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Garth Newsletter - Spring 2002
PMWS/PDNS
We have reviewed our client base and currently 70% of farms have had PMWS/PDNS diagnosed on them within the last 2 years. Of this 70%, there has been great variation in the severity of disease. Most farms' mortality due to PMWS/PDNS has, on average, increased by approximately 30-50% compared to the pre PMWS weaning to slaughter mortality.
Epidemiology
Firstly and most importantly the virus is confined purely to pigs and is highly resistant in terms of inactivation by disinfectants. The causative Porcine Circovirus 2 (PCV2) virus is between 60-80% similar to the non-pathogenic PCV1. However, analysis of the protein sequences from PCV2 is similar to some plant circoviruses yet is quite different to other animal circoviruses such as chicken anaemia virus.
PCV2 has been in the population for more than 15 years, yet it is only in the last 2-3 years that it has caused disease in Europe. Other viruses in the same family are psittacine beak and feather disease, chicken anaemia virus and pigeon circovirus.
It has been shown that clinical signs and lesions of PMWS are closely associated with the presence of antigen to circovirus. It has now been shown that PCV2 on its own can create wasting disease but this can be intensified particularly if there is a co-infection with either parvovirus or PRRS virus.
There is evidence that the virus can be transmitted to pigs at birth although this is probably uncommon. Research has also shown that colostrum antibodies give piglets protection up to at least 3 weeks of age, hence under normal farm circumstances we can expect all pigs to be resistant until then, although protection can last longer if antibody levels are high in the sow's colostrum. There is some evidence that if this is the case protection can last up to 40 days of age.
It has been reported that foetal lesions and abortions in pigs can be associated with PCV2 although it is uncommon. Experimental infections show that parvovirus and PCV2 together can cause a higher level of disease.
There is experimental evidence showing that if you have positive and negative pigs in the same airspace the disease is readily transmitted to animals within the same group.
The disease is now present in Scotland. However, the spread of disease in Scotland would appear to be slower, due to the lower density of pigs and greater distance between units.
Clinical Features
Typically PMWS occurs between 6 and 13 weeks of age. Classical signs are sudden loss of condition, breathing difficulties, jaundice, pallor, enlarged lymph nodes, yellow watery scour and lack of response to conventional treatment. In conjunction with the clinical signs of PMWS, signs of PDNS are also more common. This is normally seen as patchy, purple or ruby red discolouration of extremities, black spots on the body, enlarged inguinal lymph nodes and also legs that suddenly swell up and are oedematous (particularly the back legs). Boars are more commonly affected with PDNS than gilts.
Diagnosis
Post mortem examination is critical and the common features are interstitial pneumonia, large lymph nodes and non-collapsed lungs. Laboratory findings include lymphocyte depletion, presence of syncytial cells and granulomatous inflammatory infiltration of the lymph glands.
In PDNS enlarged lymph nodes and enlarged kidneys, sometimes with areas of haemorrhage, are observed. The virus would appear to mainly multiply in the white blood cells of the animal. However the level of multiplication of the virus can be increased if there is a further activation of the immune system by a co-infection particularly with viruses such as parvovirus and PRRS. In utero infection appears to be associated with infection in the cardiac muscles. PCV2 would appear to multiply in monocytes and macrophages and despite the production of antibodies by the pig the PCV2 can remain for at least 7 weeks after infection.
Disease Control
The scientific evidence for controlling PMWS and PDNS is very poor and we are still highly dependent on Madec's 20 rules.
On many farms we have seen other changes that have helped significantly to reduce the incidence of PMWS/PDNS. Currently many farms are trying out serotherapy. Whilst it does seem to be giving some positive results it is not consistent. We are still cautious about the use of serotherapy and are not convinced it is the absolute answer. Also there is some circumstantial evidence that giving pigs high doses of serum (i.e. 20ml) when they have just started with the disease may have a positive effect. We are still short of hard data on the effectiveness of this technique and we would appreciate hearing about people's experiences pre- and post-serotherapy. Please let Neville at the practice know if you have concrete figures.
From blood sampling sows and feeding pigs we have seen 2 patterns:
a) About one in seven sows is negative serologically for PCV2.
b) The antibody levels in finishing pigs are generally 2-4 times higher than in sows.
This varying level has lead to a belief that mixing serum from at least 2 pigs is beneficial when using serotherapy.
We have recently developed a protocol for the use of serotherapy. This was circulated to all farms currently using the technique. However, if you require a further copy please contact the practice.
Our experience on farms consistently indicates that to control wasting disease, good husbandry and hygiene are essential. Chilling pigs sets off PMWS at staggering levels. Another factor that triggers severe wasting disease is moving pigs. We advise not moving pigs at the initial 7-12 week stage. Several farms have achieved a significant reduction in disease levels by keeping pigs in the same pen from weaning to 35kg and then moving them. Whilst this is relatively easily achieved in outdoor weaner huts, reconfiguration of indoor weaner accommodation is much more difficult.
Partial depopulations
Partial depopulations have shown good results on weaner producing farms with significant drops in mortality, although this does tend to increase again over 6-9 months. Best results have been by taking all weaned pigs off the farm and then also moving the next 3-4 weeks weaners off site.
Health status
If the health status of a unit is 'marginal' prior to PMWS then in most cases the PMWS/PDNS is very severe and realistically partial or complete depopulation should be considered.
SEROTHERAPY AND PMWS
You may have read or heard of farms using injections of serum to try and control PMWS.
The use of serotherapy must be under direct veterinary control. Guidance on its use has been issued by the Veterinary Medicines Directorate. If you are currently using or considering the use of this procedure you must discuss it with your herd vet to ensure you fully comply with the legal requirements.
On farm euthanasia of pigs
The strict legal position for on farm euthanasia is that a slaughterman's licence is required. But there are a number of exemptions, including emergency slaughter to stop unnecessary suffering e.g. a broken leg. It is a legal requirement to euthanase any pig in an emergency situation if the welfare of the animal is at risk.
Whenever pigs are shot with a captive bolt they must be either pithed or bled out by cutting major blood vessels. The bolt itself may only stun the animal.
Any form of on-farm euthanasia must be humane and avoid any potential suffering.
Captive bolts are an effective and safe method of on farm euthanasia and we advise that all units have their own captive bolt pistol. There is no requirement to have a firearms certificate to obtain or use a captive bolt.
The procedure for obtaining a slaughterman's licence is to contact DEFRA and request a visit to inspect/approve use of the captive bolt. This visit is likely to be chargeable although you may be able to get it done at the same time as a statutory visit e.g. for the renewal of a weaner exemption licence, multiple pick up licence approval etc. DEFRA (if satisfied) can then issue a Certificate of Competence and for a "one off" fee this will allow you to obtain a slaughterman's licence.
It could be helpful for one or two people from every pig enterprise to obtain a slaughterman's licence.
The Food Standards Agency has recently reminded us of the legal position for carcasses of animals slaughtered on farm. There are only two situations where it is clear that the meat can be used for human consumption:
- Where it is for consumption by the owner and their immediate family only.
- Where killed on farm by a licensed slaughterman (or vet) with a prior arrangement to take the carcass to an approved abattoir for dressing. It may then go for sale and consumption in the usual manner.
If you are killing and intend to eat meat from your own pigs you must be entirely satisfied that the animal was healthy at the time of slaughter and that the carcass does not contain any evidence of disease or damage that would render the meat unfit. Although rare there are infections that can be transferred from pigs to man. If in doubt do not use the carcass meat.
MFSP'S (PRESCRIPTIONS FOR FEED MEDICATIONS)
For each farm we have a record of medications that have been agreed with the herd vet for each different feed in use. A copy of this in-feed medication table is sent to you after each visit. It is helpful to the office if you let us know of any changes to feed suppliers and the names of feeds so that prescriptions for medication to the same age group as agreed can be issued promptly. Otherwise we need to contact you to check that the mill have requested the correct medication in the correct feed and this can result in delays in obtaining the feed.
If you are a home mixer you require a valid MFSP (date and tonnage) to make or feed a medicated ration and also to hold the appropriate POM product on farm. Remember that you also need to be licensed by RPSGB and keep records of all medicines used and medicated feeds made and fed. These should include the appropriate MFSP number.
We will be supplying MFSP's for all home mixed diets on the first day of each month to enable most situations to be covered. MFSP's will not necessarily be issued when each batch of medication is supplied but will be needed if MFSP's issued on the 1st of the month are not sufficient to cover the quantity required.
As there can be oversights please let us know if you wish to make a medicated feed and do not have a valid MFSP for the entire tonnage. To make it easy we will be asking for tonnage of each ration made each month at the next visit.
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