Pathology
Infections and Infestations
Infections for MBBS and NEET-PG: malaria, cysticercosis and leprosy pathology, plus common bacterial, viral and parasitic diseases and their diagnostic features, mapped to NMC codes PA10.1 to PA10.4.
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Infections and Infestations
Infections for MBBS and NEET-PG: malaria, cysticercosis and leprosy pathology, plus common bacterial, viral and parasitic diseases and their diagnostic features, mapped to NMC codes PA10.1 to PA10.4.
This chapter covers the pathology of important infections and parasitic infestations. It focuses on malaria, cysticercosis and leprosy in depth, and surveys the tissue reactions and diagnostic features of common bacterial, viral, protozoal and helminthic diseases relevant to Indian practice.
High-yield: Infections and Infestations
- Malaria is caused by Plasmodium species transmitted by the female Anopheles mosquito, with falciparum causing the most severe disease.
- Plasmodium falciparum causes cerebral malaria through sequestration of parasitised red cells in cerebral capillaries.
- The malarial pigment haemozoin accumulates in macrophages of the spleen, liver and bone marrow.
- Blackwater fever is severe intravascular haemolysis with haemoglobinuria seen in falciparum malaria.
- Cysticercosis is infection with the larval stage of Taenia solium, the pork tapeworm.
- Neurocysticercosis is a leading cause of acquired epilepsy in endemic regions and shows cysts with a scolex on imaging.
- Leprosy is caused by Mycobacterium leprae, an acid-fast bacillus that cannot be cultured in vitro and grows best in cool tissues.
- Tuberculoid leprosy has few bacilli, well-formed granulomas and a strong cell-mediated response with a positive lepromin test.
- Lepromatous leprosy has abundant bacilli, foamy macrophages (lepra cells), weak immunity and a negative lepromin test.
- The leonine facies of lepromatous leprosy results from diffuse dermal infiltration by foamy macrophages.
- Granulomatous inflammation is the common tissue response to tuberculosis, leprosy, syphilis and certain fungal infections.
- Amoebiasis produces flask-shaped ulcers in the colon and can cause a liver abscess with anchovy-sauce pus.
- Hydatid disease is caused by Echinococcus granulosus and forms fluid-filled cysts, most often in the liver.
- Special stains help identify organisms: Ziehl-Neelsen for mycobacteria, periodic acid-Schiff and silver stains for fungi.
- Viral infections may show characteristic inclusion bodies, such as the owl-eye inclusions of cytomegalovirus and Negri bodies of rabies.
Tuberculoid versus lepromatous leprosy
- **Bacillary load:** Tuberculoid: few bacilli (paucibacillary). Lepromatous: abundant bacilli (multibacillary).
- **Immunity:** Tuberculoid: strong cell-mediated, positive lepromin. Lepromatous: weak, negative lepromin.
- **Histology:** Tuberculoid: well-formed granulomas. Lepromatous: sheets of foamy lepra cells.
- **Clinical:** Tuberculoid: few, well-defined lesions. Lepromatous: widespread lesions, leonine facies.
NMC competencies in this chapter
- **PA10.1:** Pathogenesis and pathology of malaria
- **PA10.2:** Pathogenesis and pathology of cysticercosis
- **PA10.3:** Pathogenesis and pathology of leprosy
- **PA10.4:** Pathology of common bacterial, viral, protozoal and helminthic diseases
Frequently Asked Questions
Which malaria species causes cerebral malaria?
Plasmodium falciparum, through sequestration of parasitised red cells in cerebral capillaries, which impairs microvascular flow in the brain.
How do tuberculoid and lepromatous leprosy differ?
Tuberculoid leprosy is paucibacillary with strong immunity and granulomas, while lepromatous leprosy is multibacillary with weak immunity and sheets of foamy lepra cells.
What is neurocysticercosis?
Infection of the central nervous system with the larval stage of Taenia solium, a leading cause of acquired epilepsy in endemic areas.
What tissue reaction is common to TB and leprosy?
Granulomatous inflammation, a focal collection of epithelioid macrophages, is the shared tissue response to both diseases.
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