Parasitology

Question 1

What are parasites? How many main classes are there that can cause disease in humans? What are these main classes?

Answer 1

What is a parasite?
A parasite is an organism that lives on or in a host organism and gets its food from or at the expense of its host.

There are three main classes of parasites that can cause disease in humans:

1. Protozoa
2. Helminths
3. Ectoparasites.

Question 2

What are protozoa?

Answer 2

What are protozoa?

• Protozoa are microscopic, single-celled organisms that can be free-living or parasitic in nature.
• They are able to multiply in humans allowing serious infections to develop from a single organism. •Transmission:
• Protozoa living in the human intestine can be transmitted by the fecal-oral route
• Protozoa living in blood or tissues are transmitted by an arthropod vector eg mosquitos

Question 3

What are protozoa classified by?

Answer 3

Types of protozoa –protozoa are classified by mode of movement
•Amoeba, e.g.Entamoeba
•Flagellates, e.g.Giardia,Leishmania
•Ciliates e.g.Balantidium
•Sporozoa–organisms whose adult stage is not motile e.g.Plasmodium,Cryptosporidium

Question 4

What are examples of some medically important protozoa?

Answer 4

Medically important protozoa infections
•Entamoeba histolytica
•Giardia lamblia (causes diarrhoea)
•Trichomonas vaginalis (causes infections in genital tract)
•Malaria (Plasmodium spp.)
•Toxoplasma gondii (transmitted through cats flu like)
•Cryptosporidium (causes of severe diarrhoea transmitted through faecal oral route)
•Leishmania spp. (causes ulcers)
•Trypansomacruzi
•Trypansomabrucei (gambiense/rhodesiense)

Question 5

What are helminths?

Answer 5

Helminths are large, multicellular organisms (worms) generally visible to the naked eye in their adult stages. In their adult form, helminths cannot multiply in humans. (host) depends on amount of exposure in environment
•There are three main groups of helminths that are human parasites:
1.Nematodes (roundworms)
2.Trematodes (flukes)
3.Cestodes (tapeworms)

Question 6

Name some examples of Medically important helminths -nematodes

Answer 6

Soil-transmitted helminths:
•Ascaris lumbricoides
•Trichuris trichiura
•Hookworm spp
.•Enterobius vermicularis

Filarial parasites:
•Wuchereria bancrofti
•Loa loa (travel through eye!)
•Onchocerca volvulus (causes blindness)
•Dracunculus medinensis (South Sudan, ethiopa and chad mostly see this now in Africa-  now not so common through all of Africa like it used to be) female parasites can be up to 1m long!!

Others:
•Toxocaracanis/cati
•Trichinella spiralis

Question 7

What are examples of Medically important helminths –trematodes and cestodes?

Answer 7

Trematodes:
•Schistosoma mansoni/haematobium/japonicum]
•Clonorchissinensis ( can cause carcinoma pf bladder)
•Fasciolahepatica
•Paragonimusspp. (poorly cooked crustaceans!)

Cestodes
•Taenia saginata
•Taenia solium
•Echinococcus granulosus

Question 8

What are ectoparasites?

Answer 8

Blood-sucking arthropods such as ticks, fleas, lice, and mites that attach or burrow into the skin and remain there for relatively long periods of time (e.g., weeks to months).

Question 9

What are some Medically important ectoparasites?

Answer 9

Mites:
•Scabies
•Trombiculid (found in vegetation)

Ticks:
•Hard
•Soft

Lice:
•Pediculus humanus capitis (headlouse)
•Pediculus humanus humanus (body louse)
•Pthirus pubis (pubic louse)

Flies
•Botflies (cause abscesses)

Question 10

Parasites –general comments

Answer 10

Parasitic infections cause a tremendous burden of disease in both the tropics and subtropics as well as in more temperate climates. Malaria kills ~660,000people each year.
•TheNeglected Tropical Diseases(NTDs) include parasitic diseases such aslymphatic filariasis,onchocerciasis, andGuinea worm disease, and affect >1 billion people, largely in rural areas of low-income countries
. •Parasitic infections also affect persons living in developed countries.

Parasites often have complex life cycles
Type of host:
•Intermediate –host in which larval or asexual stages develop
•Definitive –host in which adult or sexual stage occurs

Vectors:
•Mechanical when no development of parasite in vector
•Biological when some stages of life cycle occur

Question 11

What determines parasite infections? And how can this be prevented?

Answer 11

Depends on mode of transmission and opportunities for transmission
Faeco-oral: prevented by
•Household sanitation
•Access to clean water
•Personal hygiene behaviours

Food: prevented by:
•Animal husbandry
•Surveillance
•Regulations and government controls

Complex life cycles:
•Distributions of vectors and intermediate/definitive hosts

Others:
•Government resources and level of human development/per capita income
•Education
•Country-level and regional control programmes
•Availability of cheap and efficacious treatments
•Construction and building regulations (egChagas)
•Urban vs. rural residence
•Environmental sanitation:

Question 12

What are the Phases of disease?

Answer 12

Acute:
–Incubation 1-2 wks after bite
–Up to months after transfusion
–Trypanosomes in blood

Chronic:
 ‘indeterminate’
•Lifelong infection
•Generally trypanosomes not detectable but often positive for parasite DNA
•Seropositive•60-70%
•Normal ECG and X rays

Determinate’ Chronic disease
–Seropositive
–30-40% of infected 10-30 years after infection
–5-10% develop chronic Chagas immediately after acute disease

Question 13

What happens in the acute phase of disease with application to Chagas?

Answer 13

• Occurs within 3 weeks
• Generally mild or asymptomatic
• Local swelling (Romaña)
• Nodule or chagoma
• Fever
• Anorexia
• Lymphadenopathy

1-2% diagnosed
•Symptoms last 8-10 wks
•Rarely (young and IS)
•Hepatopsplenomegaly
•Acute myocarditis
•Meningoencephalitis
•Fatality <5% of symptomatic

Question 14

effects of Chagas? cardiac wise/ muscular/digestion…?

Answer 14

see arrhythmias  (ventricular fibrillation can cause disease commonly seen)
get long term damage to muscle
within digestion: Esophagus, rectum, and sigmoid colon most affected

parasympathetic system important for peristalsis so leads to megacolon - enlargement of colon
Chronic chagas:
Presentation–Constipation
•Complications:
–Faecaloma
–Obstruction
–Sigmoid volvulus
–Ulceration
–Perforation

Question 15

What is the pathogenesis within all the phases of disease within Chagas?

Answer 15

Acute:
•Tissue damage caused by inflammatory response to parasite in nests of amastigotes in cardiac, skeletal, and smooth muscle
•Parasite killing by antibodies, activated innate immune response and Th1 pro-inflammatory cytokines

Indeterminate:
•Regulatory immune response characterized by IL-10 (immune regulation) and IL-17

Chronic:
•Chronic inflammatory response to persistent parasites in muscle and nerve cells
•Autoimmune mechanisms
•May vary by parasite strain and tissue tropism
•Predominance of Th1 cytokines and CD8+ T cells

Question 16

What is the Pathogenesis of cutaneous leishmaniasis?

Answer 16

Acute lesions:
•Tissue damage caused by inflammatory response to presence of parasites in macrophages
•Parasite killing by Th1 pro-inflammatory responses and macrophage killing

.Latency:
Parasites remain present long-term. Regulatory immune response characterized by balance of Th1 and anti-inflammatory responses

Relapse (rare):
Alteration in immune response (i.echange in Th1 vs. immune regulation secondary to HIV, malnutrtition) may trigger relapse
•Mucocutaneous disease associated with strong but inadequate inflammatory response to parasites that have metastasized to mucosa
•Diffuse cutaneous leishmaniasis associated with uncontrolled parasite replication
.•Recividans–recurrence of lesions at old ulcer site.