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Emerging Threats Quarterly Report – Pig Diseases – January to March 2011

by 5m Editor
30 June 2011, at 12:00am

A report on severe cases of porcine circovirus 2 (PCV-2)-associated disease linked with the cessation of PCV2 vaccination is among the main items in the latest quarterly disease report from AHVLA.

Highlights

  • Investigations continue to determine the cause of the high mortality (22 per cent) heart failure on one pig unit
  • Multi-resistant Brachyspira hyodysenteriae identified
  • Smooth muscle hypertrophy and intestinal rupture not associated with Lawsonia intracellularis identified
  • In-coordination with low morbidity reported in four to six-week-old growers
  • A case of coal tar poisoning following the use of road planings placed under straw for eight-week-old pigs was identified
  • Severe cases of porcine circovirus 2-associated disease, linked with the cessation of PCV2 vaccination were identified

Ongoing Emerging Disease Investigation

Heart failure - 22 per cent mortality in neonatal pigs – Since QR4 2010 two further submissions of typically affected piglets were received, aged four and 21 days. The case history remains the same and piglet mortality was still high through to April 2011.

A weakness throughout the investigation has been the lack of good record-keeping by the farmer. However, an agricultural college student has recently been taken on to help with the project study and will be visiting the unit daily for five weeks to record the weights in 10 litters by weighing individual piglets on a weekly basis up to weaning.

The current thinking on factors contributing to the mortality is as follows:

  • It tends to be the heaviest piglets that are affected
  • Piglet birth weights are often high on the farm, e.g. 2.2kg
  • The sows are well fed and very ‘milky’
  • Piglets are almost invariably found dead in the sow huts in the morning
  • Mortality tends to rise with rising sow parity but deaths have occasionally occurred in piglets from gilt litters
  • There is a suspicion that mortality is partly seasonal, and is higher in warmer weather conditions
  • The sow and boar lines are hybrids producing vigorous fast growing offspring

Rapidly growing piglets are dying as a result of heart failure attributable to very good sow nutrition, strong hybrid vigour and inability of the heart to cope with these factors. Mortality may be triggered by excitement and competition when feeding from the sow.

Investigation in the herd is now focussed on more accurate on-farm recording and the submission of more typical, freshly dead piglets for post-mortem examination. Cardiac morphometry will be undertaken on affected and "control piglets". The investigation only involves this herd and to date there is no evidence to suggest a risk to other pig producers. Any further action needed will be determined when the investigation is complete.

Unusual Diagnoses

Valnemulin ‘resistance’ - An isolate of Brachyspira hyodysenteriae (cause of swine dysentery) that was ‘resistant’ to all antimicrobials used to control this organism was isolated. This included valnemulin hydrochloride which has no recognised cut off point but the MIC value was more than 64ug/ml. The farmer is being encouraged to depopulate and disinfect but is hoping to obtain compensation, from a local health scheme, for the slaughter of the herd and this has delayed depopulation. The Veterinary Medicines Directorate and the Pig Veterinary Society have been made aware of this. There is no evidence of spread from this unit but the existence of a multi-resistant strain is of concern to the industry. The Defra funded antimicrobial resistance project continues to monitor tiamulin resistance and any resistant isolates are then tested against an extended range of antibiotics that are used to control this organism, using ED1200 (Pig Expert Group) funds.

Inco-ordination with low morbidity – This started from four weeks of age in outdoor growers and was reported to AHVLA when the piglets were six weeks old. Twelve pigs had been culled due to the problem in which both fore and hind limbs were affected but pigs remained alert and keen and able to feed and drink. Pigs tended to have a wide hind limb stance with hind legs splaying outwards, at rest the hocks were held excessively straight and there was a slight body tremor. When moving, limb movements were poorly coordinated and sometimes exaggerated. One or both hind limbs appeared to be ‘left behind’ before being flexed to walk. There was occasional knuckling of the forelimbs, the stride length was long and the pigs lifted their forelegs excessively high off the ground. Histopathology in the spinal cord and peripheral nerves revealed widespread, sparse inflammatory cell infiltrates of nerve roots and, more variably, of peripheral nerves. Possibilities to consider include unusual expressions of a neurotropic viral infection, or an immune-mediated pathogenesis.

Investigations are at an early stage but include virology to determine whether the lesions relate to a neurotropic viral infection of pigs (such as sapelovirus) and histological comparisons with unaffected pigs. It is not possible to assess the impact or public health significance at this early stage. This outbreak will be the subject of a five-day investigation.

Muscular hypertrophy of the ileum

Hypertrophy of the smooth muscle of the ileum, not associated with Lawsonia intracellularis, infection was recorded on two occasions from different farms. This is an unusual finding but has been reported previously and is thought to be associated with stenosis of the ileocaecal valve, which were grossly unremarkable in these cases.

This is an incidental finding of interest only and apart from monitoring and informing the Pig Veterinary Society no further action is required.

Coal tar poisoning

Coal tar poisoning was diagnosed in eight-week-old pigs submitted with a history of high morbidity and poor condition in 10 per cent of a group of 400 pigs on a nursery unit. They had been housed on straw which was over road planings that had been obtained from the local council when a road was re-surfaced. Large amounts of tar were present in the road planings.

Appropriate steps to protect the food chain were taken and the Pig Veterinary Society and BPEX are aware of the dangers associated with allowing pigs to have access to coal tar, as a paper was presented at the Pig Veterinary Society by the AHVLA and a case report was published in the Pig Journal. No further action is therefore required.

Changes in Disease Patterns and Risk Factors

Lymphoreticular system

Several cases of unusually severe porcine circovirus associated disease (PCVD) were seen in March. Two were from the same breeding herd source which had withdrawn (sow) vaccination for PCV2. The recent severe cases are of note as they include severe systemic manifestations of PCV2-related disease, including hepatitis which had previously usually been seen in young piglets (typically three to four weeks of age) and in rare breeds, typically on small less intensive pig farms.

Some features of the presentation in these cases are more akin to those seen earlier in the course of the UK outbreak of PCV2-associated disease. In one case, affected pigs were eight weeks of age and mortality of 1.5 per cent to eight weeks was reported. The affected pigs formed one of two sources on a grower unit and pigs from another breeder source that were vaccinated and in the same airspace were unaffected. The reasons for such a severe presentation of disease so early after withdrawal of vaccinal protection – affected pigs were from sows that had received PCV2 vaccination previously and in some cases had only missed one dose – are not clear.

In another case of severe PCVD, a similar presentation was seen in five- to six-week-old pigs from a small rare breed Middle White herd which did not practise vaccination for PCV2. The carcass lesions, including a necrotising splenitis, in this case, could not be differentiated from swine fever and therefore resulted in a report case. Subsequent histology confirmed the lesions which again included a severe hepatitis to be PCV2 related.

This case was highlighted in the March disease summary report that was placed on the BPEX web site. It was used recently as part of training course on Notifiable Diseases of pigs for VIOs and VOs. Pig practitioners, BPEX and the Pig Veterinary Society are aware of the issue and the apparent risk of greater losses if pig farmers try to save money by reducing PCV2 vaccination. The PEG will monitor the situation in the months to come.

Other systems - There were no significant changes in disease patterns for other diseases or organ systems.

Horizon-Scanning

Posterior paralysis of sows as reported to the European pathology meeting in Edinburgh in March 2011 by Klaas Peperkamp of Deventer in the Netherlands – Notes from the meeting

A small study was made of cases examined in the Netherlands in 2005 to 2006. These were submissions of adult pigs from 33 farms, each with posterior paresis or sudden death as presenting signs. In a few of the farms, there was more than one case per year. Those affected were adult sows which had had one or more litters and there was a single boar. No gross pathology was found and bacteriology was unremarkable.

On histopathology, 37 of the 52 cases had a leucoencephalomyelitis with perivascular mononuclear cuffing in white matter and variable eosinophilia possible neuronophagia. Eleven of these 37 also had musculoskeletal lesions, considered probably to be secondary. In eight pigs, there were no changes but this may have been due to restricted sampling. Some neuronal necrosis without cellular infiltration was identified in the ventral horn (also possibly due to sampling restriction?).

Nineteen of the 37 were checked for virus infection. All were Teschen virus-negative (as expected as grey matter relatively unaffected and disease usually seen in younger animals). Five were PCV2-positive (three farms), four were PRRSV-positive (three farms) by PCR-IHC and in-situ hybridisation were not employed as these viral infections were considered unlikely to be the cause.

Differential diagnoses

  • Fibrocartilaginous embolic myelomalacia – a few cases are diagnosed with cartilaginous material found in lesions.
  • MCF – there was no sheep contact.
  • Porcine reproductive and neurologic syndrome – this is a clinical description, the cause is unknown.
  • Polyarteritis nodosa – different pathology.
  • Immune complex disease – Streptococcus suis has been suggested as a cause. Bacteriology was negative.
  • Post-immunisation encephalomyelitis – this has been reported in children, diffuse pathology, usually after immunisation with measles vaccine.
  • Porcine lymphotropic herpes virus – one case was positive, but it is not convincing that this virus is pathogenic.

Three references of previous reports were quoted.

  1. Vet Record 2007, 161, 552; Austrian cases, no consistent cause.
  2. J. Vet. Sci. 2005, 67, 125; Polyarteritis nodosa.
  3. Acta Neurol. 2005, 62, 1673; Human cases.

Conclusion: Posterior paresis in sows occurs sporadically.

Comment and discussion from the Edinburgh meeting

One possibility considered was a post-vaccinal reaction. SAC commented that they had seen a similar clinicopathological presentation in sows in an organic unit in northern Scotland, where vaccination was not practised, indicating that vaccination would not be the only possible cause. The SAC case was reported in the Emerging Threats Quarterly Report 3, 2010.

No further action by the PEG is envisaged but members and VLA pathologists are aware should cases, additional to the SAC case, be reported.

Influenza vaccination

There are reports from the US (Vincent et al., 2008; Gauger et al., 2011) that pigs vaccinated with influenza vaccines may show a more severe response if exposed to a virus strain not covered by the vaccine that has been administered. There were more severe pulmonary changes, clinical signs and poor performance. These findings are from experimental studies and have not been observed in the field. In the UK, a new vaccine (not that used in the experimental studies described above) has been introduced which includes avian like H1N1, H1N2 and H3N2 (the last not present in the UK). There may be cross-protection from this vaccine for pandemic H1N1.

The PEG will ensure that when investigating cases of swine influenza part of the history taking will be to determine whether vaccine to protect against influenza has been used. This information will be passed to the programme manager for swine influenza surveillance. PEG is not aware of any evidence of enhanced disease post vaccination in pigs in the UK and no swine influenza has been diagnosed by VLA or SAC in influenza-vaccinated pigs.

References

Gauger P.C., Vincent A.L., Loving C.L., Lager K.M., Janke B.H., Kehrli M.E. and Roth J.A. (2011). Enhanced pneumonia and disease in pigs vaccinated with and inactivated. Article in Press, Vaccine.
Vincent A.L., Lager K.M., Janke B.H., Gramer M.R. and Richt J.A. (2008) Failure of protection and enhanced pneumonia with US H1N2 swine influenza virus in pigs vaccinated with an inactivated classical swine H1N1 vaccine. Veterinary Microbiology 126, 310-323.

Further Reading

- You can view the full report by clicking here.

- Find out more information on the diseases mentioned by clicking here.

June 2011