DD 02-28-14 11am-Noon Introduction to Parasites - Holmes Flashcards
(66 cards)
1
Q
Parasite defn.
A
an organism which lives upon or within another living organism at whose expense it obtains some advantage
2
Q
Parasitology’s concern
A
= pathogenic protozoa (unicellular eukaryotes) & metazoa (multicellular eukaryotes)
Includes:
- helminths (worms)
- arthropods (insects)
3
Q
Life Cycles of human parasites
A
Complex life cycles
- some involve development in one or more additional host species (definitive host + intermediate hosts)
4
Q
Definitive host of a parasite
A
= the species in which the parasite undergoes sexual replication
5
Q
Intermediate host of a parasite
A
= other species besides the definitive host, in which asexual replication occurs
6
Q
Important factors in parasite’s life cycle
A
Ability of parasites to infect specific tissues (tropism)
Geographic occurrence of parasitic diseases
- often restricted by availability of host species
7
Q
Protozoan vs. Worm parasites
A
Most protozoan parasites can replicate and increase their numbers in humans.
In contrast, many worms undergo development but do not replicate in humans
- worm burden in humans reflects intensity of their exposure to infection
8
Q
Parasites & the Immune system
A
- Many parasites evade or subvert the protective immune responses of their hosts
- Damage to host tissues is often result of host immune responses
- Diseases caused by many parasites become clinically apparent when the number of parasites (the parasite burden) is high or when infection persists for long periods of time.
- Development of effective vaccines against major human parasitic diseases has been difficult & remains an important goal for world health
9
Q
Selective toxicity in parasitic disease
A
- B/c parasitic diseases are caused by EUKARYOTIC pathogens, the biological bases for selective toxicity of anti-parasite drugs are quite different from anti-bacterial & anti-viral drugs
10
Q
Dx of parasitic disease
A
- often made by direct examination of parasites in specimens or biopsy materials collected directly from patients
- Generally morphological criteria are much more helpful in parasitic disease than viral/bacterial disease
- Immunological tests (detection of specific Ags & Abs) & molecular diagnostic tests (for specific nucleic acid sequences) are sometimes helpful
11
Q
Helminth groups causing human disease
A
- Roundworms/Nematodes
- Flatworms/Trematodes/Flukes
- Tapeworms/Cestodes
12
Q
Roundworms/Nematodes - examples
A
Ascaris Pinworms Whipworms Hookworms Strongyloides Echinococcus Trichinella Filarial worms
13
Q
Flatworms/Trematodes/Flukes - examples
A
Schistosomes
Lung fluke
14
Q
Tapeworms/Cestodes - examples
A
beef tapeworm
pork tapeworm
fish tapeworm
15
Q
Protozoan groups that are human parasites
A
- Amebas
- Flagellates
- Ciliates (Balantidium coli)
- Sporozoa
- Microsporidia
16
Q
Amebas - example
A
Entamoeba histolytica
17
Q
Flagellates - examples
A
Giardia
Trichomonas
Trypanosoma
Leishmania
18
Q
Ciliates - example
A
Balantidium coli
19
Q
Sporozoa - examples
A
Cryptosporidium
Cyclospora
Plasmodium
Toxoplasma
20
Q
Microsporidia - examples
A
Enterocytozoon
Septata
21
Q
Schistosomiasis - paradigm of helminthic infection - Transmission, Basic course
A
- transmitted to humans by exposure to contaminated fresh water
- -> causes acute manifestations & progresses to chronic disease affecting intestinal or urinary system
22
Q
Epidemiology of Schistosomiasis
A
- affect ~200-300 million people a year globally
- causes 200,000 deaths per year globally
- 2nd only to malaria among parasitic diseases
Among infected people, - 120 million are symptomatic
- 20 million have severe disease
- 85% live in sub-Saharan Africa
23
Q
Schistosomes
A
= trematodes/flatworms
- 3 species are widely distributed & cause most human infections
- 2 species have more restircted distribution
- Schistosomiasis in the US is only seen in immigrants or travelers
24
Q
3 species of Schistosomes with wide distribution
A
Schistosoma mansoni
S. japonicum
S. haematobium
25
Schistosoma mansoni - location
primarily in Africa, South America and the Middle East
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S. japonicum - location
primarily in China, the Philippines and Indonesia
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S. haematobium - location
primarily in Africa and the Middle East
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2 species of Schistosomes with restricted geographic distribution
```
Schistoma intercalatum (West Africa)
Schistoma mekongi (Southeast Asia)
```
29
Life Cycle of Schistosomes
Complex life cycle
Eggs shed in feces/urine of infected humans
- -> hatch in fresh water
- -> release miracidia, which infect snails
Sporocysts develop in the snails
- -> free-swimming cercaria are released
- -> can penetrate human skin to initiate infection
Cercaria lose their tails & develop into schistosomulae in tissues of human host.
Schistosomulae gain access to circulation
- -> migrate to portal blood
- -> mature into adult worms
Adult worms migrate to mesenteric veins or venous plexus of bladder (depending on species)
--> mating --> eggs
Eggs produced circulate in venous blood to the liver & are also released into intestine or urinary bladder, depending on the anatomic location of adult worms.
--> Release of eggs into environment perpetuates infection cycle
(see pic in notes)
30
Schistosomes that migrate to & mate in the mesenteric vein
S. mansoni
S. japonicum
S. intercalatum
S. mekongi
--> eggs produces are released into intestines (or circulate to liver)
31
Schistosomes that migrate to & mate in the venous plexus of the bladder
primarily S. haematobium
| --> eggs produces are released into urinary bladder (or circulate to liver)
32
Cercarial invasion into the skin
can cause dermatitis (swimmers itch) within 2-3 days
33
Acute phase of schistosomiasis
- aka Katayama fever
= a serum-sickness like illness
- occurs 4-8 weeks after skin invasion
= occurs coincident w/ worm maturation & onset of oviposition
- often accompanied by lymphadenopathy & hepatosplenomegaly
34
chronic stage of schistosomiasis
- involves granulamatous & fibrotic changes in liver or bladder (site depending on species)
= consequences of host rxns to deposited eggs
--> leads to formation of infiltrates containing large numbers of eosinophils & eventually to scarring
35
Severity of Schistosomiasis & Symptoms
```
Light infestations = may be asymptomatic
Heavier infestations = often symptoms such as...
For intestinal schistosomiasis:
- diarrhea
- abdominal pain
- ascites
For urinary schistosomiasis:
- bloody urine (hematuria)
- bladder cancer
```
36
Dx of Schistosomiasis
- microscopic examination of stool or urine for schistosome eggs w/ characteristic sizes & shapes
- detection of similar schistosome eggs in tissue biopsies
37
Major strategies for controlling schistosomiasis:
Public health education
- avoid contact w/ infected water
- build wells & latrines to provide potable water
- avoid contamination of environmental water sources
Molluscicides & environmentasl modification to control snail intermediate
Mass treatment of populations w/ anti-schistosome drugs (such as praziquantel, etc.),
Research on diagnostic tests, improved therapeutics, & vaccines for schistosomiasis
Even in regions where control of schistosomiasis has been effective, parasite has not been eliminated & transmission continues, albeit at lower frequency
38
Malaria as an important paradigm of protozoan infection - Overview
= the most important parasitic disease
| - 4 species of protozoa in genus Plasmodium cause malaria in humans
39
Malaria - Epidemiology
- affects > 1 billion people a year
- causes 1-3 million deaths/year
- eliminated from US, Canada, Europe, Russia (except imported cases)
- Global malaria & resistance to anti-malarial drugs are still major problems
40
Four species of protozoa causing malaria in humans
Plasmodium genus
- P. vivax
- P. ovale
- P. malariae
- P. falciparum
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Plasmodium vivax
widely distributed, from tropical to temperate zones
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Plasmodium falciparum
- causes most of the malaria deaths
| - primarily in tropics / subtropics
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Life cycle of malaria parasites - Asexual Phase of Development
Infected mosquitos bite humans & inject sporozoites into the blood
After primary replication in liver...
- -> merozoites are released into blood
- -> infect erythrocytes
- -> undergo additional asexual replication
Gametes (macro- & micro-gametocytes) are
formed in some infected erythrocytes.
44
Asexual phase of development for malaria parasites occurs in...
- aka schizogony
| - occurs in humans
45
Dormant phase of malaria in humans
- Sporozoites of all malaria parasites can infect liver cells & replicate
- BUT only P. vivax & P. ovale can establish a dormant hepatic phase w/ non-dividing
forms called hypnozoites that can initiate late relapses.
46
Life cycle of malaria parasites - Sexual Phase of Development
- aka sporogony
- occurs in infected mosquito
- Gametes fuse in intestine to form zygotes.
- Development of parasite in mosquitoes eventually leads to production of sporozoites in salivary gland.
- Introduction of sporozoites into susceptible human by bite of an infected mosquito initiates a repetition of the life cycle of the parasite.
47
Acquire immune response to malarial parasites
- Acquired immune responses that help to control the development of malaria parasites in humans are also shown.
48
Symptoms of malaria – associated with…
- primarily associated w/ rupture of infected RBCS & release of merozoites
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P. falciparum – malaria pathogenesis
- invades erythrocytes of all ages & can therefore achieve highest parasitemia & mortality
50
P. vivax & P. ovale – malaria pathogenesis
- invade only young erythrocytes
51
P. malariae – malaria pathogenesis
- invades only old erythrocytes
52
Untreated patients with malaria & periodicity
Process often becomes synchronized
Fever paroxysms may have regular periodicity
- 48 hr for benign tertian malaria caused by P. vivax or P. ovale
- 72 hr. for quartan malaria caused by P. malariae
- 36-48 hr. for malignant tertian malaria caused by P. falciparum
53
Anemia in malaria
May be disproportionate to parasitemia
Results from…
- RBC lysis
- RBC phagocytosis by stimulated reticuloendothelial system
- RBC sequestration in enlarged spleen
- Depressed bone marrow function
Hemolysis can be extreme, resulting in hemoglobinuria (blackwater fever)
54
Physical examination findings in malaria patient
- jaundice
- hypotension
- tachycardia
- fever
- hepatosplenomegaly
55
P. falciparum infections – symptoms & mechanism
- vasodilation causes hypotension & inadequate blood supply to vital organs
- P. falciparum-infected RBCs bind to microvascular endothelium, which is especially significant in cerebral malaria (up to 50% mortality)
56
Malarial death - major cause in adults
- Multi-organ failure
57
With P. malariae infections – symptoms & mechanism
- immune complex deposition leading to glomerulonephritis is common
58
Immunity to malarial infection
- pt mounts immune response that makes subseqent episodes of symptomatic disease less severe
- Both B & T cell responses are involved
- W/in a few weeks of infection, stage specific anti-plasmodium antibodies are produced
- Natural immunity is short-lived
- Continual re-infection is required to maintain it
- People returning to endemic areas following long absence may thus be quite susceptible to re-infection
59
Diagnosis of malaria
- Usually by detection of asexual forms of parasites in stained thick or thin blood films
- Morphological features of intracellular parasites & infected erythrocytes are used to differentiate the 4 species from each other & from other protozoan that also can replicate in erythrocytes
- Rapid monoclonal Ab-based tests for the PfHRP2 antigen or for Plasmodium LDH antigens are also useful in diagnosis of P. falciparum infections
60
Splenic enlargement in malaria
- common in apparently healthy individuals w/ repeated infections
- often used to estimate prevalence of malaria in populations in endemic areas
61
Malaria & Selection of traits
- widespread & lethal nature of malaria has apparently selected for traits that protect against plasmodia but are otherwise undesirable
Includes:
- sickle cell anemia
- thalassemias
- glucose-6-phosphate dehydrogenase deficiency
62
Malaria & HbS (Sickle Cell)
- Parasites do not appear to thrive in Hb S associated w/ sickle cell disease, nor in certain other abnormal Hgb’s
63
Thalassemia & Malaria
- In thalassemia, there is increased production of fetal hemoglobin
- -> retards maturation of P. falciparum
64
G6PD deficiency & Malaria
- the oxidative stress may inhibit parasite growth
65
HLA-B53 & Malaria
- associated w/ recovery from falciparum malaria
| - very common in West Africa
66
Prevention of malaria
- Minimize mosquito contact in endemic areas
- Efforts at mosquito control & malaria eradication during 1950’s & 1960’s by using DDT & drugs were eventually frustrated by concurrent appearance of DDT-resistant mosquitoes, cessation of DDT use due to its environmental effects, & development of drug resistant plasmodia
- Array of drugs for prophylaxis & treatment
- Extensive efforts to develop effective vaccines against various forms of malaria
