NAVLE Bovine Herd Health Management 5

Question 1

A 450-cow commercial dairy herd has a 21-day pregnancy rate of 12% and a conception rate of 32%. The reproductive program manager wants to improve the 21-day pregnancy rate to 20%. Which metric most directly limits achieving this goal given the current data?

Accepted Answer

21-day estrus detection rate, which must be increased from approximately 38% to at least 50%

Answer

The 21-day pregnancy rate = estrus detection rate × conception rate. With a 32% conception rate and 12% PR, the estrus detection rate is approximately 12/32 = 37.5%. To achieve a 20% PR at the same conception rate: 20/32 = 62.5% EDR. Even if conception rate were improved moderately, the primary bottleneck is the low estrus detection rate. Estrus detection (now often called 'submission rate' under synchronization programs) must be the primary target. The other options are not directly calculable constraints from the given data.

Question 2

A beef cow-calf producer reports that 15% of calves born in the past two weeks have died within 48 hours of birth. Necropsy of three calves reveals fibrinous polyarthritis, fibrinous pericarditis, and pleuritis with a gram-positive coccus isolated in pure culture. The dam herd was vaccinated with a modified-live virus (MLV) respiratory vaccine at pregnancy check in mid-gestation. Which pathogen is most likely responsible, and what is the most significant management gap?

Accepted Answer

Histophilus somni; failure to vaccinate cows with a killed bacterin containing H. somni before calving

Answer

Histophilus somni causes thromboembolic meningoencephalitis (TEME) in older cattle but in neonates produces fibrinous polyserositis (arthritis, pericarditis, pleuritis) — the 'serositis syndrome.' It is a gram-negative coccobacillus, but small gram-positive cocci on Gram stain can be a field-lab artifact with H. somni due to cell morphology variation. More importantly, MLV respiratory vaccines given mid-gestation do not provide colostral protection against H. somni bacterin antigens; a pre-calving killed H. somni bacterin in the dam is required to transfer protective IgG1 in colostrum. Mannheimia typically causes bronchopneumonia, not polyserositis; Trueperella and Streptococcus do not fit the lesion pattern.

Question 3

In a 1,200-head feedlot, the nutritionist increases the dietary sulfur concentration from 0.3% to 0.55% DM to accommodate co-product inclusion (distillers grains). Two weeks later, the pen rider reports a cluster of sudden deaths and neurological signs (head pressing, blindness, recumbency) in yearling steers. Gross necropsy shows no significant lesions, but brain histopathology reveals focal symmetric polioencephalomalacia. Which mechanism best explains the pathogenesis in this outbreak?

Accepted Answer

Ruminal production of hydrogen sulfide that is eructated and inhaled, causing CNS hypoxia and secondary polioencephalomalacia via mitochondrial cytochrome oxidase inhibition

Answer

High dietary sulfur is fermented in the rumen to hydrogen sulfide (H2S) gas. During eructation and normal breathing of rumen gas, H2S is inhaled and absorbed across pulmonary alveoli, entering the bloodstream. H2S inhibits cytochrome c oxidase (Complex IV of the mitochondrial electron transport chain), causing cellular hypoxia preferentially in laminar gray matter of the cerebral cortex — producing the symmetric polioencephalomalacia seen histologically. This is distinct from thiamine-deficiency PEM, though both produce similar histologic lesions. The mechanism is sulfide-induced mitochondrial toxicity via inhalation, not direct blood-brain barrier penetration or thiamine destruction. Adequate thiamine supplementation does NOT prevent sulfur-induced PEM, which is a critical diagnostic distinction.

Question 4

A veterinarian is designing a Johne's disease (paratuberculosis) test-and-cull program for a 500-cow closed dairy herd. The herd-level prevalence is estimated at 8%. The diagnostic lab offers two ELISA tests: Test A (sensitivity 45%, specificity 99%) and Test B (sensitivity 28%, specificity 99.8%). The producer's primary goal is to eliminate all infected animals without culling excessive false-positives. Which testing strategy is most appropriate?

Accepted Answer

Use Test A as a screen, then confirm positives with Test B in series, to maximize both sensitivity and specificity

Answer

At 8% prevalence, even a 99% specific test yields a positive predictive value (PPV) of approximately 82% for Test A and 97% for Test B when used alone. However, Test A's sensitivity (45%) catches more true positives per annual round. Using Test A as a screen (higher sensitivity captures more infected animals) and Test B as a confirmatory test in series (very high specificity eliminates most false positives) optimizes the program: sensitivity of the series is limited by Test A (acceptable), but specificity approaches Test B's 99.8%, making PPV very high and preventing unnecessary culls. Fecal PCR is valuable but not superior in all low-prevalence settings and is more expensive per sample; it is not 'always preferred' below 10% prevalence. The series testing strategy is the standard of practice for Johne's control programs.

Question 5

A cow-calf producer in a selenium-deficient region is using injectable sodium selenite (0.5 mg Se/lb BW) in neonatal calves to prevent white muscle disease. He now wants to supplement pregnant cows pre-calving to improve passive transfer of selenium via colostrum. He asks about adding selenium to the mineral mix at 120 ppm. Which response is most accurate?

Accepted Answer

This is inappropriate; the legal maximum for selenium in cattle mineral supplements is 120 ppm, but actual intake must not exceed 3 mg Se/head/day, which requires calculating expected mineral consumption to avoid toxicosis

Answer

The FDA-mandated maximum for selenium in mineral supplements is 90 ppm in complete feeds and up to 700 ppm in salt-based mineral mixes (with a label restriction to 0.3 mg/kg of total diet DM), but the critical practical constraint is that total selenium intake must not exceed 3 mg/head/day to avoid selenium toxicosis. At 120 ppm Se in a mineral mix, if cows consume even 50 g of mineral/day they intake 6 mg Se — double the safe limit. Cattle do NOT self-regulate selenium intake (option C is false). Selenium CAN be supplemented via minerals (option D is false). The key is that the producer must calculate actual mineral consumption and adjust selenium concentration accordingly to stay ≤3 mg/head/day — the concentration alone does not guarantee safety.

NAVLE - North American Veterinary Licensing Examination Practice Test

NAVLE - North American Veterinary Licensing Examination Practice Test

NAVLE Bovine Herd Health Management 5