Chapter 22 - Pathogens

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Chapter 22 - Pathogens

• Objectives
• Be able to describe the difference between a
  frank and opportunistic pathogen
• Be able to list the five modes of transmission of
  pathogens
• For each of the major groups of pathogens (virus,
  bacteria, protozoa) be able to discuss relative
  minimum infective dose, survival in the
  environment, and sensitivity to disinfection
• Be able to discuss an example of each of the
  major groups of pathogens from the perspective of
  why it has been an important pathogen, what its
  mode of transmission is, what its lifestyle is
• Be able to give an example of an emerging
  pathogen

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Pathogens in the Environment

• Outbreaks of water-, air-, or foodborne disease
  have engendered study of pathogens and ways to
  protect ourselves from them
• filtration/chlorination of drinking water sources
• treatment/disposal of wastewater
• food processing/preparation
• air handling, esp. hospitals/buildings

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• Terminology
• Infection is the invasion and growth of an
  organism within a host organism
• Pathogens are infectious organisms that harm
  their host
• - Frank pathogens can cause disease in
  otherwise healthy
• individuals
• - Opportunistic pathogens can only cause
  disease in compromised
• individuals (burn victims, AIDS
  patients, the young or
• elderly, pregnant women, transplant
  patients)
• - Human pathogens include bacteria, viruses,
  and protozoa (amoebas,
• flagellates, and apicomplexans)
• Virulence is the degree of pathogenicity of a
  parasite determined in part by
• minimal infective dose, the number of
  organisms needed to cause an
• infection bacteria gt viruses gt parasites

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Five Modes of Transmission

• Waterborne transmission
• - drinking water or swimming (usually via
  ingestion)
• - fecal-oral route
• - fecal contamination of drinking water from
  municipal wastewater sources or
• animal feedlots

• Foodborne transmission
• ingestion of infectious agents in food
• poor sanitation, hygiene (fecal-oral route)
• insufficiently cooked fish and shellfish
• in US there are 76 million cases/yr with 325,000
  hospitalizations and 5000 deaths

• Person to person transmission
• requires direct physical contact between hosts
• sexually-transmitted diseases
• respiratory infections (coughing, sneezing)

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Modes of Transmission (cont.)

• Airborne Transmission
• inhalation of pathogens in aerosols
• aerosols created at wastewater treatment plants,
  land application of sludge, showers
• legionellosis, fungal infections

• Vector-borne transmission
• transmission by the bite of an animal host
• malaria, sleeping sickness, yellow fever

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• High minimal infective dose
• 104-109
• Bacterial pathogens do not remain infectious in
  the environment very long
• typical half-life less than 24 hours
• Outbreaks can be prevented with proper sanitation
  and chlorination of drinking water, proper food
  handling and preparation

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• Inhabit intestines of animals
• Gram rods, facultative, aerobes
• non-sporulating
• nonmotile, or motile with peritrichous flagella
• mixed-acid fermentation
• ferment sugars to acetic, lactic, and succinic
  acids

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Enteric bacteria -- Salmonella

• Found in particularly high numbers in the
  intestines of birds and reptiles
• Over 2000 serotypes can cause disease in humans
• serotypes differentiated by O-antigen, a cell
  wall antigen
• Serotypes Typhimurium, Enteriditis, Typhi, and
  Paratyphi cause human disease
• Genome 50 homologous with E. coli

• Salmonellosis
• caused primarily by serotypes Typhimurium and
  Enteriditis
• fever, abdominal cramps, and diarrhea (sometimes
  bloody), 5-7 days
• disease due to cell lysis in stomach and release
  of endotoxin (LPS)
• may lead to septicemia or Reiters syndrome
  (e.g., chronic arthritis)
• minimal infective dose 104 107
• 40,000 confirmed and 1.4 million estimated cases
  in US/yr, 500 fatalities
• 2 develop chronic arthritis
• Usually a foodborne disease (food poisoning), but
  may also be waterborne

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• Typhoid fever
• infection of intestines and blood caused by
  serotype Typhi
• fever, headache, constipation, malaise, chills,
  myalgia for 3-4 weeks
• Rare in industrialized nations (400 cases per
  year in the U.S. most from international travel).
  16 million cases and 600,000 deaths occur
  worldwide each year
• In 5 of cases, victims become carriers, and shed
  S. typhi for at least a year in feces

• Paratyphoid fever
• Caused by serotype Paratyphimurium
• Similar to typhoid fever, but milder

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Enteric bacteria - Escherichia coli

• Commensal enteric bacterium, but some strains are
  pathogenic
• enteropathogenic E. coli
• watery diarrhea with mucus, fever, dehydration
• common in infants in less developed countries
  (50 mortality rate)
• high minimal infective dose 106-109

• enterotoxigenic E. coli
• cramping, vomiting, profuse diarrhea, dehydration
• disease caused by the production of two toxins
• common in travellers (travellers diarrhea) and
  children in less developed countries

• enteroinvasive E. coli
• severe cramping, watery diarrhea, fever
• disease caused by invasion of epithelium of
  intestine by the bacterium, much like Shigella
• common in less developed countries

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• enterohemorrhagic E. coli (e.g. O157H7)
• severe cramping and very, very bloody diarrhea,
  5-10 days
• very young and elderly can develop hemolytic
  anemia and acute renal failure (HUS, 2-7
  incidence)
• disease due to the production of two
  prophage-encoded toxins shared with Shigella
  dysenteriae
• occurs in North and South America, Europe

• 73,000 cases of infection and 61 deaths each year
  in the U.S.
• There have been multiple outbreaks of E. coli
  0157H7 since 2003 affecting from 3 to 24 people
  each.
• waterborne outbreaks occur, but usually due to
  contaminated meat (especially hamburger), milk,
  fruit juice, leafy veggies

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Enteric bacteria - Shigella

• Four species cause shigellosis S. sonnei, S.
  flexneri, S. boydii, S. dysenteriae
• Most severe symptoms due to S. dysenteriae.
• S. sonnei most common in U.S.
• Shigellosis watery or bloody diarrhea, cramps,
  fever, malaise
• due to invasion and destruction of intestinal
  epithelium
• can cause Reiters syndrome, hemolytic-uremic
  syndrome (HUS), convulsions in children

• Estimated 440,000 cases per year in U.S.
• Epidemics in Africa and Central America have
  5-15 fatality rate
• Second most common source of waterborne disease
  outbreaks in U.S. from 1972-1985 (also foodborne)
• Infective dose 10-200 organisms !
• On August 20, 1995, 82 cases of shigellosis
  occurred at resort in Island Park, Idaho due to
  high water tables and leaky sewage lines

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Enteric bacteria - Vibrio cholerae

• Gram (-) oxidase () fermentative facultative
  aerobes
• first waterborne disease whose epidemiology was
  determined (John Snow, 1854)
• native marine microbe
• infection is usually asymptomatic or mild. One in
  20 develop cholera Cholera profuse watery
  diarrhea (and rapid dehydration), vomiting, leg
  cramps, shock
• 20-50 fatality rate within a couple of hours if
  untreated (with antibiotics) due to dehydration
• disease due to production of enterotoxin
• only 5 cases per year in U.S., but pandemic in
  India, sub-Saharan Africa, and Latin America

1991 A cholera epidemic began in Peru, spread
to Central and South America affecting 1,041,400
people with 9640 deaths.
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Waterborne Viral Pathogens

• Viruses are the leading cause of gastroenteritis
  (GE)
• inflammation of the mucous membrane of the
  intestine, usually accompanied by cramps and
  diarrhea (and dehydration)
• Half of all waterborne gastroenteritis outbreaks
  have no known etiology (cause). It is thought
  that viruses are the responsible agents
• Viruses last longer in the environment and have a
  lower minimal infective dose than bacteria
• Although many of these viruses infect animals,
  infection is usually species-specific (only human
  viruses infect humans)

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Viruses That Cause Gastroenteritis

• Astrovirus first described in 1975 by EM
  following a diarrhea outbreak in a Scottish
  hospital maternity ward
• 28 nm diameter
• ssRNA genome
• mild GE in 1-3 year old children. Seroprevalence
  studies show that more than 80 of children
  between 5 and 10 years old have antibodies to
  astroviruses
• rare in infants, young adults, adults, elderly

most common cause of GE in the immuno-compromised
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• Rotavirus
• 70 nm diameter
• segmented dsRNA genome (11 segs.)
• 2-layered protein capsid
• vomiting, watery diarrhea, mild fever for 4-8
  days
• usually spread person-to-person, but can also be
  waterborne
• Rotavirus A is endemic worldwide, and is the
  leading cause of infantile GE and diarrhea.
  Adults can be infected, but disease is usually
  subclinical
• Rotavirus B can cause severe disease in adults
  and has caused epidemics in China involving
  millions of victims
• 2.7 million cases per year in U.S., including gt
  49,000 hospitalizations and 150 deaths

Almost 1 million infants die worldwide from
rotavirus (mainly by dehydration from diarrhea)
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• Norwalk virus
• 26-35 nm diameter
• ssRNA genome
• unable to be propagated in cell culture, so not
  much is known about it
• identifiable with EM of stool samples

most common cause of waterborne viral
gastroenteritis

• Adenovirus
• 70 nm diameter
• dsDNA genome
• 49 serotypes cause human disease
• - primarily cause respiratory disease
• - also cause waterborne GE and conjunctivitis
• Resistant to drying, inactivation in tap water
  and seawater, heat, and UV
• dsDNA genome uses host cell DNA repair
  mechanisms to repair itself
• - serotypes 40 and 41 (enteric adenovirus)
  are second most common
• causes of GE in children
• - serotypes 3 and 4 cause most outbreaks of
  waterborne viral conjunctivitis

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Enteroviruses

• 27-32 nm diameter
• ssRNA genome
• includes polioviruses, coxsackieviruses,
  echoviruses, and enteroviruses
• May be waterborne or spread person-to-person
  (airborne) Some groups cause gastroenteritis, but
  mainly responsible for causing meningitis,
  paralysis, conjunctivitis, and respiratory
  diseases
• Waterborne oubreaks difficult to establish, since
  infection is usually subclinical (no symptoms)

Aggressive vaccination worldwide has nearly
eliminated paralytic polio
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Hepatitis A Virus (HAV)

• Closely related to Enteroviruses (and
  morphologically identical)
• Most common viral waterborne disease from
  1946-1994
• In the late 1990s, hepatitis A vaccine was more
  widely used and the number of cases reached
  historic lows.
• 10-50 day incubation period, during which viruses
  are shed in feces
• fever, malaise, nausea, anorexia (loss of
  appetite) abdominal discomfort, followed by
  jaundice

• Almost asymptomatic in children, with symptoms
  increasing in severity with age
• Epidemics occur both nationally and within
  communities
• One-third of Americans have evidence of past
  infection (immunity).
• Not a chronic disease, like Hepatitis B
• in fact, HAV and HBV are not related
• Very stable in environment, heat stable, and
  resistant to chlorine disinfection

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Waterborne Protozoan Parasites
Giardia lamblia (G. intestinalis, G. duodenalis)

• Phylum Zoomastigina, Order Diplomonadida, Family
  Hexamitidae
• i.e. a flagellate
• Trophozoites (active, feeding stage)
• 14 ?m long
• teardrop shaped
• 4 pairs of flagella
• ventral sucking disk
• two nuclei

• Very primitive
• no mitochondria, nucleoli, peroxisomes
• anaerobic!
• rRNA more like prokaryotes in size
• replicates by binary fission
• Giardia has 5 chromosomes, with 4-10 copies of
  each in each nucleus

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• Giardiasis, Beaver Fever
• Attaches to the epithelium of the duodenum (where
  its anaerobic) with sucker disk
• absorbs bile and other intestinal goodies
• Exudes enzymes and other substances that damage
  Na and K pumps in the epithelium, allowing
  salt, and then water to leak into the lumen,
  causing diarrhea, 1-4 weeks
• Infection can be asymptomatic, particularly in
  previously infected hosts but hosts still shed
  cysts

• Most common agent of waterborne disease outbreaks
  1972-1985
• Endemic worldwide
• CO, OR -- incidence is high as 13
• Infects many other warm-blooded animals
• Animals may infect humans, but this isnt proven
• Repeat infections are possible, but some immunity
  is acquired

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• Cysts
• victims suffering from diarrhea pass trophozoites
  in feces, which quickly die in environment
• when stools are more well-formed, trophozoites
  encyst when they reach the lower intestine

• The signal to encyst is cholesterol starvation
• cysts are 12-15?m, oval shaped, containing 4
  nuclei
• last over a month in the environment, esp. in
  cold watersheds
• resistant to chlorine disinfection, but easily
  removed by settling and filtration
• when swallowed, cysts excyst in the duodenum,
  releasing 2 trophozoites
• Readily detectable in stool and environmental
  samples with fluorescent monoclonal antibody
• minimal infective dose only 10 cysts!

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Cryptosporidium parvum

• Phylum Apicomplexa, Class Sporozoea, Order
  Eucocciida, Suborder Eimeriina, Family
  Cryptosporiidae
• i.e. apicomplexan, or coccidian
• Several life stages including oocyst/sporozoites/m
  erozoites/zygote

Oocyst is the most hardy, resistant life stage

• oocysts are 3 to 6 um in diameter
• survive for weeks in surface waters
• resistant to chlorine disinfection
• oocyst contains 4 naked sporozoites which are
  released upon excystation
• 20 of oocysts are thin-walled and excyst within
  original host
• oocysts pass from host in feces

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• Cryptosporidiosis/Coccidiosis
• 3-10 day incubation period
• 10-14 days profuse watery diarrhea, stomach
  cramps, slight fever
• after symptoms cease, may pass cysts in feces up
  to 2 months
• autoinfective (thin-walled cysts can excyst
  within and reinfect original host)

• immunocompromised may not be able to clear
  infection mortality rate is 10-15 in AIDS
  patients
• Infects all mammals, especially cattle
• Oocysts persist 6-12 months in the environment!
• Highly resistant to chlorination
• minimal infective dose 15-100 oocysts

Outbreak in Milwaukee, 1993 affected 400,000
people
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Emerging Waterborne Pathogens

• Pathogens that we are only now becoming aware of
  and linking to disease
• Many emerging pathogens were discovered due to
  the AIDS epidemic

Helicobacter pylori

• Binds to epithelium in stomach and duodenum
• produces urease that locally lowers pH,
  disrupting mucous layer and causing peptic and
  gastric ulcers
• 90 of duodenal and 80 of gastric ulcers caused
  by H. pylori infection, not spicy food, acid, or
  stress
• 2/3 of the worlds population is infected

• Most likely a waterborne disease
• In 1996, the FDA approved the use of antibiotics
  to treat (and cure!) peptic ulcers

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• Phylum Microspora
• Enterocytozoan bieneusi, E. hellem, E. cunniculi,
  E. intestinalis, Pleistophora spp., and Nosema
  corneum cause disease in humans

no mitochondria! may be closer to fungi than
protozoa!

• Spores are only 1.5 ?m in diameter!

• Contain coiled polar filament
• under certain conditions, filament explodes from
  cyst and pierces host cell
• sporoplast (contents of cyst) are injected into
  host cell cytoplasm
• reproduces asexually (merogony), but not sexually

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• E. bieneusi causes diarrhea, and is a very common
  infection of AIDS patients
• E. intestinalis can infect macrophage and can
  disseminate through body. Cysts shed in feces
  and urine
• E. cunniculi causes hepatitis, and is shed in the
  urine

• E. hellem causes conjunctivitis, uretitis, and
  pneumonia, and is shed in the urine
• If cysts are shed in the feces and urine, then
  waterborne transmission is likely. . .

The Microsporidia are all obligate intracellular
parasites and spores appear to be nearly
ubiquitous.  There are currently approximately
150 described genera of Microsporidia.
 Microsporidia parasitize animals from virtually
all groups, however, the vast majority of
Microsporidia attack insects and other
arthropods.  One microsporidian, Nosema
locustae, is even commercially marketed (as NoLo
Bait) for biological control of grasshoppers,
locusts and crickets.  However, a related
species, Nosema apis, is a serious problem for
bee keepers.