In April 2020, a free-range pig producer contacted the Riverina Local Land Services to report unusual pig losses in his herd over the previous five days. At the time of the call the producer had lost five pigs, aged 10-16 weeks from three different paddocks. In some cases, up to 75% of the litter was lost post-weaning. The pigs showed signs of staggering gait, extreme lethargy, weakness and open mouth breathing when stressed. In most cases, affected pigs died within 10-24 hours from the onset of clinical signs. Deaths were sometimes delayed for 24-48 hours when animals were treated with antibiotics and anti-inflammatories, but all affected pigs died despite treatment. The affected weaners had recently been introduced to ad lib. feeding. There had been no deaths amongst any of the older pigs or breeding stock.
The piggery has approximately 20 sows. The pigs are housed entirely outdoors with shelters in each paddock. They are vaccinated against erysipelas and porcine circovirus type 2 (PCV2) at weaning. The piggery has undergone significant improvements in management over the past 12 months. The farm uses a pig veterinary consultant for pig health management.
The Local Land Services district veterinarian conducted a property visit to examine the pigs. A live sick weaner pig on the day showed recumbency, fever (40.1°C), swollen eyelids and neck, open-mouth breathing when handled, and green frothy stools. A recently deceased pig was also available for sampling. The case history and clinical presentation of the pigs warranted including African swine fever (ASF), classical Swine Fever (CSF) and Aujeszky’s disease as differential diagnoses, along with Escherichia coli (E. coli) and pneumonia. PCV2 was also requested as the owner was concerned about its involvement in his herd as he only recently had begun vaccination. Appropriate samples were collected and sent to Elizabeth MacArthur Agricultural Institute for laboratory testing.
Laboratory testing for ASF, CSF and Aujeszky’s disease returned negative results by Taqman Assay at the Australian Centre for Disease Preparedness (ACDP) for both the live pig and deceased pig. Lung samples from the deceased pig were positive for PCV2 by polymerase chain reaction (PCR), and histology noted a histiocytic, neutrophilic, subacute, patchy, moderate bronchopneumonia. Haemolytic E. coli was isolated from the jejunum (Stx2e, AIDA, f18).
The laboratory noted that the Stx2e toxin is produced by Shiga toxin-producing E. coli (STEC) and associated with oedema disease. AIDA (Adhesions involved in diffuse adherence factor) and F18 (fimbria 18) are virulence factors involved with attachment of E. coli to the intestinal wall. AIDA may be associated with Enterotoxigenic E. coli (ETEC), Enteropathogenic E. coli (EPEC) and STEC. F18 can be associated with ETECs and STECs. All three are associated with oedema disease.
Three more weaner pigs were affected and died after the property visit.
In consultation with the pig veterinary consultant and the district veterinarian, considering the clinical signs of disease, the diagnosis of oedema disease (caused by the E. coli) was confirmed. However, the PCV2 result was also of interest as pigs on this farm were vaccinated. PCV2 is known to be associated in cases of postweaning multisystemic wasting syndrome (PMWS), which is a notifiable disease in NSW. PMWS is not currently seen in Australia despite the presence of PCV2 in Australian pig herds. Porcine circovirus-associated disease (PCVAD) has appeared in screening tests on this farm in the past. The producer has noted the occasional clinical signs (skin lesions associated with dead weaner pigs) consistent with porcine dermatitis and nephropathy syndrome (PDNS), another PCVAD.
The district veterinarian returned to the farm to sample five ill-thrifty weaner pigs from the affected paddocks and performed post-mortems on one deceased weaner pig and two of the five particularly poor weaner pigs to investigate the impact of PCV2 on this farm.
Two of the five pigs (one deceased pig and one live pig) sampled were PCR positive for active PCV2 infection in the weaner groups. In addition, the deceased pig lung had lesions consistent with PCV2 infection. PCV2 should not be detectable in tissues and blood of pigs where vaccination has been performed correctly. Upon further investigation, the farmer advised that it is possible that the batch of PCV2 vaccine he used for these pigs may have been frozen prior to use.
In the opinion of the pig veterinary consultant, this farm was likely to have PCV2 circulating on farm, which may have contributed to the recent E. coli issues. In this case, the presence of PMWS was ruled out due to the lack of supporting post-mortem lesions. There was also no evidence to support cases of PDNS. All evidence pointed towards a vaccination failure, likely due to frozen product in this case.
This investigation was the result of a cooperative effort between the pig producer, the private veterinary consultant and the Local Land Services district veterinarian. In addition, the Australian Pork Limited Evidence of Absence Surveillance Program1 along with NSW Department of Primary Industries notifiable disease funding covered the cost of the testing to allow for a definitive diagnosis to be made. The benefits of this investigation included exclusion of three notifiable diseases in a NSW pig herd, proof of freedom evidence for the pork industry and a local disease management plan to restore production and improve the health and welfare of the pigs for the pig producer.