TECHNICAL ARTICLES Swine

European Management Strategies to Control PMWS

1. PIC Europe, Fyfield Wick, Abingdon, Oxfordshire, UK
2. Eastgate Veterinary Group, Cotton Lane, Bury St. Edmunds, Suffolk, UK
3. B&M Consulting Vet Porcino, Sant Cugat del Valles (Barcelona), Spain.

Since Post-weaning multisystemic wasting syndrome (PMWS) was first identified in Europe, different strategies have been used to try to control the disease and its effects, with variable levels of success. There is no a single strategy that can claim control by itself.

Why is PMWS so difficult to control?

There are several reasons why it is proving so challenging to find a strategy to control PMWS that is successful and applicable in all pig units:

  • We are still trying to deal with the unknown: how does the causal agent(s) behave, how it spreads and how it interacts with the immune system and other diseases. This only leads to speculations and approaches by “best guess available”.
  • Diagnosis of PMWS in the field still presents difficulties, especially when it is combined with other diseases. There is over and under diagnosis. Sometimes a successful treatment for PMWS is in reality a successful treatment for another disease present at the same time.
  • PMWS is a big problem; therefore vets, farmers and managers usually try different control strategies at the same time. This makes it difficult to evaluate the efficacy of individual measures. Also when different interventions are being used, it is possible that some or all are having a beneficial effect in controlling the disease. The disease arrives at a threshold where losses are acceptable, and then there is a tendency to remember only the last one or two measures applied even if they might not have been the most effective ones (perception theory).
  • A lot of the information available is circumstantial evidence, where control groups are not the norm and where it is difficult to separate facts, conjectures, beliefs and opinions.
  • Clinical trials are the best study design to address the questions of treatment but they are very difficult to perform with PMWS under field conditions.
  • The majority of academic research into control measures are epidemiological studies (case-control studies) that have limitations as they are observational and retrospective studies. The type and quality of information about the intervention may be uneven, and the strength of evidence they provide is moderate to weak. There are also descriptive studies but they do not have a control group and therefore they provide poor evidence of efficacy. (Slater, 2003)

Strategies for PMWS control

The different PMWS control strategies are directed to a) decrease levels of infection and b) increase immunity levels, and consequently achieving a stable herd. Decrease infection + increase immunity (or decrease susceptibility) => Stable herd.

The control strategies that have been used can be classified in different groups (even if there is some overlap, it will help clarification in this paper):
1. Management
2. Nutritional
3. Genetic
4. Therapeutic
5. Immunological
6. Biosecurity

The reason to choose one strategy or another varies, and nearly all control programmes include a combination of strategies.

1. Management Strategies

1. 20 Madec points and their developments: It has been demonstrated in France (Madec et al. 1999, 2001) that management strategies can have a positive impact in controlling PMWS. Madec summarises this in 20 management points that were first published in 1999. These 20 points are nothing new, just the basics of pig management fully applied.

They are based in strictly applying all in-all out (AI-AO), limiting stress as much as possible, reduced stocking densities and strict hygiene. It is suggested that at least 16 of the 20 points must be carried out to show a significant response. The challenge is in applying all these points, especially in big units. These points have also been applied successfully in other countries. Reduction in post weaning mortality was observed from 19 to 3.7%, from 12 to 3%, from 21.4 to 10.2% and from 20.3 to 5.8%

Mike Muirhead (ThePigSite.com) published a development of Madec’s plan which summarised the key points in three golden rules; this was later updated and published (Muirhead 2002a) in conjunction with the UK Meat and Livestock Commission (MLC) and the golden rules were increased to four:

1. Limit pig to pig contact

2. “Stress” is a killer

3. Good hygiene

4. Good nutrition

Note that individual measures have not been found to be successful by themselves. For example a Danish study (Hassing 2003a) found that AI/AO did not reduce mortality in a statistically significant way.

2. Batch farrowing: changing to a batch farrowing practice every 2,3,4 or even 5 weeks has proved to be successful for some farmers. The main impact of this technique is to allow farms to undertake advantageous management techniques for PMWS which they would not otherwise be able to do (e.g. AI/AO, terminal disinfection, age segregation etc.).

This system has plenty of advantages, from improved health to faster growth but it also has its disadvantages (difficulties in achieving mating targets, boar use if not using AI, etc) and might be difficult to implement in herds over 500 sows. There is a report from a 380 sow herd where after changing to batch farrowing and implementing hygiene procedures the post weaning mortality decreased from 23.6% to 5.3% (Dennis 2002).

There is a farm where mortality was reduced from 18-20% to 6-8% when the farm was moved to a 3 weekly batch (Waddilove 2003). However other work shows that variation can be observed between batches. A farm that was moved to 3 week batch production, had batches with 9.8%, 9.7%, 8.8% mortality and others batches in between with 1.3% and 1.2% mortality (Marco 2003a).

3. Partial depopulation: Partial depopulation of the flat decks has proved successful to control PMWS for some farmers and not for others. It is not clear what the key factors for success are. It could be speculated that it will be successful once the sow herd is stable for the disease.

This raises the question: how do we know that the sow herd is stable? Currently we do not have any tools that can answer this question, so it is a matter of “best guess available” based in other parameters such as replacement rate, management system, closed herd, etc… Partial depopulations have been done combined with medication with Pulmotil®. In these all pigs that have been weaned and are under 9 months of age are removed (ELANCO EP eradication programme) and there have been reported reductions in wean to slaughter mortality from 17 to 4% (Waddilove 2003).

In most cases partial depopulation will not eradicate PMWS, but if it helps to control or eradicate other diseases like PRRS, then it can assist in decreasing the impact of PMWS in the unit.

4. Depopulation/repopulation: This technique has not been widely used for PMWS control. This is due to the fact that there is not enough understanding of the way in which the disease is spread, which biosecurity measures will protect the unit against a new infection and inadequate knowledge in how to obtain or monitor animals that are free of disease for the new stocking.

2. Nutritional strategies

1. Bioflavonoids, Vit E, Selenim and antioxidants: Bioflavonoids are phyto-chemicals that act as antioxidants (neutralise free radicals) and have anti-inflammatory effects; research suggests that they might promote cardiovascular health. There are also some claims of antiviral and antibacterial properties. There are nutraceutical products like Viramatrix® (Park Tonks Ltd, UK) that have a combination of bioflavonoids, anthrocyanins and essential oils that help the anti-oxidant and immune function. It is proposed that the physiological and nutritional changes at weaning may contribute a trigger factor for PMWS through an oxidant effect in the gut.

The producer of Viramatrix® makes no medicinal claims, but many producers have stated that the addition of Viramatrix® to their feed has helped decrease some of the symptoms of PMWS, observing a significant reduction in mortality. In some units the product has not given consistent results, especially when PMWS is complicated by severe pneumonia. In a UK farm where mortality was around 40%, partial depopulation combined with the use of Viramatrix® decreased the mortality to near 4% (Waddilove 2003). In two Spanish farms, trials were done with 459 and 201 piglets respectively (Casanovas 2003) and in both cases no benefit was observed with the use of Viramatrix®. It might be speculated that a good positive effect in the reduction of mortality is only observed in farms that are suffering a very high mortality.

Addition of Vit E and/or selenium might be beneficial in some units due to their antioxidant properties. Data from one herd in Denmark showed no correlation between the vitamin E levels in blood of postweaning pigs and the outcome of PMWS (Baekbo et al. 2003).

2. Meal versus pellets and other feed related ideas: In the Danish study where the first cases of PMWS were characterized (Hassing et al. 2003), it was observed that farms using home mixed feed (meal) for suckling piglets had lower mortality in the flat decks compared to the farms using purchased feed (pellets) for the suckling piglets.

On the other hand, in a case control study done in the UK (Cook et al. 2001) no association was detected between any of the following and PMWS: home mixing, inclusion of home-grown cereals, purchase of fee in bags or in bulk, use of mould inhibitors and use of feed acidifiers.

3. Ad libitum feeding: It has been observed in fattening units affected with PWMS, that the pigs fed ad-libitum had a higher mortality compared to the pigs on restrict feed. Observations in a farm from January 2002 until May 2003, showed that females had a mortality of 10% when restricted fed (slight restriction after 20 Kg) and 11.06% fed ad-libitum.

In the males the mortality was 13.85% in the restrict fed group and 25.75% in the ad-libitum group. In a second farm, the restricted fed group (after 20 Kgs animals were fed manually twice a day) had mortalities of 2.07% and 1.69% in batch 1 and 2 respectively (groups of 530 animals) and the batches fed ad-libitum had 7.7% and 4.2% mortality respectively (groups of 910 animals) (Casanovas 2003).

3. Genetics

1. Breed susceptibility/resistance: There are several field observations from farmers that changing the boar led to an improvement in the levels of PMWS. This does not appear to be a change towards a specific breed, even if Pietrain seems to be one of the most preferred options. So far there is no scientific paper published with objective research looking into this information, but there are institutions in Europe that are working in this area. Information has to be processed carefully as many times a change in boar implies a change in source. This can lead to confusing breed resistance with source change effects.

It could be speculated that Pietrain shows less susceptibility due to its slow growth in comparison with other commercial boars. This might have an effect when the heart is damaged by PMWS (Marco 2002 a,b), and slow growing animals might be less affected.

2. Boars with high appetite: It seems contradictory with the Pietrain observations above, but there are field observations coming from countries like The Netherlands and Italy that using boars with high appetite leads to PMWS problems being reduced. This might be because the animals are less susceptible to the general effects of the disease (not less susceptible to the infection) and they keep eating during the infection process; this might also be true for other diseases. There are no papers or research published in this area.

4. Therapeutic strategies and control of other diseases

1. Serotherapy: Serotherapy was first used as a control measure to decrease the incidence of the clinical signs (see below 5-r for serotherapy as preventive measure) but it has also been used to treat pigs that already have some signs of the disease. Waddilove and Marco reported that affected animals treated with serum showed an increased survival rate (48% in one farm, 58% in a second farm) compared with normal survival of below 10% in both farms. In this cases, higher doses of up to 10 ml of serum was injected IP.

2. Ozone: Ozone has been used in medicine for its different properties: antimicrobial, antiviral, antimycotic, improvement of the supply of oxygen to hipoxyc tissue areas and reactivation of the oxygen metabolism capacities of the cells (Jimenez et al.). An Italian product with microencapsulated ozone in vegetable oils (Beterline®) has been used in cattle to control enteric problems.

Taglietti (2003) tried the product in a farm where after applying management measures to control PMWS there was still a slight increase in mortality due to enteric problems (4%) that were not solved with the routine antibiotics. After treatment with ozone, mortality decreased to pre-PMWS levels (1.5%).

3. Congestive heart failure: Congestive heart failure (CHF) has been reported as one of the signs in PMWS (Marco 2002 a,b) and it could explain some of the signs that are observed. Any management or therapeutic measures taken that can help minimize the effects of the congestive heart failure might be useful in the control of PMWS. Part of the effect of serotherapy, aspirin, ozone and others might be due to the effect they have on CHF.

4. Aspirin and other therapies: It has been reported (Marco 2003a) that treatment with aspirin (25ppm in water) in the finishing period help reduce decrease the remaining mortality after other measures had reduced it to acceptable levels. In one unit, the remaining 3% mortality deaths that were observed in the fattening period (after other measures were applied) stopped after the treatment with aspirin. It is speculated that the aspirin could have a beneficial effect on the congestive heart failure that is observed in animals with PMWS.

Bernard et al (2002) did a field study to evaluate the efficacy of acetaminophen, orally administered for 10 days and they observed an improvement of the total morbidity rate in a herd chronically infected by PMWS, but there are not many reports of this being a very common treatment (in some countries it is not allowed in food producing species).

5. Vaccination of piglets: A specific vaccine against PMWS is not yet commercially available. Vaccine companies are working developing a PCV2 vaccine that could be useful in controlling PCV2. Experimental vaccines have been tested (Jestin et al. 2001) with promising results, but the vaccines are not commercially available yet.

There are various papers about the evolution of PMWS after vaccinations against other diseases or immunostimulation. In some, vaccination seems to have a negative effect but not in others.

Negative effect: 1- Krakowka et al (2001) reproduced clinical signs compatible with PMWS in gnotobiotic pigs infected with PCV2 and stimulated with keyhole limpet hemocyanin in Freund’s incomplete adjuvant. 2- In a trial to compare the efficacy of a EP vaccine, it was shown that the group of animals that was vaccinated at 3 weeks of age had less mortality (8.1%) compared to the group that was vaccinated at 1 and 3 weeks of age (16.07%) (Marco, 2003b). It can be speculated that the results are due to the interference with the immune system at early stages.

No effect: In Denmark, Botner et al (2001) using conventional pigs failed to demonstrate an immune-stimulation dependent difference in pigs inoculated with PCV2.

Positive effect: In a study performed to characterise the first 45 cases of PMWS in Denmark, it was found that vaccination against Mycoplasma (details of the vaccination protocol not given) showed a trend to reduce mortality in the flat decks (p=0.10) (Hassing 2003). In a small study (using 4 pigs per group) done in The Netherlands (Stockhofe-Zurwieden 2003) pigs that had repeated administration of incomplete Freund’s adjuvant and challenged with PCV2 had a reduction of clinical and pathological lesions as compared to those found in other PCV2 infected groups.

6. Control of secondary infections: In PMWS is not unusual to observe that diseases such as Glasser’s and salmonellosis do not respond to conventional treatments. Treatment for those diseases should be a priority as it will help to minimise the effects of PMWS. In some cases different medication strategies with different drugs and doses and vaccinations should be tried to obtain success.

7. Stabilisation of the herd for other diseases: Herds that are affected with PMWS and diseases like PRRS are more severely affected that herd without PRRS. Stabilising other diseases has proved to be useful.

8. Antibiotics with antiviral effect: It has been reported that antibiotics like Pulmotil® can have an antiviral effect on PRRSV, inhibiting the replication in macrophage cell cultures. It is speculated that it may reduce the viral load in PRRS infected pigs.

There are studies that showed that Pulmotil® didn’t eliminate PRRS infection but there was a tendency to lower PRRS titers, it reduced morbidity and mortality and increased growth rates reducing the economical impact of PRRS (Benfield 2002, Lehe 2003). Could it have similar effects with PMWS virus? So far there are no trials published, but it is an area to contemplate.

5. Stabilise PMWS Immunological status

In this section, emphasis will be made on PMWS immunological status and not on PCV2 status. There are publications that suggest that high level of PCV2 antibodies will protect the piglets against PMWS but not against PCV2 infection (Allan et al. 2002). Other studies showed the opposite effect and piglets born from sows with high PCV2 titres were significantly more at risk of dying from PMWS (Hassing 2003b).

It is also known that the PCV2 seroprofiles of PMWS positive and negative farms are not that different (Rodriguez-Arrioja et al 2000, Sibila et al 2001, Larochelle 2003) and that there is no apparent difference between the prevalence and distribution of PCV2 between the pigs from control and case herds, respectively (Ladekjaer-Mikkelsen et al 2003).

1. Serotherapy: Serotherapy was first described by French veterinarians as a helpful strategy in the control of PMWS in 2001 (Ferreira, 2001). It was first used in the flat decks but later it was also used at earlier stages as in the farrowing house. It is a treatment that has to be done carefully.

The serum should only be obtained from pigs from the same farm (pigs that have survived the disease), and it has to be collected and stored in the proper hygienic conditions. Usually small quantities of serum given IP are sufficient (3ml are normally sufficient, and in suckling piglets even smaller quantities have been used). The mechanism of action has not been defined and it seems that it is not a “vaccine” or antibodies effect, but it might be linked to reduction of inflammatory cytokines (Kishimoto 2001, Nakano 2002).

Success has been variable between farms. Decreases in mortality have been published: from 10.7% to 3.6% and from 22.2% to 6.4% (Waddilove & Marco 2002), from 16.2 to 4.8% (Sanchez 2002) and from 5.5% to 1.5% (Marco 2003a).

In other farms, the serotherapy has not helped to control PMWS. Some reasons for failure are: 1) the animal selected as a donor was probably viremic for other diseases like PRRS, thus creating more problems than solutions in the farm 2) the serum has been frozen and efficacy has decreased 3) when the donors are animals that were treated with serotherapy as a piglets, the serum seems to be less effective.

2. Colostrum intake: It appears that PMWS protection given by colostrum is even more critical than for other diseases. Farmers in UK and The Netherlands have reported that they have decreased the effects of the disease by taking measures to ensure that all piglets receive a proper amount of colostrum. This seems to be of critical importance.

3. Closed herd: During the peak of PMWS in the UK in 2000, many farmers decided to close their herds. Some of them did it completely, some of them stopped taking in live animals but continued with regular semen intakes. This strategy has been used usually in combination with others and has proved some success. However it is proving challenging for small farmers to manage closed herds and they have started taking in again live animals, mainly from PMWS positive sources.

4. Introduction of positive replacements: Farmers that are PMWS positive have started taking replacements from PMWS positive farms. There are no technical papers demonstrating its benefits but field experience indicates it to be of some help. There are no negative effects reported.

5. Acclimatisation of the incoming animals: A tactic followed by some farmers that are taking replacements is to prolong the quarantine period for over the traditional 30 days (for at least 6 weeks). The idea behind this process is to expose all the animals before they are admitted to the herd, so the casual agent of PMWS might stop circulating and the animals might stop shedding the agent. It is difficult to tell if this procedure works or not, but at least it has no negative effects.

6. Feed-back: This practice is used for other diseases. For PMWS material from the farrowing house and faeces has been used to help acclimatise incoming gilts or sows over 80 days of pregnancy and to produce high levels of immunity to be passed in the colostrum (Madec et al 2002). There are no technical trials indicating its efficiency, but it does not appear to cause problems if performed correctly.

6. Biosecurity 1. Biosecurity measures: So far PMWS has presented a real challenge for biosecurity. It doesn’t seem to respect the traditional biosecurity measures applied for other diseases. Clear examples are farms that have managed for over 10 years to be free of PRRS and EP, and they have broken down with PMWS while still maintaining the PRRS and EP negative status (Donadeu 2002). 2. Terminal disinfection: Measures to decrease the pressure of infection seems to be critical. Disinfection should be carried with an appropriate product (Royer 2001) at the end of each batch, during partial depopulation etc. This terminal disinfection is an integral and important part of many other control strategies.

Conclusions

The combination of two or more of the above strategies in the control of PMWS is not unusual. Batch production, partial depopulation and change of breed have been combined with success. Implementation of the Madec management principles with serotherapy has been successful in some other farms.

It can be concluded that there is no a general control strategy that will benefit all the farms, and each individual farm should try to find the strategy that will be most suitable for its situation.

Once we have a better understanding of the disease in terms of what the causative agent is, how it behaves and how it interacts with the immune system, we will have a better understanding of why some control measures work, why some control measures work better than others or only some times, and why others do not work. With this better understanding of the disease more appropriate measures could be defined.

Acknowledgements

The authors would like to thank Tom Alexander and Dan Tucker for the review of the paper and critical comments, and Josep Casanovas, Paolo Taglietti, and other practitioners who have provided data and observations.

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Date published: 2003-11-11

Author:
Meritxell Donadeu1
DVM, et al. MS; Jake Waddilove
2 MA, VetMB, MRCVS; Enric
Marco3 DVM

Publication:
N/A