Parasite Disease Flashcards
Define “parasite”.
a parasite is an organism which lives upon or within another living organism at whose expense it obtains some advantage
Explain the importance of parasitic diseases for human health from a global perspective.
Infectious disease cause about 1/3 of all deaths worldwide (48% of deaths for people under 45 yo), these deaths are potentially preventable.The also have a profound impact on morbidity and disability adjusted life years (DALYs), especially in developing countries.
Explain why the life cycles of parasites are important for the geographic distribution, diagnosis, and control of parasitic diseases.
The life cycles of parasites are complex and may require one or more hosts to reproduce and develop. The ability of parasites to infect certain tissues (tropism) is often essential for their life cycle. The geographic distribution of parasites is often restricted by the availability of their hosts. A detailed history of travel and activities is essential for diagnosing most parasitic diseases. Understanding life cycles is imperative to preventing the transmission of parasitic disease.
Describe the taxonomic groups in which the major protozoan and metazoan parasites of humans are found.
Helminths (worms/metozoan/multicellular euk.): Round worms/Nematodes Flatworms/Termatodes/Flukes (Schistosomes) Tapeworms/Cestodes
Protozoan (unicellular eukaryotic): Amebas Flagellates Ciliates Sporozoa (Plasmodium = malaria) Microsporidia
Explain the pathogenesis of schistosomiasis as a paradigm of an important diseases caused by metazoan (multicellular eukaryote) parasites.
Schistosomes contaminate fresh water. Eggs shed in the feces or urine of infected humans hatch in fresh water and release miracidia, which infect snails. Following further development as sporocysts in the snails, free-swimming cercaria are released. The cercaria are capable of penetrating the skin of humans to initiate infection (causing dermatitis - swimmer’s itch). The cercaria lose their tails and develop into schistosomulae in the tissues of the human host (4-8 weeks, serum sickness, lymph adenopathy, hepatosplenomegaly). The schistosomulae gain access to the circulation, migrate to the portal blood, and mature into adult worms that migrate to the mesenteric veins (S. mansoni, S. japonicum, S. intercalatum, and S. mekongi) or to the venous plexus of the bladder (primarily S. haematobium), where mating occurs. The eggs that are produced circulate in the venous blood to the liver and are also released into the intestine or urinary bladder, depending on the anatomic location of the adult worm (causing pathological fibrosis of bladder and liver - hepatic scarring, portal hypertension and ascites). Release of the eggs into the environment perpetuates the cycle of infection.
Explain the pathogenesis of malaria as paradigms of important diseases caused by protozoan (unicellular eukaryote) parasites.
Affects > 1 billion people and causes 1-3 million deaths/year. The symptoms of malaria are primarily associated with the rupture of infected erythrocytes and release of merozoites. Fever paroxysms occur periodically with the rupture of erythrocytes (every 48-72 hours) Anemia, results not only from the lysis of RBCs, but also from their phagocytosis by the stimulated reticuloendothelial system, their sequestration in the enlarged spleen, and depressed bone marrow function. Hemolysis can be extreme, resulting in hemoglobinuria (blackwater fever). Physical examination reveals jaundice, hypotension and tachycardia in addition to fever and hepatosplenomegaly. Especially with P. falciparum infections, vasodilation causes hypotension and inadequate blood supply to vital organs. P. falciparum-infected red blood cells bind to the microvascular endothelium, which is especially significant in cerebral malaria (up to 50% mortality). Multi-organ failure is the major cause of death in adults. P. falciparum is most likely malaria organism to cause death. With P. malariae infections, immune complex deposition leading to glomerulonephritis is common. The patient generally mounts an immune response that makes subseqent episodes of symptomatic disease less severe. Both B and T cell responses are involved. Within a few weeks of infection, stage specific anti-plasmodium antibodies are produced. Natural immunity is short-lived, and continual re-infection is required to maintain it. People returning to endemic areas following a long absence may therefore be quite susceptible to re-infection.
Describe some of the major current challenges to treatment and prevention of parasitic diseases.
Vaccines
Money
Environmental conditions
Compliance with protective protocols
Affects billions of people - how to treat/prevent them all?
Definitive Host
The host species in which parasites sexually reproduce.
Intermediate Host
Hosts in which sexual reproduction does not occur.
Malaria life cycle
Infected mosquitos bite humans and inject sporozoites into the blood. The asexual phase of development (schizogony) occurs in humans. Sporozoites of all malaria parasites can infect liver cells and replicate, but only P. vivax and P. ovale can establish a dormant hepatic phase with non-dividing forms called hypnozoites that can initiate late relapses. After primary replication in the liver, merozoites are released into the blood, infect erythrocytes, and undergo additional asexual replication. Gametes (macro- and micro-gametocytes) are formed in some infected erythrocytes. The sexual phase of development (sporogony) occurs in the infected mosquito where the gametes fuse in the intestine to form zygotes. Development of the parasite in mosquitoes eventually leads to the production of sporozoites in the salivary gland. Introduction of the sporozoites into a susceptible human by the bite of an infected mosquito initiates a repetition of the life cycle of the parasite.
Definitive Host = mosquito
