Bovine Respiratory Disease Complex: Infectious causes

By Dr B.B. Van Houten,
Technical Manager: Zoetis South Africa (Pty) Ltd

Bovine respiratory disease (BRD) is a broad definition of the complex disease process which is the single most important animal health factor affecting the health and profitability of cattle operations around the world.1 It is estimated that BRD costs the USA beef industry $ 1 billion annually due to; treatment and labour costs, production losses and death.2 There are numerous factors which predispose animals to BRD and preventative measures need to be taken to minimise their effect.

These factors involved include; management, stress (including environmental conditions), nutrition as well as viral, bacterial and parasitic infections.2 Unfortunately, no method or strategy can completely prevent BRD occurring and treatment may be necessary in certain cases. The goal of any preventative program needs to be to reduce its occurrence to an economically viable level. Prevention is always better than cure. This article will discuss the various factors, placing emphasis on the effect of viral and bacterial pathogens and their prevention.

Management
Every factor contributing to BRD in a feedlot or cattle operation hinges on the managerial decisions taken. Over and above that, training and staff management play a vital role in timeous detection and treatment of sick animals.

Stress
All animals in an intensive system, such as a dairy or feedlot operation, have massive amounts of stress placed on them. This can be in the form of environmental stress (heat, cold, dust etc.), social stress (comingling of animals of different sizes and origins) as well as the stress of production. This all has a depressive effect on the animals’ immune system and predisposes them to viral and secondary bacterial infections. Viruses will colonise and start replicating in the upper respiratory tract, damaging the ciliated epithelium of the respiratory tract and depress the ability of the physical immune system to clear infectious agents.3

All this leaves the animal extremely susceptible to a secondary bacterial infections. The bacterial infection is where the serious damage is done to the lower respiratory tract, and if left untreated, very often results in death.

Nutrition
Adequate nutrition has a large impact on the animal’s wellbeing and ability to produce. Deficiencies in macro and micro nutrients affect the body’s ability to fight disease. Their immune systems are dependent on vitamins and minerals obtained from feed to function properly. Copper, zinc and selenium are just a few minerals which are essential in terms of immune system function. Their roles are extremely complex and factors such as deficiencies and presence of antagonists in the feed play a role. The above contributing factors have an effect on the immune system’s function in one way or another, allowing the viral and bacterial pathogens involved in bovine respiratory disease to strike.

Pathogens involved in bovine respiratory disease:

Viruses:

Bovine herpes virus (IBR)-Infectious bovine rhinotracheitis: Uncomplicated cases usually show inflammation of the upper respiratory tract with a clear ocular and nasal discharge. Animals are also depressed and anorexic with a harsh cough and accompanying fever. Animals usually recover within 4- 5 days without secondary bacterial infection.4 Transmission occurs via aerosol droplets and the virus is shed for around 7- 10 days post infection, after which the infection establishes lifelong latency.5 This virus can also affect the reproductive tract of cows/ heifers and cause abortions.

Bovine viral diarrhoea virus (BVDV)6 : is an extremely important virus in any cattle production system. It affects the reproductive performance (early embryonic death, abortions and congenital defects of calves) in cows and heifers in beef and dairy operations countrywide. It also results in the birth of persistently infected (PI) animals when the mothers are infected with BVDV and there is in utero exposure to a noncytopathic (ncp) biotype prior to development of a competent foetal immune system. This cut off time is said to vary between 90 and 125 days.8 These animals constantly spread the virus within the herd or feedlot via body fluids and discharges, including the faeces.

The importance of BVDV in terms of BRD can be seen in multiple areas including;7

• The possibility of persistently infected animals being introduced into a herd, where constant spread of the virus will occur predisposing multiple animals to acute infection which results in subsequent immune suppression;

• PI’s can have reduced growth, production and a generally increased susceptibility to disease, as well as the possibility of succumbing to the inevitably fatal, Mucosal Disease;

• Research has shown that animals exposed to BVDV at any point of gestation are more at risk of developing respiratory disease later on in life, not only PI’s.

Bovine respiratory syncytial virus (BRSV):4 is a common respiratory pathogen isolated from respiratory outbreaks in dairy animals and beef calves. Transmission occurs via aerosol spread of the virus. It is usually seen in nutritionally compromised animals and young animals that have never been exposed before. These animals often have severe respiratory distress, standing with their head and necks extended, struggling to breath. On post mortem, gross lesions include a diffuse interstitial pneumonia with sub pleural and interstitial emphysema along with interstitial oedema. In older animals you often see a mild form of the disease.

Parainfluenza virus (PI3):4 is spread via ocular and nasal discharge and infects the epithelial cells of the lower respiratory tract, specifically, the trachea, bronchi and alveoli. Infection usually results in sub-clinical and unapparent infections but as with any of the respiratory infections discussed, a complicated infection with the other viruses or bacteria results in synergism towards serious disease.

As previously mentioned, most of the viruses mentioned above affect the immune system function, however a lot of their damage comes from their replication in epithelial cells and destruction of the muco-cillary clearance mechanism, which is essential for the removal of dust and infectious agent laden mucous in the respiratory tract.

Bacteria:2

Mannheimia haemolytica: previously known as Pasteurella haemolytica, is considered the predominant bacterial pathogen associated with BRD. This bacteria is normally found in the upper respiratory tract of healthy animals, but as the less pathogenic strain A2. It is suggested that Mannheimia haemolytica maintains a commensal relationship with the host until conditions change as a result of predisposing factors such as a viral infection or stress. Once this commensal relationship is disrupted, the A1 serotype quickly becomes the predominate organism and is responsible for characteristic bronchopneumonia. Type A1 is most frequently isolated from pneumonic tissue but other strains have been isolated, such as A6.2 Type A1 and A6 are very similar except for differences in capsule structure.2 A functioning muco-cillary clearance mechanism would usually help assist in clearing some of this bacteria (together with dust etc…) from the lungs in healthy animals.

There are multiple virulence factors involved with a M. haemolytica infection. Initially, the bacteria need to adhere to and colonise the lungs, they utilize the capsule and adhesins to accomplish this process. Neuramidase produced by the bacteria reduces the viscosity of the respiratory mucous and allows access to the lung cell surface. After colonisation, M. haemolytica starts to produce lipopolysaccharides (LPS) and leukotoxins. These are the most important substances involved in subsequent lung damage. LPS causes hemorrhage, oedema, hypoxemia, and acute inflammation, while leukotoxin is responsible for lysis of leukocytes (release of oxygen-derived free radicals and proteolytic enzymes) and platelets.10

The subsequent pneumonia results in severe respiratory distress, depression, anorexia and possibly death. On post mortem, damage is seen as a fibrinous bronchopneumonia in the cranio-ventral area of the lungs.

Other bacterial pathogens implicated in BRD include; Pasteurella multocida, Histophilus somni and Mycoplasma bovis. These bacteria have all been implicated as the sole pathogen in cases of BRD; however the severity of their infections is elevated substantially in combination with M. haemolytica. An important note to remember is that all these infections can look identical on post-mortem and need to be confirmed with laboratory tests. M. bovis can sometimes be differentiated on the basis of the presence of caseo-necrotic foci in the lungs. These lesions can vary from a few millimeters to a few centimeters in diameter and can easily be confused with lung abscesses.

Prevention:

Vaccination is the most cost effective way to try and help prevent the effects of viruses and bacteria within any operation. As mentioned earlier, no vaccination program is perfect. This is due to the varying response of animals to vaccination as well as other external factors. In the past we have had very little ability to effectively vaccinate young animals soon after birth, that all changed when INFORCE™ 3 (Reg. No. G4044 Act 36/1947) was launched last year. Dairy/beef farms now have the option of vaccinating animals from as young as 1 day of age.

This stimulates the production of interferon and immunoglobulin A (Ig A), providing very quick and effective local protection against IBR, PI3 and BRSV. They can then receive another vaccination of INFORCE™ 3 prior to weaning. Calves should then receive two vaccinations of a 4 way modified live vaccine, 3-4 weeks apart from as early as 3 months of age. This is to stimulate the production of systemic Ig G after maternally derived antibodies have begun subsiding below protective levels. Consider a product like Bovishield® Gold (Reg. no. G3675 (Act36/1947)).

For effective protection against M. haemolytica, consider One Shot Ultra™ 7 (Reg. no. G2818 (Act 36/1947)). One Shot Ultra™ 7 contains cellular components (bacterins) which would aid in the prevention of the initial colonisation and replication, and a leukotoxoid component for the protection against excessive lung damage caused by leukotoxins.

The Rationale for using a product with more than one M. haemolytica component to aid in prevention is clear. It also gives protection against most of the important Clostridial diseases in South Africa and is also registered for use in sheep.

Metaphylactic antibiotics can also go a long way in preventing BRD around high risk times. Weaning for dairy and beef calves are such times. Consider highly effective long acting antimicrobial products like Draxxin® (Reg. no. 05/21.1/2. (Act 101/1965)) and Excede® Sterile Suspension (Reg. no. 10/15.1.1.2/01 (Act 101/1965)) in these situations. In the prevention of BRD it is important to remember that for the most effective prevention possible, all the contributing factors need to be handled and dealt with. The chain is only as strong as the weakest link.

References:

1. M. J. Schneider, R. G. Tait Jr., W. D. Busby, and J. M. Reecy. An evaluation of bovine respiratory disease complex in feedlot cattle: Impact on performance and carcass traits using treatment records and lung lesion scores. J. Anim. Sci. 2009. 87:1821–1827.

2. Dee Griffin, DVM, M.M. Chengappa, Jennifer Kuszak, D. Scott McVey. Bacterial Pathogens of the Bovine Respiratory Disease Complex. Vet Clin Food Anim 26 (2010) 381–394.

3. Christopher Antony Paul Carrington. The role of mycoplasma species in bovine respiratory disease complex in feedlot cattle in South Africa. Submitted in partial fulfillment of the requirements for the degree M.Med.Vet (Med)(Bov). 2007.

4. Merck Veterinary Manual. Viral Respiratory Tract Infections in Cattle. Last full review/revision March 2015 by John Campbell, DVM, DVSc. Accessed on the 6th May 2016.

5. Clinton Jones and Shafiqul Chowdhury. Bovine Herpesvirus Type 1 (BHV-1) is an Important Cofactor in the Bovine Respiratory Disease Complex. Veterinary Clinics of North America Food Animal Practice · July 2010.

6. Merck Veterinary Manual. Bovine viral diarrhea and mucosal disease complex .Last full review/revision August 2014 by Walter Gruenberg, DrMedVet, MS, PhD, DECAR, DECBH. . Accessed on the 6th May 2016.

7. Julia Ridpath. The contribution of Infectious Bovine Viral Diarrhea Viruses to Bovine Respiratory Disease. Vet Clin Food Anim 26 (2010) 335 – 348.

8. R.L. Larson. Bovine Viral Diarrhea (BVD): Review for beef cattle veterinarians. The Bovine practitioner. Vol. 38 No. 1. (2004) 93 – 102.

9. Bruce W. Brodersen. Bovine Respiratory Syncytial Virus. Vet Clin Food Anim 26 (2010) 323–333

10. S.K. Maheswaran. Effects of Pasteurella haemolytica A 1 leukotoxin on bovine neutrophils: degranulation and generation of oxygen-derived free radicals. Veterinary Immunology and Immunopathology, 33 ( 1992 ) 51-68.

One Shot Ultra™ 7: Reg. No.: G2818 (Act 36/1947). Cl. chauvoei, Cl. septicum, Cl. novyi, Cl. sordellii, Cl. perfringens Type C & D, Mannheimia haemolytica bacterin-toxoid.

Inforce™ 3: Reg. No.: G4044 (Act 36/1947). Infectious bovine rhinotracheitis (IBR), bovine respiratory syncytial virus (BRSV), parainfluenza type 3 virus (PI3).

Bovi-Shield® Gold 5: Reg. No.: G3675 (Act 36/1947). BVD Type 1 & 2 (Bovine viral diarrhea virus), IBR (Infectious bovine rhinotracheitis), BRSV (Bovine respiratory syncytial virus), PI3 (Para-influenza 3).

Zoetis South Africa (Pty) Ltd (Co.Reg. No.2012/001825 /07). PO Box 783720, Sandton, 2146. Tel: (011) 245 – 3300. 0860 ZOETIS (963847). www.zoetis.co.za

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