13 December 1996

Three lines of defence in war against disease

To start our series on tackling pig respiratory disease we provide an insight into the respiratory system and its defence mechanisms.

Jonathan Riley reports

PIG performance is governed by the condition of the respiratory system and producers must aim to provide the right environment and management for it to cope with disease.

But while most producers have a rudimentary understanding of machinery, few understand how the pigs respiratory system and its defence mechanisms work, says Dr Stan Done of MAFFs Central Veterinary Laboratory.

"The respiratory system is simply the lungs plus the tubes which connect them to the atmosphere.

"Better knowledge of the system could help producers understand why, when pigs are stressed, overstocked or water supply and nutrition are poor, challenges such as dust, ammonia and diseases, cause pigs to succumb to disease.

"These challenges build up in housing as the finishing period progresses and have been added to by the introduction of swine flu and porcine reproductive and respiratory syndrome," he says.

But Mr Done explains that it is only when a mix of these challenges is present that the pigs defences are overwhelmed and clinical disease occurs.

He suggests that for a finishing pig the combined threshold is about 10mg/cu m of dust, 1m bacteria/cu m and 20-30 parts per million of ammonia.

The respiratory system has three key defences to prevent pathogens reaching the lungs. These are – hairs, mucus membranes and specialised cells.

The first defence is in the nasal cavity which has a border of large hairs to trap dust and bacteria.

"These present a barrier to large dust particles. But this barrier will be overwhelmed by large quantities of dust in poorly ventilated and overstocked houses, or evaded by small dust particles below 0.001mm in diameter.

"Dust under 0.001mm is airborne and small enough to penetrate deep into the pigs lungs," says Dr Done. The second line of defence is provided by a mucus layer covering respiratory tract surfaces. This traps dust and acts as a barrier to prevent pathogens crossing the tract lining and entering the bloodstream.

"As the tract progresses towards the pigs lungs the airways divide repeatedly into a network of branches until they measure only 0.5mm in diameter.

"The mucus layers in this network are the main line of defence and to function the pig must have access to adequate water. When water is in short supply the pig draws moisture from the surfaces of the respiratory system for other uses within its body.

"Any breakdown in mucus production allows rapid infection by enzootic pneumonia and swine flu which cause tubes to become blocked.

"Cell repair is slow in these tubes, but repair rates can be enhanced by supplying plenty of vitamins A and C in the diet," says Dr Done.

The pigs third line of defence is a specialised cell which has up to 200 projections on its surface. These line the nasal cavity and beat rhythmically to create a flow of mucus against the direction of invading bacteria and particles. The mucus traps pathogens and the flow transports them away from the lungs.

"These specialised cells also line the airways as they progress towards the lungs and their efficient functioning is vital to prevent disease," says Dr Done.

The key threat to their dysfunction is ammonia, which acts as a specific toxin to these cells and causes them to produce a thinner mucus.

"This is less effective at blocking bacteria which then cross the mucus layers and attack cells. Once these cells are destroyed there is little to stop pathogens reaching the lungs," says Dr Done.

Specialised cells called macrophages also exist throughout the animals body and are found in the 700m air sacs of the lung.

"The macrophage is designed to identify, engulf and digest pathogens. But when pigs are stressed they produce a chemical called cortisone which slows the macrophage response, allowing pathogens time to establish disease," says Dr Done. &#42

These pigs are showing classic signs of respiratory diseases with uneven pig sizes within batches resulting from loss of appetite and poor growth rates.


&#8226 Stress.

&#8226 Overstocking.

&#8226 Poor nutrition.

&#8226 Dust.

&#8226 Ammonia.

&#8226 Disease.