Evolution and policy: outline

• Antibiotic use in agriculture: risk to human health?

• Insecticide resistance

• Genetically modified crops: risks and outcomes

• Public health cost:benefit analysis – tuberculosis

Applying evolution: risks of antibiotic use in agriculture

Uses of antibiotics

Estimates of US non-therapeutic (nt) antibiotic usage (lbs / year)

Class Description Antibiotics nt useI No substitute exists Vancomycin 600,000

Erythromycin

II Used by humans, penicillin, bacitracin 12.9 x 106

but alternatives tetracyclin

III Not currently used 11 x 106

by humans

Effect of farm antibiotics on humans?

• Levy (1976) fed farm chickens tetracycline at low levels (“sub-therapeutic”)

• Monitored gut bacteria: saw resistance

• Monitored farm staff:

Do food-borne anti-biotic resistant bacteria occur?

• Case I: Denmark 1998. Salmonella. 25 patients, 2 died. 2 were nurses infected by patients. Source: Danish pigs.

• Resistant to ampicillin, chloramphenicol, streptomycin, *tetracycline, *sulfonamides ( * = used as food supplements).

• Also, partially resistant to fluoroquinolones (eg cipro)

Case 2:

• Antibiotic resistance in Minnesota• Fluoroquinolones licensed for use in poultry in 1995

(therapeutic only). • Campylobacter: bacterial gastroenteritis. Treated with

erythromycin, cipro.• 1992: 1.3% of human infections resistant to cipro

Case 3: removal of antibiotic from farm use

• Denmark: used avoparcin as growth promoter until 1995.

• Avoparcin: similar in fucntion to vancomycin• 1995: vancomycin-resistance Enterococci 72.7% in

Denmark• 2000: 5.8%

Applying evolution: studying insecticide resistance in mosquitoes, fruit flies

• DDT: discovered 1939, extensive use in WW II (3 million pounds / year), resistance by 1945

• Quite successful in malaria control– Sri Lanka: 1 million cases 1955, 24 in 1961

• 1972, anti-malaria program dead

• Agricultural use: 1986 100 million pounds of insecticide used in US; 13% of crop lost

Genetic basis for insecticide resistance

• Mosquitoes: organic phosphate resistance due to duplicated gene; dominant mutation. Cost of resistance

• Fruit flies: resistant to almost all pesticides at a single locus (p450); dominant. No detected cost of resistance

“Natural insecticides”: Bacillus thuringiensis (Bt)

• Invade insect gut

• toxins open holes

• Insect dies – bacteria reproduce

• Bt used as insecticide 1958; first resistance 1989

Nature of Bt toxicity

Strain toxins

*kurstaki HD1 CryIA(a), Cry1A(b), CryIA(c), CryIIA, CryIIB

kurstaki HD-73 CryIA(c)

aizawai CryIA(a), CryIA(b), CryIC, CryID

israelensis CryIVA, CryIVB, CryIVC, CryIVD, CytA

• most commonly used strain

Mode of resistance to Bt?

• Reduced binding of toxin - recessive

• Other modes remain unknown – but dominant (F1 test)

• Cross-resistance:

• Cost of resistance?

Applying evolution: evaluating the risk of genetically modified crops

1997: first commercial use of Bt-modified corn and cotton

Plants produce Cry1A protein

Strategy to maintain resistance

• Dosage:

• Refuge:

• Rationale:

Assumptions behind control strategies?

Concerns

Escape of transgenes?

• Many crops have wild or weedy relatives

Evolution and public health: treating tuberculosis

• Mycobacterium tuberculosis

• aerosol droplets

• 20-24 hours per division

• waxy coating

• lump of bacteria - tubercle

• daughter cells may split off to colonize elsewhere

tb prevention and treatment

• 1890: test developed. • 1921: vaccine developed

• key antibiotics: streptomycin (1944); PAS (1946); isoniazid (1952); ethambutol (1963), rifampin (1966)

• early signs of resistance: multi-drug therapy

Mechanisms of action

Isoniazid: Inhibits mycolic acid synthesis

PAS: active at low pH

Rifampin: inhibits RNA polymerase

ethambutal: disrupts metabolism

Problems in tb treatment

• Mass of bacteria difficult to penetrate:

• Slow growing

Good news for treatment

• Most anti-tuberculin drugs are used only for this purpose– Most show little cross-resistance– Exception: rifampin

• Mycobacterium tuberculosis is not exposed to most gut bacteria

Why is multi-drug resistance a major problem?

• Some natural resistance

• Not a problem in all locations– Africa:– Western Europe and US:

– Russia:

– Peru:

TB treatment in 1990s

• World Health Organization: DOTS• Direct Observed Treatment, Short-course• Premise: eliminate resistance by ensuring that patients

always take medication• Effectiveness: excellent• Cost: low ($10 / patient in India)

Unless: Peru.

Paul Farmer

Cost:benefit analysis

• Treating tb with DOTS: cheap• Testing for drug resistance: expensive and slow

• Procedure: use all four drugs.• Failure?

• Treat drug-resistant? Yes: 2+ years, very expensive ($2200 in India)

• Treat 1 MDR patient or 200 non-MDR

Study questions1. Explain the potential risks and benefits of using antiviral

drugs to treat farm animals. In your view, when would this be appropriate?

2. What sorts of antibiotics should be prohibited from non-therapeutic uses on animals (if any), and why? Explain the relative risks of different classes.

3. You are studying populations of fruit flies in Africa that are susceptible to an insecticide. A dominant mutation to the p450 promoter arises, resulting in resistance in that fly. Graph the frequency of the p450 mutant allele over time. Ignore drift and gene flow.

Study questions4. You are studying populations of cotton bollworms on

transgenic bt-producing cotton. A recessive mutation arises that confers resistance to the bt protein in the cotton. Graph the frequency of the resistance allele in the population over time. Ignore drift and gene flow.

5. Explain how migration and random mating with susceptible insects might slow the evolution of resistance to transgenic Bt crops.

– From an evolutionary point of view, would it be wise to introduce a two-toxin transgenic crop plant in areas where one of the two toxins was already present? Why or why not?

– Explain how the cost of resistance would alter the risks and benefits of halting multi-drug treatment of TB when the TB was resistant to one of the drugs.

ReferencesAvise, J. C. 2004. The hope, hype, and reality of genetic

engineering. Oxford.Bates, S.L. et al. 2005. Insect resistance management in GM crops:

past, present, and future. Nature biotechnology 23:57-62.Daborn, P. J. et al. 2002. A single p450 allele associated with

insecticide resistance in Drosophila. Science 297:2253-2256.Ellstrand, N. C. 2003. Dangerous liasons? When cultivated plants

mate with their wild relatives. John Hopkins: Baltimore.Gillespie, S. H. 2002. Evolution of drug resistance in

Mycobacterium tuberculosis: clinical and molecular perspectives. Antimicrobial agents and chemotherapy 46:267-274.

Molbak, K. et al. 1999. An outbreak of multidrug-resistant, quinolone-resistant Salmonella enterica serotype Typhimurium DT104. New England Journal of Medicine 341:1420-

Palumbi, S. R. 2001. The evolution explosion: how humans cause rapid evolutionary change. Norton: New York.

Reichman, L. B. and Tanne, J. H. 2002. Timebomb: the global epidemic of multi-drug-resistant tuberculosis. McGraw-Hill: New York.

ReferencesSmith, K. E. et al. 1999. Quinolone-resistant Campylobacgter

jejuni infections in Minnesota, 1992-1998. New England Journal of Medicine. 340:1525-1532.

Tabashink, B. E. 1994. Evolution of resistance to Bacillus thuringensis. Annual review of entomology 39:47-79.

Wedel, S. D. et al. 2005. Antimicrobial-drug susceptibility of human and animal Salmonella Typhimurium, Minnesota, 1997-2003. Emerging infectious diseases 11:1899-1906.