Lecture 13 - Parasitology and Malaria Flashcards
What are parasites?
Parasites are eukaryotic organisms that live in or on another organism (host) and that cause harm to the host. A parasitic relationship is one in which one organism, the parasite, lives off another organisms, the host, harming it and possibly causing death.
There are two main groups: protozoa and helminths (worms)
* Protozoa are all microbes - single celled organisms
Helminths are multicellular and very from very small to very large - often have small eggs.
Give examples of both protozoan and helminth parasites
Examples of protozoan parasites
* Plasmodium species: cause Malaria
* Leishmania species: cause Leishmaniasis
* Giardia
* Cryptosporidium
Examples of helminth parasites
* Schistosomes: Schistosoma species
Roundworms: Ascaris lumbricoides
How are parasitological diseases diagnosed?
Direct methods - involve visualisation of the parasite itself or detection of some specific component of the parasite such as its DNA or antigen detecting tests.
Indirect methods - involve detection of the effects of the parasite, includes clinical diagnosis, biochemical tests and serological tests.
Sample types - Blood and stool samples are the most important for parasites
Tests don’t work perfectly - sometimes they incorrectly diagnose someone as positive (false positive) or incorrectly diagnose someone as uninfected when they actually have the infection (false negative)
Prevalence - total positives (true positives and false negatives)/total population
* Sensitivity - proportion of true positives identified (small number of false negatives).
* Specificity - proportion of true negatives identified (small number of false positives).
There is no such test as the ideal test and we usually don’t know the actual prevalence of a disease in a population. In developing and evaluating diagnostic tests they are compared with the best existing tests (the gold standard)
How does clinical context influenct the choice of test?
In serious infections where it is life threatening sensitivity is more important because it is necessary to start treatment immediately even if there is a chance it is a false positive.
In conditions where the treatment side effects are significant specificity is more important as drugs shouldn’t be supplied unless it is a true positive.
What species cause malaria
There are five known species to cause malaria
* Plasmodium falciparum
* Plasmodium vivax
* Plasmodium malariae
Are the most important of these
Malaria is transmitted by the bites of female mosquitoes Anopheles spp.,
There are no significant animal reservoirs except P. knowlesi in macaque monkeys.
Describe the malaria life cycle.
Diagnosis of malaria occurs in malaria erythrocytic life cycle
1. Immature trophozoite (ring stage) in RBC
2. Trophozoites mature
They can then either become schizont and burst to repeat the cycle
Or form gametocytes
Describe the epidemiology of malaria.
About 3.3 billion people are at risk of malaria and ~247 million malaria cases occur a year 90% of which are in Africa.
The global frequency of infection = P. falciparum > P. vivax»_space; P. malariae > P. ovale»_space; P. knowlesi
P. falciparum is responsible for ~50% of cases and most of the deaths from malaria.
In non-endemic regions infection is seen in those who have travelled through or relocated from malaria regions, e.g. in the UK
Describe the indirect and direct diagnostic tests for malaria.
Indirect diagnostic tests for malaria
* Presumptive clinical diagnosis: fever, clinical algorithm
* Antibody detection (not used for diagnosis - but can determine if there has been past exposure) - ELISA and IFAT
Direct diagnostic tests for malaria
* Parasitological
○ Microscopy: thick and thin blood films = current gold standard
Thick film - sensitivity
Thin film - specificity
○ Quantitative buffy coat (QBC)- acridine orange stains DNA - RBCs don’t usually contain DNA so anything fluorescing in RBC layer is parasite
* Molecular biology methods - Polymerase chain reaction (PCR)
* Detection of specific malaria antigens
○ Paracheck-Pf
○ Binax (ICT)
○ OptiMal
Describe the clinical diagnosis of malaria.
No unique set of symptoms – overlap with other infections
* Symptoms are associated with the blood stage infection – occur after a delay of 2 weeks or longer (incubation period) following mosquito bite
* Fever is the classic symptom – can be irregular or regular with following stages:
○ Cold stage – patient shivers, then temperature rises rapidly
○ Hot stage – peripheral vasodilation (flushed appearance), rapid pulse and high temperature ~41°C
○ Sweating stage – copious sweating leading to a fall in temperature
* Other common symptoms include anaemia (low red blood cells), splenomegaly (enlarged spleen), and jaundice (yellowish appearance due to accumulation of bilirubin – breakdown product of haem)
Cerebral malaria in the brain is a serious complication of P. falciparum infection resulting in coma
