Case 1 Answers
1. What
is the mechanism of transmission of Vibrio cholerae?
Major reservoir is humans.
Transmitted through ingestion of contaminated food and water. Large inoculum (108
-
1010)
required to establish infection with normal gastric acidity.
2. What
host factors increase the likelihood of infection with V. cholerae
A- or hypochlorhydria.
3. Describe
the mechanism of action of cholera toxin.
The A1 portion of the cholera
toxin catalyzes the ADP-ribosylation of the Gs (stimulatory) regulatory
protein, "locking" it in the active state. Gs acts to
return adenylate cyclase to its active state resulting in persistent activation
of AC. Accumulation of cAMP results inactive secretion of Na, K, Cl, HCO3,
and water into the intestinal lumen.
4. Besides
toxin production, what other virulence determinant is essential for the
development of cholera?
The
ability to adhere to the mucosa. Nonadherent mutants are avirulent.
5. How
would you manage the dehydration? Why can oral rehydration fluid be used in
many cases of cholera?
For severely ill persons, TV
fluids containing approximately isotonic concentrations of Na and Cl with
super-physiologic concentrations of K and bicarbonate (or lactate). Taking advantage of a glucose-linked sodium
absorptive mechanism that is not affected by cholera toxin, less ill persons
can maintain hydration with glucose containing oral solutions of electrolytes.
6. What
advice would you provide to other travelers to Rwanda concerning avoidance of
cholera?
Drink only bottled water or beverages,
no ice, and eat only freshly prepared cooked foods. Wash hands after using the lavatory and before eating
7. List three public health solutions to the global problem of epidemic cholera.
Improved sanitation even including more effective latrines, an effective vaccine, and improved standard of living, in the third world might be suggested.
Case 2 Answers
1. What enteropathogenic type of Escherichia coli is causing this infection?
Enterohemorrhagic E. coli. These organisms are usually of serotype 0157:H7 and the disease often occurs in outbreak.
2. What complication of E. coli enteritis did this patient develop? Define the complication.
Hemolytic uremic syndrome (HUS). HUS is a systemic disease characterized by microangiopathic hemolytic anemic, thrombocytopenia, and intra renal thrombosis resulting in hematuria, renal insufficiency, and central nervous system manifestations. HUS developed in 6% of patients with EHEC from US outbreaks and the mortality was 1.2%.
3. List the virulence characteristics of E. coli that are responsible for this syndrome?The major virulence factor is the production of one or more related shiga-like toxins, so called because they are biochemically, genetically and immunologically related to the Shiga toxin of Shigella dysenteriae. The organisms also colonize the distal ileum, cecum and large bowel by means of a plasmid mediated factor, a pilus. The organisms also have lipopolysaccharide, but its role in hemorrhagic colitis or HUS is not defined.
4. Describe the pathogenic process. In your response address how the infectious microorganism might be responsible for hemorrhage, anemia, and renal failure.
After ingestion, colonization of the ileum, cecum and colon occurs via pili. The attachment resembles the attachment and effacement lesion initially described for EPEC and presumably involves a plasmid encoded "bundle forming pilus". In EPEC, the interaction between the adherent organism and host cell involves an increase in calcium ions in host cell protein phosphorylation and cytoskeletal rearrangement with the formation of dense "pedestals" of filamentous actin under the site of attachment. The shiga-like toxin is an AB type in which the B subunit binds to cell membrane glycolipid. The A subunit is internalized by endocytosis into a membrane bound vesicle. It crosses the vacuolar membrane in the trans-Golgi network and enzymatically modifies the 28S ribosomal RNA of the 60s-ribosomal subunit by removing an adenine base. This prevents elongation-factor 1-dependent binding of amino acyl t-RNA to the ribosome blocking protein synthesis, leading to cell death.
Animal experiments with Shiga toxin
and shiga-like toxins disclose capillary thrombosis and associated inflammation
of the colonic mucosa leading to hemorrhagic colitis similar to that seen in
patients. It is postulated that some of the toxins gain access to the
circulation and cause endothelial damage in renal capillary beds, other vascular
beds, or that they alter platelet function. Alternatively, a recent review of E.
coli 0157:H7 and the hemolytic uremic syndrome suggests that the colonic
vascular damage allows Ips and other inflammatory mediators to gain access to
the circulation and initiate HUS.
5. How is the infection acquired? Include in your answer an explanation of how the most common vehicle of transmission is
likely to cause large outbreaks.
Infection is usually acquired by ingestion of undercooked ground beef. Additionally more
recent outbreaks have been associated with vegetables contaminated with tainted ground beef or other cattle products and
unpasteurized apple juice distributed from California. Additionally several outbreaks associated with swimming, in fecally
contaminated lakes or consumption of contaminated municipal water supplies are reported. Secondary cases occur in as many
as 22% of contacts in households and day-care centers documenting person-to-person transmission. These high secondary
attack rates suggest that the inoculum required to cause infection is low.
One percent of healthy cattle are colonized with E. coli 0157:H7. The process of grinding beef may transfer the surface
pathogens to the interior which in turn protects the organisms from all but complete heating (cooking). In addition, commercial
ground beef includes meat from many carcasses allowing a small number of infected cattle to contaminate a large supply of
ground beef.
A.
ETEC - Enterotoxigenic E. coli is the commonest cause of travelers diarrhea.
Its mechanism involves attachment and production of one or two toxins-LT -
an -AB toxin that functions like cholera toxin and ST, a small peptide that
activates membrane-bound guanylate cyclase and ultimately resulting in watery
diarrhea.
B. EIEC - Enteroinvasive E. coli which
causes clinical disease indistinguishable from that caused by invasive shigella
(although less severe).
C. EPEC - Enteropathogenic E. coli
which produce the adherence and effacement lesions described above. They are
associated with but not proven to cause self-limited diarrhea in adults and
chronic watery diarrhea in infants.
D. EAEC - Enteroaggregative E. coli
are associated with watery diarrhea in epidemiological and volunteer studies as
well as chronic diarrhea in certain developing countries. The poorly understood
postulated mechanism is similar to that described for EPEC, i.e. adherence and
effacement.
7. Should the child be isolated in the hospital?
Yes.
Given the relatively high secondary attack rates, person-to-person spread
is likely.
8. How does one diagnose the infection?
Most strains of E. coli 0157:H7 do not ferment sorbitol whereas other E. coli strains ferment sorbitol. Sorbitol-MacConkey
agar can quickly identify sorbitol-negative strains. These strains can be confirmed with commercially available 0157 antisera.
Reference laboratories canperform the H typing. The CDC recommends that clinical laboratories screen all stools for the
organism. Many laboratories, including our own, have chosen to screen only bloody stools or on request of the physician,
however many patients will not have grossly bloody stools. Given a conservatively estimated annual US incidence of 21,000
cases, and a greater frequency than shigella, screening is strongly encouraged. Tests for shiga-like toxin or for the toxin gene
are being developed for use in the future.
9. Should antibiotics be used to treat the child? If so, which ones are used?
Trimethoprim/sulfamethoxazole (TMP-SMX) or quinolones should be used.
Case
3 Answers
1. What is the likely cause of this man's diarrhea?
Salmonella
2. Do these organisms cause diarrhea by invasion or toxin production? Describe what is known of the pathogenesis of diarrhea caused by these organisms?
Invasion. Salmonella surface factor reacts with host cell receptor to stimulate "ruffling" caused by actin rearrangement. Organisms are internalized by pinocytosis, allowing internalization of adjacent salmonellae as well. From there on it is only speculated that the process involves transport across the cell, through the basolateral membrane, giving access to the lymphatic cells of the lamina propria where the organisms induce an inflammatory response. The enterocytes of the large and small bowel are involved.
3. How did the members of the family become infected? What is the usual route of infection with this pathogen?
Probably from a raw egg in the Caesar salad.
4. What is the preferred therapy of enteritis due to salmonella?
Rehydration.
Antibiotics have not been shown to affect the clinical course of salmonellosis.
However they are indicated in severely ill individuals in whom bacteremia is
suspected or shown.
5. What other clinical syndromes are described with salmonella infection?
Septicemia, enteric fever (typhoid fever) and asymptomatic carrier state.
6. Given the present distribution systems for meat, poultry and dairy products in the United States, what measures could be adopted to reduce the spread of salmonella infection?