Infectious disease mod 1-3

Disease

harmful alteration to physiological or metabolic state of host

Infectious disease

Harmful alteration to physiological or metabolic state of host caused by pathogen or its product

Pathogen (endogenous vs exogenous)

disease causing organism/agent

* Endogenous = present in human body (cause disease when immune system becomes weak)
* Exogenous = present in external environment

Toxins

Soluble substances which alter normal metabolism of hosts w deleterious effects

Limitations of Koch’s postulates

Essentially the microorganism is found in and can be isolated from diseased organisms + when infect other organisms, causes same disease

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Limited in that M. leprae cannot be easily culture, some viruses can only be identified by molecular techniques + a disease can be caused by more than one type of pathogen.

How do infectious diseases fit into the global leading causes of death>

lower respiratory infections, neonatal conditions + diarrhoeal diseases

In low SES, infectious diseases are more spread (note that reporting of disease in low SES is dodgy)

Deaths vs DALYs of infectious diseases?

IDs are very prevalent in DALYs (more so than causes of death where chronic diseases like IHD + COPD are major contributors)

Why is infectious disease impact driven by $$$?

Disease impact is affected by environment, host + pathogen.

Environment factors including health care access, intervention control programs, communication/education, shelter, sanitation/access to clean water

Describe TB infection vs disease

TB infection - latent infection, asymptomatic, cannot spread, not sick, bacteria are alive but inactive + tests usually come back positive

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TB disease- tests come back positive w an abnormal chest X ray, TB bacteria are active + usually sick -→ spread TB

Five classes of pathogens

Bacteria

Viruses

Protzoan

Helminths

Fungi

Describe 5 sources of infection

Physical contact w person (i.e. STDs or skin infection)

Contact w fomites

Ingesting infected food or drink

Entry of soil or dust into wound

Bites by insects

Unsterile medical procedures

Infection carried by mother’s bloodstream

Self-infection - poor hygiene

Describe entry stage of infectious disease

Transmission = look here to prevent infection

* Contacts like fomites, vectors, food, water, vertical transmission, direct physical contact
* Inhalation is most frequent source= breathing, speaking etc. (1-4m) - could lead to contaminated food or fomites
* Vertical transmission - congenital, perinatal (during birth), post natal
* Horizontal - person, vector, vertebrae to humans (rabies)

Purpose of pasteurised milk

Get rid of diseases like brucella, M. bovis, coxiella burnetii by heating for 72 degrees for over 15 seconds + then cooling

Entry

Ingress (inhalation or ingestion), penetration (through epithelial or barrier/mucous membrane- influenza virus, insect bites, cuts/wounds, organ transplant)

* Reliance on virulence factors for entry + colonisation
* i.e. pili help adhere and stop clearance

Multiplication + Spread

Understanding how to prevent infection progressing

What is needed for survival?

* Temperature (particular parts of body are infected)
* nutrients (i.e. iron)

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Immune evasion methods?

* microbial sanctuary
* Virulence factors

How can micro-organisms evade the complement system?

1. Prevent activation (mask- capsular polysaccharide or cleave the C3 peptidase)
2. Inhibition complement fixation (capsules- w sialic acid, or contain other sialic acid resides on LPS)
3. Inhibit access of MAC - LPS on bacteria like E. coli prevents access to surface of bacterium

Virus immune evasion methods (multiplication + spread)

Inhibit recognition (nucleic acid decoys + viroceptors that block cytokines)

Inhibit complement components (block C3 convertase and encode proteases)

Intracellular location + prevent NK cell recognition

Prevention of infected host cell apoptosis + antigen presentation

How can microbes cause damage?

Directly- toxins + mechanical disruption/ damage

* Cholera toxin causes diarrhoea
* Clostridium botulin
* Tetanu stoxin - spastic paralysis (inhibits GABA)
* Bordetella pertussis
* IDK the mechanical disruption example….

Indirect- immune system induced (inflammation, fluid collection, swelling)

Types of toxins

AB toxins - intracellular target

Superantigens- excessive activation of immune system (ETEC enterotoxin)

Anthrax= 2A+B

Different levels of damage/disease severity

Factors affecting infection outcome

Dosage (quantity) + quality of infectious organisms, including virulence (which quasispecies)

Age of host

Immunity, immunocompetence, health status

Nutritional status

Genetics

Behaviour (seeking health care)

Social determinants (availability of health care + clean water)

Functions of innate immunity

Recognise microbes encountered by host

Prevent infection/disease by eliminating microbes or allowing them to exist on body surfaces as normal, non-harmful flora

Initiate + influence nature of adaptive immune response depending on invading microbe

Activate adaptive immune response

Innate immunity components

Cellular - epithelial cells, phagocytic cells, pro-inflammatory cells, NK cells, APCs

Humoral - antimicrobial peptides, complement, cytokine, chemokines, acute phase proteins

Inflammation - vasodilation, vascular permeability, chemotaxis

Describe barriers of the skin, gut, lungs + eyes/nose (3x categories)

Skin- epithelial cell tight junctions + air flow (mechanical), antibacterial peptides + fatty acids in sweat (chemical), normal flora (microbiological)

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Gut- epithelial tight junctions, flow of fluid (mechanical), low pH, enzymes (pepsin) + antibacterial peptides (chemical) + normal flora

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Lungs- movement of cilia + epithelial cells (mechanical), antibacterial peptides (chemical)

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Eyes/nose- mucus/hair in nose (mechanical), salivary enzymes (lysozyme) (chemical)

Benefits of normal microbiota

Prevent attachment by pathogens

Antimicrobial factors

Biochemical reactions that benefit host (i.e. metabolise food0

Metabolic breakdown/synthesis (use of vitamin K)

Promote gut associated lymphoid tissue (GALT)

What are some ways that microorganisms can switch from mutualistic/commensal to a disease associated microbe/state?

Damage to epithelium (allow entry of skin flora into blood stream)

Presence of a foreign body

Transfer of microbiota to unnatural sites (self innoculation?)

Suppression of immune system by drugs or radiation

Impairment of host defenses due to infection by exogenous pathogens

Disruption of normal microbiota by antibiotics

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3 ways innate immune system recognises dangerous vs non dangeorus microbes?

PRR (pattern recognition receptors)

Danger signals from damaged tissue

Detection of missing self

Describe common PAMPs

Highly conserved structures in microbes (not humans)

* LPS
* Peptidoglycan
* Mannose
* Flagellin
* Pilin

PRR bind, allow immediate response (responses 1000 molecule patterns)

What is a subset of PRRs?

Toll like receptors-

TLR has two subsets-

* subset one is present on membrane surface (other cell membrane PRRs include Cd14, mannose receptor etc.)
* subset two is present within cell (endosome prrs)- detect DNA and RNA

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Cytoplasmic PRRs

* cell cytoplasm - think NOD-like receptors

Soluble/secreted PRRs

* enable or enhance response to host cells microbial products

Induced innate immunity response to infection

increase production of antimicrobial peptides

secretion of mediators of inflammation

activation of complement

clotting cascade

chemotactic attraction of phagocytic cells

inflammation

What is a broad spectrum antibiotic

Covers most bacteria- use when not sure what pathogen or injury to mechanical barrier

Functions of the complement system

Opsonisation

Inflammation

Chemotaxis

Phagocytic cells activated

Lysis of some microbes directly

Anaphylatoxin

Substance produced during complement activation that is a mediator of inflammation by binding to mast cells, basophils + platelets

What is the MAC?

Membrane attack complex (MAC) -→ lyses gram negative bacteria, envelope viruses + foreign cells

Phagocytosis resistant bacteria (such as meningococci + gonococci)

Regulate complement activation

Complement system can easily destroy cells so much be regulated

* Can self-amplify but each component can be inhibited
* Suicide substrate mechanism- covalent bond formed w active site
* Proteolytic digestion of active fragments

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Chemokines vs cytokiens

Cytokines = secreted from pro-inflammatory cells (macrophages + lymphocytes)

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Chemokines = type of cytokine that has chemotactic properties (produced by macrophages, endothelial + epithelial cells)

Chemotaxis

Cells moving in response to chemotaxins

* Exogenous (produced by bacteria)
* Endogenous (complement, chemokines, leukotrienes)
* phagocytes are directly influenced by chemotaxins

Describe neutrophils

Bone marrow production

Motile + **phagocytic**

A PMN

Have azurophils w anti-microbial defensins that fuse w phagocytic vacuoles

Circulates in blood

Short lived + numerous, first at site of microbe invasion

Do not use mitochondria for energy= glycogen stores

Form pus when die

What are the PMNs?

Polymorphonuclear leukocytes = segmented nucleus

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They are BEN - basophils, eosinophils + neutrophils (also granulocytes)

Describe eosinophils

Not good phagocytes but exocytose granules

Target animal parasites (protozoa + worms- these organisms are too large to be phagocytosed)

Monocytes + macrophage immune response

Slower than PMNs, settle in tissue (resident tissue macrophages)

* share common progenitor w neutrophils
* Differentiate after leaving bone marrow
* Important in innate + adaptive immunity
* Neutrophils recruit

How are macrophages activated?

LPS on PRRs

Platelet activating factor

Cytokines (think IFN-gamma)

Fibronectin (acute phase protein)

Function of cytokines released by macrophages

IL-1beta - activates vascular endothelium inducing fever + IL-6

TNF-alpha - increase vascular permeability

IL-6- increased antibody production

CXCL-8- chemotaxin (basophils, neutrophils)

IL-12 - NK differentiation

What events does the macrophage induce?

Cytokines released induce vascular permeability -→ leukocytes move into vessels from increased adherence factors → leukocytes extravasate (move out of circulatory system) -→ blood clotting in micro-vessels

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Diapedesis - outward passage of WBC through intact vessel walls to surrounding tissues

How do monocytes and macrophages kill?

Clear remains of microorganisms and neutrophils -→ phagocytose debris

Differentiate, complement components and cytokines

Activated macrophages- phagocytose better, take up more oxygen + secrete more hydrolytic enzymes

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Activated by C3b + interferon gamma

Describe mastocytes

Mast cells - in locations that are in close contact w external environment (skin, airways, intestines)

Granules- histamine, heparin + serotonin

* Release of granules after C3a and C5a trigger = vasodilation and chemotaxis
* Can also phagocytose + use ROS

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3 mediators

* Granule releases -→ vasodilation, capillary permeability, chemokinesis + bronchoconstriction
* Lipoxygenase pathway(leukotrienes)- bronchoconstriction, chemotaxis + chemokinesis
* Cyclooxygenase pathway - prostaglandins + thromboxane = platelet aggregation + alter vasodilation

Outline the steps of phagocytosis

1. Recognition + attachment (i.e. PAMPs, chemotaxins, opsonins)
2. Engulfment (uptake into a phagosome via actin filaments)
3. Killing + degradation (lysosome fuses, lysosomal enzymes- ROIs and NO)


1. Oxygen dependent killing (hydrogen peroxide)
2. Oxygen independent killing (toxic products)

Oxygen dependent killing

In phagolysosome - NADPH oxidase (produce hydrogen peroxide= bactericidal, particularly for gram positive in skin and upper respiratory)

Oxygen independent killing

Microbe killing in anaerobic conditions - using toxic products or enzymes (lysozymes, phospholipase A, neutral proteases, cathelicidin)

Histological process of inflammation

1. Dilation of arterioles, capillaries + venules (inflammatory mediators)
2. Increased permeability + blood flow (more antimicrobial cell migration)
3. Exudation of fluids (plasma proteins)
4. Leukocyte migration into inflammatory focus

Characteristics of inflammation

Heat

Redness

Swelling

Pain

Loss of function

Function of IL-1, IL-6 and TNF-alpha

Induce acute phase response (opsonisation+ complement activation)

Neutrophil mobilisation

Increased temperature

Increase protein/energy mobilisation

Dendritic cells

Acute phase response

CRP, serum amyloid P, fibrinogen = induced by pro-inflammatory cytokines, beings after inflammation (protects host)-→ helps active complement

Sepsis

Systemic inflammatory response syndrome in reaction to infection (usually bacteria) - 30% mortality

C reactive protein

Fever

Elevation of temp due to inflammatory mediators (pyrogens)

* Some leukocytes function better at higher temperatures
* Endogens pyrogens decrease metals required for some bacterias’ growth
* Directly can inhibit some microbes (pneumococci + gonococci)

Cons

* metabolic demands, CV stress (CHF or IHD)
* Too many pyrogens leads to sepsis, tissue necrosis, organ failure, shock + death

What are the three ways a body can respond to a virus

1. Destroying virus
2. Destroying virus infected host cell
3. Protecting un infected cells from becoming infected

NK cells + how tey function

Cytotoxic activity- not specific, must be activated by adaptive responses, kill virus infected cells w lytic granules

* Detect changed self cells
* Use Natural kill receptors - bind to glycoproteins on cells
* Killer inhibitory receptors- stop killing normal host cells of can bind to MHC class I molecules

Granule released onto non-self cell

* perforin (insert itself into membrane of target cell)
* Granzymes (serine proteases that cleave and active intracellular caspases)

APCs cell types

dendritic cells + macrophages

Why is the adaptive immune system required?

Most pathogens have developed defenses to partially escape destruction by innate protective mechanism- need specific + strong mechanisms

Summary of immune innate system

What are potential outcomes of the immune response?

Successful removal of all pathogens + development of immunity

Contained infection but not eliminated (TB infection, HIV, herpes)

Pathogen or immune response or both leave tissue damage (disease)

Vectors

Living transmitters of pathogens- usually arthropods

* anopheles- transmits malaria parasites
* Tsetse flies- sleeping sickness
* Reduviid- Chagas disease

Primitive/definitive vs secondary/intermediate vs dead end (incidental) host

Primitive= host in which the parasite reaches maturity and reproduces sexually

Secondary = host that harbours parasite during a developmental stage

Dead end = intermediate host that does not allow transmission to primary host, cannot complete development

Toxoplasmosis

Primary/definitive host = domestic cats

Intermediate hosts = birds + rodents

Humans = dead end hosts, become infected by eating consuming cyst contaminated substances/blood

T. gondii - 95% of ppl are infected, very few have the disease

Infection vs disease of parasitic infections

Parasitic infections - parasites exploit hosts to survive but does not aim to kill (cannot complete lifecycle) -→ disease then will usually be prolonged or repeated + highly burdensome

Anthroponosis

reverse zoonosis (human to animal),

60% of pathogens are zoonotic

What are some parasitic mechanisms for evading host immune response?

Antigenic variation - Plasmodium species

Intracellular - plasmodium

Camouflage- schistosomes

Cleavage/destroying of Igs- Giardia

Suppression or redirection of immune response- most protozoa

Surviving phagocytosis- T. cruzi

protozoa + potential methods of reproduction

unicellular eukaryotes- occur wherever there is water (intracellular or extracellular infections)

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Asexual (mitotic divisions- binary/multiple fission) + sexual reproduction (gametocytes, gametes, fertilisation)

4 types of protozoa

Amoebae

Flaggelates

Ciliates

Sporozoa

Intracellular vs extracellular protozoan infectiosn

Intracellular (plasmodium or leishmania)- usually spread through body via RBCs or macrophages, need a vector for transmission (can’t survive well outside of host)

Extracellular (giarda) - have an active trophozoite form + a dormant cyst form (resistant to drying + acidic pH- good for transmission)

Different protozoan lifecycles

Direct

Faecal-oral

Vector-borne

Predator-prey

What cells do plasmodium infect?

Hepatocytes + erythrocytes

Why is malaria light microscopy advantageous?

Traditional method and can understand stage of divison (no sexual reproduction in the body) - can sometimes differentiate between species

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P. falciparum trophozoites and schizonts are not usually seen because they attach themselves to venular endothelium  

PCRs vs RDTs for malaria diagnosis

RDTs- variation in sensitivity, high false negative rate (gene deletions of plasmodium), need cold storage _+ cannot identify all species

PCR- not used in clinical setting, expensive, more sensitive

Discuss Malaria transmission factors

Geography and topography of land

Mosquito vector species, abundance, drug sensitivity

Temp + rainfall

Strength of immune system

Quality of housing

How ppl spend time + times when vectors are feeding

Amount + type of agriculture in area

What is the erythrocytic cycle + RBC preference of plasmodium - F., V., O.,+ M.

Erythrocytic cycle- A stage in the life cycle of the malaria parasite found in the red blood cells.

F- 36-48hrs, prefers younger RBCs but can infect all RBCs (high drug resistance, severe in non-immune)

Vivax- 48hrs, reticulocytes (immature RBCs, mild to severe)

Ovale- 48 hrs, reticulocytes (mild, no drug resistance)

Malariae- 72 hrs + older cells (mild, no drug resistance)

Asexual blood stage lifecycle of plasmodium

Ring form -→ trophozoites -→ schizont (last two can sequester in various organs)

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Malaria zoonosis

P. knowelsi (long tailed macaque can be a natural host- monkeys are asymptomatic)

* Immature trophozoites can look similar to P. falciparum
* Older trophozoites + schizonts look like P. malariae

Describe severe malaria + vulnerable groups

Usually by P. falciparum in children, travellers, co-infected + pregnant women

* 80% of deaths occur in young African children
* Malarial anaemia, blood transfusions + deaths

Why does P. falciparum cause the most mortality?

Infects RBCs of any age

*Pf*EMP1-→binds + attaches to RBCs/walls of blood vessels (rosetting, cytoadhesion + autoagglutination)

PAM

Pregnancy associated malaria-

* Can cause maternal anemia
* Fetal loss (LBW, premature, death)
* Congenital malaria (maybe, maybe not)

Malaria protection + immunity

* exposure

Repeat infections = partially protective immunity

Semi-immune = mild symptoms/asymptomatic even though can still be infected by malaria

Maternal antibodies protect newborns in first few months of life

in lower transmission areas- less infections, most ppl have no protective immunity

Malaria protection + immunity

* genetic

Protection against P. falciparum malaria

* Sickle cell trait (heterozygotes for abnormal HbS, cannot support parasite growth - 80% in SSA)
* Thalassemias (abnormal Hb formation)
* G6PD deficiency

protection against P. vivax

* Duffy blood group = vivax needs Duffy positive RBC for invasion, majority of Africans are Duffy negative (rare in SSA)

Malaria prevention strategies

Vaccines that exist (mosquirix, matrix) have poor efficacy -→ only used for children in affected areas

Insecticides (indoor residual spray, insecticide treated nets, increased drug resistance) + drugs (travellers, pregnant women take sulfaxoin + pyrimethamine (iPTP))

Issues w antimalarial drugs

Increased resistance- must use a combo

**Artemisin combination therapies** - artemisinin (short half life) { piperaquine and artemether + lumefantrine

* Cases of resistance in western cambodia
* reasons could be due to improper ACT use, substandard drugs, incomplete dosages for prophylaxis, malaria parasites w unique ability to develop any anti-malarial drug

Why is chloroquine no longer used?

Used to be a drug that was rolled out for eradication and then resistance built up

Was part of an eradicaiton program (1955-1969) but they ignored signs of resistance + relied only on one strategy

Eradication vs elimination

Eradication = permanent reduction to zero of the worldwide incidence of infection caused by human malaria parasites as a result of deliberate efforts

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Elimination = reduction to zero of incidence of infection caused by a parasite in a defined geographical area as a result of deliberate (need to continue measures to prevent re-establishment of transmission)

How to re-establish a downward trajectory in malaria incidence and deaths?

Political commitment

Substantial funding

Regional collaboration

Coordinated global response

new tools = drugs, vaccines, vector control, education + implementation strategies

Trypanosoma diseases

Trypansoma brucei/T. gambiense/T. rhodiense = African sleeping sickness (HAT)

Trypansoma cruzi = American Trypansomiasis = Chagas disease

Characteristics of HAT

Human african trypanosomias = can be caused by T. brucei gambiense (95%, chronic) or T. brucei rhodesiense (acute, severe)

* Hosts = most mammals (zoonosis- reservoirs of animals particularly in rhodesiense)
* nagana cattle disease pathogens cannot infect humans (not zoonotic)
* Bloodstream + CSF (**extracellular)**
* Transmitted by the bite of a tsetse fly (M&F)
* vertical transmission
* also sexual contact
* Mostly in Tropical Africa (DRC)
* Epidemiology- 2-4000 deaths per year + high DALY burden

Nagana Cattle disease

Also caused by T. brucei, T. congolense. T. vivax

46 million cattle are threatened, costs lots of money

Life cycle of HAT

Bite of Tsetse flie inoculates metacyclic trypomastigotes under skin→ parasite replicates in blood, lymph, spinal fluid (binary fission) as a blood stream trypomastigote → trypomastigotes in blood are taken up by tsetse flies

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*Cannot be transmitted without fly*

Within fly- parasites become procyclic trypomastigotes in midgut, divide by binary fission, become epimastigotes in salivary gland and then form metacyclic metacyclic trypomastigotes

T. b. gambiense vs T. b. rhodesiense distribution + treatment

Gambiense- most cases, found in west and central Africa, chronic cases- **takes longer to develop into the menigoencephatic stage**

* fexinidazole, pentamidine
* eflornithine, NECT, fexinidazole

Rhodesiense- eastern and southern Africa, acute severe

Human African Sleeping Sickness stages of sickness

1st (hemolymphatic stage) - weeks to months after infection, patient develops systemic illness w fever and swollen lymph nodes + trypanosomes present in blood stream (non-specific symptoms0 fever, headaches, lymph nodes, joint pain etc.)

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2nd (meningoencephatic stage) -parasites invade CNS- brain + spinal fluid

* headache, mental dullness, poor cordination, sleeping (somnolence), disturb consciousness, coma, death
* **disrupts sleep cycle**

How do trypansomas evade the immune system ?

Variable surface glycoproteins (VSGs) - alters VSGs in bouts of parasitemia in a chronic infecrtion

VSG switching is from genetic rearragenement

* VSGs stop complement binding
* Shed VSGs as decoy
* Produce TSIF - immunomodulatory factor

How to diagnose HAT?

Serological testing can be used for screening (only T.b. gambiense- cheap, simple, not 100% accurate) and/or checking clinical signs (swollen cervical glands)

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Body fluids- find parasites (invasive, expertise required) lumbar puncture for staging (is it in CSF)

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Make dx as early as possible before neurological stage

Treatment of HAT

First stage- easier, less toxic + more effective

Gambiense = pentamidine (IM- usually well tolerated, 7-10d), fexinidazole (oral- 10d)

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Rhodesiense = suramin (IV- 5-7d, causes urinary tract + allergic rxns)

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Second- cross BBB, toxic + complicated

Gambiense = Eflornithine (IV 4x a day for 14d), NECT (oral + IV, 10d + 7d), fexindazole (oral- if not severe, 10d)

Rhodiense = melarsoprol (IV, 10d, drug resistance observed- can be fatal 3-10%)

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American trypanosomiasis overview

Chagas disease- Trypanosoma cruzi - zoonotic

Bloodstream and **intracellular** in many cells (muscles)

* trypomastigotes are extracellular
* amastigotes are intracellular

Transmitted via triatomine bugs (kissing bugs), also congenital transmission

Central and south america

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Life cycle of american trypanosomiasis

Reduviid bug deposites faeces w trypomastigotes → wiped into body when it takes a blood meal → transforms into amastigote (ICF form) + invades tissue like muscle cells → cells

American trypanosomiasis phases of illness

Acute phase = 2 months after infection, mild or absent symptoms,