decrease font size
increase font size
change type face
bookmark this page
email this page
print this page
Pig Health Featured Articles
Print This Page
A PMWS update
By Jake Waddilove, Veterinarian - We still do not fully understand exactly how PMWS is moving about and how to stop it. This article looks at a a number of these issues and helps to answer a few questions.
 PMWS & PDNS Technical Zone Back to the Technical Zone Sign up for the Mailing list Go to the Discussion Forum |
PMWS is most commonly seen in pigs of 8-12 weeks of age, although the age spread can be 5-16 weeks. The prime clinical signs are progressive weight loss, inappetance, hairy pigs and in many cases respiratory distress. Scour can often be a feature. Pallor and sometimes jaundice are seen and there can be an increase in gastric ulcers. In most cases body lymph nodes are enlarged and palpable.
Many other signs can be seen and these are often related to secondary infections, including Glasser’s Disease, Salmonella, E.coli infections, Staphylococcal skin infections, Enzootic Pneumonia, bacterial pneumonias, and many others.
Mortality rates vary but farms have been reported with post-weaning mortality in excess of 60% and figures of 15-25% are common. Another feature is the persistent nature of the syndrome – some farms are still affected after two or more years and still experience similar levels of mortality.
A further cost is that the overall growth rates and feed conversion on the farm deteriorate due to the number of debilitated pigs present. It has been estimated (1) that the costs on a typical affected farm are equivalent to £10.39 per pig sold.
It is important to realise that not all outbreaks are as dramatic as this. Some farms have been positively diagnosed and have shown mortality increases of only 1-2% and have recovered in 3-4 months.
Are there risk factors for the syndrome?
Anything that stresses the pig at or around weaning and up to 12 weeks precipitates the problem. Poor environment, chilling and mixing of pigs from different farms, different age groups or even different litters can all increase the syndrome. Continuous production and failure to apply good biosecurity measures are major risk factors. Overstocking is another important area. Proximity to an affected farm increases the risk – if you are within 3 kilometres of a case you are nine times more likely to develop the syndrome.Has PCV2 been linked with any other diseases?
While PMWS remains the most important syndrome associated with PCV2, this virus has been linked with a number of other syndromes in various parts of the world. It is widely thought that PCV2 is at least involved in the epidemic form of PDNS. It has also been implicated in certain types of Congenital Tremor (aII) in piglets, nervous disease possibly leading to sudden death in weaners, Proliferative and Necrotising Pneumonia (PNP), perinatal myocarditis, low-grade reproductive problems and possibly Sow Abortion and Mortality Syndrome (SAMS).Correct diagnosis is important
Because this syndrome causes variable signs, diagnosis is not always easy. As the syndrome renders the immune system less effective other concurrent diseases are common including Glasser’s Disease, Salmonellosis, Enzootic Pneumonia, PRRS, Suppurative broncho-pneumonias and Streptococcus suis. We need to be careful not to miss lower levels of PMWS due to masking of these other diseases.Diagnosis must be based on a combination of clinical signs, multiple postmortems, and especially histology (Lymphocyte depletion, histiocytic infiltration and interstitial pneumonia are common). Finally PCV2 virus should be demonstrated in lesions by specialised techniques.
How do you control PMWS?
The most frustrating thing about this syndrome is the failure of many attempts at controlling it. If we couple this with the lack of successful treatments of affected pigs, the levels of mortality seen and the persistence of the syndrome on farm it is easy to see why PMWS is driving producers out of business.There have been many areas suggested for control, and the most successful have been based on management changes along the lines suggested in Madec’s 20-point plan (2). Essentially the aim is to reduce mixing of pigs, reduce the microbial load the pig experiences and reduce the levels of stress it is exposed to.
A major part of the plan is the cleaning and disinfection between batches of farrowing, weaners and finishers accompanied by all-in, all-out production. For the plan to be successful a minimum of 16 out of 20 points must be followed, and for real success all 20. Madec reports a reduction of wean-to-finish mortality of 20.3% to 5.8% on one farm and Dennis (3) has achieved similar reductions (23.6% to 5.3%).
One problem with the Madec 20 point plan is that many farms struggle logistically to achieve it. Initially these have to apply more general management changes targeted at reducing challenge and important risk factors to the syndrome:
- Reduce stressors – especially environmental (temperature variation, draughts and noxious gasses).
- Adopt strict all-in, all-out at whatever level possible.
- Ensure strict biosecurity between batches- use a disinfectant that is appropriate – see below.
- Do not mix pigs from different sources or age groups.
- Decrease stocking densities.
- Control concurrent infections by appropriate means including vaccination, antibiotics and management.
- Stabilise herd immune status – consider closed herds with within herd multiplication or at least good isolation and acclimatisation of incoming stock.
Finally what about vaccination? Scientific work now indicates that it is possible to produce immunity against PCV2 and that this can protect against PMWS. Unfortunately development of a commercial field vaccine is likely to be one to two years away. Some success is being achieved by serotherapy, where mortality reductions of two thirds have been achieved on farms that could not comply with the Madec plan. A different approach has been to try vaccination of weaner pigs against PPV – results have been variable, but some good results have been reported from the US.
Circoviruses are notoriously resistant to disinfectants. Yet a major part of both the Madec Plan and other ways of controlling the syndrome is to adopt a thorough terminal cleaning and disinfection programme between each batch of pigs.
The purpose of this is not only to reduce the challenge of PCV2 to the pigs, but also to reduce the challenge of other pathogens and agents that may be involved with the triggering of the syndrome (e.g. PPV and PRRS) and to reduce the burden of secondary infection. For this reason we need a disinfectant with a proven activity against PCV2 but also a proven broad spread of activity.
For best activity thorough cleaning before disinfection is essential and here the heavy-duty detergent Biosolve (DuPont Animal Health Solutions) is advised. Virkon S should also be used as a part of vehicle disinfection programmes – recent work at MIRA (the independent vehicle testing agency in the UK) shows that Virkon S is a safe disinfectant for repeated use on vehicles.
It should also be used for footdips (5), wheel dips and similar to prevent spread of the syndrome. Good water disinfection is also important and here again use Virkon S.
Many countries and businesses that are currently free from PMWS are asking how they can stay free. With our current lack of knowledge of the epidemiology of the syndrome and especially its spread, the answer is we don’t know if we can prevent it, but we do know that we must try.
All farms not showing the syndrome must maximise control measures now – waiting until the syndrome is nearer may well be too late. Prevention will be based on sound biosecurity principles:
- Maximise external biosecurity – control visitors, vehicles and animal access.
- Close your herd, or purchase stock from a source that is free from clinical signs or history of PMWS. As there is no specific test for the syndrome it is impossible to guarantee freedom. Biosecurity at the supply farm must be of the highest level.
- Have effective bird and rodent control programmes.
- Adopt management procedures that will minimise the effects of PMWS – these must include be all-in, all-out production coupled with good terminal biosecurity between batches – use Virkon S for this.
- Reduce stress to young pigs. Do not mix pigs.
References:
(1) Richardson, P. (2001) MLC Stoneleigh Conference, UK; (2) Madec F. et al (1999) La maladie de l’amaigrissement du porcelet (MAP) en France. 1. Aspects descriptifs, impact en élevage. Journées de la Rech. Porcine en France. 31 347-354; (3) Dennis, I. (2002) The Pig Journal 49; (4) Royer, R.L. (2001) Susceptibility of Porcine Circovirus type 2 to commercial and laboratory disinfectants. J. Swine Health Prod. 9 (5) 281-284; (5) Amass, S.F. et al, Evaluation of the efficacy of a peroxygen compound, Virkon S, as a footbath disinfectant. J. Swine Health Prod 2001 9 (3) 121-123.Source: Jake Waddilove - December 2002
