1. 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

2. 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
The apicomplexa is a phylum of protozoa that includes the most important parasitic protozoan pathogens in humans and livestock. These include Plasmodium, Toxoplasma, Neospora, Sarcocystis, Cryptosporidium, Eimeria, Theilaria, Babesia and others

3. Pathogens are infectious organisms that harm their host
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 > viruses > parasites
The apicomplexa is a phylum of protozoa that includes the most important parasitic protozoan pathogens in humans and livestock. These include Plasmodium, Toxoplasma, Neospora, Sarcocystis, Cryptosporidium, Eimeria, Theilaria, Babesia and others

4. 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)

5. 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

6. Bacterial Pathogens
High minimal infective dose
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

7. Enteric bacteria
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

8. 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 Reiter’s 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

9. 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

10. 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:
enterotoxigenic E. coli
cramping, vomiting, profuse diarrhea, dehydration
disease caused by the production of two toxins
common in travellers (traveller’s 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

11. enterohemorrhagic E. coli (e.g. O157:H7)
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 0157:H7 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

12. 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 Reiter’s 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 (also foodborne)
Infective dose: 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

13. 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.

14. 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)

15. 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

16. 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 > 49,000 hospitalizations and 150 deaths
Almost 1 million infants die worldwide from rotavirus (mainly by dehydration from diarrhea)

17. 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

18. Aggressive vaccination worldwide has nearly eliminated paralytic polio
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

19. Hepatitis A Virus (HAV)
Closely related to Enteroviruses (and morphologically identical)
Most common viral waterborne disease from
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

20. 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

21. Giardiasis, “Beaver Fever”
Attaches to the epithelium of the duodenum (where it’s 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
Endemic worldwide
CO, OR -- incidence is high as 13%
Infects many other warm-blooded animals
Animals may infect humans, but this isn’t proven
Repeat infections are possible, but some immunity is acquired

22. 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!

24. Cryptosporidium parvum
Phylum Apicomplexa, Class Sporozoea, Order Eucocciida, Suborder Eimeriina, Family Cryptosporiidae
i.e. apicomplexan, or coccidian
Several life stages including oocyst/sporozoites/merozoites/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

26. 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: oocysts
Outbreak in Milwaukee, 1993 affected 400,000 people

27. 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 world’s population is infected
Most likely a waterborne disease
In 1996, the FDA approved the use of antibiotics to treat (and cure!) peptic ulcers

28. 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

29. 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.