Download Soil and Human Pathogens Outline Overview Pathway to humans

2/18/2016
Outline
Soil and Human Pathogens
(Overview)
Tom Loynachan
Iowa State University
• Introduction
• Pathways to humans
• Types of soil-borne pathogens
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Viruses
Bacteria
Fungi
Protozoa
Parasitic worms
• Soil survival and movement
• Conclusions
Overview
• Pathogens cause diseases or illnesses
• May be:
– a) natural soil residents
– b) introduced to soil and survive until infection
• Relatively new science: only about 150 years
ago pathogens discovered to be living
organisms
Pathway to humans
• Passed through the skin (e.g., several
helminths such as hookworms)
• Present in surface skin abrasions (e.g.,
Clostridium tetani)
• Ingested orally (e.g., Clostridium botulinum)
• Inhaled into the respiratory system (e.g.,
Aspergillus fumigatus)
Many introduced are enteric
• Found in the intestines of humans and animals
• Must survive in the soil until contacting a new
host
• Length of survival function of soil conditions
• Mixed group: bacteria (Salmonella spp.,
Shigella spp., Campylobacter spp., Yersinia
spp.); viruses (hepatitis A, hepatitis E,
adenoviruses); protozoa (e.g., Entamoeba
histolytica); worms (e.g., Ascaris lumbricoides)
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Host and dose level
• Contact may or may not cause disease
• Dose level:
– Salmonella spp. may require 106 to 109 cells
– Shigella spp. may require as few as 10 cells
• People with compromised immune systems
are especially susceptible
• Thus, threshold varies by both disease
organism and host
Tenanus
• Caused by the toxin-producing, anaerobic, sporeforming, Gram-positive bacteria, Clostridium
tetani
• 233 tetanus cases in the United States during the
years 2001 to 2008, with a 13% fatality rate
• Tetanus is a significant cause of death worldwide,
especially in Asia, Africa, and South America. In
2006, 290,000 persons died of tetanus, of which
250,000 were neonatal deaths
Pathogens introduced to soil often
from infected humans or animals
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Night soil
Sewage sludge
Municipal wastewater
Septic tanks
Animals and animal wastes (zoonoses)
Resident pathogens
• Nasty bacteria: cause tetanus, anthrax,
botulism, gas gangrene
• Many are spore formers can survive long
periods in soil
• Helminths (worms)
• Both present in most soils of the world
Tetanus
• Once in anaerobic tissue, C. tetani spores
convert to the vegetative form, multiply, and
produce a neurotoxin, which can affect the
central nervous system (lockjaw)
• Deep wounds tend to provide the needed
oxygen-depleted environment
Viruses
• 20-300 nm (with or without tail)
• Contain either RNA or DNA surrounded by
protein coat; must have host cell
• Major cause of gastroenteritis; infected
individuals may pass 1010-1011/g feces
• Longer a virus survives in the environment,
the greater the risk of transmission to a new
host
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Bacteria
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1-3 µm
Prokaryotic cells
Live in soil or transmitted through soil
E. coli O157:H7; Campylobacter spp.,
Salmonella spp., Shigella spp., Bacillus
antracis; Clostridium tetani, Clostridium
botulinum; Clostridium perfringens; Vibrio
cholerae, Legionella spp.; Mycobacterium
leprae; Mycobacterium tuberculosis
Fungi
• Eukaryotic cells
• Relatively few cause human diseases; called
mycoses, rarely fatal
– Dermatophytes: skin, nail, or hair diseases; several
genera; Trichophyton spp., etc.
– Systematic infections: histoplasmosis;
coccidioidomycosis; Aspergillus fumigatus
Protozoa
• Single celled, eukaryotic
• 10-50 µm
• Several have a cystic phase that can survive in soil
for extended periods
• Most multiple in the gut and excreted into soil
• Transmission is mainly fecal-oral route
• Entamoeba histolytic, Giardia intestinalis,
Cryptosporidium parvum, Acantamoeba spp.,
Naegleria fowleri
Giardia
• Giardia attached to
the small intestine
• Giardia infection is
marked by
abdominal cramps,
bloating, nausea
and bouts of watery
diarrhea
• Commonly
transmitted in
water but cysts
survive in soil
Photo: Center for Disease Control and Prevention
Parasitic worms
• Nematodes (roundworms) most common
• Some are really nasty
• Reproduce by eggs that past through the stool
and persist in soil
• Flukes, tapeworms, hookworms, pinworms,
whipworms
Guinea worm
• If you are feeling low and discouraged, check
out the life cycle of the “guinea worm”
• Life is good if you don’t have this parasite!
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Survival in soil
• Resident (compete for growth requirements)
• Transient
– Survival function of timing and conditions: the fresher
the contamination, the greater the potential for
human infection
– Food grown in soil fertilized with human wastes or
effluents
– Land application is common to recycle nutrients
– May not grow but can survive (often exponential rate
of die-off)
Survival in soil
• Time—death phase often exponential with
time
• Common maximum survival times:
Pathogen
Soil
Plant
Viruses
3 months
1 month
Bacteria
2 months
1 month
Protozoa
2 days
2 days
Helminths
2 years
Survival in soil
• As a group, helminths and spores of bacteria
and fungi persist the longest
• Soil generally better medium for survival than
plant surfaces
Survival in soil
• Sunlight—UV light inactivates pathogens and
tends to dry the environment
• Moisture—usually greater die-off in drier soil
• Temperature—survive better under cooler
temperatures but not freezing
1 month
Gerba and Smith, 2005
Survival in soil
• Texture--fine-textured soil retains more
pathogens
• pH--sorption general increases as pH
decreases
• Soluble organics--often complete with
pathogens for sorption sites
Survival in soil
• Microbial type—surface charge, surface
proteins, extracellular polysaccharides, size
• Water flow rate—higher flow rates more
easily move pathogens; less movement under
unsaturated flow (greater soil-pathogen
contact)
• Preferential flow—large channels bypasses
soil contact and even large pathogens can
move quickly
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Survival in soil
• Cation exchange capacity—at neutral pH, both
soil and pathogens likely negatively charged;
divalent cations such as Ca2+ and Mg2+ may
bind until die-off or predation
• Organic matter—CEC, retains moisture,
soluble components (some viruses readily
move through organic soils)
Movement in soil
• Larger worms/eggs and protozoa often
trapped by soil matrix
• Smaller bacterial and viruses often sorbed by
physical and chemical properties on soil
surfaces
Conclusions
Movement in soil
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Helminths are ca. the size of sand particles
Protozoa the size of silt particles
Bacteria the size of fine silt and coarse clay
Viruses the size of
very fine clay
Uncultured and human health
(Future)
• Currently thought to be in the thousands of
different species per gram of soil.
• Really bad guys affecting human health have
been studied.
• Early exposure of infants to the soil microflora
may affect infant’s intestinal microbiota and
immune systems. Much yet remains to be
discovered about these uncultured organisms.
Questions/comments
• Wide variety of pathogens can live and/or survive
in soil
• Some organisms natural residents, others
introduced (often shorter lived)
• Of introduced, fecal-oral route most common
• Soil properties and types of pathogens affect
length of survival; usually decreases with time
• Soil is a good filter (viruses most likely to escape
filtering process); preferential flow can move
organisms quickly
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