36 Trypanosomiasis and Leishmaniasis (Hannah)

Question 1

What is the causative agent of leishmaniasis?

Answer 1

Leishmania species (many different species)

Question 2

What is the vector for leishmaniasis?

Answer 2

Sandflies (Plebotominae)

• As the sandflies take a blood meal, the parasite is transferred via the saliva into the host

Question 3

What is the tropism for leishmania species?

Answer 3

Intracellular parasites, occupy phagosomes of macrophages

Question 4

What is the causative agent of Chagas disease?

Answer 4

Trypanosoma cruzi

Question 5

What is the vector for Chagas disease?

Answer 5

Triatomine (kissing) bug

• Aim for the corners of eyes and mouth to feed on tears/saliva
• Transmission route is via defecation onto skin - if there are open wounds nearby (e.g. the individual scratches the itchy bite and forms a wound) or parasites reach mucosal surfaces, the parasite is transmitted

Question 6

What is the tropism for Trypanosoma cruzi?

Answer 6

• Intracellular parasite, infects many different types of cells
• Tissue tropism can also be different between strains

Question 7

What is the causative agent of human African trypanosomiasis (African sleeping sickness)?

Answer 7

Trypanosoma brucei

There are two sub-species that cause human disease:

• T. brucei gambiense
  
  • Human-infective, tends to cause chronic disease
  • Can be zoonotic
• T. brucei rhodesiense
  
  • Human-infective, tends to cause acute disease
  • Tends to progress to CNS/CSF infection, very severe symptoms and high mortality
  • Often zoonotic

Question 8

What is the vector for African sleeping sickness?

Answer 8

Tsetse flies (Glossina)

Question 9

What is the tropism for Trypanosoma brucei?

Answer 9

• No tissue tropism, extracellular parasite living in the blood

Question 10

What limits global distribution of trypanosomatid and leishmania diseases? What is this global limitation?

[EXTRA?]

Answer 10

• Distribution is defined by the distribution of the insect vectors
• Typically diseases of the tropics and sub-tropics, but distribution can be skewed by current outbreaks
  
  • E.g. outbreak of leishmaniasis currently in South Sudan and Afghanistan, HAT in Democratic Republic of the Congo

Question 11

What are the death and DALY impacts of trypanosomatid and leishmania diseases?

Answer 11

• Trypanosomatid and leishmania diseases have high DALY impact in the tropics and sub-tropics
  
  • Effect is much lower on deaths (especially when compared to malaria), but do have a large effect on ill health
• Combined DALY impact of trypanosomatid disease is approx. 50% that of malaria, but they are still a worthy target for therapeutics (and yet still receive a lot less funding)
• These conditions can have chronic effects, some potentially disfiguring (e.g. Leishmaniasis can leave disfiguring scars on the face), and these will have an impact on DALYs

Question 12

What are the different life stages of a trypanosomatid?

[EXTRA]

Answer 12

• Lots of life cycle stages are named from the morphology of the parasite, using 3 key structures:
  
  • Nucleus
  • Kinetoplast – mitochondrial DNA compacted down into a single structure
  • Flagellum – always have a single flagellum that they use to swim
• Different parasites/species/life cycle stages adapt their morphology in different ways
• Terms: trypomastigote, epimastigote, promastigote and amastigote used in particular

Question 13

What is the general, fairly vague life cycle for parasites?

Answer 13

• Can see consistent trends in activity, two stage life cycle:
  
  • Insect vector
  • Mammal/vertebrate host
• In insects, tend to have one replicative stage, then some undergo differentiation to produce a stage that is infectious to allow transferral to the mammal host
• In mammals, then have a replicative stage (parasitic replication), then some portions tend to differentiate into a transmissive stage
• Note intracellular vs extracellular parasites

Question 14

What are the symptoms of African sleeping sickness/Human African Trypanosomiasis?

[EXTRA]

Answer 14

• Common symptoms
  
  • Chancre at the site of bite (painless ulcer)
  • Headache, malaise, weakness, fatigue, pruritis, and arthralgias
  • Swollen cervical lymph nodes (CHARACTERISTIC FEATURE)
• Other clinical features (infection broken down into 2 stages)
  
  • Stage 1: Haemolymphatic stage (T. b. rhodesiense and gambiense), affects blood and lymph
    
    • Hepato-splenomegaly, weight loss
    • Intermittent fevers lasting 1-7 days with variable separation (days to months), spike after short intervals (this is due to waves of parasitaemia)
  • Stage 2: Meningoencephalitic/CNS/CSF stage, affects CSF and CNS
    
    • Can progress from a blood/lymph infection to a CSF/CNS infection, certain species more likely than others
    • T. b. rhodesiense infection after 21-60 days, more frequent
    • T. b. gambiense infection after 1+ years, less frequent (does not happen in all cases)
    • Neuropsychiatric symptoms, sleep/wake cycle disruption or reversal, mental, motor, sensory and neurological disturbances (usually more severe, causes severe symptoms)
    • Coma, often fatal

Question 15

What subspecies of Trypanosoma brucei causes chronic and which causes acute disease?

[EXTRA]

Answer 15

• Chronic: T. brucei gambiense
• Acute: T. brucei rhodesiense

Question 16

What is the distribution of animal Trypanosoma disease? What effect can this have?

[EXTRA?]

Answer 16

• Infection due to Trypanosoma brucei brucei**​
  
  • Not human-infective
  • Widespread among livestock and wild animals
  • Disease called Nagana in cattle (‘to be depressed’ in Zulu), has economic importance
  • Major obstacle to rural economic development
• Animal disease distribution not really known, but can be predicted from location of cattle
• Indicates that this disease has a huge economic effect
• If animals are susceptible to the same parasites as humans, huge risk of zoonotic infection

Question 17

Is underdiagnosis likely in HAT? Why/why not?

Answer 17

• Yes, especially as it is likely to also be under-reported
• Symptoms in early stages are usually non-specific, therefore not well reported
• Those infected are exposed to tsetse flies, and this will only really occur in rural populations that depend on agriculture, fishing, animal husbandry and hunting for survival
• Diagnosis and treatment is complex and requires specifically trained staff, making the disease difficult to control (particularly in rural populations)

Question 18

What is the methods of diagnosis for HAT?

[EXTRA]

Answer 18

• Gold standard of diagnosis: microscopy, can actually visualise the parasites
  
  • Trypomastigotes in a thin blood smear, use Giemsa stain
  • Longer and thinner than RBCs
  • This may be in chancre fluid, blood, lymph node aspirate or (for stage 2), cerebrospinal fluid (requires lumbar puncture), but is most commonly peripheral blood
  • Wet samples should be used to look for motile parasites and smears stained with Giemsa should also be analysed (but can use dry smears)
• Antigen detection is another, newer method, with both advantages and disadvantages
  
  • Several rapid diagnostic tests are in development/testing
  • Advantageous for use in rural areas
  • Some limitations
    
    • Antibody detection has variable sensitivity and specificity
    • T. b. rhodesiense seroconversion occurs after the onset of clinical symptoms, making use of a rapid diagnosis test is not useful and instead must be diagnosed clinically
    • Useful for epidemiological surveys of T. b. gambiense

Question 19

How is HAT managed, and what are the treatments at each stage?

[EXTRA?]

Answer 19

Disease management

• Three stage diagnosis/management procedure make management important
  
  • Screen for potential infection
    
    • Serological tests (T. b. gambiense only)
    • Clinical signs (often swollen cervical glands)
  • Microscopic identification of parasites
    
    • Blood smear
    • More labour intensive
  • Staging disease progression
    
    • Examination of other fluids, particularly CSF by lumbar puncture
    • Identification of CNS infection greatly complicates treatment, early diagnosis is critical if possible as therapeutics for CNS stage are more intense
    • Long and sometimes asymptomatic T. b. gambiense initial infection poses challenges – active screening of at-risk population and early identification necessary to reduce transmission, but this comes at a large human and logistical cost (making it more difficult to reach the WHO plan of stopping human-to-human transmission by 2030)

Treatment

• Differs significantly depending on disease stage
• First stage:
  
  • Drugs have lower toxicity and are easier to administer, less stringent administration regimes, early identification of infection gives far better prospects for a cure
    
    • Pentamidine: discovered in 1941, used for the treatment of the first stage of T. b. gambiense, generally well tolerated by patients despite non-negligible undesirable effects
    • Suramin: discovered in 1921, used for the treatment of the first stage of T. b. rhodesiense, provokes certain undesirable effects in the urinary tract and in allergic reactions
    • The above drugs are very old – we do not have many modern therapeutics for treating this disease
• Second stage (e.g. once parasites in the CNS):
  
  • Drugs must cross the BBB effectively, drugs are typically toxic and complex to administer
    
    • Melarsoprol: discovered in 1949, effective in both first and second stage but normally reserved for second, arsenic-containing with many undesirable side effects (including reactive encephalopathy/encephalopathic syndrome, which can be fatal in 3-10%)
    • Eflornithine: registered in 1990, less toxic than melarsoprol but only effective against T. b, gambiense (which is less likely to progress to this secondary stage of infection anyway), regimen is strict and difficult to apply, relatively new
• These four drugs are provided free of charge to endemic countries
• More recent developments:
  
  • Combination of nifurtimox and eflornithine was introduced in 2009
  • Fexinidazole: registered in 2018, developed by DNDi (drugs for neglected diseases initiative)
  • Increased drug resistance of T. b. rhodesiense to melarsoprol emerging in central Africa – this is a major concern

Question 20

Why is there no vaccine for T. brucei?

Answer 20

• This is due to its capacity for antigenic variation
• Infection appears to be cyclical
  
  • Can see cyclic increase and decrease in parasite load over time, scale is in weeks
  • Waves of fever seen, associated with peaks in parasitaemia
  • Can track this using levels of parasitaemia (parasites per microlitre of blood)
• Most parasites in a wave of parasitaemia have the same surface coat protein, antibody responses to each wave results in their elimination
  
  • To combat this, each new wave has a new coat, as nwe variants emerge in each subsequent wave of parasitaemia
  • This is a cyclic process and likely to be a little bit more complex than just simple selection pressure
• These constant changes in antigen make it very difficult to create an effective vaccine

Question 21

Describe antigenic variation between different rounds of parasitaemia in T. brucei infection.

Answer 21

Question 22

How is antigenic variation thought to be possible in T. brucei?

[EXTRA]

Answer 22

• Likely to be due to some quorum sensing in the bloodstream - when parasite load is high, selection of a new protein coat may be triggered
  
  • High parasitaemias lead to formation of stumpy bloodstream forms, which are non-replicative - possible that this process is combined with antigenic variation
• Variant surface glycoprotein (VSG) is abundant and densely packed in the protein coat
  
  • This can be seen by looking at the thickness of the parasite cell coats under electron microscopes
  • This is a membrane anchor protein and makes up approx. 10% of total cellular protein
• RNA Pol 1 is used by trypanosomes to transcribe surface coat genes
  
  • This protein is normally reserved for rRNA, but ultimately makes normal mRNA for the parasite (albeit in a slightly unusual way)
  • Co-transcribes gene groups where multiple are encoded in the same length of DNA, depending on which point of the life cycle the parasite is in
    
    • Genes expressed will be the best adapted for that particular stage
• There is a large VSG repertoire
  
  • Around 20 bloodstream-form expression sites
  • Found near the ends of chromosomes (sub-telomeric)
  • One VSG gene, and several expression site-associated genes
  • One and only one expression site is active at any one time
  • What do the expression site associated genes (ESAGs) do?
    
    • Many shared between different expression sites, some different
    • Abundant proteins to help the bloodstream form of the parasite survive in the blood?
    • Specialisation for different host species?
      
      • There are some which are specific to different locations/expression sites
    • These are co-transcribed poly-cistron units
    • Really not fully understood
• Switching active VSG site could be achieved through:
  
  • Changing transcription
    
    • Multiple expression sites, each has a different VSG unit
    • Changing active expression site changes VSG expressed
  • Using homologous recombination to replace the VSG in an active expression site
    
    • There is a huge number of VSGs that the parasite could do this with, estimated that there is >1000 VSG genes and pseudogenes within the genome
    • Replace the whole VSG with a new VSG, or create a mosaic
• This allows an enormous potential for antigenic variation for this parasite, and making production of a vaccine near impossible

Question 23

What is the specialised Pol I factory in T. brucei?

[EXTRA]

Answer 23

• Specialised factory for Pol I production of ESAG and VSG transcripts required by the parasite
  
  • This specialised factory is visible, close to the nucleolus
• Molecular machinery only recently being discovered
• This structure is critical for the survival of this parasite in the blood

Question 24

What are some global and medical consequences of Chagas disease?

Answer 24

• Approx. 6 million ongoing cases
• Mostly in Latin America
• Curable if treated shortly after infection, but chronic disease can lead to severe complications
• Historically confined to the Americas, now some spread in other continents (slightly concerning)
• Estimated 30% of chronic infections lead to cardiac alterations
  
  • These occur because the trypanosomes can actively invade cells, including cardiomyocytes leading to severe dysfunction and, eventually, sudden cardiac arrest
• Estimated 10% of chronic infections lead to digestive, neurological or other alterations (less common)

Question 25

How can transmission of Trypanosoma cruzi be controlled?

[EXTRA]

Answer 25

• Vector control is the most useful control approach (get rid of the triatomine bug)
• Blood screening is vital to prevent inadvertent transmission
  
  • Blood transfusion
  • Organ transplant
  • In some regions of Latin America it is vital to scan blood and organ donors for the disease in order to prevent inadvertent transmission
• Diagnosis/screening of pregnant women and their new-born children is essential

Question 26

Summarise the Trypanosoma cruzi life cycle.

[EXTRA]

Answer 26

• The trypanosome replicates in the gut of the kissing bug, defecated as a trypomastigote stage that is preadapted for infection
• Once inside a human host, they invade cells and enter an amastigote stage (no longer as motile) and then multiply
• Can then adapt into another trypomastigote stage that emerges from the cells and travels through the blood, before invading new cells to perpetuate the infection
• As the bugs take a meal, the trypomastigotes can be ingested from the blood and enter the gut of the bug to then replicate

Question 27

How can Trypanosoma cruzi infection be diagnosed?

[EXTRA]

Answer 27

• Microscopy/blood smears during acute infections
  
  • Only sensitive during initial infection or acute symptoms due to the life cycle of the parasite (only has a short time in the blood, mostly stays within cells to replicate)
  • Chronic disease associated with few bloodstream form parasites in circulation
• Serological tests during chronic phase
  
  • Detect antibody to the parasite
  • Single test is not sufficient, two or more complementary tests are necessary (as these tests are not overly effective)
    
    • Two different antibodies to different antigens
    • Two different detection methods
  • ELISA, IFA, rapid diagnostic tests
• USA context
  
  • Estimated 40,000 childbearing-age women infected with T. cruzi but with low mother-to-child transmission rate (1-5%) in southern American states
  • FDA requires blood donation screening (since 2006), any positive result means that donor can no longer donate (regardless of further tests), due to chronic infection

Question 28

What are the different types of leishmaniasis? What are the symptoms?

[EXTRA]

Answer 28

Ranges from silent infection to severe/lethal visceral infection

• Silent infection: no obvious symptoms
• Cutaneous leishmaniasis:
  
  • Common, causing skin sores
  • Typically develop within weeks or months of a sandfly bite, appear at site of bite (typically)
  • Can change in size and appearance
    
    • Start as papules or nodules, can develop into ulcers
    • Often scabbed or crusted
  • Sores typically painless but can be painful
    
    • Glands near the sores can be swollen, satellite lesions can arise around the sores
  • Can lead to diffuse cutaneous infection, appears like lepromatous leprosy (if left untreated or patient is immunocompromised)
  • Sores typically self-heal but can last months or years
  • Typically leaves scar, can be disfiguring (particularly as bites often occur on the face)
• Mucocutaneous (mucosal) leishmaniasis:
  
  • Less common, causing sores in the mucous membrane
  • Can be a sequela/secondary site of infection, spreading from the skin to the mucous membranes
    
    • Original site of infection may not be noticed
    • Original sores may have healed
    • Infection can still spread to mucous membranes, however
  • Most commonly nose, sometimes mouth or throat, serious risk of complications
    
    • Initial symptoms of congestion, nose bleeding, inflamed mucosa (mild)
    • Later symptoms may reach ulcerative destruction (severe)
  • Best prevented by treating initial cutaneous infection
  • Can be severely disfiguring
• Visceral leishmaniasis (aka kala-azar, ‘black fever’):
  
  • Less common, usually affects the spleen, liver and bone marrow
  • Can be life threatening, often fatal without treatment
  • Typically develops within months to years of a sandfly bite
    
    • Can arise after becoming immunocompromised, e.g. after HIV infection
  • Fever, weight loss, splenomegaly, hepatomegaly, anaemia, leukopenia and thrombocytopenia

Question 29

What is the distribution of leishmaniasis?

[EXTRA]

Answer 29

• Different species of sandflies (the vector) are likely to carry different leismania species
  
  • These are morphologically indistinguishable via microscopy
  • Differentiated by molecular diagnostics, antibody diagnostics or DNA
• There are also different species that are more likely to cause cutaneous and mucocutaneous, and those that are more likely to cause visceral infection
• There is a complex interplay between different species, which disease they tend to cause, where they tend to cause it, what vector species carries them etc
• Particular challenge is that sandflies are particularly small, and are often able to pass through standard mosquito nets, therefore control of the vector is very difficult

Question 30

Is leishmania well-reported or under-reported?

[EXTRA?]

Answer 30

• Significant under-reporting
  
  • WHO estimates 60-75% of visceral cases not reported/not recognised as leishmania infection

Question 31

Summarise the Leishmania life cycle.

[EXTRA?]

Answer 31

• Procyclic promastigotes replicate in the gut of the sandfly
• Specialised promastigotes migrate up to the pharyngeal valve and then the salivary glands
  
  • These preadapt into metacyclic promastigotes ready for transmission
• Once the sandfly bites a person, the metacyclic promastigotes are transmitted via the saliva
  
  • These parasites can then be taken up by macrophages and other monocytes
• Once inside the macrophages/monocytes, they differentiate into an amastigote lifecycle stage and replicate within the parasitic vacuole within the macrophage
• From here they can occasionally erupt from the macrophage and be transmitted
• It is thought that transmission back to the sandfly occurs from ingestion of infected macrophages within the circulation, which then release amastigotes during digestion of the blood meal and they differentiate back into procyclic promastigotes
• A lot of complexity over the type of macrophage or monocyte infected (e.g. tissue resident, circulating, do cells escape the tissue they are in to be cleared by the liver or spleen)
  
  • This may contribute to some of the differences seen in clinical presentation of infection by different leishmania species

Question 32

What cell type do Leishmania species infect?

[IMPORTANT]

Answer 32

• Macrophages and monocytes
• Depending on the species, can be visible in circulating macrophages, tissue-resident macrophages, etc
  
  • This may contribute to the different clinical presentations seen between species

Question 33

What is the treatment for cutaneous leishmaniasis?

[EXTRA]

Answer 33

• Pentavalent antimonial drugs
  
  • Sodium stibogluconate and meglumine antimoniate
  • Most widely used antileishmanial agents, but are increasingly being replaced by safer drugs
• Pentostam (sodium stibogluconate) is not approved, but is available under new investigational new drug protocols
• Oral antifungal drugs (fluconazole, ketoconazole, itraconazole) have been used to treat cutaneous leishmaniasis with variable results
• Pentamidine isethionate
• Liposomal amphotericin B
• Other treatment modalities that have shown some efficacy for cutaneous leishmaniasis
  
  • Paromomycin ointment, oral miltefosine, thermotherapy, and intralesional pentavalent antimonial drugs
• Treatment is complicated by the fact that the infection could either be contained or diffuse, with little way of knowing

Question 34

What is the treatment for visceral leishmaniasis?

[EXTRA]

Answer 34

• Case-fatality rate of visceral leishmaniasis (kala-azar) is >90% untreated cases
  
  • Mortality often due to haemorrhagic or infectious complications
  • Supportive therapy to address nutritional status, concomitant anaemia, haemorrhagic complications, and secondary infections is essential to maximise survival
    
    • May be these nutritional deficiencies that have led to the presentation of the visceral form of infection
• Liposomal amphotericin B
• The pentavalent antimonial drugs: sodium stibogluconate and meglumine antimoniate
• Paromycin and miltefosine
• Patients with infection should be evaluated for HIV co-infection, visceral leishmaniasis may emerge from dormancy due to immunocompromised state
  
  • Treat HIV aggressively
  • Treatment response is poor in co-infected patients without effective immune reconstruction
  • This has been particularly seen in India

Question 35

What are some groups that are helping to overcome the difficulties in treating ‘neglected’ tropical diseases?

[EXTRA?]

Answer 35

• Medecins sans frontieres (MSF)
  
  • Access to medicines, coordinated effort
  • Political activism
  • Pressure to release/open intellectual property
  • Drug donation schemes
  • Production of non-profitable drugs
    
    • Make drugs available so that they can reach the people who need them
• Drugs for Neglected Diseases Initiative (DNDi)
  
  • Direct research into new drugs
  • New formulations of existing drugs for neglected disease
  • Transfer of veterinary drugs to human use

Question 36

What are some gaps in neglected tropical disease drug development?

Answer 36