NAVLE Bovine Herd Health 4

Question 1

A beef cow herd in the northern Great Plains experiences a cluster of late-term abortions in January. Fetuses are autolyzed, placentas are retained, and Brucella testing is negative. PCR on placental tissue detects Neospora caninum. Which epidemiological pattern would most strongly support vertical (transplacental) rather than horizontal (postnatal oral) transmission as the dominant route in this herd?

Accepted Answer

Abortion rate significantly higher in daughters of seropositive dams compared to daughters of seronegative dams, regardless of exposure to definitive hosts

Answer

Neospora caninum is transmitted both vertically (cow to calf in utero) and horizontally (ingestion of oocysts shed by definitive hosts — canids). The hallmark of vertical transmission dominance is a familial clustering pattern: daughters of seropositive dams have dramatically higher seroprevalence and abortion risk independent of canid exposure. This dam-to-offspring clustering across generations is the epidemiological fingerprint of congenital transmission. Temporal clustering around a feed change or purchase event would suggest horizontal exposure, and removal of definitive hosts would reduce horizontal but not vertical risk.

Question 2

A 450 kg dairy cow at 10 days postpartum develops acute septic metritis with a fetid red-brown uterine discharge, fever of 40.8°C, and milk drop. Culture yields a pure growth of Trueperella pyogenes. Her BCS at dry-off was 4.0/5.0, and she had a twins delivery. You elect to treat with intrauterine cephapirin. Which additional systemic treatment is MOST pharmacologically justified given the likely concurrent metabolic status of this cow?

Accepted Answer

Propylene glycol oral drench to address subclinical ketosis compounding immunosuppression

Answer

This cow is at high risk for subclinical or clinical ketosis: she was over-conditioned at dry-off (BCS 4.0/5.0) and carried twins, both major risk factors for negative energy balance postpartum. Ketosis causes significant immunosuppression — neutrophil function is directly impaired by elevated BHBA — which underlies her susceptibility to metritis. Propylene glycol drenching addresses this by serving as a gluconeogenic precursor to restore energy balance and normalize immune function. Flunixin is reasonable for pain and anti-endotoxin effects but does not address the root immunosuppression. Oxytocin has limited efficacy beyond 48 hours post-partum once receptors downregulate. Dexamethasone is contraindicated as it will further suppress immunity.

Question 3

During a herd health visit to a 300-cow cow-calf operation, you review the spring vaccination protocol. Calves receive modified-live virus (MLV) BVDV vaccine at branding (approximately 3 months of age). Several producers in the region report that calves vaccinated with MLV-BVDV at this age occasionally fail to seroconvert. Which mechanism BEST explains this vaccine failure in the context of herd health?

Accepted Answer

Colostral maternal antibody interference neutralizing vaccine virus before an immune response is mounted

Answer

Maternal antibody interference is the classic and most well-documented explanation for MLV-BVDV vaccine failure at branding age. Calves acquire passive immunity through colostrum, and maternally derived antibodies (MDA) to BVDV can persist for 3–6 months or longer depending on colostrum quality and dam titer. These MDAs neutralize the attenuated vaccine virus before the calf's own immune system can mount a primary response, resulting in seronegative calves despite vaccination. This is why some protocols recommend delaying BVDV vaccination until weaning (~6–7 months) or using killed vaccine initially. PI calves shedding high-titer wild-type BVDV can cause immunotolerance in herd-mates but do not mechanistically interfere with vaccine virus the same way MDA does.

Question 4

A feedlot veterinarian is evaluating a pen of 200 steers. Over 10 days, 18 steers have been pulled and treated for bovine respiratory disease (BRD). The pen morbidity is 9%. The consulting nutritionist notes the ration has a high silage inclusion with a low-quality corn silage (pH 4.9, but ammonia-N/total-N ratio = 28%). Which consequence of this silage quality is MOST likely to be contributing to the BRD outbreak?

Accepted Answer

Reduced dry matter intake secondary to poor palatability, causing negative energy balance and impaired pulmonary macrophage function

Answer

An ammonia-N/total-N ratio >15% indicates poorly fermented (clostridial or secondary fermentation) silage with significant protein degradation to non-protein nitrogen. The most direct herd health consequence is severe palatability reduction, causing steers to eat far below their energy requirements. This negative energy balance directly impairs pulmonary alveolar macrophage function and neutrophil activity, dramatically increasing susceptibility to the Mannheimia-Pasteurella-Histophilus complex driving BRD. High butyric acid is also a marker of clostridial silage but is less directly linked to respiratory immunity than energy balance. Mycotoxins are associated with Fusarium contamination, not clostridial fermentation. Ammonia absorption occurs but hepatic urea cycle capacity typically prevents acute neutrophil dysfunction at silage ammonia levels.

Question 5

A herd of 120 commercial beef cows has a pregnancy rate of 78% after a 63-day breeding season using natural service at a 1:25 bull-to-cow ratio. Breeding soundness evaluations (BSE) were performed on all bulls 45 days before turnout and all passed. Cow body condition averaged 5.5/9 at breeding. Progesterone assays on 20 randomly selected non-pregnant cows at preg-check reveal 14 of 20 have luteal-phase progesterone levels (>1 ng/mL). What is the MOST likely diagnosis for the reproductive failure in these cows?

Accepted Answer

Embryonic loss between days 16–45 of gestation after normal conception

Answer

The key finding is that 14/20 non-pregnant cows (70%) have luteal-phase progesterone at preg-check, indicating they were cycling and had been bred (they are in a luteal phase cycle, not anestrus), yet failed to maintain pregnancy. BSE-passed bulls in adequate numbers with cycling cows of good BCS points toward the problem occurring AFTER fertilization — early embryonic or fetal loss between days 16–45, after maternal recognition of pregnancy would have been expected but before the conceptus reached a size detectable at preg-check. BVDV is plausible but would require more evidence. Subclinical endometritis could cause repeat breeding but would more typically present with cows that cycle back quickly without sustained luteal activity. The high proportion with active corpora lutea suggests conception occurred but embryos were lost before or around preg-check, and they've cycled back.

NAVLE - North American Veterinary Licensing Examination Practice Test

NAVLE - North American Veterinary Licensing Examination Practice Test

NAVLE Bovine Herd Health 4