Comprehensive Study Notes on Protozoan Infections and Diseases

MALARIA

• Definition: Malaria is an acute and chronic life-threatening parasitic protozoan disease characterized by fever, anaemia, splenomegaly, and other symptoms. It is transmitted from one person to another through the bite of female Anopheles mosquitoes.

• Aetiologic Organism: The causative organism is a protozoon of the genus Plasmodium.

• Human Parasitic Species: There are four species naturally parasitic in humans: P. falciparum, P. vivax, P. malariae, and P. ovale. These species differ in morphology, immunology, geographical distribution, relapse pattern, and drug response.

PLASMODIUM FALCIPARUM

• Alternative Names: Known as malignant tertian, subtertian, or aestivoautumnal malaria. It is the most virulent of the human Plasmodium species.

• Distribution: It has been eliminated in the United States of America but remains the greatest killer of humanity in the tropical zones. It extends into temperate zones in areas such as the Balkans (Yugoslavia, Albania, Kosovo, etc.) and around the Mediterranean.

• Liver Cycle: Exoerythrocytic schizonts grow in liver (hepatic) cells; these schizonts are more irregularly shaped than those of P. vivax.

• Recrudescence vs. Relapse: Relapses do not occur, but recrudescences may follow remission some years after the initial infection.

• Erythrocyte Selection: Merozoites can invade erythrocytes of any age, including reticulocytes, resulting in much higher levels of parasitaemia compared to other types.

• Maurer's Clefts: Infected erythrocytes develop irregular blotches called Maurer's clefts, which are larger than the Schuffner's dots found in P. vivax.

• Sequestration: Infected red blood cells are withdrawn (sequestered) into smaller blood vessels. They are antigenically distinct from uninfected cells, which may decrease clearance by phagocytes in the spleen.

• Rosettes: Infected erythrocytes can bind to uninfected red cells, forming "rosettes" that may contribute to the clogging of venules.

• Developmental Timing: The early ring-stage trophozoite is the smallest of any human Plasmodium species. Sexual stages (gametocytes) require about $10$ days to develop.

PLASMODIUM VIVAX

• Causative Agent: Causes benign tertian malaria, also known as tertian ague.

• Terminology: The name "tertian" refers to the paroxysms recurring every $48$ hours; derived from the ancient Roman custom of calling the day of an event the first day, making $48$ hours later the third day.

• Distribution: Flourishes in temperate zones but has disappeared from many due to eradication. Currently found mostly in Asia and North Africa. It drops to low levels in tropical Africa due to the natural resistance of Black people to this species.

• Global Impact: It is estimated that approximately $43\%$ of global malaria is caused by P. vivax.

• Initial Cycle: Sporozoites invade liver parenchyma $1$ to $2$ days after mosquito injection. By the seventh day, they mature into exoerythrocytic schizonts.

• Hypnozoites: Some parasites remain in hepatic cells as hypnozoites ("sleeping animalcules").

• Relapse Pattern: Relapses can occur up to $8$ years after initial infection, believed to result from genetic differences in original sporozoites or the immune state of the host. The patient appears healthy during the latency period.

• Cell Specificity: Merozoites invade only young erythrocytes (reticulocytes) and require mediated receptor sites.

• Morphology Changes: Following invasion, the cytoplasm becomes actively amoeboid, throwing out pseudopodia. The erythrocyte enlarges, loses pink color, and develops Schuffner's dots.

• Erythrocytic Schizogony: Takes about $48$ hours. Early in the disease, two populations may mature on alternate days, causing daily (quotidian) periodicity.

• Gametocytes:

  • Maturation: Takes about $4$ days (twice the time of schizonts).

  • Appearance: Mature macrogametocytes (female) fill most of the enlarged erythrocyte. Microgametocytes (male) are smaller and usually do not fill the cell.

  • Ratio: Macrogametocytes often outnumber microgametocytes by $2:1$.

• Mosquito Stages: Macrogametocytes are fertilized by microgametes in the mosquito gut. The zygote transforms into a motile ookinete, penetrates the peritrophic membrane to the haemocoel, and becomes an oocyst.

PLASMODIUM MALARIAE

• Causative Agent: Causes quartan malaria characterized by paroxysms every $72$ hours.

• Distribution: Cosmopolitan but discontinuous; found in tropical Africa, Myanmar (Burma), India, Bangladesh, Sri Lanka, Malaya, Europe, Brazil, Guyana, Panama, and Guadeloupe.

• Biological Characteristics:

  • Exoerythrocytic schizogony completes in $13$ to $16$ days.

  • Ring forms are less amoeboid than P. vivax.

  • Nucleus usually divides into $6$ to $12$ merozoites.

  • Parasitaemia levels are characteristically low because merozoites can only invade aging erythrocytes.

• Longevity and Relapse: Recrudescences can recur up to $53$ years after initial infection. Due to this longevity in blood, it is the most important cause of transfusion malaria.

PLASMODIUM OVALE

• Causative Agent: Causes ovale or mild tertian malaria.

• Distribution: Rarest species; found mainly in West Africa, India, Philippines, and Vietnam (scarce in Central and East Africa). Reported occasionally in Europe and the USA.

• Morphology:

  • Ring stage has a large round nucleus and a small vacuole that disappears early.

  • Mature schizont is oval or spherical.

  • Usually forms $8$ merozoites (range is $4$ to $16$).

PATHOGENESIS OF MALARIA

• Haemolysis: Destruction of both parasitized and non-parasitized erythrocytes. In P. falciparum, intensive haemolysis causes haemoglobinuria and blackwater fever.

• Spleen Involvement: Parasitized erythrocytes are phagocytosed by macrophages in the spleen. Destruction of platelets is also caused by sequestration.

• Anemia Factors: Chronic cases may have refractory anemia due to four factors:

  1. Continued destruction of parasitized and non-parasitized erythrocytes.

  2. Failure of the liver to recycle iron (haem) bound in haemozoin.

  3. Inadequate erythropoietic response of the bone marrow.

  4. In P. vivax, selective parasitization of reticulocytes.

• White Blood Cell Changes: Chronic malaria is marked by leucopaenia with an increase in monocytes.

• Haemozoin: Malaria pigment produced from host haemoglobin. It is taken up by polymorphonuclear leucocytes, monocytes, and reticuloendothelial cells (liver, spleen).

  • Appearance: Single, round, dark brown blocks (characteristic of P. falciparum) or small black masses.

  • Distinction: Lighter in color than iron pigment.

• Organ Changes:

  • Liver: Hepatomegaly (enlarged) and dark in color.

  • Spleen: Soft and congested in acute malaria.

  • Brain: Lead-colored due to pigment; cerebral capillaries are engorged.

• Margination: In vessels larger than capillaries, parasitized erythrocytes containing older forms contact the endothelial lining while non-infected cells occupy the lumen.

CLINICAL CHARACTERISTICS AND COMPLICATIONS

• Salient Features: Periodic fever, splenomegaly, anaemia, and leucopaenia.

• Incubation Periods:

  • P. vivax: $11$ to $15$ days.

  • P. falciparum: $11$ to $14$ days.

  • P. ovale: $14$ to $26$ days.

  • P. malariae: $5$ days to $4$ weeks (Transcript Note: Likely implies $3$ to $4$ or $18$ to $40$ days; recorded as "5 to 4 weeks" verbatim).

• Prodromes: Malaise, muscle pain, headache, anorexia, and slight fever.

• Paroxysm Stages:

  1. Rigor (Cold Stage): Lasts $20$ to $60$ minutes; sensation of extreme cold.

  2. Hot Stage: Intense heat, severe headache, flushed face, nausea, and vomiting.

  3. Sweating Stage: Profuse diaphoresis for $2$ to $3$ hours, followed by exhaustion and sleep.

  • Total Duration: Averages $10$ hours for P. vivax; $11$ hours for P. malariae.

• Complications of P. falciparum:

  • Cerebral Malaria: Sudden onset; progressive headache, coma, uncontrollable fever above $42.2^{\circ}C$ ($108^{\circ}F$), and convulsions. Initial stages may mimic acute alcoholism.

  • Pulmonary Oedema: Often fatal; can result from over-administration of intravenous fluids.

  • Algid Malaria: Rapid development of shock, markedly low blood pressure, cold/clammy skin, and vascular involvement of the GI tract.

  • Blackwater Fever: Acute massive lysis of erythrocytes; dark-red to black urine due to haemoglobin. Renal failure is the common cause of death.

  • Hypoglycaemia: Reduced blood glucose concentration.

DIAGNOSIS AND PROPHYLAXIS OF MALARIA

• Microscopy: The "gold standard." Quantifies parasite densities to assess hyperparasitaemia. Cost: $USA\$0.12-0.40$ per slide.

• Rapid Diagnostic Tests (RDTs):

  • Histidine-rich protein II (HRP-II): Detects P. falciparum only.

  • Parasite lactate dehydrogenase (pLDH): Produced by sexual and asexual stages; detects all four human species.

  • Aldolase: "Pan malarial" antigen for non-falciparum species.

• Individual Prophylaxis: Regular chemoprophylaxis (chloroquine/proguanil, doxycycline, malarone), insecticide-impregnated nets, and repellents.

• Vector Control Terms:

  • Anthropophily/Zoophily: Preference for human vs. animal blood.

  • Endophagy/Exophagy: Biting indoors vs. outdoors.

  • Endophily/Exophily: Resting indoors vs. outdoors.

• Control Methods: Residual indoor spraying (DDT still effective) and larval control (Paris green, larvicidal oils, Gambusia fish).

TOXOPLASMA GONDII AND TOXOPLASMOSIS

• Introduction: Discovered in $1908$ by Nicolle and Manceaux in a desert rodent, the gondi, in Tunis. It is a cosmopolitan obligate intracellular parasite.

• Asexual Reproduction Methods:

  • Endodyogeny: Budding process where two daughter cells form inside the mother cell.

  • Merogony: Multiple fission producing merozoites.

• Life Cycle Stages:

  • Tachyzoites: Rapidly dividing cells in acute infection.

  • Bradyzoites: Slowly multiplying cells in chronic infection.

  • Zoitocysts: Tissue cysts containing bradyzoites (found in brain, heart, skeletal muscle).

• Human Infection Routes: Direct ingestion of oocysts (from cat faeces) or tissue cysts (raw/undercooked meat). Congenital transmission via transplacental route.

• Clinical Classification:

  • Acute: Painful, swollen lymph glands (inguinal, cervical, supraclavicular), headache, and muscle pain.

  • Subacute: Extensive lesions in brain, eyes, and heart due to slow immunity development.

  • Chronic: Relates to cyst rupture; can lead to chronic encephalitis, paralysis, myocarditis, or blindness.

  • Congenital: Can cause hydrocephaly, microcephaly, cerebral calcification, mental retardation, and spontaneous abortion. Sheep are particularly susceptible.

• Diagnosis: Isolation via intraperitoneal injection into mice. Serology includes the Sabin-Feldman dye test (cytoplasm-modifying test). Active diagnosis requires a four-fold rise in titre.

• Treatment: Combination of pyrimethamine and sulphonamides.

LEISHMANIASIS

• Etiology: Protozoans of the family Trypanosomidae and genus Leishmania, transmitted by Phlebotomine sandflies.

• Syndromes:

  1. Cutaneous: Ulcers or nodules; generally heal with disfiguring scars. Produced by L. tropica.

  2. Muco-cutaneous (Espundia): Progressively destructive ulcerations of mucosa (nose, mouth, pharynx). Most cases caused by L. braziliensis.

  3. Visceral (Kala-azar): Systemic and fatal if untreated. Caused by L. donovani (East Africa/India) or L. infantum (Europe/Latin America).

  4. Post-Kala-azar Dermal Leishmaniasis (PKDL): Complication appearing after treatment or in immunosuppressed individuals; characterized by nodular rash.

• Transmission Cycles:

  • Zoonotic: Animal to vector to human (L. infantum, dogs are reservoirs).

  • Anthroponotic: Human to vector to human (L. donovani).

VISCERAL LEISHMANIASIS (KALA-AZAR)

• Clinical Signs: Long-duration irregular fever, emaciation, anaemia, leucopaenia, and hepatosplenomegaly.

• Distribution Types:

  • Indian: Endemic in Assan, Bengal, Bihar, Madras, Sikkim, Nepal, Bangladesh.

  • Mediterranean (Infantile): Southern Italy, France, Spain, Russia, North China.

  • African: Sudan, Ethiopia, East Africa.

  • American: Brazil (Northeast), Argentina, Colombia.

• Pathogenesis: Promastigotes (flagellar form in fly) transform into amastigotes (intracellular in human) within phagolysosomes. Dissemination occurs through lymphatics to the reticulo-endothelial system.

• Diagnosis:

  • Non-leishmanial: Pancytopenia (reduction in RBCs, WBCs, platelets) has $98\%$ specificity but only $16\%$ sensitivity. Aldehyde test (formol gel test) has poor sensitivity ($34\%$).

  • Parasite Detection: Microscopy of splenic aspirates ($93-99\%$ sensitive) is more effective than bone marrow ($53-86\%$) or lymph nodes ($53-63\%$).

• Treatment: Pentavalent antimonials (sodium stibogluconate), Amphotericin B, Miltefosine (oral anticancer drug derivative), Paromomycin, and Sitamaquine.

HUMAN AFRICAN TRYPANOSOMIASIS (HAT)

• Definition: Acute and chronic disease produced by haemoflagellates of the genus Trypanosoma, transmitted by Glossina (tsetse flies).

• Species:

  • T. brucei gambiense: West/Central Africa; chronic disease; transmitted by the Palpalis group.

  • T. brucei rhodesiense: East/Southern Africa; acute zoonosis; transmitted by the Morsitans group.

• Life Cycle: A cycle of approximately $30$ days. Trypomastigotes transform to procyclic stages in the fly midgut, then metacyclic trypomastigotes in the salivary glands.

• Antigienic Variation: To evade the immune system, parasites express different Variant Surface Glycoproteins (VSG), one at a time.

• Clinical Stages:

  1. Haematolymphatic (First Stage): Characterized by a chancre, fever, and Winterbottom's sign (swollen posterior cervical lymph nodes).

  2. Meningoencephalitic (Second Stage): Circadian rhythm disappears; tremors, mental confusion, and coma occur as the CNS is invaded.

• Diagnostics:

  • CATT: Card Agglutination Test for Gambiense; latex agglutination providing results in $10$ minutes.

  • mAECT: Miniature anion-exchange centrifugation; most sensitive parasitological method.

• Vector Control: Use of Pyramidal, Biconical, or Vavoua traps. Olfactory attractive baits include acetone, urine phenols, and host skin secretions.

• Treatment: Pentamidine (early-stage gambiense), Suramin (early-stage rhodesiense), Melarsoprol (late-stage), and Eflornithine.

AMERICAN TRYPANOSOMIASIS (CHAGAS DISEASE)

• Aetiology: Trypanosoma cruzi, transmitted by Reduviid (triatomine) bugs, also called "kissing bugs."

• Distribution: South and Central America, and the USA. Reservoirs include dogs, cats, wood rats, and guinea pigs (in Peru/Bolivia).

• Morphology: Trypomastigotes have a characteristically large oval kinetoplast situated posteriorly (the largest of any trypanosome).

• Biology: Bugs defaecate while feeding; metacyclic trypanosomes enter through bite wounds or mucous membranes. Intracellular amastigotes form pockets in muscle called pseudocytes.

• Pathogenesis Stages:

  • Acute: Chagoma (red nodule at inoculation site) and Romana's sign (unilateral eyelid oedema).

  • Chronic: Megaoesophagus or megacolon due to destruction of autonomic ganglia.

• Diagnosis: Demonstrated via fresh blood films or Xenodiagnosis (allowing uninfected laboratory-bred bugs to feed on the patient, then examining hindgut contents after $10$ to $30$ days).

• Treatment: Nifurtimox and benznidazole.

GIARDIA DUODENALIS

• Introduction: Discovered by Antonie van Leeuwenhoek in $1681$ in his own stool. Belong to the family Hexamitidae.

• Epidemiology: Highly contagious via swallowing mature cysts. Reservoirs include beavers, dogs, cats, and sheep.

• Morphology:

  • Trophozoites: Pear-shaped, rounded anteriorly, tapering posteriorly. Dorso-ventrally flattened and concave at the ventral surface.

  • Anatomy: Contains two parallel rods along the median line, two nuclei, and eight flagella.