Porcine respiratory disease complex (PRDC)

ABSTRACT:

Porcine Respiratory Disease Complex (PRDC) is one of the most common and costly clinical conditions encountered in veterinary medicine in commercially kept pigs. In this introductory article (sponsored by Intervet/Schering Plough AH), the clinical presentation in both the individual pig and groups will be described, along with the pathological lesions encountered. The multiple aetiological factors involved in the syndrome, ranging from primary and secondary infectious agents to environmental and managemental factors will be explored. Diagnostic and monitoring methods are also discussed. Subsequent articles will look in detail at specific elements of the syndrome.

Respiratory disease in the form of rhinitis, bronchitis, pneumonia and pleurisy has long been a major problem for the growing pig. Whilst historically, viral infections such as Classical Swine Fever and Aujeszky's Disease were implicated, these agents have long been eradicated from Britain. However, over recent decades a sequence of pathogenic viruses have emerged in the pig population that individually and collectively have combined with other enzootic agents such as Mycoplasmas, Bordetella and Actinobacillus pleuropneumonia to produce a disease complex which requires detailed diagnostic investigation and a multi‐disciplinary approach to control. In some cases, parasites such as migrating ascarids and lungworm may also complicate the picture.

In this initial article, an overview of PRDC will be given and subsequent articles will describe specific organisms and their role in PRDC, control, environmental influences and costs of disease.

Table 1 describes the most important pathological agents contributing to PRDC in the growing pig and classifies them as either primary or secondary agents. The latter are opportunist bacteria that will invade the damaged lung following previous infection with a primary agent. However, in a highly complex disease picture several primary agents can act together and subsequently to produce a population disease picture that can be very difficult to unravel.

Table 1.

Major aetiological agents of PRDC (*are key agents in PRDC)

Agent	Type	Primary/Secondary	Notes
Porcine Reproductive and Respiratory Syndrome Virus *	Virus	Primary	Since 1991
Porcine Circovirus Type 2 *	Virus	Primary	Since 1999
Swine Influenza Virus *	Virus	Primary	Since 1986 and evolving
Porcine Respiratory Coronavirus	Virus	Primary	Since 1986
Mycoplasma hyopneumoniae*	Mycoplasma	Primary	Combines with secondary bacteria to become enzootic pneumonia
Other Mycoplasmas	Mycoplasma	Primary or secondary	Generally mild
Actinobacillus pleuropneumoniae*	Bacteria	Primary	Many serotypes
Haemophilus parasuis*	Bacteria	Primary or secondary	Many serotypes
Bordetella bronchiseptica	Bacteria	Primary
Streptococcus suis	Bacteria	Secondary	Several serotypes implicated
Pasteurella multocida	Bacteria	Secondary	(Toxin producing strains are primary pathogens in progressive atrophic rhinitis)
Actinobacillus suis	Bacteria	Primary
Salmonella cholerae‐suis	Bacteria	Primary	Rare
Ascaris suum	Worms	Primary	Migrating larvae
Metastrongylus apri	Worms	Primary	Lungworm (only where pigs have earthworm contact)

CLINICAL PRESENTATIONS

Whenever a herd disease picture is encountered, it is necessary to identify both clinical effects in the individual pig (Fig. 1) and the overall behaviour of the disease in the population.

Figure 1.

Clinical respiratory disease in a growing pig.

Individually, the principal problem for the pig is one of pneumonia (lung damage), which will cause:

• • coughing
• • laboured breathing
• • fever
• • lethargy/recumbency
• • anorexia
• • discoloration of the extremities (Fig. 2)
• • weight loss/slowed growth
• • nasal/ocular discharges
• • death.

Figure 2.

Discoloration of the extremities – especially the ears – can be seen with any respiratory distress.

The effect within a population will depend upon the size and structure of the population. In fixed populations such as isolated batches, the disease may affect the group over a short period, leaving chronically affected individuals that later die or require euthanasia. In larger dynamic populations, particularly where pigs of sequential ages are present, the disease can become a rolling problem over a longer period of time, limiting growth and productivity.

Affected animals will variably suffer lung damage and adhesions of the lung to the chest wall (pleurisy), both of which are clearly evident post mortem (Figs. 3 and 4).

Figure 3.

Intense colour changes to the lung with acute pneumonia with overlying pleurisy.

Figure 4.

The roughened surface of the chest lining typical of pleurisy.

COMPLICATING FACTORS

The quality of the environment, particularly the air quality, will play a major part in the development of PRDC, with aerial pollutants such as ammonia, carbon dioxide, methane and dust all very important. Temperature variations, draughts and relative humidity can also act as major triggers of disease. Additionally, group size, social structure, comfort, feed and water availability, stocking rates and existing disease (e.g. swine dysentery) can all play a part in the development of PRDC.

CONTROL

With such a highly complicated picture involving interaction between the pig, the environment, the stockmen and primary and secondary pathogens, it is unsurprising that control of PRDC will often require a multi‐directional approach over time. This may require amendments to the environment and management, medication and vaccination – the latter often the long‐term solution to problems. Specific disease elimination programmes may also be required. These various techniques will be covered in detail in future articles.

DIAGNOSIS AND MONITORING

The BPEX Pig Health Scheme, involving assessment of lung damage to the pig, is a unique approach available to most pig farmers, but which is sadly underused. Over time, BPEX has provided vital information on the developing picture of respiratory system damage and, when combined with clinical assessment, performance figures and specimen on‐farm post mortem examination of new cases, provides the basis for the monitoring of this complex disease picture. Unfortunately at present, Food Chain Information (CCIR) reports arising from meat inspection – designed to determine fitness of meat for human consumption – is not a reliable indication of health.

Where required, extensive diagnostic testing can be performed on individuals in the herd to identify the different agents involved and often their relative importance in the overall disease picture. Tests available include:

• • bacterial culture
• • virus identification including PCR tests and immunohistochemistry
• • histopathology (microscopic examination of the lungs)
• • cross‐sectional serology (blood samples) in which the behaviour of different agents can be examined by looking simultaneously at different ages of pigs within the growing cycle.

It is important that where material is collected from dead pigs these should be selected alive and euthanased, and represent new cases rather than pigs that have been affected for days or weeks and subject to long‐term medication.

Intervet Schering Plough Animal Health's diagnostic tool, RESPIG, brings together the clinical, recording and laboratory components of an investigation into PRDC.

CONCLUSION

In this brief introduction to PRDC it can be seen that the disease is highly complex, requiring detailed understanding if satisfactory control is to be achieved. In future articles the disease will be broken down into its component parts, leading to overall control strategies.

CONTINUING PROFESSIONAL DEVELOPMENT SPONSORED BY NORBROOK PHARMACEUTICALS WORLDWIDE

These multiple choice questions are based on the above text. Answers appear as supporting information in the online version of this article.

1. What is PRDC:

• a. a notifiable viral disease of pigs
• b. another name for disease caused by Porcine Circovirus Type2
• c. a complex respiratory disease of multiple aetiology
• d. a form of enzootic pneumonia.

2. Which of the following pathological processes is not typically found in PRDC:

• a. pneumonia
• b. arthritis
• c. pleurisy
• d. rhinitis

3. Which of the following statements is true of PRDC when seen in a continuous production breeder feeder farm:

• a. The disease picture can be continuous over a long period of time.
• b. Sows will be the most severely affected class of pigs on the farm.
• c. The disease is usually epizootic, resolving within 1–2 weeks.
• d. PRDC does not affect growth of pigs.

4. Which of the following sources of data are not appropriate for diagnostic purposes for PRDC:

• a. clinical appraisal and use of production records
• b. use of abattoir data derived from Food Chain Information reports
• c. post mortem examination of typical cases in the early stages of disease.
• d. laboratory testing including bacteriology, serology, virology and histopathology

Supporting Information

Additional Supporting Information may be found in the online version of this article:

S1: Continuing Professional Development – Answers

Supporting information

Please note: Wiley‐Blackwell are not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing material) should be directed to the corresponding author for the article.

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