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NADIS Commentary: APHA GB Emerging Threats Quarterly Report – Quarter 3, 2017

In the autumn of 2017 the UK pig industry was charged with the task of drastically reducing overall antibiotic usage. A baseline use of 263.5 mg/kg PCU (the EU preferred methodology for recording use) was established for 2015, representing the highest use pro rata of all the main farmed species of animals in the UK.

The target is to achieve staged reduction over a five-year period such that by the end of 2020 the industry average use will be below 99 mg/kg - an overall reduction of 63%. It is believed that substantial reductions in antibiotic use in pigs has already occurred (in 2016 and 2017) such that the industry is now well on its way towards achieving what is a challenging target.

However, averages inevitably disguise detail and whilst the target is for the industry as a whole, individual farms' use can be vastly different. Some farms are "high users" and face considerable challenge to reduce usage without compromising health and welfare of the pigs and financial viability of the farm. It is clear from the overall RUMA Targets Task Force antibiotic reduction plan that reducing use of antibiotics in farm animals must be achieved without compromising the animals' wellbeing. Reductions should occur as a result of improving the health of the pigs rather than simply withdrawing essential treatments. It is therefore obvious that the focus of attention should be on health management and control in the herd and the third quarter APHA emerging threats report for 2017 serves as a timely reminder of the significance of two major viral diseases facing the UK pig industry which lead directly to health challenges with secondary and opportunist bacterial infections requiring antibiotic treatment.

Porcine Circovirus type 2 (PCV2) Infection

The disease syndrome caused by PCV2 infection is now referred to as Porcine Circovirus Associated Disease (PCVAD) and a full clinical description of the historical and current disease manifestations is available at /bulletins/pmws,-circovirus-disease-and-pdns.aspx

The virus typically infects young pigs and leads to loss of condition associated with either respiratory (coughing etc.) or enteric (scour) signs. At the height of the disease in the early part of the century, herds experienced ongoing losses in weaned pigs in excess of 10% (much higher in the early stages of the disease) but over the last ten years, effective vaccines have become available. Industry estimates suggest that 95% of pigs produced in the UK are now vaccinated against PCV2 - either with standalone vaccine applied in the first three to four weeks of life or in vaccines combined with Mycoplasma hyopneumoniae given at a similar age. (In a small number of farms control of PCVAD is successfully achieved by vaccinating the sows rather than young pigs directly.) However, PCV2 vaccines are designed to control the clinical disease syndrome and reduce viral excretion - they are not expected to eliminate the virus from populations. Testing of vaccinated pigs frequently identifies that low levels of virus are still present in the population. As such, the challenge to a herd is continuous.

The APHA report highlights an episode of respiratory disease in a nursery-finisher site affecting 50% of the pigs. Diagnostic investigation confirmed the disease to be due to PCVAD and investigations revealed an accidental omission of vaccination at weaning.

Other experiences over the years consistently indicate that in most farm situations attempts to withdraw the vaccine, as a cost-saving measure, have proved to be unsuccessful and in most cases disastrous. Not only does disease reappear causing mortality and lost growth but expensive and extensive therapeutic programmes are required to control secondary disease.

Commentary F1

Commentary F2

Fig 1&2 Pigs either deliberately not vaccinated or where vaccine is omitted in error are vulnerable to disease outbreaks.

It is therefore clear that the vast majority of pigs need protection against PCV2 infection and subsequent PCVAD in the form of vaccination, and removal of this either deliberately or by accident is extremely unwise. PCV2 has major effects on the immune system of the pig, allowing secondary bacterial infection to take hold. If PCVAD is allowed to resurface on farm, supportive antibiotic therapy to control the secondary diseases will be needed, compromising the ability of the farm to reduce use as well as suffering the effects of primary PCVAD.

It was also highlighted in the APHA reports that PCVAD can on occasions cause reproductive disease and where this has been seen, vaccination of adult animals (especially replacement gilts) is appropriate. Experience suggests that this procedure can be an essential part of the acclimatisation process for replacement animals on some farms.

(It should be noted that improving diagnostic and laboratory tests are progressively identifying additional agents which may prove relevant in pig disease. A variant PCV type 3 has been widely found but at present its role in disease is unclear and it would not be expected to be controlled by PCV2 vaccines which are virus specific. Where vaccine is consistently and correctly administered, disease is extremely rare and any occurrence should trigger full diagnostic investigation.)

Porcine Reproductive and Respiratory Syndrome (PRRS)

Porcine Reproductive and Respiratory Syndrome virus (European type 1 virus) has been present in the UK pig herd since 1991 and remains one of the most significant pig pathogens. As its name indicates it causes both reproductive disease in adults (a full description of which is available at /bulletins/porcine-reproductive-respiratory-syndrome-(prrs)-in-the-breeding-herd.aspx ) but also  a respiratory syndrome in young growing pigs as part of the respiratory complex ( /bulletins/porcine-reproductive-and-respiratory-syndrome-(prrs)-in-weaners.aspx ).

The virus destroys cells in the lungs which normally protect against inhaled or circulating infections and as such pigs challenged with PRRS often suffer complex pneumonias and pleurisy with multiple primary and opportunist pathogens that require intensive treatments.

Commentary F3

Fig 3 Pleurisy at slaughter can be associated with uncontrolled PRRS in the young pig

Vaccines have been available since the late 1990s for use in adults and young pigs but, despite the widespread occurrence of the disease, estimates suggest that only approximately 35% of piglets are vaccinated against PRRS.

The APHA emerging threats report for Quarter 3, 2017 highlights the seasonal pattern of diagnosis of PRRS disease with submissions typically declining in the summer and rising in winter, and more than 75% of submissions relate to disease in weaners of which over half the incidents are reported as respiratory disease.

If a herd is suffering ongoing respiratory challenges, particularly in younger growing pigs, PRRS infection should always be suspected and fully investigated with either saliva or blood testing of affected pigs or full post mortem examination and tissue analysis. Where the virus is identified vaccination of subsequent batches is an option which should be considered.

Conclusions

Respiratory diseases present a major challenge in pig production and are responsible for the most significant health problems occurring in pigs. They are often complex infections which require intensive treatment and are frequently underpinned by immune modulating viruses such as PCV2 and PRRS. Effective vaccines are available and should be used as part of a herd strategic plan to control the disease syndrome, minimising both the welfare harm and the economic consequences and reduce the need for long-term high level antibiotic use.