Introduction to Environmentally Transmitted Pathogens, Part 2

Envr 133

Mark D. Sobsey

Spring, 2006

Some Important Pathogens of Aquatic and Terrestrial Environments

• Microbial Agents:

– Most are from feces or other excreta (urine, respiratory exudates, etc.) of humans and/or other animals

– Some are of natural origin

• either exclusively or in addition to fecal contamination

• Enteric Microbial Agents:– Infect the human and/or animal gastrointestinal (GI) tract

• Some enteric microbes also infect or invade other sites in the body

IMPORTANT HUMAN ENTERIC VIRUSES

Viruses/Groups Animal Feces

Enteroviruses: no

(polios, echos*, coxsackies*, etc.)

Hepatitis A virus no (primates)

Hepatitis E virus pigs, rats, others

Reoviruses yes

Rotaviruses yes**

Adenoviruses* yes**

Noroviruses*: maybe**

Norwalk, Snow Mountain, etc.

*On EPA’s candidate contaminants list (CCL).

**humans & animals usually infected by different ones; but perhaps not always.

IMPORTANT HUMAN ENTERIC BACTERIAL PATHOGENS AND OTHER SOURCES OF THEM

Bacterium/Group AnimalsOther SourcesSalmonella spp. yes no

(except S. typhi) no noCampylobacter spp. yes yesEscherichia coli yes noHelicobacter pylori* unknown unknownAeromonas hydrophila* yes yesYersinia enterocolitica yes yesVibrio cholerae; other Vibrio spp. yes yesLeptospira yes noMycobacteria spp. (non-tubercular) very rare yesShigella spp. no no

*On EPA’s candidate contaminants list (CCL).

IMPORTANT HUMAN ENTERIC PROTOZOAN PATHOGENS

Parasite Animal Feces

Cryptosporidium parvum yesCyclospora cayetanensis* unknownGiardia lamblia yesEntamoeba histolytica rareBalantidium coli yes (pigs)Microsporidia* yes

(Enterocytozoon and Septata) unknown

Toxoplasma gondii* yes

*On EPA’s candidate contaminants list (CCL).

Helminths (Worms): Some of the Important Ones

Most acquired from ingestion of or contact with feces-contaminated soil or food

• Nematodes (Roundworms):– Ascaris lumbricoides 1 GI illness; pneumonitis– Trichuris trichuria 1 chronic GI

• Hookworms:– Ancylostoma duodenale 1 chronic anemia– Necator americanus 1 chronic anemia– Strongyloides stercoralis1 chronic anemia

• Cestodes (tapeworms):– Hymenolepis nana 1 GI illness

Some Non‑fecal Bacterial PathogensBacteria:Legionella spp. >20 Pneumonia; febrile illness

L. pneumophila, etc. Legionnaires’ diseaseMycobacterium spp. many Upper respiratory illnessM. tuberculosis 1 tuberculosisM. avium-intracellulare several Upper respiratory illnessOther bacteria many Variable; "opportunistic"

or "conditional pathogens

Some Non‑fecal Protozoan Pathogens

Free living amebas:

Naegleria fowleri 1 Primary amebic menino-encephalitis

Acanthamoeba spp. few eye infections; encephalitis

Some Non-fecal HelminthsAgent: No. Illness and Sites• Dracunculus medenensis (N) 1 Tissue infections (subcutaneous & deep;

esp. foot and/or leg• Schistosoma (T; blood fluke) 3 Liver, intestine, colon

S. haematobium, S. mansoni bladder & rectum fromand S. japonicum colonization of venous

vessels.• Schistosoma spp. (T). few swimmers itch: larvae birds and

fish penetrate skin; not in bloodstream; no maturation in human

Occurrence of Microbial Pathogens in Humans

• Microbial pathogens usually are not “normal flora” of humans; opportunities for pathogenicity are possible– Some are “normal flora” of animals:

• Salmonella enteriditis and Campylobacter jejuni in poultry– “Normal flora” for local populations may be pathogenic

for visitors and transient populations:

• “Traveller’s diarrhea” due to local strains of E. coli– “Some “normal flora” are pathogenic for sensitive

populations, such as immunocompromised persons:• Example: Pneumocystis carinii (a protozoan or fungus)

– causes fatal pneumonia in AIDS patients– immunocompetent people get asymptomatic infections

Occurrence of Enteric Microbial Pathogens in Humans and Pathogen Shedding

• Enteric (gastrointestinal) illnesses are second only to respiratory illnesses in the population

• Most people get 1 enteric illness per year in the developed world:– Annual illness rates are higher in infants, children, the elderly,

child caregivers, health professionals, the poor, male homosexuals and other high risk groups & developing countries

• Not all enteric infections produce illness (asymptomatic or sub-clinical infections)– So, rates of infection are even higher (by perhaps 2 to 100 times)

• People (and animals) with enteric infections fecally excrete high concentrations of pathogens for days, weeks, months or longer.

• Pathogen concentrations can be >106 to >109 per gram of feces.• Community pathogen shedding is often 1-10% at any time.

Disease Due to Some Important Waterborne Enteric Virus Pathogens

Norwalk Virus Gastroenteritis: A Localized Infection• Fecal‑oral transmission• Localized infection of small intestine• Damage to microvilli of intestinal epithelium

– “blunting” of the microvilli• Incubation period 1‑3 days • Illness 1‑3 days • Major symptoms: diarrhea, vomiting, nausea, abdominal pain and low

grade fever • Fecal shedding from onset of illness for several days.

– Virus concentration in feces as high as 108/gram• Low infectious dose; perhaps as few as 10-100 virus particles

– Virus has not been cultured in laboratory animals or cell cultures

Response of Human Volunteers to Norwalk Virus Infection via the Oral Route

Disease Due to Some Important Waterborne Enteric Virus Pathogens

Hepatitis A Virus and Infectious Hepatitis: A Systemic Infection• Fecal‑oral transmission• Systemic (generalized; disseminated) infection• Liver as "target organ" • Incubation period 2‑6 weeks; average 4 weeks • Illness for several weeks or months• Destruction of liver hepatocytes• Jaundice (in some but not all cases) and severe "flu‑like"

symptoms, including gastrointestinal symptoms. • Virus shed fecally from 2 weeks before to a few weeks after onset

of symptoms.

Disease Due to Some Important Waterborne Enteric Bacterial Pathogens

Salmonella gastroenteritis: (S. enteriditis): localized infection• Fecal‑oral transmission• Localized infection of intestines• Damage and inflammation to lamina propria• 0.5‑2 day incubation period• Watery diarrhea, nausea, vomiting, abdominal cramps,

low grade fever, lasting several days• Bacteria shed fecally at billions per gram

• Infectious dose is relatively high: >103 ID50 for many strains

Disease Due to Some Important Waterborne Enteric Bacterial Pathogens

Typhoid fever: (S. typhi and S. paratyphi): Systemic Infection• Fecal-oral transmission• Systemic infection:

– Macrophages, reticuloendothelial system (esp. liver, spleen and bone marrow), gallbladder and intestines as major sites of damage

• 1.5‑2 week incubation period • Symptoms: fever, headache, malaise, anorexia, then bloody

diarrhea • Mortality rate 10%, if untreated• Carrier state possible

– "Typhoid Mary”: infamous food handler; infected hundreds• Fecally shed at billions/gram by ill persons and carriers

Disease Due to Some Important Waterborne Enteric Protozoan Pathogens

Giardiasis (Giardia lamblia): localized enteric infection• Fecal‑oral transmission; hardy cyst ~10 m diameter• Human and numerous non‑human animal reserviors• Infectious dose: low: ID50 ~10 cysts• Infection: cysts excyst in small intestine; trophozoites attach to microvilli

of intestinal epithelium, tissue damage and• Interference with transport processes • Profuse watery to semi‑solid, greasy, bulky, malodorous diarrhea;

abdominal cramps,nausea, vomiting, anorexia, low grade fever, headache

• 1‑1.5week incubation period• Duration of Illness: few days to months• Subchronic infection possible

Disease Due to Some Important Waterborne Enteric Protozoan Pathogens

Cryptosporidium and cryptosporidiosis• Cryptosporidium parvum: coccidian (sporozoan) parasite• Numerous animal reservoirs: feral, domestic and agricultural• Fecal-oral transmission of hardy oocyst, ~5 m diameter• Infectious at low dose: ID50 ~10 oocysts for some strains• Excysts in small intestine; trophozoites attach to epithelial cells• Complex life cycle; 6 major stages, some asexual, other sexual• Infection and illness in immunocompetent hosts: similar to giardiasis:

diarrhea, nausea, vomiting, anorexia, fever, malaise– Incubation period ~1 week; duration ~1.5 weeks, range 1-4 weeks

• Infection in immunocompromised hosts (ex, persons with AIDS):– Life threatening, excessive fluid loss, chronic, no drug therapy– ISpread to extra-intestinal sites: respiratory tract; pneumonia.

Constitutive Defenses: Physical Barriers to Infection

System or Organ Cell Type Clearing Mechanism

SkinMucousmembranes

SquamousColumnar nonciliated(e.g., gastrointestinal tract)Columnar ciliated(e.g., trachea)Cuboidal ciliated(e.g., nasopharynx)Secretory

DesquamationPerstalsis

Mucociliary movementTears, saliva, mucus,sweat

Flow of liquids

Examples of Encounters and Disease PreventionType ofcontact

Example Type ofAgent

Source Strategy forPrevention

Preventive Aim

Inhalation Common cold Virus Aerosol frominfectedpersons

None Difficult to avoidcontact

Ingestion Coccidiodo-mycosisTyphoid feverSalmonella foodpoisoning

Fungus

BacteriumBacterium

Soil

Water, foodFood

None

SanitationSanitation

Hard to avoidcontactLower infectingdose

Sexualcontact

Gonorrhea Bacterium Person SocialBehavior

Avoid contact

Wound Surgicalinfections

Bacterium Normal florasurroundings

Aseptictechniques

Avoid contact

Insect Bite Malaria Protozoan Mosquito Insectcontrol

Eliminate vector

Constitutive Defenses: Chemical Barriers to Infection

System or Organ Source Substances

SkinMucous membranes

Sweat, sebaceous glandsParietal cells of stomachSecretionsNeutrophils

Organic acidsHydrochloric acid, Low pHAntimicrobial compoundsLysozyme, peroxidase,lactoferrin

Lung A cellsSalivary glandsNeutrophils

Pulmonary surfactantThiocyanateMyeloperoxidaseCationic proteinsLactoferrinLysozyme

Small bowel and below Liver via biliary treeGut flora

Bile acidsLow molecular weight fattyacids