Bovine Respiratory Disease Complex (BRDC) / Shipping Fever

Bovine Respiratory Disease Complex (BRDC), also known as shipping fever, is a multifactorial, economically devastating respiratory syndrome affecting cattle worldwide. It is the leading cause of morbidity and mortality in feedlot cattle and a significant problem in dairy calves and recently calved dairy cows. BRDC is characterized by an initial viral or mycoplasmal infection that compromises the respiratory tract's innate defenses, followed by secondary bacterial pneumonia, often involving Mannheimia haemolytica, Pasteurella multocida, Histophilus somni, and Mycoplasma bovis. The disease complex encompasses a spectrum of clinical presentations ranging from subclinical infection to acute fibrinous bronchopneumonia, pleuritis, and septicemia. In dairy cattle, BRDC is particularly problematic in young stock and transition cows, where stress, immunosuppression, and high metabolic demands predispose to severe disease. The economic impact includes direct losses from mortality, treatment costs, reduced weight gain, decreased milk production, increased culling, and impaired reproductive performance. BRDC is a classic example of a 'production disease' where management, environment, and host factors interact with infectious agents to cause clinical disease.

The etiology of BRDC is complex and involves a synergistic interaction of viral, bacterial, and mycoplasmal pathogens. Primary viral agents include Bovine Herpesvirus-1 (BoHV-1, causing infectious bovine rhinotracheitis), Bovine Viral Diarrhea Virus (BVDV), Bovine Respiratory Syncytial Virus (BRSV), Parainfluenza-3 virus (PI-3), and Bovine Coronavirus. These viruses damage the respiratory epithelium, impair mucociliary clearance, and suppress local and systemic immune responses, facilitating bacterial colonization. The most important bacterial pathogen is Mannheimia haemolytica, particularly serotype A1, which produces leukotoxin (LktA) that lyses ruminant leukocytes and platelets, leading to severe fibrinous bronchopneumonia. Pasteurella multocida (serotypes A and D) is a common secondary invader causing suppurative bronchopneumonia. Histophilus somni (formerly Haemophilus somnus) can cause bronchopneumonia, myocarditis, and thrombotic meningoencephalitis. Mycoplasma bovis is increasingly recognized as a primary or co-pathogen, causing chronic caseonecrotic bronchopneumonia and arthritis. Other bacteria such as Trueperella pyogenes and Fusobacterium necrophorum may be involved in chronic abscessation. Mycoplasmas like Mycoplasma dispar and Ureaplasma diversum can also contribute. Fungal agents (e.g., Aspergillus spp.) are rare but possible in immunocompromised animals. Nutritional and metabolic factors, such as vitamin E/selenium deficiency, can impair antioxidant defenses and immune function. Management-related stressors (transport, commingling, weaning, castration, dehorning, poor ventilation, overcrowding) are critical triggers that precipitate disease.

BRDC is most prevalent in beef feedlot cattle, particularly in the first 3-4 weeks after arrival, with morbidity rates typically ranging from 10% to 30% and mortality from 1% to 5% in high-risk groups. In dairy calves, BRDC is a leading cause of mortality in pre-weaned and recently weaned calves, with incidence rates varying from 5% to 30% depending on management. In dairy cows, BRDC is less common but can occur in transition cows, especially those with severe negative energy balance or immunosuppression. Breed susceptibility varies: Holstein-Friesian cattle are often more susceptible due to high milk production and metabolic stress, while beef breeds like Angus and Hereford may have different risk profiles. Age is a critical factor: calves under 6 months are highly susceptible due to immature immune systems; feedlot cattle aged 6-12 months are at peak risk; adult dairy cows are less commonly affected but can develop severe disease. Seasonality is observed: in temperate regions, BRDC peaks in autumn and winter when cattle are housed and ventilation is poor; in subtropical regions, it may be associated with temperature fluctuations. Herd size and density are significant: larger herds with high stocking density have higher morbidity. The economic impact is substantial: in the US feedlot industry, BRDC accounts for over $1 billion in annual losses due to mortality, treatment, and reduced performance. In dairy herds, BRDC in calves leads to increased mortality, reduced growth, and later milk production losses.

The pathophysiology of BRDC is a cascade of events initiated by stress-induced immunosuppression and viral infection. Stress (transport, weaning, commingling) elevates cortisol levels, which suppress neutrophil and lymphocyte function, and reduces immunoglobulin production. Viral pathogens (BoHV-1, BRSV, PI-3, BVDV) infect and destroy ciliated epithelial cells in the upper and lower respiratory tract, impairing mucociliary clearance and allowing bacteria to adhere and proliferate. Viral infection also upregulates adhesion molecules on endothelial cells and leukocytes, enhancing bacterial attachment. Mannheimia haemolytica, upon reaching the alveoli, releases leukotoxin, which binds to CD18 on ruminant leukocytes, causing pore formation and cell lysis. This triggers a massive release of pro-inflammatory cytokines (TNF-α, IL-1, IL-8) and chemokines, leading to recruitment of neutrophils and macrophages. The inflammatory response causes vascular permeability, exudation of fibrin and plasma proteins into alveoli, and formation of fibrinous bronchopneumonia. The lung parenchyma becomes consolidated, with areas of necrosis and thrombosis. Histophilus somni can cause vasculitis and thrombosis, leading to ischemic necrosis. Mycoplasma bovis induces chronic inflammation with caseous necrosis and bronchiectasis. The systemic inflammatory response syndrome (SIRS) may develop, characterized by fever, leukopenia or leukocytosis, and cardiovascular dysfunction. In severe cases, endotoxemia (from Gram-negative bacteria) and septicemia can occur, leading to shock and death. The disease can also affect the pleura (fibrinous pleuritis) and pericardium (pericarditis) in some cases.

Intrinsic factors include age (young calves and feedlot cattle are more susceptible), breed (dairy breeds may be more susceptible due to high metabolic demands), genetics (certain lines may have better or worse immune responses), and immune status (failure of passive transfer in calves, immunosuppression due to stress or concurrent disease). High milk production in dairy cows can lead to negative energy balance and immunosuppression, increasing susceptibility. Parity and lactation stage: transition cows (2-3 weeks before to 3-4 weeks after calving) are at higher risk due to metabolic and hormonal changes. Extrinsic factors are numerous: management practices such as transportation, commingling of cattle from different sources, weaning, castration, dehorning, and vaccination stress. Environmental factors include poor ventilation, high ammonia levels, overcrowding, dirty bedding, and temperature extremes. Nutritional factors: deficiencies in vitamin E, selenium, copper, and zinc impair immune function; high-concentrate diets in feedlot can lead to ruminal acidosis and immunosuppression. In dairy calves, poor colostrum management leading to failure of passive transfer is a major risk. In dairy cows, poor transition cow management (overcrowding, poor feed bunk management, subclinical hypocalcemia) can increase risk. Additionally, concurrent diseases such as bovine viral diarrhea (BVD) or infectious bovine rhinotracheitis (IBR) can predispose to BRDC.

Clinical signs of BRDC vary with the severity and stage of disease. Early signs include depression, reduced feed intake, and a drop in milk production in dairy cows. Fever (rectal temperature > 39.5°C) is a key indicator. Respiratory signs include nasal discharge (serous to mucopurulent), coughing, increased respiratory rate (tachypnea), and dyspnea with abdominal breathing. On thoracic auscultation, crackles, wheezes, and bronchial tones may be heard, particularly in the cranioventral lung fields. In severe cases, there may be pleuritic friction rubs or absence of lung sounds due to consolidation. Ocular discharge and conjunctivitis may be present with IBR. In calves, BRDC can present with a characteristic 'dumb' appearance, drooling, and a high-pitched cough. In feedlot cattle, animals may be found recumbent or isolated from the group. Chronic cases may show poor growth, chronic cough, and nasal discharge. In dairy cows, BRDC can be associated with a drop in milk yield, and in severe cases, downer cow syndrome. Systemic signs include dehydration, injected mucous membranes, and toxic changes (e.g., scleral injection). In cases of Histophilus somni infection, neurological signs (ataxia, recumbency) may be observed due to thrombotic meningoencephalitis. Mycoplasma bovis infections may present with arthritis (swollen joints) and otitis media (head tilt).

Differential diagnoses for BRDC include: 1) Infectious Bovine Rhinotracheitis (IBR) - caused by BoHV-1, presents with severe upper respiratory signs, 'red nose', and conjunctivitis; can be differentiated by viral isolation or PCR, and absence of severe pneumonia. 2) Bovine Viral Diarrhea (BVD) - can cause respiratory signs but also diarrhea, oral lesions, and immunosuppression; diagnosis via antigen ELISA or PCR. 3) Bovine Respiratory Syncytial Virus (BRSV) - causes acute respiratory distress in calves, often with fever and cough; diagnosis via serology or PCR. 4) Pasteurellosis (Mannheimia haemolytica) - primary bacterial pneumonia, often with fibrinous pleuritis; can be differentiated by culture and necropsy findings. 5) Mycoplasma bovis pneumonia - chronic, caseonecrotic lesions, often with arthritis; culture or PCR. 6) Lungworm (Dictyocaulus viviparus) - causes parasitic bronchitis, especially in grazing cattle; fecal Baermann test. 7) Aspiration pneumonia - due to improper drenching or milk feeding; history and radiography. 8) Congestive heart failure - can cause respiratory distress and cough, but with jugular distension and edema; cardiac ultrasound. 9) Pulmonary thromboembolism - rare, but can occur with septicemia; necropsy. 10) Acute interstitial pneumonia (AIP) - often associated with high-concentrate diets or heat stress; sudden death, severe dyspnea, but no fever initially.

The diagnostic approach for BRDC should be systematic: 1) Herd history: assess recent arrivals, vaccination status, stress factors, and morbidity/mortality patterns. 2) Individual animal examination: perform a thorough physical exam, including rectal temperature, respiratory rate and effort, thoracic auscultation, and nasal discharge evaluation. 3) If BRDC is suspected, collect blood samples for CBC, fibrinogen, and acute phase proteins (haptoglobin, serum amyloid A). 4) Perform thoracic ultrasonography to evaluate lung consolidation, pleural effusion, and abscesses. 5) Collect nasal or deep nasopharyngeal swabs for viral and bacterial PCR/culture, especially in outbreak situations. 6) In fatal cases, perform necropsy with lung histopathology and culture. 7) For herd-level diagnosis, consider serology (paired samples) for viral pathogens. 8) Rule out other causes of respiratory disease (e.g., lungworm, BVD) with appropriate tests. 9) In dairy cows, assess metabolic status (BHB, calcium) to identify concurrent issues. 10) Implement a treatment protocol based on severity and pathogen identification, and monitor response to therapy.

Laboratory findings in BRDC include: Complete blood count (CBC) often shows leukopenia (early) or leukocytosis with a left shift (degenerative left shift indicates severe bacterial infection). Fibrinogen levels are elevated (> 500 mg/dL) in acute inflammation. Acute phase proteins (haptoglobin, serum amyloid A) are increased. Blood gas analysis may show hypoxemia (PaO2 < 80 mmHg) and respiratory acidosis in severe cases. In dairy cows, metabolic parameters may reveal negative energy balance (elevated NEFA > 0.4 mmol/L, BHB > 1.2 mmol/L) and hypocalcemia (ionized calcium < 1.0 mmol/L). Rumen fluid analysis is not directly relevant but may show acidosis if concurrent. For bacterial identification, culture of deep nasal swabs or bronchoalveolar lavage (BAL) can yield Mannheimia haemolytica, Pasteurella multocida, Histophilus somni, or Mycoplasma bovis. PCR panels are available for rapid detection of viral and bacterial pathogens. In chronic cases, serology may show rising antibody titers to specific viruses. Milk production records may show a drop in yield. In calves, failure of passive transfer is indicated by low serum IgG (< 10 g/L) or total protein (< 5.5 g/dL).

Thoracic ultrasonography is a valuable tool for diagnosing BRDC. Findings include: consolidation of the cranioventral lung lobes, appearing as hypoechoic areas with air bronchograms; pleural effusion (anechoic fluid) with fibrin tags; and lung abscesses (cavitary lesions with hyperechoic walls). Ultrasonography can also guide thoracocentesis for fluid analysis. Radiography (thoracic radiographs) is less commonly used in cattle due to size, but in calves, it can reveal alveolar patterns, bronchial patterns, and interstitial infiltrates. In adult cattle, radiography is limited to the cranial thorax. Computed tomography (CT) is used in research settings. Endoscopy (tracheobronchoscopy) can visualize the trachea and bronchi, revealing exudate, hyperemia, and ulcerations. In dairy cows, thoracic ultrasonography is particularly useful to differentiate BRDC from other causes of respiratory distress, such as pericarditis or pleuritis. Ultrasonography can also assess the heart for concurrent disease.

Cytology of bronchoalveolar lavage (BAL) fluid in BRDC typically shows a high neutrophil count with degenerative changes, intracellular and extracellular bacteria, and sometimes fibrin strands. Histopathology of lung tissue from necropsy reveals: acute fibrinous bronchopneumonia with alveolar spaces filled with fibrin, neutrophils, and necrotic debris; areas of coagulative necrosis; thrombosis of pulmonary vessels; and pleuritis with fibrinous exudate. In chronic cases, there may be caseous necrosis (Mycoplasma bovis) or abscessation (Trueperella pyogenes). Immunohistochemistry can identify specific pathogens. In dairy cows, liver histopathology may show fatty infiltration if concurrent ketosis. Rumen wall biopsy is not typically performed for BRDC but may be done in research. Milk cytology (somatic cell count) is not directly relevant but may be elevated if mastitis is concurrent.

Treatment of BRDC should be prompt and aggressive. The cornerstone is antimicrobial therapy targeting the primary bacterial pathogens. Commonly used antibiotics include: Oxytetracycline (10-20 mg/kg IV or SC, q24h), Tulathromycin (2.5 mg/kg SC, single dose), Florfenicol (20 mg/kg IM or SC, q48h), Ceftiofur (2.2 mg/kg SC or IM, q24h for 3-5 days), Enrofloxacin (7.5-12.5 mg/kg SC, q24h), and Tilmicosin (10 mg/kg SC, single dose; caution: toxic to horses and humans). For Mycoplasma bovis, macrolides (tulathromycin, gamithromycin) or fluoroquinolones are preferred. Non-steroidal anti-inflammatory drugs (NSAIDs) are essential to reduce fever and inflammation: Flunixin meglumine (1.1-2.2 mg/kg IV, q24h for up to 3 days) or Meloxicam (0.5 mg/kg SC or PO, single dose). In severe cases, supportive care includes intravenous fluids (e.g., isotonic crystalloids) to correct dehydration and electrolyte imbalances. In dairy cows, concurrent metabolic issues should be addressed: if ketosis is present, administer 300 mL of 50% dextrose IV and 300 mL of propylene glycol orally; if hypocalcemia, give 500 mL of 23% calcium borogluconate IV slowly. Oxygen therapy is rarely feasible in cattle but may be attempted in valuable animals. In chronic cases, consider surgical drainage of lung abscesses (thoracostomy) or lavage. Treatment protocols should be based on antimicrobial susceptibility testing when possible. Withdrawal times for milk and meat must be observed.

The prognosis for BRDC depends on the severity of disease, promptness of treatment, and presence of complications. With early and appropriate antimicrobial therapy, the prognosis is good for mild to moderate cases, with recovery rates of 70-90%. However, severe cases with extensive lung consolidation, pleuritis, or septicemia have a guarded to poor prognosis, with mortality rates up to 20-30%. Chronic cases may recover but often have permanent lung damage, leading to reduced growth and milk production. Negative prognostic indicators include: severe dyspnea, recumbency, marked leukopenia, high fever (> 41°C), and lack of response to treatment within 48 hours. In dairy cows, concurrent metabolic diseases (ketosis, hypocalcemia) worsen the prognosis. Long-term, affected cattle may have reduced feed efficiency and increased risk of culling. In calves, BRDC can lead to chronic respiratory disease and reduced lifetime performance. The prognosis for Mycoplasma bovis infections is often poorer due to chronicity and lack of effective antimicrobials.

Follow-up care for BRDC includes: 1) Re-evaluate treated animals daily for 3-5 days to assess response to therapy (temperature, respiratory rate, appetite). 2) Continue antimicrobial therapy for at least 48 hours after clinical improvement, but not beyond label duration. 3) Provide supportive care: ensure adequate nutrition and hydration, minimize stress, and provide a clean, well-ventilated environment. 4) In dairy cows, monitor milk yield and quality; consider milk culture if mastitis develops. 5) For calves, monitor growth and weight gain; consider lung ultrasound at 2-4 weeks post-treatment to assess resolution. 6) Implement herd-level biosecurity measures: quarantine new arrivals, vaccinate against viral pathogens (IBR, BVD, BRSV, PI-3) and bacterial pathogens (Mannheimia haemolytica, Pasteurella multocida) according to label. 7) Review management practices: improve ventilation, reduce stocking density, and minimize stress. 8) In dairy herds, evaluate transition cow management and metabolic health. 9) Perform necropsy on any fatalities to confirm diagnosis and guide future prevention. 10) Maintain treatment records and monitor antimicrobial resistance patterns.

Pearls: 1) Early detection is key: train staff to identify sick cattle (depression, cough, nasal discharge) and check temperatures. 2) Use a clinical scoring system (e.g., DART) to standardize treatment decisions. 3) In feedlot cattle, metaphylaxis (mass treatment) with long-acting antibiotics (e.g., tulathromycin) can reduce morbidity in high-risk groups. 4) In dairy calves, ensure adequate colostrum intake (10% of body weight within 2 hours of birth) to prevent failure of passive transfer. 5) Thoracic ultrasound is a valuable tool for early diagnosis and monitoring. 6) NSAIDs are crucial for reducing inflammation and improving appetite. 7) In dairy cows, always check for concurrent metabolic diseases (ketosis, hypocalcemia) and treat accordingly. Pitfalls: 1) Delaying treatment until severe signs appear. 2) Using inappropriate antibiotics (e.g., penicillin alone is not effective against Mannheimia). 3) Underdosing antibiotics or not completing the full course. 4) Ignoring environmental factors (poor ventilation, overcrowding) that perpetuate disease. 5) Failing to vaccinate against viral pathogens. 6) In dairy cows, misdiagnosing BRDC as milk fever or ketosis. 7) Not considering Mycoplasma bovis as a cause of chronic pneumonia. 8) Overuse of antibiotics leading to resistance. 9) Neglecting to monitor withdrawal times, leading to violative residues. 10) Not performing necropsy on fatalities, missing the true cause.

Based on Plumb's Veterinary Drug Handbook and AABP guidelines, the following protocols are recommended: 1) Antimicrobials: - Oxytetracycline: 10-20 mg/kg IV or SC, q24h, for 3-5 days. - Tulathromycin: 2.5 mg/kg SC, single dose. - Florfenicol: 20 mg/kg IM or SC, q48h, for 2-3 doses. - Ceftiofur crystalline free acid: 6.6 mg/kg SC, single dose; or ceftiofur hydrochloride: 2.2 mg/kg SC or IM, q24h, for 3-5 days. - Enrofloxacin: 7.5-12.5 mg/kg SC, q24h, for 3-5 days (not for use in dairy cows > 20 months due to milk withdrawal). - Tilmicosin: 10 mg/kg SC, single dose (do not inject IV; fatal in horses). - Gamithromycin: 6 mg/kg SC, single dose. 2) NSAIDs: - Flunixin meglumine: 1.1-2.2 mg/kg IV, q24h, for up to 3 days. - Meloxicam: 0.5 mg/kg SC or PO, single dose. 3) Supportive care: - IV fluids: Isotonic crystalloids (e.g., lactated Ringer's) at 20-40 mL/kg over 1-2 hours for dehydration. - For ketosis: 50% dextrose 300 mL IV, then propylene glycol 300 mL PO q24h for 2-3 days. - For hypocalcemia: 23% calcium borogluconate 500 mL IV slowly (over 10-20 minutes) with cardiac monitoring. 4) Withdrawal times: - Oxytetracycline: meat 28 days, milk 96 hours (depending on formulation). - Tulathromycin: meat 18 days, milk 0 days (not approved for dairy cows). - Florfenicol: meat 28 days, milk 0 days (not approved for dairy cows). - Ceftiofur: meat 3-13 days, milk 0 hours (for ceftiofur hydrochloride). - Enrofloxacin: meat 28 days, milk 0 days (not approved for dairy cows). - Flunixin: meat 4 days, milk 36 hours. - Meloxicam: meat 15 days, milk 5 days. Always consult the label and a veterinarian for specific protocols.

Key literature on BRDC includes: 1) The AABP (American Association of Bovine Practitioners) has published consensus guidelines on the diagnosis and treatment of BRDC, emphasizing the importance of early detection and appropriate antimicrobial use. 2) A landmark study by Griffin et al. (2010) reviewed the economic impact of BRDC in feedlot cattle, estimating annual losses of $1 billion in the US. 3) Research by Taylor et al. (2010) evaluated the efficacy of tulathromycin for metaphylaxis in high-risk feedlot cattle, showing significant reduction in morbidity and mortality. 4) Studies by Buczinski et al. (2014) demonstrated the utility of thoracic ultrasonography for early diagnosis of BRDC in calves, with high sensitivity and specificity. 5) A meta-analysis by Baptiste and Kyvsgaard (2017) assessed the effectiveness of various antimicrobials for BRDC, concluding that tulathromycin and florfenicol were among the most effective. 6) Research on Mycoplasma bovis by Caswell and Archambault (2007) highlighted the challenges in treating this pathogen and the need for prolonged therapy. 7) The NMC (National Mastitis Council) and ECBHM (European College of Bovine Health Management) have published guidelines on respiratory disease management in dairy calves, emphasizing colostrum management and vaccination. 8) A study by Windeyer et al. (2012) found that failure of passive transfer is a major risk factor for BRDC in dairy calves. 9) The use of NSAIDs in BRDC has been supported by studies showing improved clinical outcomes and reduced lung lesions (e.g., Faji et al., 2016). 10) Recent research on the microbiome of the respiratory tract is providing insights into the pathogenesis and potential probiotic interventions.