Part 2 - Chronic Respiratory Disease(2)

Introduction

The first two papers within this series provided a detailed outline of the causes of both short term (acute) and long term (chronic) respiratory disease in pigs and the various factors which trigger problems.  The relationship between the causative organisms and the pigs' immune system was explored.   In this third paper the aim is to develop some of these concepts to be able to understand how  the  environment,  (both  physical  and  social) plays a critical role in disease development.  In the second part we will look at pharmaceutical control of long term disease within a population both by use of immunological products (vaccines) and antibiotics.

Environment

As a general principle, the requirements of the growing pig for health can be summarised as; clean buildings, clean air, clean water and food and a temperature that maintains the pig within its required range - known as thermo-neutrality.

Hygiene

The cleanliness of a building is integral to health generally.  The greater the level of contamination by pigs, whether the result of faecal or aerial excretion, the higher  the level of microbes within the building, increasing the challenge either to the existing population or to those pigs following on.

The  ideal  situation  might  be  deemed  to  be  an isolated room or building which is filled in one go with pigs of the same age; the pigs remain within that group for the requisite time during which contamination is removed (e.g. via slats or physical removal plus fan ventilation) before all exiting at the same  time  allowing  full  cleaning,  washing, disinfecting  and  drying  of  the  building  before  the next batch enter.  This is the classic All In All Out concept (AIAO).

If  any  of  these  features  fails  then  hygiene breakdown and disease can result. Examples will include:-

1)  Filling the room with mixed ages of pigs.  As detailed in the first paper of this series, growing pigs go through a challenge, multiplication, excretion and immune development cycle (with or without disease) as they grow.   This means that older pigs act as a source of infection for younger pigs exacerbating disease.

2) Rolling populations - permanently occupied buildings in which young pigs enter regularly and older pigs are removed (e.g. weekly) creates a constant cycle of disease with the naïve young pigs "feeding the fire" of infection.   Scrape through dunging systems exacerbate the situation, encouraging spread of disease whilst permanent occupation of the building often procludes sufficient cleaning.

3)   Cleaning failures - even when AIAO policies are followed a failure to clean, disinfect and rest buildings between groups undermines any benefit.

Aerial Pollutants

Pigs and their excreta generate a range of aerial pollutants which can damage the respiratory tract or impede appetite leading to chronic disease.

Fig 1: Naturally ventilated straw yards for finishing pigs can lead to ammonia build up in hot weather, exacerbating respiratory disease.

Ammonia

In the paper on Acute Respiratory Disease (Paper I) a broad outline of the protective mechanism of the respiratory system was provided including the escalator  system  for  removing  mucus  (phlegm). This depends on cells lining the airways which move the escalator.  High ammonia levels paralyse these cells and so contaminated mucus gets trapped in the lung allowing microbes to multiply.

In cold weather mechanical air movement may be reduced   (to  maintain   temperature)   and   in   hot weather  particularly  in  naturally  ventilated  straw yards the ammonia levels can increase explaining the often seen respiratory disease in midsummer in yards (Fig 1).

Dust

Inhaled dust gets trapped in the nasal chambers and on  the  mucus  escalator.     If  excessive,  it  can overload the escalator - thickening the mucus - making removal difficult thus leaving inhaled microbes in the lung, some of which will be carried on dust particles.  Thus, dusty conditions (including those in straw yards) can exacerbate respiratory disease (Fig 2).  In addition, dust will contain foreign protein (allergens) which can damage the structure and the immune mechanism of the lung increasing vulnerability to infection.

Fig 2: High dust levels affect lung clearance mechanisms exacerbating respiratory disease.

Carbon Dioxide (CO2)

Raised CO2 levels are soporific and will reduce feeding activity.  They are often indicative of poor aerial quality and may well rise in parallel with ammonia levels.

Reduced  feed  intake  especially  in  cold  weather (when fans may not be removing foul air) can have a long term stressing effect on the pig increasing susceptibility to disease.

Temperature

All pigs exist within a thermoneutral zone, i.e. a temperature range, in which their body temperature can be maintained with minimal effort and energy expenditure. Outside of this range - which will be determined by age/size, group size, insulation levels and bedding, feed levels and air flow - pigs expend more energy.  Over a prolonged period (6-12 hours or more) this can increase susceptibility to disease.

There is a very delicate balance required between temperature control and air flow - which itself will be influenced by insulation levels and heat provision - to provide satisfactory conditions for pigs.  Building provisions  must  be  reviewed  as  part of  any  long term health control programme.

Group size and stocking rates

As a rule of thumb, the more pigs there are within a given airspace (i.e. greater group size) the greater will be the generation of microbes within the population and the greater spread.   This will be exacerbated by high stocking density i.e. the number of pigs per square and cubic metre in the building and mixing of ages and sources.

Statutory minimum floor space requirements exist (Fig 3) but the key to these figures is that they are LEGAL MINIMUM levels not optimum.   Practical stocking will be determined by building type e.g. pigs in fixed straw yards need approximately 2.25 times the space that pigs in fully slatted accommodation require.

The  unobstructed  floor  area  available  to  each weaner or rearing pig reared in a group must be at least-

(a) 0.15 m² for each pig where the average weight of the pigs in the group is 10 kg or less;

(b) 0.20 m² for each pig where the average weight of the pigs in the group is more than 10 kg, but less than or equal to 20 kg;

(c) 0.30 m² for each pig where the average weight of the pigs in the group is more than 20 kg but less than or equal to 30 kg;

(d) 0.40 m² for each pig where the average weight of the pigs in the group is more than 30 kg but less than or equal to 50 kg;

(e) 0.55 m² for each pig where the average weight of the pigs in the group is more than 50 kg but less than or equal to 85 kg;

(f) 0.65 m² for each pig where the average weight of the pigs in the group is more than 85 kg but less than or equal to 110 kg; and

(g) 1.00 m² for each pig where the average weight of the pigs in the group is more than 110 kg.

Fig 3 from The Welfare of Farmed Animals(England) Regulations 2007 Schedule 8

Where chronic respiratory disease occurs it may be appropriate to reduce stocking density by anything up to 30%.  However, such a reduction obviously incurs a financial penalty and must be carefully costed out.   Furthermore, in cold weather the balance between temperature maintenance, air flow and stocking rates will often require some compromise.

Hospital Pens

A full discussion of hospital pen requirement is beyond this paper but where disease occurs it is vital that facilities exist to deal with compromised pigs.   The same principles apply here as to the mainstream accommodation i.e. warmth, hygiene, AIAO, but extra care is needed as these represent the  most  diseased  pigs  and  have  the  greatest danger to healthy pigs. Returning hospital pigs to the mainstream can be a high risk strategy as many sick pigs are chronically affected and act as generators and sources of infection. As a rule  of  thumb,  do not  return  sick  pigs  to  the mainstream system.

Vaccination

The idea of vaccination is to expose the pig to a controlled dose of a particular harmful organism (which has been altered to reduce risk of it causing disease) to allow the young pig to develop immunity such that it is protected against that infection when it meets it later in its growing phase.  Whilst some diseases of growing pigs can be controlled by vaccinating the sow - relying on transfer of immunity to piglets soon after birth - most of the respiratory pathogens are controlled by direct vaccination of the piglets.   Table 1 lists those diseases for which vaccines are currently available.

Unfortunately at present, even though some farms require multiple vaccinations, complex combination vaccinations (such as those available for cattle, dogs and cats) are not available, but they are slowly being developed.

A carefully drawn up programme is required for each individual farm and care is needed when more than one disease requires control as it is often not appropriate to give 2 different vaccines simultaneously.   This is particularly relevant with PCV2 vaccines.

Some general rules regarding piglet vaccines and their administration can be drawn up:-

• Store unused vaccines in a fridge (2-8°C)
• Discard part used bottles
• Only use clean equipment to administer
• Adhere  to  product  licence  advice  or  veterinary advice (if different) regarding dose and timing of vaccine administration
• Do not combine vaccines in the same syringe
• Only vaccinate healthy animals
• Vaccinate young piglets (slaughter generation) in the neck rather than the ham

Vaccination against specific diseases e.g. Enzootic pneumonia does not protect against all types of pneumonia.  It is always necessary to undertake a proper diagnostic investigation and tailor vaccine provision to the specific farm requirements.   The monitoring programme outlined in the second paper in this series (CRD Control Part I) must then be continually used to monitor the effectiveness of the vaccine  and  adjust  the  programme  where necessary.

By way of example, abattoir examination (under BPHS) identifies Enzootic Pneumonia type consolidation in a known infected farm, with an SEP score of 8.   Vaccination is applied to piglets at weaning (using a commercial Mhyo vaccine) and 12 months later the SEP score has reduced to 2.  On farm data would be expected to show an increase in growth of c45gms/day between 30-100kg reducing days to slaughter by 5 days saving 8 kg of feed i.e. £1.18/pig.

Table 1

Respiratory Pathogen Vaccines for pigs

1. Mycoplasma hyopneumoniae (Enzootic Pneumonia)

2. Porcine Reproductive Respiratory Syndrome (PRRS)

3. Porcine Circovirus Type II (PCV2)

4. Haemophilus parasuis (Glässers Disease)

5. Actinobacillus pleuropneumoniae*

6. Strep suis*

7. Pasturella multocida type D (Atrophic Rhinitis)

8. Aujeszky's Disease**

*only available under Special Import Certificate

**applies to Northern Ireland only