Emerging Threats Quarterly Report – Pig Diseases - October-December 201307 May 2014
This report from the Animal Health Veterinary Laboratories Agency (AHVLA) in Great Britain includes reports on a single episode of piglet deformities on outdoor unit, the increased incidence of Glassers disease and swine influenza outbreaks in combination with salmonellosis. Overviews are also included of Porcine Epidemic Diarrhoea in North America and African Swine Fever in eastern Europe.
These reports aim to identify emerging animal disease-related threats. Their production is underpinned by a large amount of surveillance data and information compiled as part of the Defra Food and Farming Group animal disease surveillance programme. Some of these data can be viewed on the AHVLA web site.
- Spread of Porcine Epidemic Diarrhoea virus within North America
- Single episode of piglet deformities on outdoor unit
- Increase in Haemophilus parasuis disease incidents
- Swine influenza outbreaks with salmonellosis
- African Swine Fever detected in Lithuania
Spread of virulent Porcine Epidemic Diarrhoea virus within North America
"These results suggest a high degree of susceptibility of the national pig herd to PEDv."
Porcine epidemic diarrhoea (PED) emerged earlier in 2013 in a virulent form in the United States of America (USA) and, in January 2014, the first diagnosis of the disease in Canada was made on a breeder-finisher unit in Ontario in a relatively pig-dense area. There are thousands of livestock movements between the US and Canada each year and Canada had been concerned about the risk of introduction of PED virus either by live pigs or on vehicles contaminated with infected faeces. The route of introduction of infection into the USA has still not been established although the virus is believed to be of east Asian origin. By January 2014, 23 US states had reported at least one confirmed case of PED.
BPEX funded an assessment of seroprevalence to PEDv in GB pigs using samples collected for a recent abattoir survey in pigs jointly funded by Government and industry. Antibody to PED virus was detected in approximately 10 per cent of slaughter pigs, suggesting that the national pig herd is largely naïve to PEDv, whether endemic or virulent. Caecal samples from just the seropositive pigs were tested for PED virus by PCR and no viral nucleic acid was detected.
The low seroprevalence indicates that PEDv serology could be a useful tool to detect exposure to virus at herd level but PEDv PCR on individual faecal samples remains the method of choice for diagnosis. Batch PCR testing of routine diagnostic samples submitted to AHVLA from pigs with diarrhoea through a combination of BPEX funding and Defra-funded surveillance has not detected PED virus so far.
These results suggest a high degree of susceptibility of the national pig herd to PEDv, and the limited PCR testing of diagnostic samples has not implicated endemic PED viruses as a significant current cause of diarrhoea in pigs in England and Wales.
Funding for diagnosis of suspected virulent PED outbreaks, should they occur, remains available from the Defra-funded pig scanning surveillance project (ED1200).
Unusual Diagnoses or Presentations
There were a number of unusual diagnoses this quarter; details of these have been included in monthly AHVLA and SACCVS reports and AHVLA highlights to BPEX, BPA and Pig Veterinary Society. These will be kept under review to assess whether they are seen on other units and justify initiation of emerging disease investigations.
Otitis with bacterial and mycoplasmal involvement in pigs with swine influenza
Further to the unusual outbreak of head tilt in outdoor gilts described in the last 'Emerging Threats' report, another outbreak of middle and inner ear disease causing nervous signs was diagnosed in 10-week-old growing pigs being reared in 'cosikennels'. Thirty of 700 were affected and five died.
Clinical signs were principally head tilt, head tremor and circling, and there was also weight loss and some respiratory disease in the group. The previous batch of pigs weaned three weeks earlier from the same breeding herd had also been affected. The pigs were vaccinated for PRRSV, PCV2 and Mycoplasma hyopneumoniae.
Five typical cases were submitted live (Figure 1) showing head tilts and poor body condition. Sectioning of the cranium in all the pigs revealed exudate accumulation in the region of middle and inner ears with softening, fragmentation and pale irregular discolouration of the adjacent cancellous occipital bone (Figure 2). Exudate was also present in the adjacent ear canal in two pigs. Pasteurella multocida and Trueperella pyogenes were both isolated from these lesions. Lesions were well established and involved bone tissue, thus it was not surprising that a poor response was reported to recent antimicrobial treatment.
Figure 1. Pigs with head tilt due to suppurative otitis
Figure 2. Section through cranium - suppurative otitis lesions and adjacent osteomyelitis
All the pigs submitted also had severe pneumonias from which Pasteurella multocida was mainly isolated, with Trueperella pyogenes from the lung of one pig. Active swine influenza virus infection was detected in two pigs by PCR and would have exacerbated the clinical disease.
Histopathology also revealed lung lesions suggesting possible mycoplasmal involvement and Mycoplasma hyorhinis was detected in the lung of two pigs together with Mycoplasma hyopneumoniae in the lung of one.
The case was discussed with colleagues in Europe and the possibility of involvement of Mycoplasma species infection as an initiating cause of the otitis was raised.
Further testing of samples from the otitis lesions yielded both Mycoplasma hyorhinis and Mycoplasma hyopneumoniae, supporting their possible involvement. Infection may have spread to the ears from the tonsils by ascending infection via the Eustachian tubes; alternatively haematogenous spread may have occurred.
Possible mycotoxicosis causing paresis in growing pigs
Mycotoxicosis was considered a possible differential for nervous disease in seven-week-old weaners in one shed on a multisource single-age indoor straw-based nursery-finisher. Approximately 10 per cent of pigs developed an unusual ungainly gait with apparent paresis of fore and hindlimbs although the pigs could, once standing, walk but the hocks appeared over-extended. The pigs were alert, without tremor and there was no history of meningitis. One pig was submitted live showing the clinical signs described and is shown in Figure 3.
The pig was in good body condition with no significant gross lesions but histological findings were very unusual, the most spectacular change being found in spinal cord motor neurons where there was very extensive chromatolysis.
Overall, there was a very limited inflammatory response, strongly suggesting a neurodegenerative process as a consequence of toxic or of intrinsic or acquired metabolic cause.
Porcine breed-related (presumptive inherited) motor neuron disease has been reported in Yorkshire and Hampshire pigs (Montgomery and others, 1989), although the pathology in this case was somewhat different. As the pigs were a commercial hybrid of Large White, Landrace, Duroc and Hampshire breed, an inherited cause was not considered likely.
The pathology had some overlap with that reported by Wohlsein and others (2012) in cases of sudden onset neurological signs associated with extensive motor neuron degeneration suggestive of a toxic cause, although their toxicological and microbiological investigations failed to identify the cause. By extrapolation from findings in other species, possibilities raised by the histopathologist to consider included Aspergillus clavatus mycotoxicosis, aluminium toxicity, tin toxicity, copper deficiency, vitamin E deficiency and plant intoxication (Chrysocoma tenuifolia). Biochemistry ruled out copper and vitamin E deficiency and selenium status was also satisfactory.
Interestingly, other affected pigs recovered after showing transient signs and the problem did not recur or spread and no more pigs were submitted to investigate further. Given this, the acute nature of the clinical disease and pathology, and other epidemiological features of the outbreak, an extrinsic cause was probable and toxicity due to Aspergillus clavatus mycotoxicosis from fungal contamination of the environment, particularly the straw bedding was suspected to be the most likely cause but was not proven and a source was not evident from an on-farm visit by the attending veterinarian.
Single episode of piglet deformities on outdoor unit
Piglets with head deformities were born into eight of 100 litters in a single batch of sows farrowing on an outdoor unit. One or two piglets were deformed in each affected litter with littermates appearing to be unaffected. Some mummified piglets were also delivered in the affected batch. The problem did not occur in the subsequent batch to farrow, nor on a sister outdoor breeding unit nearby, and no other health problems were reported in the herd.
Meningo[encephalo]coele (Figure 4) and cyclopia were the main deformities seen although the one piglet submitted to AHVLA had deformity of the lower jaw and oral cavity (Figure 5).
Histopathology and virus microarray (Gurrala et al., 2009) were undertaken on tissues from the submitted piglet and neither provided evidence to support viral involvement in the problem. Familial associations to meningo(encephalo)coele are reported in some breeds (Wijeratne and others, 1974; Vogt and others, 1986). The piglets were Landrace cross Duroc and farm records indicated that more than one boar had sired litters, and that all of these boars had normal litters before and after the event making a genetic cause unlikely and exposure of the sows to a transient environmental factor affecting embryos early in development was considered most likely.
Hemlock toxicity remains a possibility. Human neural tube defects are generally considered to result from a combination of genetic and environmental factors, as well being influenced by folic acid supplementation, and exposure to an environmental factor which acted as a folic acid antagonist is another possibility. The piglets derived from matings in mid-June 2013 and it was known that the sows had been subject to significant heat stress in mid to end of July, the significance of this in the clinical problem was unclear.
Figures 4. Meningocoele (image kindly provided practitioner)
Figure 5. Developmental abnormality of oral cavity by and mandible
Changes in Disease Patterns and Risk Factors
Increased rate of diagnosis of Haemophilus parasuis disease incidents
In 2013, AHVLA saw the highest annual rate of diagnosis of respiratory and systemic disease due to Haemophilus parasuis (Hps) since 2002 (Figure 6). In spite of reduced submission numbers in July to September 2013, the diagnostic rate was highest in quarter 3 and the trend continued into the fourth quarter of 2014 as illustrated in Figure 7.
Hps diagnoses are a combination of Glässer’s disease and pneumonia due to Hps. All the disease incidents were diagnosed in pigs aged three to 15 weeks, apart from one case of Glässers in an adult.
There were outbreaks where Hps disease occurred concurrent with viral disease as illustrated in Figures 8 and 9. However it was streptococcal disease and salmonellosis which were the two most frequent concurrent diagnoses, together with pasteurellosis with Hps pneumonia.
A presentation on this increased rate of Hps diagnosis was given at the November 2013 Pig Veterinary Society conference ('Increase in Haemophilus parasuis disease incidents - Laboratory anomaly, myth or reality?' by Susanna Williamson) to disseminate the findings and obtain the views of practitioners as to whether these laboratory observations reflect a true increasing disease trend in the field.
The view was that more disease was being seen although discussion did not reveal why this was occurring. To investigate the possibility of a shift in the Hps serotypes involved in disease, MSD UK Ltd kindly funded serotyping of all AHVLA Hps isolates from 2011-2013 and the serotypes detected by year are shown in figure 9.
The results did not provide evidence to indicate that a change in serotype accounts for the increased Hps diagnostic rate. However, the analyses showed that serotypes 4 and 5 together comprise 34 per cent of the Hps isolates identified between 2011 and 2013 and remain, in 2013, the two most commonly identified serotypes (nine of 24 isolates) identified. However, they do not represent the majority and a significant proportion of isolates (24 per cent) from 2011-13 were not typeable.
Commercial vaccines currently available are based on serotypes 4 and 5 and the degree of immunity offered by vaccines to serotypes other than 4 and 5 is uncertain. It is important that AHVLA continue to isolate Hps and retain isolates stored. Veterinarians considering the use of vaccine may wish to fund serotyping before implementing this intervention as a control measure.
Swine dysentery shows an annual decline but outbreaks persist in Yorkshire
The diagnostic rate of swine dysentery (SD) by AHVLA and SACCVS has declined since 2011 as illustrated in Figure 10.
SD was identified by the British pig industry in 2011 as one of the top four diseases to be targeted for control as indicated in the vision for the 20:20 Pig Health and Welfare strategy and embraced by BPEX’s Pig Health Improvement Project (PHIP). Significant progress has been made in sharing information about SD outbreaks and provision of advice. More recently, BPEX have funded multi-locus sequence typing on SD isolates from AHVLA and SACCVS to assist in understanding the epidemiology of outbreaks.
In spite of these initiatives, there are some areas where SD persists and, at the end of 2013, infection spread to newly infect three pig units in Yorkshire which declared their infected status and are working to contain the infection. Tiamulin resistance occurs in a high proportion of B. hyodysenteriae isolates in some European countries and has been detected in a minority of isolates in GB, some of these have been multi-drug resistant. Spread of a multi-drug resistant B. hyodysenteriae isolate is particularly serious and has resulted in depopulation of some pig units as the only means of control. Surveillance for tiamulin, and other antimicrobial resistance, in B. hyodysenteriae is vital and AHVLA undertakes tiamulin minimum inhibitory concentration testing on Brachyspira hyodysenteriae isolates when isolated from diagnostic submissions.
Swine influenza outbreaks with salmonellosis in weaners
Since the emergence of pandemic H1N12009 virus in GB pigs and the introduction of the m gene PCR for routine diagnosis, both in 2009, the annual rate of swine influenza diagnoses increased significantly and in 2013 was 4.9 per cent, similar to 2011-12.
Strains H1N2 and pandemic H1N1 2009 were the predominant strains circulating in GB pigs in 2013 with just one detection of avian-like H1N1.
A feature of some recent swine influenza outbreaks in weaners has been the diagnosis of salmonellosis at the same time or in slightly older pigs. Although swine influenza alone does not generally cause mortality in pigs, it may affect the ability of young weaners to establish normal feeding and drinking when infection occurs immediately postweaning, which could predispose them to salmonellosis and other enteropathogens.
In addition, where antimicrobials are used to treat the respiratory disease seen, these tend to favour colonisation of salmonella organisms, which are resistant to the commonly used antimicrobials for respiratory disease. This has been highlighted in a previous Emerging Threats report and active swine influenza infection and its clinical impact through predisposing to other disease may not be recognised without diagnostic investigations at the appropriate time.
Descriptions of clinical scenarios involving swine influenza have been included in AHVLA’s monthly highlights to the pig industry and Pig Veterinary Society. AHVLA offers free of charge PCR testing for swine influenza under a Defrafunded surveillance project, details are given in the link: http://www.defra.gov.uk/ahvlaen/ publication/vet-si/
African Swine Fever detected in Lithuania
African Swine Fever (ASF) virus was recently reported to have been detected in wild boar in Lithuania. There is no evidence of disease yet being reported in the commercial pig sector in Lithuania, however, this is the first report of spread into the EU of ASF from neighbouring Eastern European countries.
A preliminary outbreak assessment published by AHVLA indicates that spread was by movement of infected animals from affected regions in Belarus, (link for further details).
Wild boar are notoriously difficult to control and the assessment emphasised the need to maintain vigilance in terms of movement of livestock, transport, workers around and out of the region and entering commercial pig farms.
The quantification of risk from illegal trade is difficult to measure and it is therefore considered that there is a constant low risk of introduction of various exotic animal diseases into an uninfected area through the illegal trade of infected meat or contaminated products/fomites; this list of diseases includes ASF.
BPEX has compiled an ASF web page for pig producers with a wealth of background information . A consortium of experts working on ASF (ASForce) has produced an on-line training course on ASF. In the light of the recent spread of African Swine Fever into the EU, this was circulated to AHVLA veterinary staff to refresh their knowledge of the virus and disease.
The key to control of ASF is early detection, and a disease alert was included in AHVLA surveillance centre newsletters to private practitioners asking veterinary surgeons with clients who keep pigs to remind them of the requirement to report any clinical signs suspicious of ASF, and of what those signs look like. Links were provided to information on ASF on the Defra website and a useful ASF leaflet.
Brachyspira hampsonii isolated from pigs in Germany
A new potentially pathogenic Brachyspira species, B. hampsonii, was described in 2012 in US and Canadian pigs.
This species has not yet been identified in diagnostic submissions to AHVLA or SACCVS, although there has been no comprehensive survey for it and there is no knowledge of the incidence of B. hampsonii in Europe.
Rhode and others (2013) recently reported detection of B. hampsonii in pigs showing mild to moderate diarrhoea imported to Germany from Belgium.
Several Brachyspira species were cultured from the pigs and identified as B. murdochii, B. innocens and B. intermedia. Three of six B. intermedia isolates proved to be negative in a species-specific PCR and sequencing of the nox-gene of these isolates revealed that the sequences were 99 per cent identical to B. hampsonii clade I strain KC35 and EB106.
The paper describes the phenotypic characteristics of the three isolates and these indicate that the bacteriological methods used by AHVLA and SACCVS would have identified them as B. hampsonii. The authors noted that B. hampsonii might be phenotypically confused with indole-negative B. hyodysenteriae, which are common in Belgium and Germany, but are unusual in GB.
This finding emphasises the importance of not relying solely on specific (molecular) tests for known pathogens, and the need to maintain a range of clinical microbiology methods that are non-specific and enable detection of emerging pathogens, not previously identified.