Summary
Highlights
The lecture begins by introducing Sporozoa as a continuation of previous discussions on coccidians and miscellaneous protozoa. These organisms primarily target blood and tissue. The phylum Apicomplexa, to which Sporozoa belong, is characterized by an apical complex that aids in host infection, despite lacking apparent means of locomotion. They are obligate intracellular parasites, replicating within host cells. The life cycle involves sexual reproduction (sporogony) in the definitive host and asexual reproduction (schizogony) in the intermediate host. The main focus is on Plasmodium species, which cause malaria, and Toxoplasma gondii. Malaria, meaning 'bad air,' was historically believed to originate from contaminated soil, but is now known to be caused by a parasite. It remains the leading parasitic disease worldwide, causing significant mortality and morbidity.
While Plasmodium has many species, only five infect humans: P. falciparum, P. vivax, P. ovale, P. malariae, and recently P. knowlesi. P. falciparum and P. vivax are responsible for 95% of worldwide infections. In the Philippines, P. falciparum is the most common and deadliest, capable of infecting all stages of red blood cells. P. vivax, while widespread globally due to its ability to survive in various climates, causes benign tertian malaria. P. ovale and P. malariae are less common but important. The primary drug of choice for malaria has long been chloroquine, though resistance has led to the use of alternative treatments. The vector for malaria is the female Anopheles mosquito, with Anopheles minimus var. flavirostris being the main vector in the Philippines. The infective stage for humans is the sporozoite, while gametocytes are infective to mosquitoes. Malaria can also be transmitted through blood transfusions, organ transplantation, needle sharing, and congenitally.
Plasmodium species exhibit various morphological stages in blood samples, including ring forms (early trophozoites) characterized by a blue cytoplasmic circle and a red chromatin dot. As they develop, trophozoites enlarge, and brown 'hemozoin' pigment (malarial pigment) may appear as a breakdown product of hemoglobin. Immature schizonts show active chromatin replication, and mature schizonts contain merozoites, which are the asexual stages. The number and arrangement of merozoites vary by species and aid in identification. Gametocytes, the sexual stages, develop from merozoites, with microgametocytes (male) being colorless with diffuse chromatin, and macrogametocytes (female) having more concentrated chromatin and cytoplasmic material.
The asexual cycle (schizogony) occurs in humans, our intermediate host. Sporozoites, injected by mosquitoes, travel to the liver, infecting hepatocytes (exoerythrocytic stage). They develop into cryptozoids, then maturer metazoids, which eventually rupture from liver cells. These metazoids then infect red blood cells, becoming ring trophozoites, and later schizonts containing merozoites. Some merozoites develop into gametocytes. Notably, P. vivax and P. ovale form hypnozoites, dormant stages in the liver that can reactivate years later, causing relapses. The sexual cycle (sporogony) occurs in the Anopheles mosquito, the definitive host. When a mosquito ingests gametocytes, they fuse to form a zygote, which develops into a motile ookinete. The ookinete insists on the gut wall to form an oocyst, where sporozoites mature and eventually migrate to the salivary glands, ready to infect a new human host.
Key features for diagnosing specific Plasmodium species are emphasized. P. vivax causes benign tertian malaria and is characterized by amoeboid trophozoites, Schüffner's dots (condensed hemoglobin), and exclusively infects reticulocytes, which become enlarged. P. malariae causes quartan malaria, features Ziemann's dots, infects mature RBCs (normal size), and has trophozoites in a band shape, with merozoites arranged in a fruit-pie or rosette form. P. ovale causes ovale malaria, presents James' dots (or Schüffner's dots), infects enlarged and fimbriated (serrated) RBCs, and also forms hypnozoites. P. falciparum, the deadliest, infects all RBC stages, commonly shows applique forms (rings at the RBC edge) and multiple ring forms per RBC. Its gametocytes are distinctly crescent, banana, or sausage-shaped. Round/ovoid gametocytes are typical for the other three species. P. knowlesi, a zoonotic species from Southeast Asia, is microscopically indistinguishable from P. malariae, making patient travel history crucial for diagnosis.
A tabular summary from Belizario highlights the specific features of each Plasmodium species, comparing infected RBC size, ring forms, trophozoite characteristics, schizont merozoite counts and arrangements, and gametocyte shapes. P. falciparum's rapid asexual cycle (36-48 hours) contributes to its deadliness. Mnemonics are introduced to remember the distinctive dots associated with each species: Falciparum-Maurer's (FM), Vivax-Schüffner's (VS), Malariae-Ziemann's (MZ), Ovale-James' (OJ). The speaker shares a personal mnemonic involving "Ferdinand Marcos, Vilma Santos, Manila Zoo, Orange Juice" to relate these dots. The importance of extensive training and experience for accurate morphological identification of malaria parasites to the species level is underscored.