MC3-4: Eukaryotic microbial pathogens Flashcards
Define ‘facultative parasite’
An organism that lives and can complete its lifecycle independent of a host, but may occasionally be parasitic under certain conditions, e.g. when immunocompromised
Define ‘obigate parasite’
A parasite that cannot lead an independent, non-parasitic existence
Define ‘opportunistic parasite’
A parasite that takes advantage of certain opportunities to cause disease. Many lie dormant in the host until the immune system is reduced
What are the three ways in which a fungus can infect a human? Which are the most common?
- Superficially (on the skin) – more common
- Subcutaneously (under the skin) – less common
- Systemically (multi-organ) – more common
What are dermatophytes?
How are dermatophyte infections acquired by humans?
Name two examples of dermatophyte infections.
Dermatophytes are moulds that grow on keratin (e.g. on the skin)
Infection is acquired from people, animals, or the environment, depending on the species
Examples of diseases: athlete’s foot, ringworm
Why do dermatophyte infections cause a circular wound?
They eat the keratin and then move outwards when they run out
What is candidiasis and what causes it?
Oral or vaginal thrush
Opportunistic Candida albicans following antibiotics and T-lymphocyte deficiencies
Define ‘commensalism’
A relationship between two organisms where one organism benefits from the other without affecting it
Define ‘polymorphic’ and give an example of a polymorphic fungus.
An organism that can grow as both a yeast and as filamentous cells
e.g. Candida albicans
What percentage of humans are infected with Candida and where are the most common sites of infection?
80%
Intestine, colon, mouth
What percentage of women have had candidiasis and what is the most common cause?
75%
Following antibiotic treatments
How can Candida albicans be pathogenic if they are commensal?
If the host has low immunity or if there is a change in natural microbiota of physiology
How does Candida albicans cause disease?
It attacks the skin or mucosae and invades tissue by puncturing skin with its hyphae
If nutrient-rich environment, spores are produced and the population doubles in an hour
Biofilms can grow on implantable medical devices
What is Cryptococcus neoformans, what does it cause, and how is it acquired?
A yeast, causing chronic meningitis
Probably acquired from the environment, possibly pigeon droppings
How does Pneumocystic carinii grow, what does it cause, and what is the route of infection?
Grows intracellularly
Produces serious pneumonia in immunocompromised
Route of infection uncertain, but endogenous and exogenous routes likely
Define ‘endogenous’
When an organism already carries a pathogen
Define ‘exogenous’
When an organism does not already carry a pathogen and acquires it from outside
What are Aspergillus and what do they cause?
Ubiquitous mould
Causing pulmonary and systemic infections, e.g. allergic aspergillosis (triggering asthma and COPD)
Can cause aflotoxin (a neurotoxin) and aspergilloma (lumps)
Why are effective antifungal drugs hard to find?
Many drugs do not differentiate between fungi and humans
What are the three ways in which antifungal drugs work? Give examples of each type.
- Cell wall integrity, e.g. nystatin, amphotericin B
- Cell wall biosynthesis, e.g. fluconazole, itraconazole, miconazole, terbinafine
- RNA synthesis: flucytosine (5-fluorocytosine)
What diseases can protoxoal pathogens cause in humans?
- Malaria
- Toxoplasma
- Intestinal and vaginal infections
Where are Naegleria found?
In warm fresh water, such as cooling towers in nuclear power stations
How does Naegleria affect humans?
Infects humans through their olfactory nerve (through the nose)
Ultimately causes a fatal brain disease
(primary amoebic meningoencephalitis or PAM/PAME)
Known as the brain-eating amoeba
What is the life cycle of Naegleria?
-
Flagellate stage:
- A small pear-shaped organism with two flagellae
- Mobile
- Stage that infects people who are exposed through water
-
Amoeba form:
- Occurs after infection
-
Cyst stage:
- Occurs in response to unfavourable environmental conditions
- Resistant to harsh environmental conditions
Where are Naegleria fowleri usually found?
In warm-water environments
What is one of the most common amoebae found in soil, fresh water, and the natural environment?
Acanthamoeba
How big are Acanthamoeba?
15-35μm
Where are Acanthamoeba found?
In fresh water (e.g. contact lens solution) and salt water
How do Acanthamoeba affect humans?
- They infect humans through contaminating contact lenses/contact lens cases
- Can also infect broken skin and cause cutaneous disease or even spread to the brain via the blood
- Can also infect the mucosa and then the brain in a similar way to Naegleria
What is the life cycle of Acanthamoeba?
- Cysts
- Trophozoite (infective)
- Amoebae which can enter humans in many ways
What diseases can Acanthamoeba cause?
- Acanthamoeba keratitis (AK) – eye disease
- Cutaneous amoebiasis – skin disease
- Granulomatous amoebic ancephalitis (GAE) – brain disease
What is acanthamoeba keratitis (AK)?
- Progressive sight-threatening corneal disease
- Leading risk factor is contact lens water
- Most common type of Acanthamoeba infection
- Treatment is not always successful and corneal transplants often required
- Reactivation occurs if in transplanted cornea
What are coccidia?
Obligate intracellular parasites that infect the intestinal tracts of mammals
What are common characteristics of coccidia?
Sexual cycle occurs in epithelial cells (gut in most species)
Oocysts (infective stages) secretion after sexual cycle. Environmentally resistant
Define ‘trophozoite’
The feeding stage of a protozoan parasite
Define ‘schizogony’
The process of asexual reproduction during which the nucleus undergoes division preceding cell division
What are ‘merozoites’?
The daughter cells produced by schizogony. They can develop into gametocytes or enter new host cells and undergo another cycle of schizogony
Define ‘gameocyte’
Cells that are capable of developing into gamete. They are derviced from merozoites
Cryptosporidium were once thought to have many species. What is the current view?
They contain only a few species that have broad host specificity
What is the lifecycle of Cryptosporidium?
Direct faecal-oral life cycle with no intermediate hosts
Why did diseases caused by Cryptosporidium come to light in the 1980s?
A large number of AIDS patients were suffering from the disease
What is Cryptosporidium parvum and what are the symptoms of the disease it causes?
- A protozoan parasite that infects the intestine of the host
- It is a problem in the young, old, and immunocompromised
- Symptoms:
- Diarrhoea (10-15 litres per day)
- Abdominal cramps
- Headache
- Fatigue
- Mild fever
Why are Cryptosporidium less accessible to factors that the host would normally use to control intracellular pathogens?
They live in an intracellular but extracytoplasmic location
What are Cryptosporidium’s unique features?
- Parasite develops just under the host epithelial cell membrane in an intracellular but extracytoplasmic position
- The oocyst stages are fully formed when expelled from the host
- There is no need for sporulation and therefore oocysts are immediately infective (auto-infection)
What is the common coccidian sexual life cycle?
- Multiple rounds of schizogony (depending on parasite species)
- Microgamete (male) and macrogamete (female) form a zygote
- Oocysts can sporulate in host or externally, depending on species
What is Toxoplasma gondii, how is it spread, and what are the symptoms in humans?
An opportunistic parasite
Usually asymptomatic in adult humans
Toxoplasmosis is usually spread by eating poorly cooked food that contains the cysts
Can also contract it directly from cats
What are the different life cycles of Toxoplasma gondii?
- Takes place in the epithelium of the cats
- Similar sexual cycle to Cryptosporidium
- Direct (faecal-oral) life cycle can occur in cats
What is the life cycle of Toxoplasma gondii in an intermediate host?
- Any warm-blooded animal that injests oocysts can become infected
- The parasite does not undergo sexual multiplication in the gut of intermediate hosts
- Instead, it undergoes asexual (extra-intestinal) multiplication in any cell of the host as the tachyzoite stage for around 14 days
- At 14 days post infection, the tachyzoites transform into bradyzoites. These form cysts in the muscle, brain, and eye
- These cysts stages are long-lived and can initiate the sexual cycle when eaten by a cat or the asexual cycle when ingested by any other carnivore
Why are incidences of Toxoplasma gondii higher in pregnant women in Paris than they are in pregnant women in Glasgow?
Parisian women eat a lot of almost-raw meat
What are the differences in the Toxoplasma gondii symptoms in immunocompetent and immunocompromised humans, and congenital disease?
Immunocompetenet
- Disease is usually slef-limiting but mild flu-like symptoms are common at onset
- Cyst stages persist for life
Immunocompromised
- Disease commonly manifests as severe encephalitis
- Due to a new infection or reactivation of cyst stages from historic infection
Congenital disease
- Women infected during pregnancy can transfer parasite to foetus
- Congenital disease can be severe hydrocephalus, ocular lesions, mental retardation
What is the treatment for infections of Toxoplasma gondii?
- Sulphadiazine
- Pyrimethamine
- Spiromycin
What causes malaria?
The obligate parasite Plasmodium
How many cases of malaria were there in 2010? How many of these died?
- 219-550 million cases
- 660,000 - 1.24 million deaths
Which species of Plasmodium infect man? Which is the most severe, and which is the most common?
- P. falciparum (most severe)
- P. vivax (most common)
- P. ovale
- P. malariae
- P. knowlesi
What does Plasmodium require?
Two hosts (obligate requirement)
How does malaria spread between hosts?
- Female Anopheles mosquite infected during blood meal
- Male and female gametocytes ingested
- Sexual reproduction occurs in stomach of mosquite
- Infective sporozoites accumulate in the salivary glands of mosquito
- Mosquito injects sporozites into next host during subsequent blood meal
Explain the geographic distribution of malaria infections.
- Limited but extensive geographic distribution
- Dictated by natural habitat of Anopheles host
- Population density, prevalence of childhood infection, ambient temperature and rainfall all play an important role
- 80% of cases originate from sub-Saharan Africa
What are the differences between P. falciparum and P. vivax infections?
P. falciparum
- Potentially fatal – cerebral malaria
- Very high parasitaemia
- Drug resistance
P. vivax
- Common, usually benign
- Recurrent
How can malaria be managed and controlled?
- Diagnosis by microscopy of blood films
- Treatment with chloroquine or quinine (++)
- Eradicate liver forms in P. vivax
- Prophylaxis for travellers
- Avoid mosquito bites
- Mosquito control
- ?Vaccination – trying to develop
Give a brief overview of the current state of malaria research.
- Malaria funding has increased nearly 10-fold
- Major gains have been made in controlling the disease in developing nations
- Through interventions, including timely diagnosis and treatment, improved diagnostics, effective drugs, indoor spraying with long-lasting insecticides, bed nets treated with long-lasting insecticides
- Current tools and treatments are insufficient to achieve elimination in many countries
- Cost of maintaining these interventions is billions of dollars a year
- Malaria parasite is developing resistance
- Infected individuals who are asymptomatic remain an ongoing source of transmission
- Microbiome impacts on mosquito gut
