Chapter 22 - Pathogens

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

3. 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) • Virulenceis 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

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 • 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

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 • enteropathogenicE. coli • watery diarrhea with mucus, fever, dehydration • common in infants in less developed countries (50% mortality rate) • high minimal infective dose: 106-109 • enterotoxigenicE. 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 • enteroinvasiveE. 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. enterohemorrhagicE. 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 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

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. 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 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

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 1972-1985 • 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: 15-100 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.