Wk#11: Diseases Transmitted by the Fecal-oral Route

Common features of fecal-oral route

• Portal of entry: mouth

•  Portal of exit: feces

•  Human as the principle host, if not the only host (no animal reservoir)

•   Transmitted from human to human, directly or indirectly

•  Low infectious inoculum of the etiological agents

  • Thus highly contagious 

Transmission of fecal oral route

1. Direct contact: caretakers, nursing homes, food handlers, family members 

2. Contaminated water

3. Contaminated food

Human-Digestive System Parts

• Mouth

• Stomach

• Small Intestine

• Large Intestine

• Normal Microflora

Defense of the Human Digestive System

• High pH

• IgA

• Phagocytic cells and tissues rich in lymphocytes underneath the mucosal membrane

• Microbiome

Hepatitis A: Virology

Family: Picornaviridae

(+), ss RNA virus. One serotype (vaccine effective)

Genome is 7,478 nucleotides, codes for 4 proteins

Infectious inoculum: 10-100 viral particles

Hepatitis A: Epidemiology

Worldwide

After vaccine developed, incidence has been reduced

Hepatitis A: Clinical Features

• 15-45 day incubation

•  Symptoms

• Fever, Headache, Diarrhea, Nausea, Vomiting, Jaundice. Occasional death

• Hepatitis 

• Self-limiting (2 weeks to several months)

•  Diagnosis

  • IgM to HAV

  • Life long immunity

Noroviruses: Virology

• Genus: Norovirus

• Family: Caliciviridae

• Group of related ss RNA, non-enveloped viruses. 7,654 bases.

• Genetically and antigenically diverse

  • Five geno-groups (I, II, IV infect human) and over 20 genetic clusters

• Highly contagious 

Noroviruses: Epidemiology

• Very common; highly contagious 

• Major cause for stomach flu (viral gastroenteritis) 

• Believed to cause >50% of food borne outbreaks of gastroenteritis

Noroviruses: Clinical Features

**No serious long-term sequela 

Incubation: 12 – 48 hours

Acute gastroenteritis. Symptoms last 24-60 hours.

Complications: dehydration 

Primary Symptoms: Vomiting, watery, non-bloody diarrhea, abdominal cramps, nausea 

Secondary Symptoms: Fever, Headache, Chills, Muscle aches, Tiredness

Norovirus: Treatment

• Supportive only

• Recovery complete, no long term consequences 

• Follows iceberg phenomenon; asymptomatic infections high 

Norovirus: Transmission

• Fecal-oral route

• Viral shedding

• Highly contagious 

• Environmental / fomite contamination

  • Aerosolized vomitus

Poliomyelitis Other Name

polio or infantile paralysis

Poliomyelitis: Virology

• Family Picornaviridae

• Ss (+) RNA genome about 7,500 nucleotides

• Non-enveloped with only RNA and a non-enveloped icosahedra protein coat

• Three serotypes, PV1, PV2 and PV3

Poliomyelitis: Clinical Manifestations

• Follows iceberg (95% asymptomatic, 5% fever/headache, & <1% paralytic poliomyelitis) 

• paralytic poliomyelitis

• post-polio syndrome

paralytic poliomyelitis

Muscle paralysis, skeletal deformity, and movement disability

post-polio syndrome

25% of in paralytic poliomyelitis survivors develop post-polio syndrome decades later

including muscle weakness, extreme fatigue, or paralysis

Poliomyelitis: Transmission

• Fecal-oral route

• Viral shedding

• Highly contagious 

Poliomyelitis: Vaccines

• IPV: Inactivated Vaccine, developed by Jonas Salk, 

  • Protects against paralysis, but not transmission of PV

• OPV: attenuated vaccine, developed by Albert Sabine

  • Able to replicate in the gastrointestinal tract but not the central nervous system.

IPV vs. OPV

OPV: Attenuated, Able to replicate in the GI tract but not the CNS

• Used for global campaigns because of its (1) immunity in GI tract (2) ease of administration.

IPV: Inactivated, Protects against paralysis, but not transmission

*** OPV was discontinued in the US in 2000 to avoid vaccine-associated poliomyelitis. IPV is used exclusively.

SARS-CoV-2: Virology

• Genome: 29,903 bases, 27 open reading frames

• Classification: (+) RNA virus, coronaviridae

• Receptor: ACE2

SARS-CoV-2 and COVID-19: Symptoms 

• GI symptoms

  • occur in 20 to 30% of patients

• usually mild

• Viral RNA detected in fecal samples

  • not been shown to transmit through fecal-oral route

Parasitic GI Diseases-Common Features

• Infections by parasites including protozoans and parasitic helminths

• Life cycle: taking different forms to complete a life cycle (eggs or cysts, trophozoites, larva, adult forms, etc). 

  • Some need additional hosts before transmitting to humans

• Incubation period: 1-3 weeks

•  Can be treated

  •  No permanent damage in most cases

Giardiasis Protozoa

Giardia intestinalis

Giardiasis: Life cycle 

Cyst form survive, but not trophozoites (due to harsh environment) 

Cyst -> trophozoites -> cyst

Cyst: infective stage

Trophozoites: diagnostic stage

Giardiasis: Clinical features 

**Leading intestinal parasite

Symptoms: diarrhea, bloating, cramps, fatigue

Incubation period: 1 – 3 weeks

Treatment: chemotherapy (metronidazole)

Amebiasis Protozoan

Entamoeba histolytica

Amebiasis: Life Cycle

Excystation: trophozoites - can either multiply into more or turn into cysts

• Passed in feces

Amebiasis: Clinical Features 

Common in tropical areas with poor sanitary conditions

Symptoms: often mild and can include loose stools, stomach pain, and stomach cramping 

• Amebic dysentery is a severe form.

Incubation period: 2-4 weeks

Complications: liver abscess and intestinal blockage

Amebiasis: Treatment

Treatment: chemotherapy (metronidazole, paromomycin)

Protozoan GI Diseases and their protozoa

Giardiasis: Giardia intestinalis

Amebiasis: Entamoeba histolytica

Balantidiasis: Balantidium coli

Cryptosporidiosis: Cryptosporidium

Cyclosporiasis: Cyclospora cayentanensis

Gastrointestinal Nematodes

Trichinosis

• Trichinella spiralis

 Hookworms

 Ascariasis

• Ascaris lumbricoides

 Trichuriasis

• Whipworm

• Trichuris trichiura

 Strongyloidiasis

• Strongyloides stercoralis

 Pinworms

• Roundworm

• Enterbius vermicularis

Ascaris lumbricoides: Life Cycle

• Fertilized egg 

• Unfertilized egg will not undergo biological development 

Ascaris lumbricoides: Clinical Manifestations

Largest intestinal round worm. Worldwide spread.

Symptoms: abdominal pain, vomiting

Incubation period: weeks

Complications: no permanent damage

Ascaris lumbricoides: Treatment

Treatment: chemotherapy (mebendazole, albendazole, or pyrantel pamoate)

Gastrointestinal Helminths

• Flukes

• Tapeworms

Guinea Worm (Dracunculus medenisis): Life cycle

1. Water fleas ingest worm larvae.

2.  People drink water contaminated with water fleas, and the larvae resist digestion and migrate to the small intestine and penetrate the intestinal wall into the body cavity where they grow up and mate. 

3. Then they move through connective tissue through various areas of the body (usually lower limbs) 

4. Worm forms a painful blister near skin surface and blister burst exposing worm 

Guinea Worm Name

Dracunculus medenisis

Helicobacter pylori: Microbiology

• Gram (-), spiral shape, highly mobile

• Genome size: ~1.6 Mb, highly AT rich 

• Displays high interstrain diversity

• major species causing human diseases

• **Produces large amount of urease to neutralize stomach acid (so bact can survive in stomach) 

Helicobacter pylori: Clinical Features

• Follows iceberg 

• Stomach or duodenal ulcers (10-20%) 

• Cancers (1-2%) 

Helicobacter pylori: Transmission

currently unknown

Helicobacter pylori: Diagnosis + Treatment

Diagnosis (none is fool-proof; many methods to confirm)

• blood test for antibodies

• urea breath test

• biopsy combined with urease test and histological exam

 Treatment: triple therapy

• antibiotics amoxicillin and clarithromycin

• proton pump inhibitor such as omeprazole

Ulcers 

• Ulcers were believed to be caused by stress, spice and excessive acid in the stomach

• Barry Marshall and Robin Warren (nobel prize 2025) 

  • observed small curved bacteria in lower stomach of biopsies of ulcer patients

  • No one believed them so barry marshall drank culture of Helicobacter and treated himself with antibiotics to prove