BCSE - Basic and Clinical Sciences Examination Veterinary Microbiology and Infectious Diseases Questions and Answers Flashcards

A 4-year-old Quarter Horse presents with progressive ataxia, dysphagia, and muscle wasting over 6 months. Necropsy reveals multifocal, nonsuppurative encephalomyelitis with intraneuronal protozoal organisms. Immunohistochemistry confirms Sarcocystis neurona. Which of the following best explains why cell-mediated immunity is the primary defense against this organism rather than humoral immunity?

Answer: S. neurona is an obligate intracellular parasite that evades antibody neutralization by residing within host cell cytoplasm

As an obligate intracellular parasite, S. neurona completes its life cycle within host cell cytoplasm (primarily neurons and macrophages), where circulating antibodies cannot reach the organism directly. Cell-mediated immunity — particularly CD8+ cytotoxic T lymphocytes and activated macrophages — is critical for eliminating infected cells. While serum antibodies are detectable and used diagnostically (IFAT, SAG ELISA), they are insufficient for clearance of established intracellular infection.

During a canine parvovirus-2 (CPV-2) outbreak at a shelter, a veterinarian notes that three adult vaccinated dogs seroconverted (showing 4-fold titer rises) but remained completely asymptomatic. Which mechanism MOST accurately explains subclinical infection in these vaccinated adults compared to fatal disease in unvaccinated puppies?

Answer: Pre-existing memory B cells in vaccinated adults enable anamnestic IgG responses that neutralize virus before it reaches the rapidly dividing crypt epithelium and bone marrow progenitors

CPV-2 selectively infects mitotically active cells (intestinal crypt epithelium, lymphoid tissue, bone marrow). In vaccinated adults, vaccine-primed memory B and T cells mount a rapid anamnestic response, producing neutralizing IgG within 24–48 hours of exposure — before viral titers reach levels sufficient to deplete the crypt stem cell population. Unvaccinated puppies lack this memory response; their naïve immune system responds too slowly, allowing massive viral replication in the crypt epithelium, leading to villous blunting, protein-losing enteropathy, and bacteremia.

A sheep flock in the western United States experiences repeated abortions. Fluorescent antibody testing of placental cotyledons is positive for Chlamydophila abortus. The owner successfully vaccinated against Campylobacter fetus and Brucella ovis the previous year. Which unique feature of Chlamydophila's life cycle most complicates vaccine-based prevention compared to those bacterial pathogens?

Answer: The biphasic life cycle — with infectious elementary bodies (EBs) converting to metabolically active reticulate bodies (RBs) intracellularly — means antibody targets the EB but cannot access the replicating RB

Chlamydophila abortus has a distinctive biphasic intracellular life cycle. The elementary body (EB) is the extracellular infectious form — metabolically inert, resistant to environmental stress, and the target of neutralizing antibodies. Once inside host cells (trophoblasts), EBs convert to reticulate bodies (RBs), which are metabolically active and replicating but completely shielded from circulating antibody within the inclusion vacuole. Vaccines can reduce initial infection by neutralizing EBs but cannot eliminate established intracellular RBs, explaining partial vaccine efficacy and persistent flock-level abortion storms.

A veterinarian submits a feline nasal biopsy to the laboratory. Histopathology reveals broad-based budding yeasts with narrow-neck attachments between parent and daughter cells, and organisms ranging from 5–15 µm with thick, refractile walls. GMS stain highlights the organisms. Culture on SDA at 25°C produces a mold; at 37°C, the same isolate produces yeast colonies. This thermal dimorphism and the histologic morphology are MOST consistent with which organism, and what is its primary ecological niche?

Answer: Blastomyces dermatitidis; moist acidic sandy soil near waterways in endemic regions

The combination of broad-based budding (as opposed to the narrow-necked multiple buds of Paracoccidioides), large thick-walled yeast forms (5–15 µm), thermal dimorphism, and feline nasal involvement is characteristic of Blastomyces dermatitidis. Blastomycosis is endemic in the Ohio and Mississippi River valleys and the Great Lakes region, associated with moist, acidic, sandy or loamy soils near waterways. The 'broad-based bud' is the pathognomonic histologic feature. Histoplasma is smaller (2–4 µm, intracellular) and has narrow-necked budding; Sporothrix has cigar-shaped yeast forms and is rarely isolated from nasal biopsy in cats with systemic disease.

A research team studying bovine respiratory disease complex (BRDC) in feedlot cattle finds that Mannheimia haemolytica leukotoxin (LKT) kills bovine alveolar macrophages but has minimal cytotoxicity against ovine or caprine macrophages at equivalent concentrations. Which receptor-based mechanism BEST explains this host species specificity of LKT-mediated cytotoxicity?

Answer: LKT binds CD18 (β2-integrin) on leukocytes, and bovine CD18 has a species-specific extracellular domain conformation that creates a high-affinity binding site absent in ovine/caprine CD18 orthologs

Mannheimia haemolytica leukotoxin (LKT) is a member of the RTX (repeats in toxin) family. Its cytotoxicity is exquisitely species-specific to ruminants and is mediated through binding to CD18, the β2-integrin subunit present on leukocytes. Critically, bovine CD18 has a unique extracellular loop conformation in the I-domain that creates a high-affinity LKT-binding site. Ovine and caprine CD18 orthologs differ at key residues in this domain, markedly reducing LKT-binding affinity and subsequent pore formation, explaining why M. haemolytica causes severe fibrinous pneumonia in cattle but is far less pathogenic in sheep under natural exposure conditions.

A 7-year-old intact male Labrador Retriever is presented with a 3-week history of draining skin nodules over the muzzle, with satellite lymphadenopathy. Cytology of exudate reveals pyogranulomatous inflammation with intracellular gram-positive, filamentous, weakly acid-fast organisms. Culture on Lowenstein-Jensen medium at 37°C yields slow-growing orange-pigmented colonies after 3 weeks. Which species is MOST likely, and why does its pigmentation differentiate it from Mycobacterium tuberculosis complex?

Answer: Mycobacterium scrofulaceum; a scotochromogen producing pigment in both light and dark conditions, classified in Runyon Group II and primarily causing cervical lymphadenitis

The clinical presentation (facial nodules with satellite lymphadenopathy, pyogranulomatous exudate with weakly acid-fast filamentous rods) combined with slow growth (3 weeks), orange colonies at 37°C regardless of light exposure (scotochromogen), and canine host is most consistent with Mycobacterium scrofulaceum, a Runyon Group II scotochromogen. Unlike M. tuberculosis complex, which is nonchromogenic (no pigment in any light condition), scotochromogens such as M. scrofulaceum produce carotenoid pigment constitutively in both light and dark conditions. Photochromogens (Runyon Group I, e.g., M. kansasii, M. marinum) only produce pigment after light exposure. This pigment classification (Runyon system) is a critical phenotypic tool for preliminary identification of nontuberculous mycobacteria.