Viral + protozoal causes of gastroenteritis Flashcards
What is the most common cause of gastroenteritis world wide?
Viral infections
Discuss viruses
Small entities that require a host cell to reproduce
Main component = small genome, either single/double stranded DNA/RNA
Viral genomes are packaged to promote infection of a new host cell
Viruses are variable + generally evolve quickly
Viruses are classed w/ their host cell
Viruses are not cells
What is the first option for treatment of viral infections?
Vaccinations
Viruses are found in either 2 states.
What are they?
Extracellular + intracellular
What is the extracellular virus called?
Virion
A virion contains a protein coating called a capsid, which surrounds the core of the virus (viral genome) containing the nucleic acid.
Can have an outer membrane
The outermost layer of the virion:
- protects viral genome
- contains proteins that recognise + bind to host cell
Outside of the virus evades adaptive immunity = most variable part of virion
What happens when the extracellular virus enters the cell?
After the virus has entered the cell:
- The capsid and/or envelope are removed, the nucleic acid containing the full viral genome is still called the virus, but it is not a virion anymore
- The virus uses its own few genes to control the cell’s gene expression
- The virus undergoes either one of the 2 modes of replication.
What are the 2 modes of replication?
Lytic: the virus directs the synthesis of many copies of its genome and capsid/envelope and the new virions are released, killing the cell
Lysogenic (bacteriophages) or latent (animal viruses): the virus allows or directs the cell to divide, replicating at a similar rate to the cells. It eventually switches to lytic mode, depending on conditions
What are the classifcations of viruses?
Viruses are classed depending on their outside coverings + genomic structure
- Enveloped viruses - membranous envelopes; can surround capsid
- Naked viruses - lack envelope; rely on capsid proteins; tend to be smaller + have simpler genomes
Viruses can also be classed depending on their nucleic forms of their genome
- dsDNA viruses - carry genome on double stranded DNA in the viroid. Mostly use host DNA polymerase for replication + host RNA polymerase to make viral mRNAs
- ssDNA viruses - carry genome on a single stranded DNA in the viroid. Most use DNA polymerase for replication.
What are most viruses are classed under?
RNA viruses
Name the different types of RNA viruses
- +ssRNA - carry genome on a positive single stranded RNA molecule. Can be used for protein synthesis. Because mammalian cells do not have an enzyme to copy RNA from RNA (e.g. DNA polymerase), the virus has to encode an RNA dependent RNA polymerase. (poliovirus, rhinovirus)
- -ssRNA - carry their genome on a negative single stranded RNA molecule. The virus needs to encode an RNA dependent RNA polymerase to make messenger RNA + to synthesise genome. (Measles, Flu)
- dsRNA - carry genome on double stranded RNA molecule. Also encode an RNA dependent RNA polymerase. (Rotavirus)
- Retroviruses - carry their genome on a positive strand RNA molecule. It encodes a reverse transcriptase which makes a DNA copy of the RNA. mRNA + new genome are transcribed from the DNA copy + integrated into the host genome. (AIDS/HIV)
Are viruses alive?
They can reproduce + evolve but…they are completely dependent on host cells for all metabolic steps.
Seen as life reduced to its most basic form.
What does rotavirus cause in children (6-24 months)?
What type of RNA virus is rotavirus?
What is its genome composed of?
What is the virus packaged in?
Gastroenteritis
Rotavirus is a naked dsDNA virus with a 70nm diameter
Genome is made of 11 double stranded RNA molecules
Virus is packaged in capsid proteins
The capsid proteins can bind receptors on enterocytes + induce import of the virus via lysosomes
How can rotavirus be contracted or transmitted?
How long is the incubation time + what are the initial symptoms?
How can you treat it?
Oral-faecal transmission
Child to child
- stool of an infected chiled contain 1014 virions per gram, only 100 are required for infection
- Incubation is 2 days - vomiting after 4 - 8 days of severe watery diarrhoea
- Most children affected once then immunity develops
- Infection is self-limiting = oral rehydration is sufficient
- Vaccines are available
What is the leading cause of gastroenteritis in adults?
Describe this.
Norovirus
Norovirus = +ssRNA, small (35nm)
Encodes 3 proteins (including 2 capsid proteins)
Virus is highly variable. Strains called after the place they were identified.
Incubation period = 1 - 2 days; forceful vomiting, watery diarrhoea + abdominal pain lasting 1 - 5 days
Infectious - < 20 virus particles can cause diease
Infection is between person - person; aerosol generated during vomiting can cause.
Virus can survive days outside human host
Handwash with soap + chlorine can reduce infections.
Immunity lasts up to 6 months (including same strain)
Genetic variation in human gene can make some people less susceptible
What is protozoa?
How many kingdoms of protozoa are there?
Unicellular eukaryotes that lack a cell wall
10 kingdoms
Most protozoa live in moist ecosystems + not cause disease
Protozoa that cause disease are usually passed on from a human/animal carrier
What are the protozoal causes of gastroenteritis?
How do the causes infect humans?
3 different kinds of protozoa that can cause gastroenteritis:
- Giardia intestinalis
- Entamoeba hystolitica
- Cryptosporidium sp.
These organisms infect humans through ingestion of their cysts on food, water or hands contaminated with faecal matter
Cysts are activated by stomach acid leading to excystment in the small intestine where proliferation of the motile trophozoite form occurs
Some trophozoites persist in the small intestine, while those entering the colon form new cysts
Discuss Giardia intestinalis
Where do they live?
What is the common route of infection?
How long is the incubation time?
What is the treatment?
Single-celled protozoan with four pairs of flagella and a ventral adhesive disk (trophozyoite form)
Giardia lives in intestinal tracts of both humans and animals worldwide and infection can be asymptomatic
Drinking or letting a pet drink from mountain streams is a common route of infection
Incubation is 1-2 weeks and symptoms include severe watery diarrhoea, abdominal pain, vomiting and low grade fever. Stools smell foul (“rotten egg”). Giardiasis usually resolves in 1-4 weeks.
Treatment other than rehydration is rarely required, but in severe cases metronidazole is effective
What is Cryptosproidum?
Another organism that forms cyst
The 2nd biggest cause of diarrhoea + infant death in Africa + Asia
Transmitted by faecal contact/water contaminated by faeces
Used to be known as a cause of diarrhoea in AIDs
What are the main concerns over gastroenteritis?
Firstly, it is to maintain hydration
Drinking larger amounts than normal in mild cases
Using oral rehydration salts in more severe and longer lasting cases
Hospitalization may be required for vulnerable patients (oral or IV rehydration) or if symptoms are severe
Food can be taken as soon as the patient feels able
Dysentery (bloody or mucous diarrhoea) does not always need doctor’s attention unless it lasts more than a few days or symptoms are very severe
Secondly, it is prevent passing of infection to others
- recommend strict hygiene
- Thorough hand washing
- Avoid contaminating your home or surface water with stool or vomit if possible
- Cleaning with chlorine containing disinfectant
- Patients should avoid food preparation for others until at least 24 hours after symptoms are gone
- Isolating affected toddlers from age mates
