Enzootic pneumonia of calves

30/08/2022

Pneumonias of the calf characterized by slight tissue destruction have been variously termed pneumoenteritis,1 enzootic bronchopneumonia,2 enzootic pneumonia,3 cuffing or atypical pneumonia,4 pneumonitis,5 and virus pneumonia 6; considerable variations in the pathology and other aspects of the disease have been recorded. Possible multiplicity of aetiological agents7 and the influence of extrinsic factors may be responsible.* A satisfactory terminology still awaits conclusive elucidation of the causative factors. In recent years the authors have noted the widespread occurrence in this country of a distinctive entity in bovines, in which pulmonary and, to a lesser extent, intestinal involvement are cardinal features. Proliferative cellular inflammatory phenomena of a chronic nature, rather than exudation and tissue destruction, predominate. The complex may easily be overlooked, as appreciable lesions are frequently lacking and clinical symptoms usually assume an insidious character.

05/03/2022

Calf rearing, calf health tips, calf rearing tips. Pneumonia in calves

05/03/2022

Treating a Sick Calf (Pneumonia)

05/03/2022

Just a brief description of how I treat and prevent calf pneumonia. The antibiotic we use for pnemonia is called Resflor, it's expensive but works and works quickly. For other viral infections I

07/08/2021

An outbreak of pneumonia associated with bovine respiratory syncytial virus occurred in July 2012 in a calf-rearing farm in Flores County is described. Morbidity and mortality were 6.71% and 6.57% respectively. Main clinical symptoms were cough, dyspnea, tachypnea, nasal and ocular discharge and fever. Necropsy was performed in two animals which died from respiratory failure. Gross findings in both were antero-ventral consolidation of lungs with caudodorsal emphysema and edema. Histopathological study showed bronchointersticial pneumonia, necrotizing bronchiolitis and exudative alveolitis with syncytial cells in lungs. Lung sections, fixed in formalin, were analyzed by immunohistochemistry. Using this technique it was found positive labeling with anti-BRSV in bronchiolar epithelial cells and syncytial cells, confirming the presence of virus-associated pneumonia lesions.

16/06/2021

16/06/2021

16/06/2021

Pneumonia can affect calves of any age. Most of the pathogens that cause lung infections are always present in the calf’s respiratory tract and become a problem only when his immune defenses are compromised by stress. Stress may be due to bad weather, extreme changes in temperature, a long truck haul, overcrowding in a dirty environment, or nutritional stress due to deficiencies of an important mineral like copper or selenium. A newborn calf in a drafty or humid barn (with saturated bedding, ammonia fumes that irritate lungs and airways) may get pneumonia.

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A primary viral pneumonia may be mild, but secondary bacterial invaders may move in after tissues are damaged by a virus. For instance, a viral infection often destroys the tiny cilia on the lining of the windpipe and bronchi, so foreign material (including bacteria) can no longer be moved up out of the airways.

Bacterial pneumonia is generally more apt to kill the calf than is viral infection. Viral pneumonia may be insignificant and run its course without treatment — unless a secondary bacterial infection (such as Pasteurella) turns it into an outbreak of pneumonia that may go through a group of calves.

Young calves are most susceptible to pneumonia after their temporary immunity (antibodies from the dam’s colostrum) begins to wane. Calves that do not get colostrum or not enough (or not soon enough) have less defense against pathogens. Calves stressed by a hard birth or calves that become chilled immediately after birth may not get up and nurse soon enough, or can’t absorb enough maternal antibodies due to stress (which hastens thickening of the gut lining).

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Calves of any age may develop pneumonia when weather conditions are stressful. Particularly dangerous are extremes of temperature in fall or spring with hot days and cold nights. Robert Cope, DVM (Salmon, Idaho) says these cases are often due to Pasteurella. “Those don’t need a virus to get started. When you get temperature extremes, we see a lot of Pasteurella pneumonias. What seems to be the key is a temperature differential of five to 10C or more (such as 15C afternoons, dropping to six degrees at night). With these conditions you may get primary Pasteurella infections and don’t need a virus to set them off. A virus vaccine (IBR, BVD etc.) won’t protect calves in this instance, but a Pasteurella vaccine will,” he explains.

SIGNS OF ILLNESS
A calf with pneumonia usually goes off feed, is dull and depressed, and may spend a lot of time lying down. He may stand hu**ed up. Ears may droop, he may have a snotty or crusty nose, and respiration may be fast or laboured. He moves slowly because he is in pain. He may have a cough or noisy breathing. In severe cases the calf may have difficulty breathing and might breathe with his mouth open or with a grunting sound as the air is forced out of impaired lungs.

There are a number of pathogens that can cause respiratory disease. What we used to call “shipping fever” is now called BRD (bovine respiratory disease). There are more than 20 bacteria and viruses that can be involved in lung infections, according to Cope.

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EARLY TREATMENT CRUCIAL
There are several good drugs available now for treating pneumonia, including Baytril, Nuflor, Micotil, Excede, Draxxin, and others that are all more effective for bacterial infections than the standard penicillins or oxytetracyclines and sulfas, says Cope, but the key to successful treatment is catching them quickly, and that’s not always easy. “Often by the time a calf is obviously sick, there’s permanent lung damage,” he says.

“The main thing with Pasteurella pneumonias, especially, is treating the calf before lung damage can become permanent. There have been studies that indicate the damage can become permanent within less than eight hours. At that point there will be scarring, even if you clear up the infection,” says Cope.

Whether the calf can make it to adulthood depends on how much scarring there is. “Once permanent damage exceeds 20 to 25 per cent of lung tissue, the calf won’t do well even if he survives. Those are the ‘chronics’ that never grow, don’t do well, and die about a week before you were going to haul your calves to market,” he says. The calf, even if he seems to be doing ok, will eventually outgrow his lung supply. He may not grow well, or outgrows his lung supply and then he suddenly dies, Cope explains.

Supportive care is important. Make sure the calf is warm and dry, protected from bad weather. If he’s not eating or drinking, you may need to force feed fluids, or give fluids with nutrients (milk to a pre-weaning age calf). Anything you can do to help a calf will help him fight the battle more successfully. If he has a high fever and does not feel like eating or drinking, anti-inflammatory drugs (like Banamine) can lower the fever and make him feel better — often enough that he’ll start eating and drinking again. Do not give Banamine to a dehydrated calf or there is risk for kidney damage. Using it in conjunction with fluid therapy, however, can be helpful.

Antibiotics are usually given, even if pneumonia may be viral, since secondary bacterial infection may occur. Be diligent with treatment and don’t quit too soon. Even if the calf is feeling better and breathing better, eating and drinking again and his fever is down, keep antibiotic levels high for at least two full days after all symptoms are gone, or he may relapse and be much harder to save the second time around.

PREVENTION BETTER
There are some good vaccines available for helping prevent respiratory disease in calves. The best prevention in early calf-hood is to make sure each calf gets adequate colostrum, soon enough. If the cow’s vaccinations are up to date, she will pass some of that protection to her calf via her colostrum. “There is no substitute for good colostrum. If a calf doesn’t get it, he is vulnerable to everything,” says Cope.

“This protection is probably waning by the time most calves are three weeks old, however, and many cows’ immunity to Pasteurella isn’t that great. If you have cases of spring and summer pneumonia in calves, it may pay to vaccinate calves at around a month or so of age. It’s also important to see if there are any underlying problems on herd health. The cows may need a higher nutrition level. Energy, protein and trace minerals are things you should check, if calves are getting pneumonia. A cow that’s not in good shape herself will not have good colostrum,” he says.

If the cow does not have a properly functioning immune system — due to being low on selenium, energy, protein, etc. or has BVD — the calf will not get good colostrum. If you’re having a problem in calves, look at the cows. If you’re getting pneumonia in calves less than three weeks old, it’s generally less the calf’s fault than the cows’, says Cope.

Most calves that have good colostrum and nutrition can handle weather stress without becoming sick, especially if they have shelter. We have better treatments (more effective drugs) for treating pneumonia than we used to have, but prevention is still preferable. A total herd health program will go a long way toward preventing pneumonia, and involves more than just vaccinations.

PERMANENT DAMAGE
Bruce Anderson, DVM, at the Caine Veterinary Teaching Center, Caldwell, Idaho says antibiotics may not always pull a calf through, because you may not realize how sick the calf is until it’s too late.

“Studies at the University of Nebraska’s Great Plains Veterinary Education Center, where groups of calves were followed from birth through feedlot and slaughter showed the calves that did not get good passive transfer (from colostrum) 18 months later performed less well in the feedlot. These calves had less functional lung tissue than normal calves, probably because they had respiratory infections and pneumonia early in life. It is really the sub-clinical pneumonias that hurt you, because you never know these calves were really sick,” he explains.

These are calves that may have been a little dull or off feed but they never get treated because they don’t appear sick — or you can’t catch them to treat them. They end up with compromised lungs, however, and do not perform as well as the rest of the herd. “At slaughter we find that some of these cattle have 10 to 15 per cent dead lung, or some old adhesions, and we know that they had a respiratory problem as a calf,”

16/06/2021

Tilmicosin, a new macrolide antibiotic, 20-deoxo-20-(3,5-dimethylpiperidin-l-yl)desmycosin, formerly identified as EL-870, has been evaluated in three experiments as a single subcutaneous injection at dosages of 10, 20 or 30 mg/kg for the treatment of naturally occurring pneumonia in neonatal calves. Male Holstein calves, under five days of age, were shipped from Wisconsin and housed in pens. They were assigned sequentially to a treatment group when their temperature was greater than or equal to 39.7 degrees C for two consecutive days or greater than or equal to 39.7 degrees C and signs of respiratory disease were present. Clinical signs were evaluated daily for 14 days after the tilmicosin treatment. Calves that died and those that survived for the 14 day experimental period were examined post mortem. Treatment with tilmicosin was effective at all dosage levels, as determined by significant (P less than or equal to 0.05) reductions in body temperature within 24 hours, in the number of animals that died, in the incidence and severity of clinical signs, in the number of Pasteurella species found in lung tissue and in the severity of the pneumonic lesions. In two of the three experiments severe outbreaks of cryptosporidiosis resulted in significant mortalities within a few days after the arrival of the calves. Treatment with tilmicosin was effective against respiratory disease even in the presence of this severe concurrent disease.

16/06/2021

Calf Pneumonia
Also known as: Enzootic Pneumonia

Calf pneumonia is a respiratory disease caused by inflammation in the lungs, primarily the alveoli (air sacs). It is a multifactorial disease caused by a range of organisms including viruses, bacteria and mycoplasmas. Environmental factors are also extremely crucial in managing the disease. Calf pneumonia can potentially be a significant economic burden to a farm, due to the costs of treatment, mortalities, reduced growth rates, additional labour and housing requirements (Andrews, 2000; van der Fels-Klerx et al., 2001). Respiratory diseases in young animals were ranked very low in importance in a survey of British organic beef and dairy farmers (Roderick and Hovi, 1999). Late weaning, a whole milk diet, the requirement for good housing standards and a closed herd policy reduce the risk factors for respiratory disease in calves.
Enzootic pneumonia in young calves is a multifactorial disease that occurs mainly in two different systems:

In housed dairy calves reared for replacement
In housed calves reared for beef either in the herd of origin (Assié et al., 2004), or after sale to a finisher.
What Causes Calf Pneumonia?
Control and Prevention
Risk Factors
Treatment Options
Welfare
Good Practice

Dairy calves are likely to suffer from the disease at any age, with it manifesting itself as a chronic, coughing pneumonia, or as a more acute, enzootic calf pneumonia. Older dairy calves are also vulnerable after housing in the autumn. Suckler calves are more likely to suffer from respiratory disease between two and five months of age, following weaning or transport from one herd to another. Outdoor reared beef suckler calves can also be severely affected by pneumonia (Peters, 1986; Scott, 1997).

In older calves, mainly in weaned suckler calves aged six months to two years, respiratory disease is likely to occur after transport or other environmental stress and is often called shipping or transit fever. (see: Pasteurellosis). Similarly, a respiratory disease caused by lung parasites, (Dictycaulus viviparous, also known as husk), occurs in older calves. However, it should be remembered that lungworm can occur in young calves, even when housed if exposed to significant challenge from carrier dams or contaminated bedding (Crawshaw and Smith, 2003; SAC Veterinary Services, 2006). A viral respiratory disease caused by the infectious bovine rhinotracheitis virus (IBR), is also more significant in older animals.

Enzootic Pneumonia in Calves

Duchy College calf in calf shed
Duchy College calves in the calf shed. There should be enough space in the calf pen to allow calves to groom themselves, lie down and stretch their limbs and rise without any difficulty.

Enzootic pneumonia in young calves can be chronic and be present with very few clinical signs apart from a dry cough and slightly increased respiratory rate. The acute form of the disease usually manifests itself in an outbreak involving several calves going down with the disease within a 48-hour period. Fever, dullness, inappetance and coughing, often combined with nasal discharge, are the most common symptoms.

There are very little data available on the prevalence of enzootic pneumonia in UK cattle herds, as recording of calf diseases is seldom carried out. Respiratory diseases are, however, considered the second most important cause of death and ill -thrift in calves. The condition is farm related, with some farms suffering serious losses due to calf pneumonia, while on others the disease is either very mild or non-existent. Sporadic outbreaks can, however, be experienced by farms that normally see very little respiratory disease in calves.

Causes of Calf Pneumonia
Pneumonia in calves is a multifactorial disease. Infectious agents (pathogens), environment, management and the immune status of the calves are factors in determining the outcome of an infection.

Pathogens Causing Calf Pneumonia
A multitude of pathogens, including viruses, bacteria and Mycoplasma spp (M. bovis, M. dispar and M. canis), are involved in different combinations on different farms (Ayling et al., 2004; Bryson et al., 1978; Caldow et al., 1988; Dyson and Linklater, 1976; Uttenthal et al., 1996). It is often suggested that the viruses and mycoplasmas are the primary infections and the bacteria cause a secondary infection in an animal whose defences have been weakened by the first infection. The most common viruses isolated from enzootic pneumonia cases are:

Respiratory syncytial virus (RSV)
Parainfluenza III virus (PI3)
Infectious bovine rhinotracheitis virus (IBR)
Bovine viral diarrhoea virus (BVD), has also been implicated in the bovine respiratory disease complex, not as a primary pathogen, but as a disease agent causing immunosuppresion.
Mycoplasmal agents are usually considered to be the most common agents causing the chronic form of enzootic pneumonia, even though Mycoplasma bovis has been identified as the causative agent in many acute outbreaks as well.

The most commonly isolated bacterial organisms are:

Mannhaeimia spp.
Pasterurella multocida
Hemophilus (Histophilus) subspecies (H. somnus).
Risk Factors for Calf Pneumonia
Inadequate ventilation of calf barns increases the risk of disease due to the buildup of humidity, noxious gases, dust and bacteria content (Lorenz et al., 2011). The main environmental risk factor predisposing calves to respiratory disease is poor ventilation in calf housing (Pritchard, 1982). Cold, humid conditions, sudden changes in air temperature, stress due to different causes and change in the environment have also been associated with outbreaks of pneumonia in young calves (Phillippo et al., 1987; Roe, 1982; Scott, 1995; Webster et al., 1985).

Inadequate intake of colostrum or poor quality colostrum will affect the calves’ defence against respiratory agents and make them more susceptible to infection (Virtala et al., 1999). Rearing systems where calves of different origin are mixed together at a young age suffer from high levels of respiratory diseases (Gibbs, 2001). Large, shared air spaces, calves from different age groups and poor sanitation between calf batches often make these systems even more vulnerable (Losinger and Heinrichs, 1996). The stress associated with management procedures such as disbudding and castration may also be associated with a high respiratory disease incidence.

Control and Prevention of Calf Pneumonia
Management of calf pneumonia is reliant on a good understanding of the causes and risk factors (See table). The incidence and severity of calf pneumonia is closely allied with management and disease patterns on farms and so the best approach is to implement a control programme tailored to the individual farm’s needs. (Gibbs, 2001). These approaches can be divided into three areas:

Maintenance of disease resistance
Reduction of stress
Minimising exposure of infection
1. Maintenance of Disease Resistance

Dairy calf sucking from its mother
Antibodies within good quality colostrum will boost the calf’s immune system and make them more resistant to infection

Passive immunity is the transfer of antibodies from one individual to another and can be in the form of maternal antibodies in utero crossing the placenta, or lactationally in the colostrum. In order to provide the calf with passive immunity protection before its own immune system is fully functional, the calf needs to receive adequate amounts of colostrum containing a sufficient quantity of antibodies (also known as immunoglobulins). An amount of 3-4 litres of colostrum that contains 50-150 g/litre of Immunoglobulin IgG within the first 24 hours of life has been recommended (Besser et al., 1991).

While the transfer of maternal immunity is important, it does not protect the calf well against respiratory pathogens after two to three months of age because the concentration of maternal serum antibodies is low and the calf’s own immune system is still not fully functional (Corbeil et al., 1984). The highest incidence of respiratory disease occurs in many herds during this period, and prevention should concentrate on reducing stress at this time.

A further factor limiting immune protection against respiratory diseases is that some of the pathogens are immunosuppressive (Woldehiwet and Sharma, 1992). Mycoplasma bovis, RSV and BVD belong to this category and, when endemic in a herd, can reduce the calves’ ability to fight off disease. Eradication of M. bovis from a dairy herd has been shown to improve calf health (Byrne et al., 1998).

Vaccines are widely used to boost the calf’s immunity against many respiratory pathogens. Vaccines are available in the UK for RSV, PI3, IBR, BVDV and Pasteurella, including several in combination. The vaccines should be used as a part of a disease prevention programme that also addresses the environmental and management factors on the farm (Richardson, 2004). Vaccines can also be used to eradicate some respiratory diseases in problem herds (Pospísil et al., 1996). However, the efficacy of the vaccines varies significantly between farms, and it is difficult to establish control programmes that work continuously (Van Donkersgoed et al., 1990; Howard et al., 1987; Straub and Mawhinney, 1988). Some modified live vaccines can also be used successfully in the face of an outbreak once the first affected animals have been diagnosed and the agent identified. It is important to check the datasheets for all vaccines before use as only some are live and all have different protocols for administration, requirement for second vaccinations and duration of immunity. Vet advice should always be sought and vaccination use should be part of the herd health plan.

Prophylactic use of antibiotics has also been successful in calf rearing units, with antibiotics being administered to all arriving animals. This practice is, however, should not be relied upon in any system.

Disbudded calf
In order to reduce stress disbudding and castration could be timed so they don’t coincide with weaning or autumn housing

2. Reduction of Stress
Gradual weaning, timing of disbudding (or dehorning) and castration not to coincide with weaning, minimisation of transport, avoidance of cattle markets and avoidance of cold stress have all been recommended as ways to reduce stress in young calves. Respiratory diseases often follow a pattern in a herd. Recording disease incidences and treatments can be helpful in establishing whether such a pattern exists. If a pattern is identified, potential stressful events around disease peaks can be recognised and management practices changed to reduce stress.

Overcrowding, poor ventilation and high humidity all cause respiratory stress in calves and make them more susceptible to respiratory disease. Reasonable space allowance and good ventilation will reduce this stress and make pneumonia less likely.

3. Minimising Exposure to Infection
Close contact with other animals allows respiratory pathogens to spread easily. Individual housing of dairy calves either indoors or outside is generally linked to improved calf health (Lorenz et al., 2011). There is long-term recognition of the benefit to dairy calf health of outdoor housing in hutches especially for the prevention of diarrhoea and respiratory disease (Davis et al., 1954).Similarly, keeping age groups separate and group sizes small has been shown to reduce respiratory disease (Lorenz et al., 2011).

Introduction of animals from other herds carries a risk of disease transfer, even in virtually closed herds, where only occasional replacement animals are brought in. Keeping recent purchases separate from the herd for 2-3 weeks to ensure that they are not incubating a respiratory disease is an adequate control measure.

Treating Calf Pneumonia
In the face of an outbreak of enzootic pneumonia in a closed herd or when a chronic problem is recognised, it is important to attempt to identify the causative agents and management and environmental factors in order to target preventive measures in the future. There are a number of investigative techniques that can be used in the face of a pneumonia outbreak. These include:

Taking nasal swabs and / or bronchioalveolar lavage (BAL) samples for bacterial culture and virus isolation
Paired blood samples can be taken to assess serological immune response to respiratory pathogens. However, reliance on paired serological testing must be done with caution as false negatives (failure to seroconvert) can occur in young calves less than 3 months old (Virtala et al., 1999).
For the acute form of enzootic pneumonia, nursing and isolation of affected calves is important in all treatment regimes. Vitamins can be administered as a supportive therapy, and a multitude of treatments from antihistamines to expectorants have been found effective in alleviating the condition. Antimicrobials and both steroid and non-steroid anti-inflammatory drugs are usually the main line of treatment. Antibiotics can be used in in-contact animals (metaphylaxis) as well as in the affected ones. Vaccines can occasionally be used for the in-contact animals, and have been shown to provide protection when the causative agent is identified fast enough (Gourlay et al., 1989; Laven and Andrews, 1991).

In all cases antimicrobial treatment should be under veterinary guidance and should be outlined in the farms’ herd health plan.

The chronic form of calf pneumonia is normally not treated unless the calf has a severe cough. Antibiotics, when chosen carefully, may be effective, although severe lung damage may not resolve. It should, however, be noted that chronic enzootic pneumonia in calves is a herd problem. When the condition becomes severe enough to require treatment, it is necessary to attempt to identify the causes of the condition. Improvement of the management and environment or eradication of the causative agent/agents with the help of a comprehensive disease control plan is required.

Calf Pneumonia and Welfare

Calf and teats
Single suckled calves reared in outdoor systems are at lowest risk of pneumonia

Prior to weaning, single suckled beef calves reared in outdoor systems are at lowest risk of pneumonia (Lorenz, 2011). For suckler calves born and reared indoors the incidence of pneumonia can be considerable (Assié et al., 2004). Therefore, the isolation of affected calves, effective treatment with antibiotics and supportive therapy and provision of dry and warm conditions are vital in the treatment, in order to avoid further suffering. The fact that many respiratory problems can be avoided by good management adds to the need to tackle calf pneumonia problems if they exist in herds.

Good Practice Based on Current Knowledge
Prevention and control of calf pneumonia should be an important part of a farm’s herd health plan. Good practice to control and prevent enzootic pneumonia in young calves can be divided into three areas of action

1. Building immunity:
Do not separate the dam until 24 hours after calving
Provide adequate bedding to allow the calf to stand without difficulty
Ensure early colostrum feeding; assist if needed, and monitor the intake as closely as possible and record it (“maximum supervision, minimum interference”). Calves should consume approximately 10% bodyweight in colostrums in the first 6 hours of life (i.e. approximately 4-5 litres for an average calf, or, a minimum of “6 pints in 6 hours”)
Keep a supply of frozen colostrum in case the dam leaks colostrum before calving
Avoid stress and exposure to new disease agents during “poor immune capacity” at 2-4 months of age
If immunosuppressive respiratory pathogens are a known herd problem (i.e. BVDV), consider eradication or vaccination
Vaccinate strategically as a part of a comprehensive disease control plan
2. Minimisation of exposure:
Provide adequate numbers of calving pens and clean and disinfect them between batches
House calves of different ages in different rooms or with adequate separation
Quarantine bought-in animals for 2-3 weeks before introduction to the rest of the herd
Provide good ventilation in the calf housing (a minimum of 6m³ airspace for calves up to 6 weeks old; 10m³ for calves up to 12 weeks and 15m³ for calves up to 16 weeks old)
There should not be excessive humidity in the calf house
In difficult circumstances, consider calf hutches (tethering not acceptable under UK organic standards).
3. Reducing stress:
Avoid overcrowding in calf pens
Provide good ventilation (There should be 6 air changes per hour)
Make sure that calves are well bedded during cold weather and do not suffer from draught (the minimum critical temperature for new-born calves in a straw bedded pen is 6ºC)
Provide shelter during prolonged cold and wet conditions on pasture
Avoid transport and introduction to new environment between 2 and 4 weeks of age
Time castration and disbudding away from weaning
Time autumn housing away from castration and disbudding
Introduce diet changes gradually, particularly when housing calves

16/06/2021

Mannheimia haemolytica–associated Bovine Respiratory Disease
Etiology:
Mannheimia haemolytica serotype 1 is the bacterium most frequently isolated from the lungs of cattle with BRD. Although less frequently cultured, Pasteurella multocida is also an important cause of bacterial pneumonia. Histophilus somni is being increasingly recognized as an important pathogen in BRD; these bacteria are normal inhabitants of the nasopharynx of cattle (see Histophilosis). When pulmonary abscessation occurs, generally in association with chronic pneumonia, Trueperella pyogenes is frequently isolated.

Under normal conditions, M haemolytica remains confined to the upper respiratory tract, in particular the tonsillar crypts, and is difficult to culture from healthy cattle. After stress or viral infection, the replication rate of M haemolytica in the upper respiratory tract increases rapidly, as does the likelihood of culturing the bacterium. The increased bacterial growth rate in the upper respiratory tract, followed by inhalation and colonization of the lungs, may occur because of suppression of the host’s defense mechanism related to environmental stressors or viral infections. It is during this log phase of growth of the organism in the lungs that virulence factors are elaborated by M haemolytica, such as an exotoxin that has been referred to as leukotoxin. The interaction between the virulence factors of the bacteria and host defenses results in tissue damage with characteristic necrosis, thrombosis, and exudation, and in the development of pneumonia. The pathogenesis of pneumonia caused by P multocida is poorly understood. This organism may opportunistically colonize lungs with chronically damaged respiratory defenses, such as occurs with enzootic calf pneumonia or existing lung lesions of feedlot cattle, and cause a purulent bronchopneumonia. H somni may invade the lung and cause pneumonia after damage to the respiratory defenses. This organism is capable of systemic spread from the lung to the brain, myocardium, synovium, and pleural and pericardial surfaces; often, death can occur later in the feeding period (40–60 days after arrival) from involvement of these additional organ systems.

Clinical Findings:
Clinical signs of bacterial pneumonia are often preceded by signs of viral infection of the respiratory tract. With the onset of bacterial pneumonia, clinical signs increase in severity and are characterized by depression and toxemia. A combination of clinical signs of depression and fever (104°–106°F [40°–41°C]), without any signs attributable to other body systems, are the classic components of a case definition for early cases of BRD. Serous to mucopurulent nasal discharge; moist cough; and a rapid, shallow respiratory rate may be noted. Auscultation of the cranioventral lung field reveals increased bronchial sounds, crackles, and wheezes. In severe cases, pleurisy may develop, characterized by an irregular breathing pattern and grunting on expiration. The animal will become unthrifty in appearance if the pneumonia becomes chronic, which is usually associated with formation of pulmonary abscesses.

Lesions:
M haemolytica causes a severe, acute, hemorrhagic fibrinonecrotic pneumonia. The pneumonia has a bronchopneumonic pattern. Grossly, there are extensive reddish black to grayish brown cranioventral regions of consolidation with gelatinous thickening of interlobular septa and fibrinous pleuritis. There are extensive thromboses, foci of lung necrosis, and limited evidence of bronchitis and bronchiolitis.

P multocida is associated with a less fulminating fibrinous to fibrinopurulent bronchopneumonia. In contrast to M haemolytica, P multocida is associated with only small amounts of fibrin exudation, some thromboses, limited lung necrosis, and suppurative bronchitis and bronchiolitis.

H somni infection of the lungs results in purulent bronchopneumonia that may be followed by septicemia and infection of multiple organs. H somni is associated with extensive fibrinous pleuritis in feedlot calves.

Pulmonary abscessation can occur as the pneumonia becomes chronic. Abscesses develop in ~3 wk but do not become encapsulated until 4 wk. T pyogenes is frequently cultured from these abscesses.

Diagnosis:
Generally, neither serologic testing nor direct bacterial detection are performed, and diagnosis relies on gross necropsy findings and bacterial culture. Because the bacteria involved are normal inhabitants of the upper respiratory tract, the specificity of culture can be increased by collecting antemortem specimens from the lower respiratory tract by tracheal swab, transtracheal wash, or bronchoalveolar lavage. Lung specimens can be collected for culture at necropsy. If possible, specimens for culture should be collected from animals that have not been treated with antibiotics to permit determination of antimicrobial sensitivity patterns. A multiplex PCR has been used to identify a number of bacterial agents implicated with bovine respiratory disease, including M hemolytica.

Treatment:
Early recognition by trained personnel skilled at detecting the early clinical signs of disease followed by treatment with antibiotics is essential for successful therapy. Treatment protocols should be established so the producer has a standardized approach to identifying and treating cases. Long-acting antimicrobials such as tulathromycin, tilmicosin, florfenicol, and enrofloxacin have label claims to treat BRD and are commonly used as first- or second-line treatment options in feedlot calves. These long-acting antimicrobials allow the feedlot producer to avoid commingling sick animals in a hospital pen, and treated animals can return directly to the home pen. NSAIDs have been shown to be a beneficial ancillary therapy in controlling fever in cases of BRD, but data are lacking in terms of effect on relapse and mortality outcomes. If selection for treatment is late and pulmonary abscessation has occurred, it is difficult to achieve resolution with antimicrobials, and use of a convalescent pen or culling of the animal should be considered.

Control:
General principles of control are discussed under {blank} Enzootic Pneumonia of Calves and Shipping Fever Pneumonia. The value of M haemolytica and P multocida bacterins is questionable, and some reports indicate they may even exacerbate the disease. Newer vaccines, which include live culture and subunit vaccines (leukotoxin), show much more promise for disease prevention and may reduce morbidity in high-risk feedlot calves given one dose of vaccine on arrival by as much as 25%; however, trials have not been consistent in all risk categories of feedlot cattle. Ideally, vaccination should be done 3 wk before transport to the feedlot and can be repeated on arrival. In dairy calves, vaccination of the dam may be of benefit by providing passive immunity to the calf. H somni bacterins are available, and there is some evidence they are effective in control of BRD in feedlot calves even when only one dose is given on arrival.

Mycoplasmal Pneumonia
Mycoplasma bovis is an emerging cause of respiratory disease and arthritis in feedlot cattle and in young dairy and veal calves. Experimental infections usually result in inapparent to mild signs of respiratory disease, but virulent strains have been identified that cause severe lung disease in calves. However, this does not preclude a synergistic role for mycoplasmas in conjunction with viruses and bacteria in BRD. Mycoplasmas can be isolated from the respiratory tract of nonpneumonic calves, but the frequency of isolation is greater in those with respiratory tract disease. M bovis has been associated with otitis media in young calves and a syndrome involving chronic pneumonia and polyarthritis in feedlot cattle. These cattle invariably have a pneumonic lesion, and 40%–60% may also develop a polyarthritis and tenosynovitis that causes severe chronic lameness. The condition results in a chronic disease that does not respond to antimicrobial therapy. A significant proportion of these animals are euthanized because of the chronic nature of the disease. Lesions include chronic bronchopneumonia with caseous and coagulative necrosis. In severe cases, >80% of the lung tissue may be involved. Culture of these organisms requires special media and conditions; growth of the organisms may take up to a week. PCR tests are now available that can detect the mycoplasma within hours, thus greatly speeding up diagnosis. Immunohistochemical tests can also be done on fixed tissue that link the mycoplasma antigen directly with the lung lesion. Vaccines are commercially available for M bovis, but their efficacy has not been demonstrated conclusively.

Chlamydial Pneumonia
Chlamydial agents have been implicated in a number of diseases of cattle, including pneumonia. Clinical signs and lesions of bronchopneumonia have been produced by experimental infections. A synergism between Chlamydia and Mannheimia haemolytica has been demonstrated experimentally. Because this pathogen is infrequently tested for, its overall importance remains undetermined. The organism can be tested for by staining sections of lung lesions with Gimenez stain or by fluorescent antibody. Isolation requires inoculation of yolk sacs of embryonating chicks. Chlamydial agents are sensitive to tetracyclines. (See also Chlamydiosis.)