Malarial treatment and control Flashcards
Is malaria due to a single organism?
No it is due to a complex range of species with different biologies
What malarial parasite is the main cause of human malaria in Malaysia?
P. Knowlesi
Its mosquito vector doesnt behave like other vectors, in that it doesnt enter dwellings to the rest and feed, so it tends to get missed by many of the standard control methods
What is P. knowlesi often misdiagnosed as?
misdiagnosed by microscopy as P. falciparum or P. vivax, but because its biology is different, with a 24-hour replication cycle, it can rapidly progress from an uncomplicated to a severe or fatal infection.
What else does P. knowlesi do that is unsimilar to p. vivax and p. falciparum?
unlike vivax and falciparum, it has a reservoir in other primates, you are not going to eliminate it in humans without controlling it in other primates as well.
It is not the only one we need to worry about.
Recent molecular studies have revealed a large diversity of Plasmodium spp. among African great apes. Some of these species are related to Plasmodium falciparum and others to Plasmodium ovale, Plasmodium malariae and Plasmodium vivax. So, what is the possibility of them jumping host to humans?
Quite high so it seems.
In 2016, Ngoubangoye and colleagues used molecular techniques to analyse blood samples from captive, non-human primates living in Gabon to evaluate the risk of Plasmodium spp. transfers between host species, and crucially also included blood samples from the human workers taking care of those primates to assess whether primate-human parasite transfers occurred.
They detected four transfers of Plasmodium from gorillas towards chimpanzees, one from chimpanzees to gorillas, three from humans towards chimpanzees and one from humans to mandrills. No simian Plasmodium was found in the blood samples from humans working with primates.
But there is clearly a lot of plasticity in the system and other simian species may yet make the jump.
Moreover, we probably still don’t fully know what is out here. In 2015, Faust and Dobson published an analysis of primate malaria species which predicted at least three undescribed primate malaria species exist in sampled primates, and many more likely exist in unstudied species.
P. falciparum belongs to a family of parasites which infect what?
African apes
In their native habitat, what does p. falciparum exhibit?
Whats one potential mechanism behind this?
In their native habitat, these parasite species exhibit strict host tropism, and human P. falciparum has never been recorded in wild-living chimpanzees and gorillas.
One mechanism behind this strict host specificity is the interaction between a parasite protein (reticulocyte binding protein homologue 5 - RH5) and its erythrocyte cell surface receptor (Basigin - BSG). This interaction is species specific and mirrors the observed host–parasite tropism.
Studies by Wanaguru and colleagues in 2013 investigated this interaction.
TOP LEFT
Binding efficiency studies show that P. falciparum RH5 bound chimpanzee BSG with a significantly lower affinity than human BSG and did not bind gorilla BSG, mirroring the known host tropism of P. falciparum. (Cd4 binding = control)
TOP RIGHT
Kinetic analysis of the interaction of human (Upper) and chimpanzee (Lower) BSG with PfRH5 – see from the scale that rate of interaction with chimp BSG is 10-fold less than with human.
They then used site-directed mutagenesis to identify residues on chimp BSG and “humanised” mutant BSG that are important for species-specific discrimination of PfRH5 binding
BOTTOM A
Single base change at residue 191 of chimp BSG caused very significant increase in binding to PfRH5
BOTTOM B
Also, much lower dissociation kinetics
BOTTOM C
Then gorrilorised human BSG to see what changes prevented PfRH5 from binding and found several residues important to the interaction.
BOTTOM D
Again, the mutants showed much lower reaction kinetics
BOTTOM E
The predicted protein structure of BSG locates all of these in exposed positions close to the join between the 2 BSG domains.
Author’s take home message was that point mutations in BSG that could preserve its function but prevent PfRH5 binding to make erythrocytes refractory to invasion potentially could be exploited therapeutically.
An alternative take home message might be that it might not take much, by way of mutations to allow host switching and new malaria species to infect humans.
P. vivax is absent from Central and West Africa because of the blood chemistry of the indigenous people, in particular the absence of the duffy glycoprotein. However, what are there reports of?
What does this suggest?
There are reports of duffy positive Europeans persons returning from these areas infected with this parasite and reports of duffy negative individuals carrying P. vivax. These suggest both the existence of active transmission (parasite presence) and of duffy negative status not being fully protective (the parasite being able to enter red cells without the duffy glycoprotein on the red cell surface).
Among the possible explanations for this apparent paradox, it has been proposed that there may be an as yet undiscovered zoonotic reservoir of P. vivax. It certainly can infect non-human primates, both marmosets and owl monkeys have been used as laboratory hosts. A recent molecular study by Rondon et al in Columbia (so Central America) detected parasites indistinguishable from P falciparum, P vivax and P malariae in blood and faecal samples from several non-human primates and in multiple anopheles species.
The mitochondrial and nuclear genetic diversity of P.vivax parasites were isolated from greatapeas and analysed. They were compared to parasites isolated from travellers returning from these regions of Africa, as well as to human isolates distributed over the world.
What was observed?
Overall, the P. vivax sequences from parasites of great apes formed a clade that is genetically distinct from the parasites circulating in humans BUT this clade’s parasites can be infectious to humans - a traveller returning from the Central African Republic was infected with one of them. YELLOW STAR
Moreover, natural transfers of P. vivax from humans to apes was also detected as among ape P. vivax for which complete mitochondrial genomes were characterized, one clearly clustered with the human clade, exhibiting its three specific SNPs (DRCG) RED STAR
Nevertheless, great ape-to-human transmission cannot apparently account for all observed cases because some human P. vivax infections were observed in areas where great apes are absent.
So, it is a very plastic and volatile situation and with fragmentation of tropical rain forests, logging, agriculture, ranching and mining activities and global travel and ecotourism – the chances of malaria species jumping hosts is ever present.
malarial life cycle
When looking at current and future treatments and control strategies for malaria, what are some of the things we want to reduce?
- Number people being bitten
- Number of people becoming infected
- Number of people transferring disease back to vector
- Numbers of mosquito’s numbers carrying disease
- Parasite load in humans
- Parasite load in mosquitos
- Severity and length of disease in humans – vaccines, drugs etc
- Reproductive capacity of parasite.
- Etc.
When did the drugs for malaria first come about?
Drug treatment for malaria in S. America goes back some 400 years when Spanish Jesuit missionaries in Peru were taught the anti-fever properties of the bark of the cinchona tree (fever tree) by natives, around 1620 to 1630.
A specific treatment for malaria did not become available in Europe until the 1630s, when bark of the cinchona tree was introduced into Spain from Peru.
The lifesaving drug became much more widely available by the mid 1800s, after the active ingredient of cinchona, quinine, was successfully isolated and the Dutch and English began to cultivate the cinchona tree in plantations on the island of Java and in India and Sri Lanka.
What are some examples of drugs that are used against malaria?
Tell me abit about them if necessary
Quinine is still used as a drug of last resort, and cinchona tree bark remains the only economically practical source of supply
Chloroquine (1934) previous drug of choice but much resistance Pf and Pv, skin itching in dark skinned individuals reduces compliance $0.08 (synthetic derivative of quinine)
atovaquone
proguanil
artemether
lumefantrine
mefloquine (Lariam®) vomiting in young children, unpredictable uptake, seizures, severe psychiatric problems $1.92
quinine – severe malaria, multidrug resistant malaria drug of last resort, side effects reduce compliance $1.51
quinidine / quinidine gluconate – often not available, can harm heart – injected – severe malaria
doxycycline (used in combination with quinine)
artesunate (not licensed for use in the United States, but available through the CDC malaria hotline) – injected
artemesin (quinghaosu) - severe malaria $1.50 - $3.40
Primaquine – hypnozoites – vivax and ovale – not pregnant women, not G6PD deficient
Amodiaquine - chloroquine resistance, some cross resistance, toxic hepatitis if used prophylactically $0.14
Artemesinin multidrug resistant Pf $1.50 - $3.40
Halfantrine – for multidrug resistant Pf, not pregnant, variable uptake, fatal cardiotoxicity $5.31
What is an issue that has arisen from using Chloroquine?
Chloroquine was the mainstay of modern antimalarial drugs. As it is very cheap BUT now there is much resistance and side effects associated with it
The path of chloroquine resistance
P. vivax infection acquired in some areas has been shown to be resistant to chloroquine and/or primaquine (hypnozoites).
Why are drug cocktails now being used for malaria?
P. vivax infection acquired in some areas has been shown to be resistant to chloroquine and/or primaquine (hypnozoites).
mefloquine and artesunate / artemesinin - non-severe resistant Pf $3.90
sulfoxidine and pyrimethamine (SP) and artesunate / artemesinin non-severe resistant Pf, safety in 1st trimester not established $1.12
tetracycline / doxyxcycline only used in combination with fast acting schizonticide like quinine, not < 8 years, not pregnant
clindamycin only used in combination with fast acting schizonticide like quinine - not renal disease or GI disease / colitis
lumfantrine and artemeter – non severe resistant Pf, safety in pregnancy not established $7.30 $57 (Europe)
atovaquone and proguanil (Malarone®) – no approved paediatric formulations but reported safe in pregnancy and young children, expensive $35
mefloquine-sulfadoxine-pyrimethamine (MSP) used in Thailand - 3 components have completely different pharmacokinetics wrt elimination half-liveslives:P= 80-100h, S=100-200h, M >430h
Consequently, drug cocktails now being used, but only a matter of time until additional resistance selected for.
Resistance to antimalarial drugs has been described for P. falciparum and P. vivax. P. falciparum has developed resistance to nearly all antimalarials in current use, although the geographical distribution of resistance to any single antimalarial drug varies greatly.
Each year about 1700 people return to the UK with malaria – 6-10 deaths. Mainly from West Africa (2/3) – primarily people returning from visits to family, but 10% from tourism.
Is the treatment for malaria complex?
Yes
What is the treatment for malaria based on?
The infecting Plasmodium species: Determination of the infecting Plasmodium species for treatment purposes is important for three main reasons
What can infection by p. falciparum or p. knowlesi cause?
Rapidly progressive severe illness or death
What does p.vivax and p. ovale infections also require treatment for?
The hypnozoite forms that remain dormant in the liver
What does p.falciparum and p.vivax species have that differ across the geographic regions
Their drug resistance patterns
When patients are diagnosed with malaria, how are they categorised?
As either having uncomplicated or severe malaria
How are patients with uncomplicated or severe malaria treated?
Patients diagnosed with uncomplicated malaria can be effectively treated with oral antimalarials.
However, patients who have one or more of the following clinical criteria (impaired consciousness/coma, severe normocytic anemia [hemoglobin < 7], renal failure, acute respiratory distress syndrome, hypotension, disseminated intravascular coagulation, spontaneous bleeding, acidosis, hemoglobinuria, jaundice, repeated generalized convulsions, and/or parasitemia of ≥ 5%) are considered to have manifestations of more severe disease and should be treated aggressively with parenteral (IV) antimalarial therapy.
Tell me about the drug susceptibility of the infecting parasites
Knowledge of the geographic area where the infection was acquired provides information on the likelihood of drug resistance of the infecting parasite and enables an appropriate drug or drug combination and treatment course to be prescribed
if in doubt – assume the worst and treat as if chloroquine-resistant P. falciparum.
BUT ALSO:
- Any accompanying illness or condition
- Pregnancy
- Drug allergies
- Other medications taken by the patient
- Genetic background of patient (G6PD deficiency X linked recessive disorder– predisposes individual to erythrocyte breakdown in response to exposure to certain foods, drugs, infections, or stress - relatively asymptomatic – quinine based antimalarials can cause acute haelolysis)
Why is treatment failing/ resistance developing?
incorrect dosing
non-compliance with duration of dosing regimen
poor drug quality
drug interactions
poor or erratic absorption
misdiagnosis
immune and health status
drug chemistry
epidemiology
parasite biology
mosquito biology
Why is incorrect dosing an issue for failing treatment?
Thailand initially recommended 2 tablets (adult dose) of sulfoxidine and pyrimethamine (SP) for treating malaria based on studies suggesting that this was effective. Within a few years, this was no longer effective, and the programme increased the regimen to 3 tablets.
Although unproven, sub curative doses may have contributed to the rapid loss of SP efficacy.
Ditto in Africa, to simplify large scale programmes, dosed by age rather than body weight so may have been under dosing some groups.
Ditto failure or inability to follow up patients with re-diagnosis and further treatment till shown to be cured to decrease risk of transmission of resistant parasites.
Vaccines…
In fact, we still have very few vaccines for any parasitic disease in man or animals, and the vast majority of those comprise live attenuated or killed whole or disrupted parasites
By far most of the spending on malaria research goes on vaccines.
literally billions of dollars spent over decades
full genome sequence of Pf since 2002
BUT we still have nothing out there in routine use in man! But hold the front page
What are the main things we want vaccines against malaria to do?
What would the ideal malaria vaccine look like?
Depends on what you might want to achieve:
Prevent infection
Prevent or decrease severity of disease?
Block transmission to mosquito?
Global malaria vaccine pipeline
No single antigen is going to be able to meet all of the requirements. But there are many vaccine candidates being worked up
Which kind of brings us back to where we started, using live attenuated parasite.
What if we try the equivilent for malaria…
PfSPZ vaccine:
It is possible to achieve more than 90% protection against malaria in humans:
by immunization with radiation-attenuated Plasmodium falciparum (Pf) sporozoites inoculated by mosquitoes
by immunization with non-attenuated Plasmodium falciparum (Pf) sporozoites inoculated by mosquitoes in volunteers taking chloroquine or mefloquine as a parallel chemotherapeutic
by intravenous injection of aseptic, purified, radiation-attenuated, cryopreserved Pf sporozoites (‘PfSPZ Vaccine’)
some data known since 1970s but only recently been revisited!
Need 4 doses
Efficacy is about 27% from 15,000 kids that received the 4 doses
WHO recommended long-lasting insecticidal nets
What been done to try and reduce vector population density?
Historically, a mainstay of malaria control globally has been the widescale use of broad-spectrum insecticides to kill adult mosquitos in and around transmission sites, to minimise transmission rates and breeding sites, to reduce vector population density.
Not surprisingly, such widespread insecticide use has led to widespread insecticide resistance in vector populations.
Since 2010, a total of 60 countries have reported resistance to at least one class of insecticide, with a total of 49 of those countries reporting resistance to two or more classes
Why is the situation now getting worse?
And the situation is getting worse, with resistance to 3 or 4 different classes of insecticide now being seen.
So, we have a real problem with insecticide resistance, without even considering wider ecological impacts,
Almost all public health insecticides are also used in agriculture and vectors may be exposed to the same or similar insecticidal compounds when they breed within or close to agricultural crops, which will select for resistance. This situation is of relevance for malaria vectors.
Also spraying water bodies is very nonselective with respect to the aquatic fauna it kills, not just mosquitos etc.
Many efforts are being made and developed as alternative natural chemicals, in ethiopia, there is significant resistance of mosquitoes to many of the traditional commercial insecticides. What naturally occuring plants have been identified?
Neem and Chinaberry have been identified. The seeds are collected and crushed to a powder which is then soaked overnight and sprayed onto waterbodies. Both contain multitudes of active ingredients with different modes of action, which lessens the chance of resistance developing in mosquito populations. This plant based larvicides are also environmentally sound and locally accessible.
The larvicidal effect of the plant extracts could be due to the presence of limonoids and Azadirachtin. These block the synthesis and release of molting hormones, leading to incomplete ecdysis in immature insects are a potent antifeedant to many insects and destroy the structure of integument and the alimentary canal.
Fourstar® Briquet- 180
Very recently (Dec 2016), new formulations of Bti have been announced which promise much more long-lasting effects, up to 6 months from a single treatment.
LHS, semi natural open tank treatments
RHS field conditions / natural habitats
EG. In natural highland habitats, during the first 2 months no pupae were detected from any of the treated habitat sites, and Anopheles spp. pupal density was reduced by 60–90% in the next 3–5 months
What is Wolbachia?
Wolbachia is an obligate endosymbiotic bacterium that is present in up to 65% of all insects and some arachnids, freshwater crustaceans, and filarial nematodes.
What are some ways in which Wolbachia could be used for population suppression of anophelines?
A What makes Wolbachia interesting and valuable is that it induces a phenomenon called cytoplasmic incompatibility in its hosts: this manifests itself as embryonic lethality after matings between Wolbachia-infected males and uninfected females or females infected with a different Wolbachia strains, thus leading to population reduction.
B Vectors harbouring a Wolbachia infection have been shown to inhibit the growth of pathogens. If females harbouring a Wolbachia infection are released, all offspring will carry the symbiont and exhibit reduced vector competence. Owing to the action of cytoplasmic incompatibility, this type of Wolbachia infection will spread.
c | If a particular strain of Wolbachia is released via females, it will not only provide pathogen blocking and spread via the action of cytoplasmic incompatibility, but also reduce insect lifespan. This has the potential to decrease pathogen transmission, as only older insects transmit disease.
So, lots of potential for future uses. What make Wolbachia especially interesting is that it can spread through the population, so is self-sustaining, only small numbers of individuals need to be released and it doesn’t rely on genetically manipulated organisms which may make it more acceptable to the authorities.
