Parasitology What are parasites and what do they cause?
Parasitology • Protozoans – • These are one-celled organisms belonging to the kingdom Protista. • Of the 20,000 species, only a few cause disease. • They were traditionally classified into phyla based upon their means of motility. • Sarcodina - consists of amoebas that move by extending out projections, called pseudopods, from the cytoplasm. The rest of the cell flows towards the pseudopod • Mastigophora – possess flagella which are capable of whip-like movements that pull the cell through the medium.
Parasitology • Ciliata – possess projections called cilia around the whole cell. The cilia move to propel the cell through the medium. Cilia are similar to, but shorter than flagella. • Sporozoa – these are incapable of independent movement. • Biology of protozoans • Nutrition – aerobic heterotrophs which live in areas with a large water supply. • Reproduction –
Parasitology • Asexual - can occur by fission, budding or schizogony (this is where multiple divisions of the nucleus occur before the cell divides). • Sexual – may be used by ciliates • Encystment – under adverse conditions, some protozoans are capable of producing a protective capsule called a cyst. • The cyst allows the organism to survive without food, water, or oxygen, and under unsuitable temperature or chemical conditions. • For parasitic species, it allows the organism to survive outside the host.
Parasitology • Medically important protozoans • Sarcodina (amoebas) • Entamoeba histolytica – • Causes amoebic dysentery (diarrhea with blood and mucous). • This organism can actually bore through the intestinal wall to cause ulcers in the liver as well as the intestine. • Consists of 2 stages – trophozoite (motile stage ) and cyst. • It is transmitted from human to human via cysts passed in the feces and ingested by the next host. • Diagnosis – finding the cyst or trophozoite in the feces.
Parasitology • Naegleria fowleri • Causes an often fatal meningitis or encephalitis. • Humans acquire it by jumping into contaminated, usually stagnant water. • The organism gets in when waster containing the organism rushes up the nose.
Parasitology • Mastigophora – some have an undulating membrane consisting of a highly modified flagella. • Giardia lamblia • Is found in the small intestine and causes acute or chronic enteritis – more common in children. • Consists of two stages – trophozoite and cyst. • Acquired through ingestion of the cyst passed in the feces. • Diagnosis is based on finding the trophozoite or cyst in feces.
Parasitology • Trichomonas vaginalis • Causes vaginitis. • Only has a trophozoite stage and must be transferred from host to host very quickly. • The organism inhabits the vaginal tract and the male urinary tract. • It is transmitted via sexual intercourse or by toilet facilities or towels.
Parasitology • Hemoflagellates – transmitted by blood sucking insects and are found in the circulatory system of the bitten host. • Trypanosoma brucei (gambiense or rhodescience) – • Cause african sleeping sickness • Transmitted by the tsetse fly.
Parasitology • Trypanosoma cruzi • Causes Chagas’ disease • Transmitted by the reduviid bug (“kissing bug”) which bites and defecates at the same time. The feces, containing the organism contaminates the bite wound and the organism gets in to cause the infection. • Cells in almost all of the internal organs can be invaded. • Cardiac problems and meningoencephalitis are the leading causes of death. • This is a major problem in Central and South America.
Parasitology • Ciliata • Balantidium coli • Causes a severe, rare type of dysentery. • Consists of two stages – trophozoite and cyst. • Humans acquire the organism by ingestion of the cysts that are passed in the feces of an infected individual.
Parasitology • Sporozoa • Mature forms are obligate, intracellular parasites. • They have complex life cycles as they are transmitted from one host to another • Plasmodium species cause malaria and have a complex life cycle: • Infected Anopheles mosquito bites and injects sporozoites (infective stage for humans) • Sporozoites go to the liver and undergo schizogony which produces thousands of merozoites (exoerythrocytic portion of the life cycle). • The merozoites are released and can either reinfect new liver cells or they can infect RBCs to begin the erythrocytic cycle. • When the merozoite invades the RBC, it first transforms into a young trophozoite that looks like a ring consisting of a nucleus and cytoplasm.
Parasitology • The ring stage enlarges and divides repeatedly to form many merozoites. • The RBCs rupture and the merozoites and their waste products are released. • This causes fever and chills (a febrile paroxysm). • Most of the merozoites reinfect new RBCs and undergo a new cycle of asexual reproduction. • The length of time required for schizogony and production and release of merozoites is characteristic for each species and determines the length of time between febrile paroxysms.
Parasitology • Instead of invading RBCs, some of the merozoites will develop into male and female sexual forms called gametocytes. • Gametocytes are picked up by an Anopheles mosquito when it bites an infected host. • They enter the intestine of the mosquito to begin their sexual cycle. • The male and female gametocytes unite to form a zygote that forms a cyst in the wall of the mosquito gut. • Cell division occurs and asexual sporozoites are formed. • The cyst ruptures and the sporozoites migrate to the salivary glands of the mosquito from whence they are injected into a new human host by the mosquito.
Parasitology • Helminths (worms) – contains 2 phyla: platyhelminths and aschelminths • Biology of helminths • Multicellular and most are free-living. A few are parasites. • Reproduction – the reproductive systems of the parasitic species are highly developed. • Adult worms can be either dioecious (male and female reproductive organs in different individuals) in which reproduction occurs when 2 species of the opposite sex are in the same host. • Adult helminths may also be monoecious or hermaphroditic in which one animal contains both male and female reproductive organs.
Parasitology • Life cycle – may be extremely complex and involve many different hosts. • Definative host – harbors the adult, sexually mature • helminth • Intermediate host – harbors a larval or developmental stage of the parasite.
Parasitology • Platyhelminths - Flatworms – are dorsoventrally flattened and either lack or have an incomplete digestive system. Consist of 2 classes: • Trematodes (flukes) – have ventral and oral suckers that hold the organism in place and are also used to suck fluids from the host. They can also absorb fluid through their outer covering called a cuticle. • Paragonimus westermani – the lung fluke. • The adult lives in the bronchioles of the lungs of humans and other mammals. • They are hermaphroditic and liberate eggs into the bronchi. Sputum containing the eggs is swallowed and eggs are excreted in the feces.
Parasitology • If excreted into a body of water, a miracidium develops and enters a suitable snail host (very specific type of snail required). • Inside the snail asexual reproduction results in the formation of rediae that develop into cercariae that bore out of the snail and penetrate the cuticle of a crayfish. • There the parasite encysts as a metacercaria in the muscles and other tissues. Humans acquire the organism by eating undercooked, infested crayfish. The metacercaria is released in the intestine and migrates to the lungs to mature into an adult. • Diagnosis is by finding eggs in the feces.
Parasitology • Clonorchis sinensis – liver fluke. • Has a similar life cycle but has fresh water fish as the second intermediate host and is acquired by humans by eating undercooked, infected fresh water fish. • Fasciolopsis buski – intestinal fluke. • Similar life cycle, but has fresh water vegetation as the second definative host and is acquired by humans by ingestion of fresh water vegetation. • Schistosoma – causes schistosomiasis. • Adults live in the mesenteric and pelvic veins of humans. • Cercaria are not ingested but can burrow through the skin of the human host and enter the circulatory system. Fresh water snails serve as the intermediate host. • Diagnosis is based on finding the eggs in the urine and feces.
Parasitology • Cestodes – tapeworms. • Are intestinal parasites. • Have a head or scolex that has suckers and possibly hooks for attachment to the intestinal mucosa. • They completely lack a digestive system and absorb food through their cuticle. • Their body consists of segments called proglottids that are continually produced by the neck region of the scolex. Each proglottid contains both male and female organs. Those furthest from the scolex are filled with eggs (gravid proglottid) • Humans may either be the definative or the intermediate host.
Parasitology • Humans as the definative host – Tania saginata (beef tapeworm) and Tania solium (pork tapeworm). Adults are found in humans. Mature proglottids with eggs are released into the feces (diagnosis) Upon ingestion by cattle (T. saginata) or pigs (T. solium), the larvae hatch and encyst in the muscles as cysticercaria which are then ingested by humans eating undercooked beef or pork. • Humans as the intermediate host – Echinococcus granulosis. Dogs and cats are the definative host. Eggs are excreted in the feces and humans can be infected by contaminating their hands with dog or cat feces or by saliva from the animals’ tongue. The eggs hatch and migrate to the lungs or liver. The host will form a cyst around the larva called a hydatid cyst. Diagnosis in humans – X-ray or autopsy.
Parasitology • Aschelminths – roundworms. • Have complete digestive systems and most are dioecious. The class nematodes has human parasites. Don’t have complicated life cycles and can be divided into 2 categories. • Eggs infective for humans • Enterobius vermicularis – pinworm. Spends its entire life in humans. Adult worms are in the large intestine in humans. The female migrates to the anus to deposit her eggs. This causes the itching that is the most common symptom. Eggs are then ingested by the host or another human (commonly transmitted in young children who are not very hygienic). Diagnosis – sticky tape method.
Parasitology • Ascaris lumbricoides – large – 30cm. in length. Adults live in the small intestine of humans, pigs and horses. Eggs are excreted in the feces and can survive in the soil. Infection occurs by ingesting the eggs and diagnosis is by finding eggs in the feces. • Larvae infective • Necator americanus – hookworm. Adults live in the small intestine and eggs are excreted with feces. They hatch in the soil and the larva can enter a new host by penetrating the skin. The go to the blood and the lungs where they are swallowed and get to the small intestine to mature into an adult. Diagnosis is by finding eggs in the feces. To prevent infection – wear shoes
Parasitology • Trichinella spiralis – causes trichinosis. • Acquired by ingestion of contaminated pork containing the encysted larva. • The larvae mature into adults in the human digestive tract. • They sexually reproduce and give birth to live nematodes that migrate throughout the body and become encysted in muscles and other tissue. • Diagnosis is by muscle biopsy.