Lecture 23: Epidemiology introduction Flashcards
26th November 2024
What is epidemiology?
The study of infectious disease in populations (humans and animals).
Can parasites (pathogens) cause infectious disease?
Yes
Describe microparasites
- They are small, difficult to count, and replicate inside their host.
Give some examples of microparasites
Viruses
Bacteria
Fungi
Protozoa
Describe macroparasites
- Large, can be counted. Multiply external of the host.
Give some examples of macroparasites
Endoparasites - Worms (helminths)
Ectoparasites (ticks, flees, lice)
What 2 ways can infectious disease be transmitted?
Through one to one contact, and through non-contact.
What are some examples of one to one contact method of infectious disease transmission?
- Direct contact
- Indirect contact (i.e touching contaminated surfaces).
- Droplets (contact, but transmission is through air, for example with respiratory ilnesses).
What are some examples of non-contact method of transmission?
- Airborne (transmission through air but is carried further, for example measles, chicken pox).
- Vehicle (single contaminated source, like poison, which infects multiple hosts).
- Vector borne
What does DALY stand for?
Disability Adjusted Life Year
What is DALYs?
The number of healthy years of life lost due to premature death and disability (combination of mortality and morbidity).
What type of infectious disease has the greatest impact on health and life expectancy?
Lower respiratory infections
Give an exmaple of an infectious disease that used to have a massive impact on quality of life and longevity of life
The plague
What bacteria causes the plague?
Yersinia pestis
What vector carried the plague?
Fleas
What was the carrier of the vector, the fleas?
Rodents
What are the 3 types of plague and what do each of them infect?
Bubonic plague – infection of the lymphatic system
Septicaemic plague – infection of the blood
Pneumonic plague – infection of the lungs
How many deaths did the black death of the 14th century cause?
The ‘Black death’, 14th Century causing 25-50 million deaths in Europe (*but not relying on rats)
What does the untreated mortality of the plague range from?
Untreated mortality ranges 30% - 100%.
What is effective treatment for the plague?
Early antibiotic treatment is effective
Describe bubonic plague
Painful lymph nodes (buboes)
Fever
Headache
Chills
Weakness
Mortality = 30%
Describe pneumonic plague
Fever
Headache
Weakness
Pneumonia
shortness of breath, chest pain, cough, sometimes bloody or watery mucous
Mortality = 80%
Describe septicaemic plague
Fever
Chills
Abdominal pain
Shock
Possible bleeding into the skin and other organs. Skin and other tissues may turn black and die, especially ectremities
Mortality = 100%
What does cycle 1 of the plague transmission cycle mean?
Cycle 1 means that the pathogen is endemic in nature.
How can humans spread pneumonic plague?
Via aerosol
Is it true that once plague is pneumonic, it can be transmitted from human to human?
Yes
What is the number of new cases called?
Incidence
In Madagascar in 2017, how many plague cases were there and how many deaths were there?
2417 confirmed (probably and suspected) cases. 209 deaths.
What is meant by the fact that plague is endemic in nature?
It is constantly present
Give some examples of arthropod vector-borne protozoan microparasites
- Trypanosoma cruzi causes human Chagas disease in central and S. America
- Trypanosoma brucei spp causes African sleeping sickness in humans in Africa
- Plasmodium spp (P. falciparum, P. malariae, P. vivax) causes malaria on many continents
What are vectors?
Living organisms that can transmit infectious disease between humans or from animals to humans. (WHO)
What causes Chagas disease?
Trypanosoma cruzi (T. cruzi
)
Describe Chagas disease
Endemic in 21 countries
6-7 million persons infected
10,000–14,000 deaths per year
100 million people at risk
Is Trypanosoma cruzi a protozoa?
Yes, it is a protozoan parasite.
Describe the routes of transmission of Trypanosoma cruzi (Chagas disease).
Vector-borne by “kissing bugs” – Triatominae subfamily (80%)
Transfusion of infected blood (20%)
Congenital: mother to foetus (Vertical transmission)
Accidental ingestion of infected sources
Generally describe the transmission cycle of chagas disease
When the protozoan parasite takes a blood meal, it causes itching ans scratching.
This causes you to rub defication under the skin
The protozoan will consequently replicate within your cells, causing cells to die and burst
Another kissing bug will take this blood from the infected person and transmit the pathogen to someone else.
Describe acute chagas disease
- pseudocysts form (replication sites)
- Ruptured pseudocysts release inflammatory mediators
- Localised cell damage and inflammation
- Acute myocarditis
Describe chronic chargas disease
Type III Hypersensitivity
Kidney disease
Chronic myocarditis
Fibrosis and necrotic damage (black area)
70 to 80% of people infected remain asymptomatic for life and may be unaware they are infected.
Describe the possible link between Darwin and Chagas disease
Triatoma infestans, a type of kissing bug, carries T. cruzi (the parasite that causes Chagas disease), and 70% of these insects were infected.
Darwin was bitten in 1835 at the age of 26, during his travels in South America.
After returning from Argentina, he suffered from:
- Gastrointestinal pain
- Nausea & vomiting
- Sleeplessness
- Myocarditis (heart inflammation)
- Darwin died in 1882 (aged 73), possibly due to a heart attack linked to chronic Chagas disease.
- Chagas disease was only identified in 1909, meaning it was unknown in Darwin’s time.
Chagas Summary
Caused by Trypanosoma cruzi
Transmitted by Triatomine (kissing) bugs when taking a
blood meal
Acute disease characterised by inflammation and acute myocarditis
Chronic disease characterised by kidney disease and chronic myocarditis
Epidemiology definitions Summary
Infection – presence of the pathogen/parasite in the host
Disease – clinical state of the host
Vertical transmission – passage of infection/transmission from mother to offspring
Endemic – regularly/constantly found/native
Describe macroparasites
- Causes Chronic recurring infections (little/no immunity).
- High morbidity, low mortality
- Endemic in nature, constantly reintroduced
- Continual reinfection (limited post-recovery immunity)
- Age-related exposure, burden, pathology
Describe the Ascariasis lumbricoides cycle
- Eggs are layed in the gut
- These eggs end up in the environment
- The eggs are reingested through dirty water, the eggs go back to the gut, move to the lungs, and cause respiratory problems
- As eggs hatch and grow into adult worms, they go up and humans swallow them.
- Mature eworms in the gut feed on human food,
Describe the Schistosoma mansoni life cycle
- \intermdiate host = snails.
1) The parasite eggs are in the enbirnment.
2) The eggs hatch and release miracidin
3) Ends up in snails (penetrate tissue).
4) Cycle in snails
5) ingested by humans from water
what causes schistosomiasis?
Schistosoma mansoni
Where is schistosomiasis most common?
Globally in tropical and subtropical areas (Neglected Tropical Disease (NTD)), especially in poor communities without access to safe drinking water and adequate sanitation.
Where do 90% of those requring treatment for schistosomiasis live?
In Africa
What is the clinical impact of Schistosomiasis?
Bowel Schistosomiasis: Caused by S. mansoni or S. japonicum, leading to symptoms like bloody diarrhea, abdominal pain, ulcers, and cramping.
Urinary Schistosomiasis: Caused by S. haematobium, associated with blood in the urine (haematuria), urinary tract damage, and a higher risk of bladder cancer.
Which form of Schistosomiasis is more common?
Chronic more common than acute form.
What are the symptoms of Schistosomiasis caused by?
Symptoms are caused by the body’s reaction to the eggs.
Does the chronic form have high mortality rates?
Yes
Fibrotic changes results from…
Fibrotic changes results from egg-retention induces granuloma formation
what are the fatality estimates for Schistosomiasis
Fatality estimates vary between approx. 24,000 and 200,000 globally per year.
Is it true that most infectious ilnesses/diseases are driven by behaviour, except for those with macroparasites?
Yes
Summary: Macroparasites
Endemic disease
Chronic disorders
Limited long-term immunity
High percentage of re-infection
Multiple worms in infected individuals
We have looked only at endoparasites
Soil-transmitted helminths and schistosomiasis
Predominantly oral / faecal transmission
Most ‘adult’ macroparasites reside in GIT
Eggs released in faecal matter
Behaviour-related exposure
