NAVLE Bovine Herd Health 5

Question 1

A dairy herd has an elevated somatic cell count (SCC) averaging 450,000 cells/mL despite a rigorous post-dip protocol and dry cow therapy. Bulk tank cultures repeatedly grow Streptococcus agalactiae. Which intervention is MOST likely to eliminate this pathogen from the herd?

Accepted Answer

Segregate and cull all chronically infected cows, treat all culture-positive cows with intramammary penicillin, and implement herd-level blanket dry cow therapy

Answer

Streptococcus agalactiae is a contagious mastitis pathogen that is obligately associated with the bovine mammary gland. It is exquisitely sensitive to penicillin and can be eradicated from a herd through a combination of treating all infected cows (culture-positive), applying blanket dry cow therapy, segregating or culling chronic cases, and strict milking hygiene. Switching teat dip chemistry (B) addresses teat-end exposure but does not treat existing infections. There is no licensed S. agalactiae bacterin shown to eliminate an active herd infection (C). Selective dry cow therapy (D) would leave infected cows untreated at dry-off, perpetuating the reservoir.

Question 2

You are investigating a cluster of sudden deaths in a cow-calf beef herd on lush spring pasture. Three 6-month-old calves died within 24 hours; post-mortem findings include bloat, hemorrhagic abomasitis, and a characteristic smell of recently mown grass from rumen contents. Clostridium septicum is isolated from the abomasal wall. What is the MOST likely predisposing factor that allowed this organism to become pathogenic?

Accepted Answer

Abomasal mucosal damage from rapid dietary change causing hemorrhagic abomasitis that created an anaerobic microenvironment

Answer

Braxy (caused by Clostridium septicum) classically occurs when sudden ingestion of large quantities of cold, lush, or frosted forage causes abomasal mucosal inflammation and focal necrosis. The resulting damaged, poorly perfused mucosa creates the anaerobic, protein-rich microenvironment that permits C. septicum spores (normally ingested without consequence) to germinate and proliferate. The toxins produced cause local and systemic disease. While copper deficiency (A) and BVDV (D) can cause immunosuppression, they are not the primary predisposing mechanism for braxy specifically. Failure of passive transfer (C) is relevant to neonatal septicemia, not braxy in 6-month-old calves.

Question 3

A commercial feedlot is experiencing a higher-than-expected prevalence of respiratory disease in newly arrived cattle. Bovine Respiratory Disease Complex (BRDC) risk modeling suggests the cattle are high-risk. The veterinarian is evaluating metaphylaxis protocols. Which of the following statements BEST reflects evidence-based practice regarding metaphylactic antimicrobial selection?

Accepted Answer

The antimicrobial selected for metaphylaxis should be a different class from the anticipated treatment antimicrobial to preserve treatment options

Answer

Evidence-based antimicrobial stewardship in feedlot medicine recommends that the drug class used for metaphylaxis (mass treatment at arrival) should differ from the class reserved for treatment of clinical cases. If the same class is used for both, animals that fail metaphylaxis and develop BRD are then treated with a related drug to which the pathogen may already have reduced susceptibility. Using a different class preserves efficacy at the treatment step. Option A overstates tulathromycin's efficacy — a significant proportion of animals still require re-treatment. Option B is not supported by evidence as a primary reason to avoid florfenicol. Option D is misleading: enrofloxacin (a fluoroquinolone) is a critically important antimicrobial class and its use in metaphylaxis is controversial from a stewardship standpoint, and it is not the most effective choice for this reason.

Question 4

In a dairy herd investigation of reproductive failure, you find a 28% first-service conception rate and multiple cows with endometritis diagnosed via cytology (>18% PMNs at 21–33 days post-partum). Mycoplasma bovigenitalium is cultured from uterine swabs of several affected cows. Which of the following is MOST accurate regarding treatment of this herd?

Accepted Answer

Mycoplasma species lack a cell wall and are therefore intrinsically resistant to all beta-lactam antibiotics; systemic fluoroquinolones or tetracyclines are more appropriate, but treatment success is poor

Answer

Mycoplasma spp. lack a peptidoglycan cell wall, which confers intrinsic resistance to all beta-lactam antibiotics (penicillins, cephalosporins, and carbapenems) regardless of generation or spectrum (rules out A and C). Drugs targeting cell wall synthesis have no activity against cell-wall-free organisms. Tetracyclines and fluoroquinolones have in vitro activity, but clinical treatment success for Mycoplasma genital infections in cattle is notoriously poor, often requiring culling of chronically infected animals and biosecurity measures. Prostaglandin (D) supports uterine clearance in bacterial endometritis but does not address Mycoplasma specifically and is not curative.

Question 5

A veterinarian is performing a herd-level investigation on a purebred beef operation with a history of recurrent Theileria orientalis (Ikeda genotype) outbreaks following tick exposure. Several cows in late gestation are showing pallor, jaundice, and decreased milk production. Which hematological finding would BEST differentiate Theileria orientalis hemolytic anemia from iron-deficiency anemia in these animals?

Accepted Answer

Presence of piroplasms within erythrocytes on Giemsa-stained thin blood smear, along with hemolytic indices

Answer

The definitive differentiating finding for Theileria orientalis hemolysis is the identification of intraerythrocytic piroplasms (characteristically appearing as 'signet ring,' 'rod,' or 'maltese cross' forms) on a Giemsa- or Wright-stained thin blood smear, combined with hemolytic indices (hyperbilirubinemia, hemoglobinuria, elevated indirect bilirubin). Iron-deficiency anemia presents as microcytic hypochromic anemia (A) — the opposite pattern to the normocytic or macrocytic regenerative anemia of hemolysis. Both conditions may show regenerative responses (B), making this non-differentiating. Thrombocytopenia (D) may occur in Theileria but is not the primary differentiating criterion — visualization of the parasite is definitive.

NAVLE - North American Veterinary Licensing Examination Practice Test

NAVLE - North American Veterinary Licensing Examination Practice Test

NAVLE Bovine Herd Health 5