Midterm 1
Session 6:
Bacteria - prokaryotes - classification based on shape, membrane coating, etc. (target for antibiotics) - may be free-living, obligate, obligate/intracellular - few are pathogenic (some are pathogenic in the "wrong location") Eukaryotes and prokaryotes: - similarities: metabolism, DNA genetic material, diversity - differences: nucleus, complexity of genome, organelles, size, protein modification, sexual reproduction Commensals vs Symbionts - symbionts are dependent on each other for survival - commensals can live independently but benefit from living together Vector-borne bacteria - Common vectors: ticks, fleas, lice (usually not mosquitos) - no definitive host (no sexual life cycle) Viruses - very diverse - not quite alive, not quite dead - always obligate intracellular, can survive but not grow on surfaces until they find a cell to infect - genomes may be DNA or single/double-stranded RNA - life cycle: 1. attachment (similar to other intracellular pathogens) 2. penetration 3. uncoating 4. replication 5. release (host cell may or may not die -- always does for intracellular parasites and bacteria) - have no machinery to make proteins, energy molecules, etc.; must hijack host cell - made of: membrane, genetic material, proteins necessary for invasion/hijacking - very few are species-specific (dengue is one) - variety of vectors - No definitive host (no sexual life cycle)
Session 1:
Definition of vector-borne disease Intro to malaria, dengue, lyme Important to success: - climate conducive to vector survival and to encounters between vector and host (transmit and acquire infection) - pathogen able to infect both host and vector (more difficult infection route than contagious pathogens) - pathogen infects host for a long enough time to be acquired by vector without killing host too often/too quickly **BEHAVIORS OF PATHOGEN, VECTOR, HOST
Session 7:
Dengue - 4 serotypes - larger proportion of humans at risk for infection with Dengue than with Malaria; cases are becoming more common - virus found in liver, spleen, lung, kidney, skin - Transmission cycle: mosquito acquires virus during blood meal; virus infects midgut and migrates to salivary glands; mosquito immune response protects from sickness but not from infection; virus transmitted to humans in saliva; virus replicates in target organs, new virions released into blood - illness ranges from asymptomatic to non-specific fever (Dengue fever (DF), Dengue Hemorrhagic Fever (DHF), Dengue Shock Syndrome (DSS, severe fluid imbalance, vessel leakage)) - genus = flavivirus (also yellow fever, west nile, zika) - dengue outbreaks used to predict zika's spread - pandemic: urbanization and population growth, resurgence of vector, four serotypes, Disease in children: - usually asymptomatic - in endemic area, 60% have antibody by age 6 - If they do get DHF, more susceptible to DSS Disease in adults: - Antibody-dependent enhancement Zika virus - not host-specific (more reservoirs) - symptoms are not severe (not true for immuno-suppressed individuals), only develop in 1/4 Why have numbers deceased so much for zika, but not for Dengue? - it's all about statistics - zika was always a small percentage of people infected - mosquito eradication programs have much more influence, proportionally, between Zika and Dengue, because the magnitude of Dengue cases is just so much larger Emerging and re-emerging diseases - viruses most numerous --> also most difficult to treat (*antivirals must be developed by identifying functions of viruses that don't also affect host cells - very difficult) - urbanization, deforestation, agriculture (esp. consider how these affect vector populations) The world-wide distribution of vector-borne diseases is not the same for bacteria, viruses, and parasites
Session 5:
From the ring stage, the parasite may develop into a trophozoite or a gametocyte - mechanism of signaling unknown Gametocytes are the form passed to mosquitos during blood feeding Gametes mature, form zygote in the midgut of the mosquito; eventually leave the gut and travel to the salivary glands Mutations that confer malarial resistance: - Sickle cell - thalassemia Mechanisms of resistance: - anemia - changes in membrane - shortened life span of RBC - less hemoglobin per cell - changes in percentage of young and old RBC - reduced parasite survival in RBC - affected parts of the life cycle: invasion, growth, cytoadherence Types of pathogens (most primitive to most advanced = prions, viruses, bacteria, parasites/fungi) Fungal diseases - most similar to plants - less known about pathogenic-mechanism - usually chronic, self-limiting disease in mammals Prions - almost nothing about them is understood - "infectious proteinaceous material" - not vector-borne Parasite = an organism that grows, feeds, and is sheltered on or in a different organism while contributing nothing to the survival of its host (in microbiology, refers specifically to eukaryotes) - two taxonomic groups: protozoa, helminths (both eukaryotic, the majority of species are non-parasitic, free-living in the environment) Protozoa parasites - single-celled organisms - most are obligate organisms - only some are vector-borne - Malaria is an example - three classes - cryptosporidia (fecal-oral transmission, not vector-borne, obligate intracellular) Helminth parasites - multi-celled primitive animals - most are obligate organisms - only some are vector-borne - many classes Definitive vs. intermediate vs. incidental hosts Vector-borne helminths - Lymphatic filariasis (elephantiasis) (vector = anopheles mosquitos, may be co-infected with malaria; infection usually occurs in childhood, does not manifest until many years later, humans are definitive hosts) - onchocerciasis (river blindness) (vector = simulium black flies) - Loiasis (eye worms) (vector = deer flies) Vector-born protozoa - plasmodia - babesia (vector=ticks, also intracellular, infects RBC like malaria) - Case: toxoplasma is 99% genetically identical to plasmodia; fecal-oral transmission, not vector-borne; obligate intracellular, infects any cell with a nucleus (ie. almost everything EXCEPT for RBC) -- difficult to determine how it recognizes cells to invade, i.e. which proteins; sexual lifecycle in the gut of domestic cat, immune after one infection; not species-specific; well-adapted, few symptoms except for immuno-suppressed individuals **GENES DO NOT INDICATE BEHAVIOR - flagellates - have no sexual life cycle SUMMARY: (similarities and differences) - Protozoan species, very closely related to Plasmodia on a molecular and cellular level, can have very different environmental life cycles. ***Consider: free-living/obligate/parasite; intra/extracellular; vector-borne/food-borne/fecal-oral; definitive/intermediate/incidental/dead-end host - Treatment and prevention strategies depend on: life cycle of the host, the vector, the pathogen; molecular and cellular biology
Session 11:
Goals of the innate immune system: - halt infection - clean up debris - recruit other cells Mechanisms of the innate immune system: - kill pathogens - phagocytose foreign cells and debris - travel to lymphoid organs - secrete signaling molecules Anatomy of an immune response to malaria: - sporozoites get deposited - phagocytes eat some, others escape to the liver - phagocytes travel to nearest lymph node and present the foreign antigen to helper T cells (few T cells are activated -- they are antigen-specific, so there are thousands of different types of helper T cells, but not very many of each type) - cells activated by the helper T cells usually travel back to the site where the phagocyte captured the pathogen... BUT sporozoites are no longer at the site of the mosquito bite! - phagocytes in the liver... but sporozoites are no longer free-living, they are inside the liver cells (harder to find) - cell-mediated immunity: sacrifice individual cells to save the organism - merozoites are released from liver, some may be eaten by phagocytes before entering RBC - infected RBC cannot present on MHC-I, so once the parasite has reached the erythrocyte stage, T cells can do nothing to help T cell receptor (TCR) - Never recognizes intact pathogens; sees only antigens presented by MHC molecules - Each T cell expresses only one version of a TCR - No two people have the same set of TCRs - recognizes only peptides (MHC only presents peptides) Compare: each phagocyte has many different receptors that allows it to interact and recognize many pathogens; T cells are specific to each antigen -- even multiple T cells per pathogen NK Cells - notice when MHC molecules disappear from the surface of cells, indicating that there is a pathogen attempting to limit antigen presentation - not antigen-specific --> innate response B cells - make antibody - B cell receptor (BCR) = antibody - Each B cell has a unique BCR (like T cells) - BCR recognizes intact pathogen in its native form (i.e. compare with TCR can only see fragment as antigen on MHC) - Once activated in the lymph node or spleen, they may stay in place and secrete antibodies or travel to the site of infection - May also display antigens on MHC-I or MHC-II **Still only present protein antigens on MHC, even though BCRs can recognize non-protein antigens Antibody - Variable end: binds to antigen - Constant end: binds to receptor on phagocytes (IgM, IgG, IgA, IgE, IgD -- when, what, where they target) - may bind to pathogens or possibly to an infected cell if a pathogen protein is expressed on the surface. (***cannot recognize antigens presented on MHC -- this is only a fragment, BCRs recognize intact antigens) - can recognize non-protein factors - mechanisms: directly kill pathogen, bind to pathogen and bring to phagocyte that has receptors for the antibody (improves efficiency of phagocyte), block interactions between cells and pathogens ("neutralize") Innate antibodies - innate B cells produce antibodies that can weakly recognize a range of pathogens -- similar to receptors on phagocytes - production does not require T cell help Adaptive immunity: - humoral: B cells secrete antibody - cell-mediated: cytotoxic T cells kill infected cells - helper T cells: help B cells produce more antibody, help activate CTLs, help activate more phagocytes Checks and balances: antigen presentation, helper T cell activation, B cell activation, CTL activation The real benefit of adaptive immunity is generating a memory response - faster and more efficient - B cells and CTLs are "ready to go" - Helper T cells need very little antigen presentation to become fully activated Immune system failures - immunopathology: overexuberant response, disregulation, inflammation - lack of control/protection/elimination Pro-inflammatory cytokines Pathogens actively try to avoid/suppress the immune response - disregulation = redirect immune response so that it doesn't effectively eliminate pathogen - antigenic variation = change protein expression to avoid immune responses that develop - Active interference: i.e. reduce presentation of antigens on MHC-I - Exhaust the immune response: i.e. over-proliferation Our immune systems exist to protect us from getting sick The innate immune response eliminates most infections before we get sick
Session 2:
Intro to and Overview of Malaria and Lyme Malaria - "mal air" Disease caused by plasmodia PARASITES (eukaryotic) Single-celled, intracellular, host-specific Distribution - almost half the world's population threatened WHO numbers vs. Gates numbers PARAMETERS Where is Malaria endemic? -mostly Africa, some Asia, some south america Where is infection possible? - climates where mosquitos can survive Are they the same areas? - No, infection could be possible on a seasonal basis in some areas Are humans the intended host? - Yes, the parasite is host-specific Are humans necessary for the parasite's life cycle? - yes, humans are like reservoirs What do you need to know to study malaria? - behavior of mosquito and parasite (host, vector, pathogen!) Lyme borreliosis -Lyme, CT late 1970s - bacterial (prokaryote) spirochete - life cycle approx. 2 yrs Area of infection for Lyme disease and malaria are almost mutually exclusive - ticks require winter hibernation period that doesn't exist in tropical climates --> temperate climates Where is Lyme disease endemic? -For humans, it is not endemic anywhere because we can only be infected during certain seasons Where is infection possible? -where the ticks live, must have well-defined seasons Are humans the intended host? -no, we are dead-end, larvae could never acquire What do you need to know to study Lyme disease? - Behavior of host (human), vector (tick), and pathogen (borrelia burgdorferi)!! Density of ticks used as infection risk LYME VS MALARIA -vector -climate (distribution) -incidental host for lyme, intended host for malaria -malaria is host-specific, lyme is not
Session 8:
Lyme Borreliosis - 3 (maybe 4) subspecies that cause lyme disease - sensu lato vs senso stricto - spirochetes Infection: - tick must be attached for approximately two days before the bacteria is transmitted - no evidence of resistance to treatment - reservoir = white-footed mouse - small numbers -- almost never seen in blood, only occasionally in tissues - more effective method of determining infection is looking at the tick - extracellular; interact with host cells as a "transit system" Manifestations - early on: bullseye rash (1 wk - 1 mo after bite; may be distant from actual bite site -- must be product of the immune response) - Nonspecific immune response - lyme arthritis (asymmetrical, can come and go; not associated with bacteria in the joint, mechanism unknown) - neuroborreliosis (facial palsy - inflammation around the nerve; aseptic meningitis, short-term memory loss, nerve pain) - carditis (inflammation of the heart, irregular beating; complication is rare; highest risk for individuals age 15-45 - associated with maturity of immune system; responds to antibiotic therapy) For borrelia specifically, the disease would probably be much less severe without the immune response Vector = ixodes ticks - transmits other pathogens: anaplasma phagocytophilum (bacteria), babesia microti (parasite), borrelia miyamotoi (bacteria), pawassan virus, ehrlichia muris-like (bacteria) - co-transmission - most are not passed on to eggs of infected ticks (B. miyamotoi is the exception) Anaplasmosis - obligate intracellular - infects neutrophils - transmitted after 24 hrs of attachment - survival advantage --> freeze resistance - severity of symptoms - broad range (general acute illness is worse than Lyme) Babesia - similar to malaria - intracellular, infects RBC - not human-specific - no known exo-erythrocytic stage yet still not seen in RBC for one week after infection - each infected RBC produces fewer merozoites than do malaria parasites - humans are a dead-end host - most infections are asymptomiatic --> problematic for blood donations/transfusions - affects not just the immune-suppressed -- severe disease seems to be getting more common, even in otherwise-healthy people Pawassan virus - transmitted within 15 min of tick attachment - 6% of mice infected in Hudson R Valley - no treatment (virus**) - neuro invasive --> much scarier than others B. miyamotoi - hard-body tick relapsing fever (odd--most relapsing fevers are transmitted by soft-body ticks) - non-specific symptoms, can have neurological involvement - extracellular, lives mostly in blood, very high numbers Types of ticks: Hard-body ticks (i.e. ixodes) - feeding cycle over two years, defined stages (larva, nymph, adult) - pathogen life cycle: larvae are usually clean, acquire pathogen from reservoir host; - ticks cannot stop and start feeding; attach for 2-7 day; chemical aids: local anesthetic, anti-coagulent, "glue", immunosuppressants - questing (wait for potential host to pass by) - can go months without blood meal Soft-body ticks (i.e. ornithodoros) - fast feeders COMPARE: B. burgdorferi do not cause severe by growing to high density or directly damaging human tissues; anaplasma, miyamotoi, and babesia are associated with higher pathogen burdens
Session 12:
Lyme Borreliosis - most, if not all, disease manifestations caused by immune response --> symptoms can linger even when the infection is eliminated - you never have overwhelming numbers of bacteria Anatomy of immune response to borrelia burgdorferi: - phagocytes encounter bacteria in the skin (initial response is weak because very few bacteria are present) - borrelia in the bloodstream may be cleared by encountering phagocytes or by passing through the spleen (this is why they must not linger in the blood) - borrelia sits in the tissue and tries to hide (esp. removing outer surface proteins) - antibody much more important than T cells (T cells not required) - spleen can be important - phagocytosis necessary for pathogen numbers but not disease - T cells make inflammatory cytokines that contribute to disease manifestations Protective immune response to relapsing fever borrelia: - spleen plays critical role (time spent in blood) - Antibody plays critical role, T cells not important - macrophages control pathogen burden (DOES impact disease development) - spirochetes rely extensively on antigenic variation Anatomy of dengue immune response: - the virus infects phagocytes at the site of infection, impairs activation of adaptive immunity - NK cells are principle actors in innate immune response + clearance of infected cells - infection by any serotype produces long-lasting immunological memory - protection occurs if second infection is same serotype as first - if second infection is different serotype: memory B cells somewhat recognize the related virus, bind but do not kill; bound antibody enhances uptake of still-active virus; memory T cells produce more and more cytokines, which increases vascular permeability --> much higher incidence of DHF and DSS With so many targets and so many immune mechanisms activated, why is malaria such a huge health threat? - avoidance, interference, corruption *Life cycle *sequestration *antigenic variation
Session 4:
Malaria life cycle - sporozoites formed in mosquito, transmitted during feeding, unable to infect RBC - sporozoites infect liver cells within 2 minutes - very few sporozoites in blood after one hour - sporozoites express proteins on their surface that can recognize molecules on the surface of liver cells --> parisitophorous vacuole - schizogeny inside the liver cell (multinucleated) - parasites exit the liver cell as merozoites -- the form that infects RBC (P. falciparum yields the highest number of merozoites per schizont) - merozoites express proteins on their surface that can recognize molecules on the surface of RBC (a family of responsible proteins, able to recognize several RBC proteins) - P. vivax: must bind to Duffy antigen to invade cells, not always expressed - confers resistance - Asexual cycle: merozoites invade, develop ring form, develop trophozoite (w/ heme crystal), develop into schizonts, lysis and release Relapse and recrudescence - relapse --> vivax; dormant parasites in the liver, may not become active for decades - recrudescence --> falciparum; parasite levels re-elevated after having been lower than clinically-observable levels (small amounts remaining in the blood cause re-establishment of infection) Hemazoin Rosetting Cytoadherence, sequestration
Session 3:
Malaria species that cause human disease P. falciparum - most widespread distribution - responsible for most infections - most severe disease - majority of malaria-related deaths P. vivax - second-most prevalent - found in more temperate regions than falciparum **duration of mosquito developmental stage PARAMETERS Disease in children - uncomplicated --> undifferentiated febrile illness - symptoms are nonspecific; mucks up determination of prevalence **Use enlarged spleen as indicator - Severe malaria: greater than 5% mortality, altered consciousness, hypoglycemia, acidosis, anemia, convlusions - Cerebral malaria: seizure, coma, neural sequelae - severe anemia: may be measured by a deficit in hemoglobin or in hematocrit (i.e. how much of the blood is made up of RBC) Disease in adult naive travelers - can experience the same effects seen children, but usually with fewer long-term effects - liver and/or kidney damage - complications can develop in a span of 24 hours Age vs exposure susceptibility - severity of anemia decreases with age - severity of cerebral malaria increases with age - pathological response is triggered at a lower parasite density threshold among older non-immune adults, and the absolute risk of clinical disease at a given parasite density is higher in older individuals Pregnancy malaria
Session 13
Many players of the innate and adaptive immune systems are activated by malaria parasites The parasites have mechanisms for avoiding and interfering with immune responses Over exuberant and disregulated immune responses are responsible for severe immunopathology Maturity of immune system as well as level of exposure affect development of NAI Immune response possibilities: - Immune system controls pathogen and eliminates disease - Immune system eliminates pathogen but only after severe illness and also provides protection from re-infection - First infection is cleared, but protection from re-infection does not develop - The first infection isn't eliminated but it still serves to protect from re-infection - Infection overwhelms/exhausted the immune system - Immune Response is deadly
Session 9:
Soft-body ticks - feed within one hour - use anticoagulents; no glue, immunosuppressants, or local anesthetic - may feed multiple times at each stage (i.e. can stop and start feeding) - able to go for YEARS without a blood meal transovarial transmission of pathogens --> longer life cycle - actively seek out hosts (moreso than hard-body ticks) - pathogens always reside in salivary glands - species-specific Relapsing fever spirochetes - high pathogen numbers in blood - extracellular Co-infection of malaria and relapsing fever - similar symptoms, difficult to diagnose - immune response: spirochete burden is much higher than it would be alone, plasmodium burden is much lower - higher rates of anemia, (immune response is hyper-focused on parasites, even starts killing healthy RBC) - higher fatality rates - secondary malaria infection can reactivate RF infection Mosquito vectors - only adult females take blood meals and are therefore able to transmit disease - pathogen usually not passed to eggs (sometimes happens in Dengue, but viability is questionable) Feeding behaviors: - anthropophagic/anthropophilic - preference for: time of day, endophagic/exophagic, temperature - sensitive to: warmth, CO2, skin scent - feed for up to one minute Mosquito behavior determines: - most at-risk populations - most effective preventative measures - impact of human behavior Gonotrophic cycle = egg production and blood feeding - transmission of malaria and dengue - can be repeated several times by one female - rest between feeding and laying eggs - generation interval = egg to adult; 1-3 wks - consider: compare parasite development cycle (in mosquito - usually at least 10 days) and length of gonotrophic cycle (efficiency of transmission; explains difference in distribution between vivax and falciparum; dengue even more efficient - approx. 7 days) Factors affecting mosquito life cycle and pathogen transmission: - mosquito density (per human) - avg. # bites per mosquito per day - probability of mosquito survival per day - life expectancy of female mosquito - pathogen maturation time (in mosquito) - temperature and humidity underly these factors anopheles mosquitos --> malaria aedes mosquitos --> dengue anopheles vs. aedes environmental impacts on different mosquito stages
Session 10:
Success of an immune response: - elimination of pathogen --> protection from subsequent infection - asymptomatic infection --> control pathogen for life of host Branches: innate and adaptive - cells and soluble factors 1. barriers prevent pathogens from entering the body (skin, gut, lungs, eyes/nose) 2. Innate immunity - not pathogen species-specific - early response - distinguishes self and non-self - functions: identify nefarious invaders, kill invading pathogens, alert the rest of the immune system 3. Adaptive immunity - pathogen-specify immune response - memory for stronger subsequent response Lymphoid organs: bone marrow, spleen, lymph nodes, thymus, liver (esp. fetuses) - bone marrow makes blood cells and acts as repository for memory cells Phagocytes = first players in innate immunity - neutrophils, macrophages, dendritic cells - eat pathogens and produce soluble factors to interact with other cells - professional antigen-presenting cells Lymphocytes = B cells, T cells, and NK cells Thymus - "educates" the T cells, eliminates non-functional Lymph nodes - initiation of response - home of lymphocytes and myeloids - swell during infection due to rapid division of immune cells - "speed dating bar" Spleen = filters the blood Innate immune cells are located at barriers, at sites of infection, at sites of damage, in places where they can interact with other immune cells (i.e. lymphoid organs) Antigen presentation is required only for adaptive immunity -- intersection of innate and adaptive - MHC-I presents antigens that were made within the cell (i.e. the cell has been infected) and is expressed on almost all nucleated cells (**not RBC--consider implications for Malaria) - MHC-II presents antigens that were acquired from outside the cell (i.e. eaten) and is expressed only on professional antigen-presenting cells (APC) - diversity of MHC molecules within and between people APC = phagocytes, B cells Helper T cells: read antigen expression on MHC-II - activate macrophages, killer T cells, B cell antibody secretion Killer (cytotoxic) T cells: kill antigen-expressing infected cell There are also Regulatory and NK T cells