The host’s immune system can be altered by pathogens as well. Most viruses, and some bacteria, have been shown to interfere with the function of macrophages and/or to destroy them. Some, such as PRRS, also alter the subpopulation of T-lymphocytes, thereby impairing the host’s ability to fight infections. 

- Ammonia reacts with the moisture in the respiratory tract to form ammonium hydroxide, a highly irritating substance. It causes inflammation, stimulates the immune response and suppresses the cough reflex and is thus recognised as one of the major atmospheric pollutants that affect health and productivity of farm animal.

Immune Response: The immune defence mechanisms involved in PRDC include both humoral immunity (antibodies) and cell mediated immunity. These same mechanisms are also involved in the appropriate response to any vaccination regimes against the causative agents of PRDC that may have been instituted.

When a pathogen enters the deeper respiratory tract, the cell mediated defence mechanism will be activated causing inflammation, which can be local (for example irritation and build-up of fluids, resulting in a cough), or systemic (e.g. fever, anorexia and depression). At the same time the white blood cells will be stimulated to produce antibodies, which will start to appear approximately 2 weeks after the initial infection. They will also form memory cells that will immediately start antibody production the next time the pathogen enters the body.

The host immune system is influenced by many factors. Pig genetics are known to affect immune function traits. Other ongoing inflammation or disease may reduce the immune system’s capacity to fight a new infection. As already mentioned, elevated ammonia levels will reduce the cough reflex, impairing the elimination of particles and mucus. An insufficient calorie or vitamin E/Selenium and/or other trace elements intake will also affect the host’s ability to fight infection, as will certain mycotoxins that might be present. Other causes of stress, such as mixing of animals and fluctuating temperatures, may also suppress the immune response.

The immune system has a high energy demand and thus any significant stimulation, even in the absence of clinical signs, may result in either reduced growth and/or increased food conversion ratio. 

Accurate diagnosis is essential when faced with a PRDC outbreak, in order to determine which particular pathogens are (most likely) involved and thus help the herd veterinary surgeon to implement the most appropriate treatment and prevention plan. The sooner in the disease process that an investigation is started, the more likely it is that an accurate diagnosis can be made. This diagnosis is likely to be based on a combination of clinical examination, post mortem examinations of representative, early, untreated PRDC cases, combined with histology and microbiological laboratory, and blood sampling for serology, and PCR testing. 

Treatment regimes are likely to be tailored to the particular characteristics of PRDC in the unit. They may consist of antimicrobial therapy, either by injection, in-feed or via water in order to control bacterial infections and/or non steroidal anti-inflammatories, in order to mitigate the effects of the infections. 

Vaccination is a key part of the prevention protocol. It will not prevent infection, but it primes the immune system by the creation of memory cells so it can fight infections and thus help prevent signs associated with the PRDC. Vaccination may also help to lower the infection pressure by reducing pathogen load. As mentioned above, for optimal immune function, there should not be any other significant infections ongoing, since concurrent infections at the time of vaccination can negatively affect the immune response (e.g., Ascaris suum or PRRS infection at the time of M. hyopneumoniae vaccination, the animals should not be subjected to significant stress. The age of the pig at vaccination is also important since maternally derived antibodies can impair the host response to vaccination. In other words, to get most out of vaccination, the environment, management and general health of the pig must be optimised as well. 

Monitoring of the pigs will be essential to evaluate current treatment and prevention protocols. Recording of clinical cases and response to treatment will give you an idea about the number of pigs involved and the success rate of the current treatment protocol. Average daily weight gains and feed conversions will give tell you how the pigs are really doing, since any stimulation of the immune system (with or without clinical signs) will have an impact on these.

Abattoir monitoring, via the BPHS or WPScotland scheme, is a useful way to monitor the condition of the respiratory tracts in finishing pigs. However it should be born in mind that intermittent, quarterly monitoring may only provide a “snapshot” of that particular group of pigs and may not represent the general situation. In addition veterinary interpretation is required to a) assess the significance of the particular lesions observed, since many are not specific for individual infections, and b) assess trends in their prevalence in conjunction with the clinical situation on the unit. In many cases analysis of abattoir reports will provide an indication as to the level of recent PRDC in a particular group of pigs and indicate whether further investigation is required.