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

A dog presents with progressive neurological signs including ascending paralysis. CSF analysis shows albuminocytologic dissociation. Rabies is suspected but the owner reports the dog was vaccinated 3 years ago. Which post-mortem finding would most specifically confirm rabies encephalitis over other viral encephalitides?

Answer: Negri bodies in Purkinje cells of the cerebellum and hippocampal neurons

Negri bodies are pathognomonic eosinophilic intracytoplasmic inclusion bodies formed by rabies virus nucleocapsid accumulation, found most reliably in Purkinje cells and hippocampal (Ammon's horn) neurons. While perivascular cuffing and non-suppurative inflammation occur in many viral encephalitides, and chromatolysis is nonspecific, Negri bodies are specific to rabies. Note: vaccine failure or waning immunity can still allow rabies infection, making post-mortem histopathology essential.

A flock of sheep develops contagious ecthyma (orf). The veterinarian must advise the farm workers. Which statement about Orf virus (ORFV) zoonotic risk is most accurate?

Answer: ORFV is a poxvirus that can infect humans but is self-limiting; reinfection can occur due to short-lived immunity

Orf virus (Parapoxvirus) is a significant zoonosis, causing proliferative nodular lesions on human hands/arms after direct contact with infected animals or fomites. Unlike orthopoxviruses such as vaccinia, ORFV infection does not produce durable sterilizing immunity — reinfection is well documented in farm workers. The infection remains localized and self-limiting in immunocompetent humans. ORFV uses heparan sulfate proteoglycans and VEGF signaling for entry, not HVEM (which is used by herpes simplex virus).

Rhodococcus equi pneumonia is diagnosed in a 2-month-old foal. Which virulence factor is essential for intracellular survival and is targeted by diagnostic PCR assays used in clinical laboratories?

Answer: Vapors-associated protein A (VapA) encoded on a large conjugative virulence plasmid

VapA (virulence-associated protein A) is encoded on a large conjugative plasmid (85–90 kb) found in virulent equine strains of R. equi. VapA is essential for inhibition of phagosome maturation and lysosomal fusion within alveolar macrophages, enabling intracellular survival. Clinical PCR assays specifically detect the vapA gene to distinguish virulent from avirulent environmental strains. R. equi lacks a T3SS homologous to Salmonella; its capsule plays a minor role compared to VapA in virulence.

A cat develops feline infectious peritonitis (FIP). The pathogenesis involves mutation of feline enteric coronavirus (FECV) to feline infectious peritonitis virus (FIPV). Which molecular change is most consistently associated with this phenotypic conversion?

Answer: Deletion in the 3c gene leading to altered macrophage tropism

The conversion from FECV to FIPV most consistently involves mutations or deletions in the 3c gene (and sometimes the 7b gene). Loss-of-function mutations in 3c are strongly correlated with the shift from enteric replication to systemic macrophage-tropic infection, the hallmark of FIPV. While spike protein mutations affect receptor affinity, the 3c gene alteration is the most consistently identified genetic correlate of virulent conversion in sequencing studies. Recombination with CCoV has been documented but is not the dominant mechanism of FIP conversion.

Neospora caninum abortion storms occur on a cattle farm. The farmer asks whether culling all seropositive cows will eliminate the infection from the herd. Which statement best explains the limitation of this strategy?

Answer: Vertical (transplacental) transmission perpetuates infection in offspring of seropositive dams, and some persistently infected calves remain seronegative early in life

The primary challenge in Neospora caninum control is highly efficient vertical (transplacental) transmission — congenitally infected calves born to seropositive dams are themselves infected and will eventually seroconvert. Additionally, congenitally infected calves may be seronegative in the first weeks of life before maternal antibody wanes and their own response develops. Culling seropositive adults removes the most obvious reservoir but misses already-infected offspring. N. caninum does not complete its sexual cycle (oocyst production) in cattle — only in canids — so cattle-to-cattle fecal-oral transmission does not occur. Serology reliably distinguishes N. caninum from T. gondii.

Glanders (Burkholderia mallei) is suspected in an imported horse. Which characteristic most reliably distinguishes B. mallei from its close relative Burkholderia pseudomallei (melioidosis agent) in laboratory identification?

Answer: B. mallei is non-motile and arginine dihydrolase-negative, whereas B. pseudomallei is motile with a polar tuft of flagella

B. mallei and B. pseudomallei are genetically nearly identical (B. mallei is essentially a host-adapted clone of B. pseudomallei), making differentiation challenging. The key phenotypic distinctions are: B. mallei is non-motile (lacks flagellar genes deleted during host adaptation) and is arginine dihydrolase-negative, while B. pseudomallei is motile via a bipolar tuft of flagella and is arginine dihydrolase-positive. Both are oxidase-positive, Gram-negative, and both fail to ferment lactose. Neither produces violet pigment. Both are Tier 1 select agents requiring BSL-3 containment.