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

A 4-year-old intact male dog presents with scrotal edema, fever, and reluctance to breed. Brucella canis is suspected. Which serological test has the highest sensitivity for initial screening but is known to produce false positives due to cross-reactivity with other gram-negative bacteria?

Answer: Rapid slide agglutination test (RSAT)

The Rapid Slide Agglutination Test (RSAT) is highly sensitive and used for initial screening, but it cross-reacts with other gram-negative organisms such as Pseudomonas, Bordetella, and Moraxella, leading to false positives. AGID targeting the cytoplasmic protein antigen is more specific. TAT is also prone to cross-reactivity. Blood culture is the gold standard but slow and requires BSL-2 precautions.

A 6-month-old Holstein calf in a herd in the midwest presents with profuse watery diarrhea, dehydration, and metabolic acidosis. Cryptosporidium parvum oocysts are identified on Ziehl-Neelsen staining of fecal smears. Which property of C. parvum makes it uniquely resistant to standard water chlorination protocols?

Answer: Thick two-layered oocyst wall with sporulation occurring prior to shedding

C. parvum oocysts are immediately infective when shed (sporulated) and possess a thick, two-layered wall that renders them highly resistant to chlorine-based disinfectants at concentrations used in municipal water systems. This is why filtration rather than chlorination is the primary water treatment strategy. The extracytoplasmic location is a biologically interesting feature but is not the reason for chlorine resistance.

During a necropsy of a sheep flock with increased abortion rates, placentomes show intercotyledonary edema and leathery discoloration. Chlamydia abortus (formerly Chlamydophila abortus) is confirmed by PCR. Which of the following statements most accurately distinguishes C. abortus from Coxiella burnetii in this epidemiological context?

Answer: C. abortus is transmitted primarily via direct contact with infected aborted material, while C. burnetii is predominantly transmitted via aerosol from contaminated parturient fluids

C. abortus is transmitted primarily through direct contact with infected aborted fetuses, placentas, and uterine discharges, with limited aerosol transmission. In contrast, C. burnetii is extraordinarily resistant in its spore-like small cell variant form and is predominantly transmitted via aerosol, making it a far greater zoonotic aerosol hazard. C. burnetii also replicates in phagolysosomes using acid-activation; C. abortus replicates in inclusions that intercept the exocytic pathway and avoids lysosomal fusion.

A 3-year-old Quarter Horse presents with rapidly progressive cellulitis and crepitus of the left hindlimb following a wire laceration. Gram stain of tissue aspirate reveals large gram-positive rods. Which toxin produced by Clostridium septicum plays the primary pathogenic role in the characteristic rapid tissue necrosis seen in malignant edema?

Answer: Alpha toxin (a pore-forming cytolysin)

Clostridium septicum alpha toxin is the major virulence factor and is a pore-forming cytolysin (similar in mechanism to C. perfringens alpha toxin, though structurally distinct) that disrupts cell membranes, causing rapid myonecrosis and edema. It has leucocidal, hemolytic, and necrotizing activity. Beta toxin is a DNase. Epsilon and iota toxins are associated with C. perfringens types D and E, respectively, not C. septicum.

An 8-year-old spayed DSH cat tests positive on a point-of-care ELISA for FIV antibody. The owner asks whether the cat could be a false positive. Which specific population of cats would be most likely to yield a TRUE false-positive FIV antibody result on ELISA that does NOT reflect active infection or vaccination?

Answer: Kittens under 6 months born to FIV-positive queens retaining maternal antibodies

Kittens born to FIV-positive queens passively acquire maternal antibodies via colostrum, which can persist and cause positive ELISA results until 6–14 weeks of age (occasionally longer). These kittens are not truly infected. This is the classic and most clinically important cause of true false positives. FeLV co-infection does not cause cross-reactive false positives on FIV ELISA. FIP antigens do not share epitopes with FIV. Transfusion from an FIV+ donor could theoretically transfer antibodies, but this is not a recognized significant clinical cause of false positives.

A commercial turkey flock experiences sudden high mortality. Gross necropsy reveals multifocal hepatic necrosis and typhlitis with caseous cores. Histomonas meleagridis is confirmed. A key challenge in managing this outbreak is that the traditional treatment was withdrawn from the US market. Which aspect of H. meleagridis biology most critically complicates eradication efforts within a flock once clinical signs appear?

Answer: H. meleagridis is transmitted cloacally via direct bird-to-bird contact as well as within the eggs of the vector nematode Heterakis gallinarum, enabling persistent soil contamination

H. meleagridis is transmitted via the embryonated eggs of the cecal nematode Heterakis gallinarum, which can survive in soil for years, and also via direct cloacal contact (cloacal drinking). This dual transmission route — especially the long-lived Heterakis egg reservoir in soil — makes environmental decontamination virtually impossible and sustains flock-to-flock and year-to-year transmission. H. meleagridis does not form cysts, does not exhibit significant antigenic variation, and while it does invade cecal and liver tissue via macrophages, it is not strictly an obligate intracellular parasite throughout its lifecycle.