NAVLE Respiratory · ⏱ 10 min read · 📅 Mar 28, 2026 · by NAVLE Exam Prep Team · 👁 1

Bovine Respiratory Disease (BRD): NAVLE Study Guide

BRD is the most economically important disease in the beef cattle industry and one of the highest-yield topics on the NAVLE. Know the pathogens, know the scoring system, know when to use tulathromycin versus florfenicol, and know what metaphylaxis means. That's most of what the exam tests here.

The disease is a synergistic process. Viral pathogens hit first — damaging the mucociliary escalator, suppressing local immunity, and priming the lung for bacterial invasion. The bacteria that follow are the ones that kill. Understanding that two-step sequence explains most of the clinical picture and most of the exam questions.

The Pathogens

BRD involves a predictable cast of bacterial and viral agents. The exam expects you to know each one's specific niche in the disease complex.

Pathogen	Type	Role in BRD	NAVLE Clue
Mannheimia haemolytica A1	Bacterial	Primary bacterial killer; shipping fever	Leukotoxin + cranioventral fibrinonecrotic pneumonia
Pasteurella multocida	Bacterial	Secondary invader; milder fibrinopurulent pneumonia	Less fibrin, less necrosis than Mannheimia
Histophilus somni	Bacterial	Vasculitis → TME + myocarditis + pleuritis	Neurological signs + chest = think H. somni
Mycoplasma bovis	Bacterial	Chronic caseonecrotic pneumonia + arthritis + otitis	No cell wall — beta-lactams don't work
BHV-1 (IBR)	Viral	Upper respiratory viral primer; latency in trigeminal ganglia	"Red nose," nasal plaques, abortion after reactivation
BRSV	Viral	Severe interstitial pneumonia in calves <6 months	Subcutaneous emphysema + syncytial cells on histo
BVDV	Viral	Immunosuppression; no specific respiratory lesion alone	PI cattle are persistent shedders; erosive mucosal lesions
Parainfluenza-3 (PI-3)	Viral	Mild disease alone; sets up bacterial secondary invasion	Damages mucociliary escalator — key primer

NAVLE PearlMannheimia haemolytica serotype A1 is the most commonly isolated bacterial pathogen from feedlot shipping fever deaths. When you see "recently weaned calves, 7–14 days post-arrival, cranioventral fibrinonecrotic pneumonia," that's your answer.

Pathogenesis: Why Shipping Fever Kills

Calves arrive already stressed from weaning, transport, and commingling. Stress elevates cortisol, which hammers neutrophil and macrophage function. Viral pathogens (most commonly PI-3, BRSV, or BHV-1) damage the respiratory epithelium and further suppress local defenses. This leaves the lower airway open to colonization by Mannheimia haemolytica, which normally lives as a commensal in the upper respiratory tract.

Once Mannheimia reaches the lung, it produces leukotoxin (LktA) — an RTX-family pore-forming toxin specific to bovine ruminant leukocytes. At high concentrations, LktA lyses neutrophils and alveolar macrophages. Those dying cells dump lysosomal enzymes and reactive oxygen species into lung tissue. The result is the classic pathology: cranioventral fibrinonecrotic bronchopneumonia with distended, gelatinous interlobular septa and fibrinous pleuritis.

NAVLE TipThe exam often tests leukotoxin mechanism directly. LktA kills bovine neutrophils and macrophages by pore formation at high concentrations. The result is impaired bacterial clearance plus tissue destruction from leukocyte contents. Vaccines that target leukotoxin (like those including M. haemolytica bacterin) can reduce disease severity by about 50–70%.

BRD Scoring: The DART System

BRD scoring systems allow pen riders to flag cattle for treatment without needing a thermometer on every animal. The most commonly referenced system on the NAVLE is based on scoring depression, appetite, respiratory signs, and temperature — often remembered as DART.

Sign	Score 0	Score 1	Score 2
Depression	Alert, responsive	Mildly dull	Recumbent/unresponsive
Appetite	Normal	Off feed slightly	Not eating
Respiratory	Normal rate/effort	Increased RR or cough	Labored, open-mouth
Temperature	<39.5°C (103.1°F)	39.5–40.0°C	≥40.0°C (≥104°F)
Total score ≥4 → pull and treat. Score 5–8 → high severity, consider additional NSAIDs.

No single parameter has adequate sensitivity. The combination of depression + fever ≥40°C + abnormal respiration gives the best sensitivity for catching BRD in pen-riding scenarios. The exam has tested this directly — single-sign scoring misses too many cases.

Classic NAVLE TrapThe question asks which single sign has the highest sensitivity for BRD. None of them do — single-sign scoring is the wrong approach. The correct answer is always a combination of signs: depression + elevated temperature + respiratory abnormality.

Antibiotic Selection

The exam tests antibiotic choices for both treatment and metaphylaxis. Know the drug classes, their mechanisms, and when each one is appropriate.

Drug	Class	Mechanism	Route / Dosing	Notes
Tulathromycin	Macrolide (triamilide)	50S ribosomal inhibition	2.5 mg/kg SC, single dose	#1 for metaphylaxis; long t½; 18-day WDT
Florfenicol	Phenicol	50S ribosomal inhibition (bacteriostatic)	40 mg/kg SC single, or 20 mg/kg IM q48h	Active vs. macrolide-resistant strains; 38-day WDT
Enrofloxacin	Fluoroquinolone	DNA gyrase inhibition (bactericidal)	7.5–12.5 mg/kg SC once	Critically important antimicrobial — use judiciously
Ceftiofur (CCFA)	3rd-gen cephalosporin	Cell wall synthesis inhibition (bactericidal)	6.6 mg/kg SC single (CCFA)	Use when macrolide resistance documented
Gamithromycin	Macrolide	50S ribosomal inhibition	6 mg/kg SC single	Cross-resistance with tulathromycin expected
Oxytetracycline	Tetracycline	30S ribosomal inhibition	11 mg/kg IV/IM	Older option; increasing resistance; oral form unreliable in ruminants

NAVLE PearlMacrolide resistance is class-wide. If tulathromycin fails and culture shows resistance, do not switch to gamithromycin or tilmicosin — they will fail too. Switch to a different class: ceftiofur (CCFA) is the standard next-line choice for BRD after macrolide failure.

Metaphylaxis

Metaphylaxis means treating an entire group of high-risk animals at arrival, before most of them develop clinical disease. It is not prophylaxis — it targets animals already likely incubating infection. The classic population: newly arrived feedlot calves that are recently weaned, transported long distances, commingled from multiple sources, and not preconditioned.

Tulathromycin is the agent of choice by network meta-analysis. Single SC injection at 2.5 mg/kg. Its long half-life gives sustained tissue concentrations across the highest-risk period (first 45 days post-arrival). Gamithromycin is an acceptable alternative. Oral medications (oxytetracycline in feed or water) perform poorly in this context because sick animals don't eat or drink, and ruminant oral bioavailability for tetracyclines is poor.

NAVLE TipVaccinating at feedlot arrival is less effective than pre-weaning vaccination because calves arrive during peak stress-induced immunosuppression. The cortisol from weaning and transport blunts the immune response. Pre-weaning vaccination 2–4 weeks before stress events is the right timing. The exam tests this often.

Distinguishing the BRD Pathogens on the NAVLE

Most BRD questions give you a signalment and a cluster of findings. Here's how to rapidly sort them:

Mannheimia haemolytica: 7–14 days post-arrival. Acute, high fever (40.5–41.5°C). Cranioventral consolidation with marbled interlobular septa. Fibrinous pleuritis. The leukotoxin question answers itself.

Histophilus somni: 40–60 days post-arrival. Watch for the multi-organ hit: bronchopneumonia plus myocarditis plus thrombotic meningoencephalitis (ataxia, blindness, opisthotonus). Fibrinous pleuritis over relatively normal lung. Vasculitis is the mechanism — not leukotoxin, but direct endothelial adhesion and thrombosis.

Mycoplasma bovis: Chronic, relapsing. Resistant to multiple antibiotics. The classic triad is caseonecrotic pneumonia + polyarthritis + otitis media. Round, white caseous foci in the lung at necropsy. No cell wall = beta-lactams are useless. Macrolides and fluoroquinolones have the best activity, though resistance is rising. PCR from deep nasopharyngeal swab is the fastest diagnosis.

BRSV: Calves under 6 months. Severe interstitial pneumonia, subcutaneous emphysema, failure of lungs to collapse at necropsy, rubbery texture. Syncytial cells on histopathology. Inactivated vaccines can cause vaccine-enhanced disease through a Th2-biased response on subsequent natural infection — the exam has asked about this.

BHV-1 (IBR): Upper respiratory, not lower. "Red nose" — inflamed, crusty nares. Nasal mucosal plaques (necrosis). Conjunctivitis. High fever. Latency in trigeminal ganglia — stress reactivates it. BHV-1 in a pregnant cow = abortion risk 15–60 days later. IBR seropositive animals shed virus on stress even without clinical signs.

Classic NAVLE TrapA question gives you a calf with respiratory disease, head tilt, and circling. Don't jump to Listeria — that's ruminant listeriosis (brainstem microabscesses). If there's a recent history of respiratory disease or feedlot arrival + neurological signs + possible myocarditis, the answer is Histophilus somni causing TME.

Diagnostics

Culture from nasal swabs is not specific — Mannheimia, Pasteurella, and Histophilus all live in the upper respiratory tract of healthy cattle. Transtracheal wash (TTW) or bronchoalveolar lavage (BAL) samples the lower airway directly and is the most specific antemortem bacterial culture technique.

For rapid multi-pathogen ID, multiplex PCR panels beat everything else. One nasopharyngeal sample, same-day results, identifies viral and bacterial agents simultaneously. Paired serology (0 and 21 days) is retrospective — useless for acute management, useful for outbreak investigation.

Thoracic ultrasound shows lung consolidation as hypoechoic, hepatized tissue replacing the normally echogenic, artifact-rich aerated lung. Useful for following response to treatment and identifying chronic non-responders.

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Practice Questions

Test yourself before moving on. Click an answer to reveal the explanation.

Question 1 A 350-kg beef steer develops acute fever (41°C), mucopurulent nasal discharge, and labored breathing 10 days after arriving at a feedlot. Necropsy reveals cranioventral fibrinonecrotic bronchopneumonia with distended, gelatinous interlobular septa and fibrinous pleuritis. Which bacterial pathogen is most likely responsible?

Question 2 Which virulence factor produced by Mannheimia haemolytica is primarily responsible for severe pulmonary tissue destruction in bovine respiratory disease?

Question 3 A feedlot veterinarian receives 400 high-risk beef calves — recently weaned, transported 18 hours, commingled from multiple sources, not preconditioned. Which metaphylactic strategy is most strongly supported by evidence to reduce BRD morbidity in the first 45 days post-arrival?

Question 4 A pen rider scores a feedlot calf using a BRD scoring system and records: marked depression (2), completely off feed (2), labored breathing with expiratory grunt (2), rectal temperature 41.2°C (2) — total score 8/8. What is the appropriate immediate action?

Question 5 A feedlot calf treated with tulathromycin initially improves, then relapses 3 weeks later with fever and respiratory signs. Transtracheal wash culture yields Mannheimia haemolytica with documented resistance to tulathromycin, gamithromycin, and tilmicosin. What is the most appropriate treatment?