoot of the epidemic.

You might take this as an argument in favor of delaying antiviral treatment until well into an epidemic.

Generally not a good idea to delay antiviral use.

Delaying use •  may not be ethical •  prevents any chance of early eradication•  risks wasting doses if use begins too late

Correctly ordering the use of multiple drugs may be feasible.

Compare  Wu  et  al.  2009  PLoS  Medicine  

SummaryAnthropogenic evolution in wild populations has profound costs—and benefits—for society. ������But its consequences are diffuse and rarely captured by economic markets.

We can manage anthropogenic evolution—but we need the tools and theory from public choice economics to implement effective policy.

This is our chance as evolutionary biologists to be societally relevant. Let’s do this!

Mark Tanaka Ben Althouse Ted Bergstrom Marc LipsitchUNSW IDM USCB Harvard

Participants in the 2011 Heron

Island Summit on Evolutionary Challenges in

Food, Health, and the Environment