NAVLE Veterinary Public Health 6

Question 1

A state veterinarian is investigating a cluster of human Salmonella Heidelberg cases linked to a poultry processing facility. Whole-genome sequencing (WGS) reveals the human isolates match a strain found in broilers from Farm A, but the processing plant draws from 14 farms. Which epidemiological concept best explains why WGS alone is insufficient to establish regulatory action against Farm A without additional trace-back data?

Accepted Answer

Misclassification of exposure due to cross-contamination during slaughter

Answer

Cross-contamination during slaughter is the critical confounder. A single processing line can transfer a strain from one farm's flock to carcasses originating from other farms, meaning WGS identity between human cases and Farm A broilers could reflect contamination at the plant rather than Farm A as the true reservoir. Without lot-specific trace-back confirming the matched carcasses came exclusively from Farm A and were not co-mingled on the same line, regulatory action targeting that farm specifically is scientifically unsupported. The other options describe real biases but are not the primary mechanistic explanation in a multi-farm slaughter scenario.

Question 2

During a PRRS (Porcine Reproductive and Respiratory Syndrome) outbreak investigation, a veterinary epidemiologist calculates a basic reproduction number (R₀) of 3.8 for the index swine production system. The system achieves 70% sow vaccination coverage with a vaccine that has 60% efficacy against transmission. What is the approximate effective reproduction number (Rₑ) after vaccination, and does it cross the epidemic threshold?

Accepted Answer

Rₑ ≈ 2.2; above threshold — epidemic continues

Answer

The effective reproduction number after vaccination is calculated as Rₑ = R₀ × (1 − vaccine efficacy × coverage) = 3.8 × (1 − 0.60 × 0.70) = 3.8 × (1 − 0.42) = 3.8 × 0.58 ≈ 2.2. Since Rₑ > 1, the epidemic threshold has not been crossed and transmission will continue, though at a reduced rate. To achieve herd immunity (Rₑ < 1), the product of efficacy × coverage would need to exceed 1 − 1/R₀ = 1 − 1/3.8 ≈ 0.74 (74%), meaning either higher coverage or a more efficacious vaccine is required.

Question 3

A veterinary public health officer receives a report of a ranch hand who developed cutaneous anthrax after handling a cow carcass. Soil samples from the pasture confirm Bacillus anthracis spores at 4 × 10⁴ CFU/gram at depths up to 15 cm. Which regulatory authority and specific action is the MOST appropriate immediate response under U.S. law?

Accepted Answer

Notify USDA-APHIS and state animal health officials; impose a state-level quarantine, vaccinate remaining animals with Sterne strain vaccine, and safely dispose of the carcass by deep burial or incineration on-site

Answer

Anthrax is a state-regulated foreign animal disease reportable condition in most U.S. states. The immediate veterinary public health response involves notifying USDA-APHIS and state animal health officials, imposing quarantine to prevent movement of potentially exposed animals, vaccinating the surviving herd with the licensed Sterne 34F2 strain (the only USDA-licensed veterinary anthrax vaccine), and disposing of the carcass by on-site deep burial (≥6 feet with lime) or incineration — never necropsy or movement off-site, as opening the carcass sporulates the organism. EPA Superfund is not applicable here; CDC select-agent regulations apply to laboratories, not farms with natural anthrax; OSHA bloodborne pathogens standard does not cover B. anthracis (a Category A bioterrorism agent covered separately).

Question 4

A One Health investigation links a series of human Cryptosporidium parvum cases to a municipal watershed used for drinking water. Environmental sampling finds oocysts at 2.7 oocysts/10L in the raw water intake. The watershed is shared by a confined dairy operation upstream. Which characteristic of C. parvum oocysts most limits the effectiveness of standard municipal water chlorination as the sole treatment barrier?

Accepted Answer

The oocyst wall confers extraordinary chlorine resistance, requiring CT values far exceeding those achievable with standard disinfection doses

Answer

Cryptosporidium oocysts are highly resistant to chlorine disinfection due to their thick, proteinaceous oocyst wall. Achieving a 3-log (99.9%) reduction requires CT (concentration × time) values of >7,200 mg·min/L at 25°C — far beyond what is operationally feasible in municipal systems (typical CT for Giardia is ~55 mg·min/L). This is why the EPA Surface Water Treatment Rule requires filtration (which physically removes oocysts) rather than relying on chlorination alone. C. parvum does not acquire chlorine resistance through plasmid-mediated mechanisms (it is a protozoan, not a bacterium), and the pH sensitivity described is incorrect — UV irradiation and filtration are the validated control strategies.

Question 5

A veterinarian conducting a pre-export health certificate inspection for cattle destined to a country in the EU discovers one animal with an ear-tag discrepancy — the tag number does not match the USDA premises registration record but the animal otherwise appears healthy. Under the USDA-APHIS Export Verification Program and EU import requirements, what is the MOST appropriate immediate action?

Accepted Answer

Reject the animal from the export lot, document the discrepancy on Form VS 17-140, and notify the APHIS District Office — the animal cannot be certified without verified identity

Answer

Under USDA-APHIS Export Verification (EV) programs and EU Regulation (EU) 2016/429 requirements, traceability — including verified official identification linked to premises of origin — is a non-negotiable prerequisite for export certification. An animal whose tag cannot be matched to a USDA premises record has an unverifiable identity chain; certifying it would constitute a false official statement on a federal document (VS 17-140), which carries both regulatory and criminal liability. The correct action is to reject the animal from the consignment, document the discrepancy, and notify the APHIS District Veterinarian. Re-tagging and retroactively updating records at the time of certification is not permissible under federal traceability rules — the identity must have been established prior to certification, not created to enable it.

NAVLE - North American Veterinary Licensing Examination Practice Test

NAVLE - North American Veterinary Licensing Examination Practice Test

NAVLE Veterinary Public Health 6