Intestinal Parasites Flashcards
1
Q
Giardia lamblia
A
- G. lamblia trophozoite (vegetative form) is pearshaped and approximately 13-19 x 8-11 microns in diameter
- The trophozoite is symmetrical with 2 median rods dividing it in half longitudinally
- It has 2 nuclei and on the ventral surface is a concave sucking disk
- The metabolism is anaerobic and the organism has no capability of synthesizing nucleic acids, so its dependent on pyrimidine and purine nucleotide salvage
2
Q
Giardia lamblia - Epidemiology
A
- Two forms; infectious cyst and a vegetative trophozoite
- Cyst is relatively resistant to the environment and gastric acid of people ingesting the cyst
- Infection occurs by ingestion of contaminated water (backpackers, contaminated municipal water), fecal-oral transmission (daycare centers, anal sexual transmission), and less commonly from contaminated foods
- Infects dogs, cats, and beavers and other mammals, animal to human infections not clear, beavers provide an important reservoir for human infections acquired from mountain waters
- Dogs and cats are usually infected with Giardia that looks identical to those that infect humans, but with different genotypes, animals to humans transmission is controversial
3
Q
Giardia - Pathogenesis
A
- Disease manifestations appear related to intestinal malabsorption of fat and carbohydrates
- Disaccharidase deficiency with lactose intolerance, altered level of intestinal peptidase and decreased vitamin B12 absorption have been demonstrated
- Mechanical blockade of the intestinal mucosa by large number of Giardia, damage to the brush borders of the microvilli , deconjugation of bile salts, altered intestinal motility, accelerated turnover of mucosal epithelium, mucosal invasion have been suggested
- Trophozoites have been demonstrated in the submucosa
- T cell mediated damage suggested
4
Q
Giardia - Immune Response
A
- Antibody mediated response is suggested by increased severity in patients with hypo-gamma globulinemia, probably with selective IgA deficiency
- Intestinal IgA response probably important in elimination of infection.
- Data from animal models suggests importance of the cell-mediated immune response
5
Q
Giardia – Clinical Disease
A
- Mostly asymptomatic, especially those with prior infection
- Symptoms include diarrhea consisting with loose foul-smelling stools, abdominal cramps, bloating and weight loss
- The duration of the illness is usually greater than a week and sometimes lasts for many weeks
- The minority of patients will have chronic diarrhea and malabsorption
- The mechanism by which Giardia causes diarrhea is unknown
6
Q
Giardia Diagnosis
A
- Stool PCR (Biofire 20)
- Stool exams:
-O & P (ova & parasite)
– Up to three are examined to increase the sensitivity of the test.
•Giardia antigen (ELISA)
- Fluorescent antibody detection
- A single FA or ELISA has approximately the same sensitivity as 3 O & Ps.
•String test:
-The patient swallows a capsule on the end of a string. The string is removed after several hours to overnight and the end of the string examined microscopically for Giardia trophozoites
7
Q
Giardia Treatment
A
- Metronidazole - the most commonly used drug in the U.S.
- Tinidazole is a related drug that can be given as single dose therapy and is now available in the U.S. It is widely used around the world
8
Q
Entamoeba histolytica
A
- E. histolytica is a protist (protozoan) ameba
- It has a trophozoite that colonizes the large intestine forms a cyst that is passed in the feces for transmission thru contaminated food or water
- The metabolism is anaerobic, and like the other anaerobic protozoan, there is no nucleotide synthesis
- More prevalent in lower socioeconomic area
- Fecal-oral transmission, sexual, chiropractic colonic irrigation
- Low prevalence in the U.S., patients with asymptomatic amebiasis are usually treated
9
Q
Entamoeba - Life Cycle
A
- After ingestion, the cyst travels to the intestine where it excysts into the trophozoite form
- The trophozoite replicates and causes disease in the large intestine
- Some of the trophozoites will then encyst into cysts and are passed in the feces where they are environmentally stable enough to establish infection in a new host
- E. histolytica is strictly a human infection, so there are no animal reservoirs
10
Q
Entamoeba - Pathogenesis
A
- Invades through the mucosa and into the intestinal wall, where tissue necrosis is a major feature of the pathogenesis
- Trophozoite kills neutrophils causing minimal neutrophilic infiltration
- Red cells also provide significant nutrition and frequently ingested red cells are found when trophozoites are examined microscopically
- Trophozoites spread to other parts of the body; the liver by retrograde travel through the portal vein. More rarely, the trophozoites disseminate to lungs and brain
- Invasive amebiasis is rare, colonization in colonic mucosa
- Cell-mediated immunity prevents invasive infection
11
Q
Entamoeba - Clinical Disease
A
- Asymptomatic infection may occur, may also be a noninvasive symptomatic infection with diarrhea and abdominal pain, controversial
- Symptomatic form of amebiasis: Amebic dysentery with abdominal pain and bloody diarrhea referred to as acute rectocolitis. Other causes of rectocolitis include shigellosis and Crohn’s or ulcerative colitis
- The illness is severe and may result in toxic megacolon with colon perforation and peritonitis, or dissemination outside the colon if not treated
- Most cases of extraintestinal amebiasis consist of liver abscess, with right upper quadrant pain, fever and elevated liver enzymes
- Liver abscess/lesions are seen on CT/MRI
12
Q
Diagnosis of amebiasis
A
- Biofire PCR Test
- Fecal diagnosis
- Histologic diagnosis
- Serologic diagnosis
- Exclude competing diagnoses (e.g. pyogenic liver abscesses, other causes of colitis)
13
Q
Treatment of amebiasis
A
• Systemic treatment
-Metronidazole
• Luminal treatment: asymptomatic amebiasis in non-endemic areas should be treated with a luminal agent (iodoquinol, paromomycin or diloxanide furoate) to eradicate infection
14
Q
Naegleria
A
- Naegleria fowleri is the principle amebic microorganism in freshwater that invades the CNS
- N. fowleri is normal microbiota of warm, freshwater ponds worldwide (often limestone reservoirs) and can become a pathogen when forcibly compressed against the cribriform plate when individuals dive into a body of water, often at great height
- Cysts, presumably are forced by water pressure across the cribriform plate, excyst and begin invasion of the central nervous system
- This disease process is known as Primary Amebic meningoencephalitis
- It is a rapidly lethal meningoencephalitis with elevated cerebrospinal fluid pressures, low glucose in the CSF and large numbers of PMNs
15
Q
Naegleria Clinical Presentation
A
- The CSF profile is often confused with suppurative meningitis, such as occurs with Streptococcus pneumoniae
- An astute observer, aware of the exposure to freshwater pond water, may be able to discern the amebic movement in saline preparations of CSF
- Monocyte/macrophages may have the same appearance until watched closely.
- Treatment is abysmally poor, with institution of amphotericinB, miltefosine and other agents
16
Q
Acanthamoeba
A
- This amoeba can be a cause of infection in normal hosts, usually a blinding keratitis
- Acanthamoeba is part of the normal microbiome of the human oral cavity. if not diagnosed early
- The process of pathophysiology is peculiar to the use of contact lenses which are not changed or discarded on a regular basis
- A severe keratitis ensues, and loss of vision may be a consequence if the process is not diagnosed early and addressed
- It is easily diagnosed by scraping the cornea and plating the scraping onto Petrie dishes prepared with a lawn of Escherichia coli (a natural food source of the amoebae, and the tracks of the amoebae can be observed on the plate as they move about to ingest the bacteria
- This amoeba also causes Granulomatous Amoebic Meningoencephalitis (GAE) which is seen primarily in immunocompromised patients and is often a chronic or sub-chronic process
- Therapies include amphotericin-B, miltefisone and several azoles. Therapy for keratitis is topical
17
Q
Trichomonas vaginalis
A
- Flagellated protozoa, anaerobe, lacks nucleotide synthesis, does not have a cyst stage
- Infects only humans
- Found in the vagina and urethra/prostate
- Sexually transmitted disease, mother to babies transmission, and institutionalized patients
- Survives in the moist state several hours outside the host
