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The 3rd Congress of Asian Pig Veterinary Society Review

by 5m Editor
20 May 2007, at 12:00am

Pigs Today Magazine report - The Third Asian Convention of Pig Veterinary was held in Hongshan Hall 22nd to 25th of April, 2007. The topic of this congress was "Efficient and Healthy Pig Production in Borderless Asia". Some influential veterinary scientists and experts around the world gave lectures in the congress. Intensive communications were carried out among veterinarians, pig health experts and pork industry manufacturers from Asia as well as other countries and areas.

Lectures were given in plenary session, breakout sessions as well as enterprise sponsored satellite sessions. PRRS, PCV-2, PMWS, PRDC, enzootic pneumonia and ileitis remain to be the hot topics in the congress this time.

1. PCVAD

An update on PCV-2 researching progress was given by Charreyre. PCV-2 is the main causal factor for PMWS, which also causes respiratory diseases in pigs, such as PRDC.

PCVAD causes clinical signs in pigs of all ages as follows: poor growth, respiratory difficulties, enlarged inguinal lymph nodes, scour, sometimes jaundice. All the signs might not be able to be observed in the same case. Post mortem examinations usually show enlarged lymph nodes, marble pattern in lungs. Main pathological changes include lymph tissue lesion, decrease in number of lymph follicles, infiltration of tissue cell and multinucleated giant cell in lymph tissue, and inflammatory cellular infiltration of lymph cells in general tissue. High level of PCV antigen or nucleic acid confirmed by laboratory test is required for a determined diagnosis. Note that this does not always apply to emaciated pigs. Consensus is yet to be reached regarding to the diagnosis criteria of PMWS/PCVD, and right now it is based on sporadic deaths in the herd. For this purpose, the EU has come up with a PMWS diagnosis criteria which is based on mortality rate and post mortem examinations. But this criteria does not apply with cases where mortality rate is lowered through management measures or antibiotic treatment to control secondary bacterial infection.

  1. If a dramatic increase in weaner mortality is observed in a farm compared to its history record, then PMWS can be considered active in this farm. There are two kinds of standard for the purpose of determining whether there is a dramatic increase. First, current mortality rate is higher than average historical mortality rate plus 1.66¡ÁSD, or current mortality rate is statistically higher than history using Chai-square test, if there is a history record available in the farm; Second, current mortality rate is more than 50% higher than national or regional average, if there is no history record available.
  2. At least 5 diseased pigs should be examined post mortem, among which at least 1 should be diagnosed both pathologically and histologically to be PMWS.
  3. High level of PCV-2 is detected in lymph tissue lesion of infected pig by imuno histo-chemical methods or situ hybridization.
PCV-2 can be transmitted from sero-positive sows to sero-negative sows via nasal inoculation, and can be transmitted from sows to piglets via the placenta, and thus be transmitted from infected herds to uninfected herds, while the infectivity is rather low, - Charreyre et. al. Susceptibility to PCVAD varies among different lines of pigs, especially those from different sire lines.

2. PRRS

Yoon KJ from Iowa State University of the USA says PRRS is still a main issue for high density pork production area. As a result of continuous mutation, a highly pathogenic strain emerges every 5-6 years. The last highly pathogenic strain strain outbreak was in 1998, which has whittled away and turned into subclinical infection by now. It can remain in pigs for quite a long time. A new challenge we are facing about PRRS is how to boost the cross protection ability of available vaccines, as cross protection of a vaccine is difficult to verify in vitro. Biosecurity still remains to be the key measure against PRRS.

Tong Zhiguang from Harbin Veterinary Research Institute, China gave a lecture on the isolation and identification of PRRSV variation strains. Via indirect IFAs separately using anti-PRRSV N, M and GP5 protein monoclonal antibody, the institute was able to prove that the viruses isolated in farms with "Unknown Fever" from different provinces to be PRRSV. ORF5 serial analyses showed these samples of viruses to be highly homologous, whereas they were very different from the strain isolated in 1996. Animal infection experiments showed this strain is more pathogenic to piglets. It is yet to be found as to what kind of role this isolation played during the 2006 unknown fever outbreak in China.

3. Classic Swine Fever

Tsuda T from Japan introduced the experiences of CSF eradication in Japan. It was in 1992 when the last CSF outbreak took place in Japan, and an eradication program was carried out in 1996. 10 years later in 2006, the program was completed and CSF vaccination abandoned throughout the country.

Commenced in 1996, the program consisted of three stages. Stage 1, April, 1996 to March, 1999: On one hand, vaccination was carried out in every farm; On the other hand, suspicious cases were tested for pathogen, and serological investigations were carried out among every CSF susceptible species including farm pigs and wild hogs. During this stage, vaccination rate among national pig population had increased from 82.8% to 84%. Stage 2, April, 1999 to September, 2000: Vaccination was abandoned in administrative regions within which all pig farmers had reached a consensus. Stage 3, October 2000 to May 2006: CSF live vaccine abandoned in all 47 administrative regions in Japan. New policy was issued on May 31st, 2006 requiring CSF vaccine entirely abandoned within Japan.

Right now, the Chinese rabbit-adapted CSF attenuated vaccine is still considered the best CSF vaccine, which can provide full protection one week post vaccination. Sanipa Suradhat from Thailand studied maternal antibody of CSF, appropriate age for first CSF vaccination and effect of PRRSV infection on CSF vaccine protection. As is generally accepted, when maternal antibody level in piglet exceeds 32, immunological effect of CSF vaccination would be compromised. In Sanipa Suradhat's study however, it was found that although maternal antibody levels in piglets are high, no antibody neutralization was observed after vaccination, and all test pigs survived after proper amount of virus attack. Because of the spread of sub acute and chronic CSF, a lot of pigs carry the virus without showing any clinical sign, which causes a lot of trouble for the control of this disease. Therefore, conventional serological and pathological diagnosis is very crucial for the monitor and control of this disease.

Study on first vaccination age shows, 5 week is a better age to vaccinate than 3 week, when both IFN-y and neutralizing antibody titres are relatively high. It was also found that vaccination protection result is not as good in finisher herds as in breeder herds. During the time of outbreak, emergency vaccination can be applied to piglets with larger dose of vaccine, but, emergency vaccination should be put to an end ASAP when the outbreak is over.

Study on the effect of PRRSV infection on CSF vaccination protection shows PRRSV infection significantly affects specific immunity stimulated by CSF vaccine, which can even result in immunity failure. This can partly explain the effect of PRRSV's highly efficient imuno-adaptability on pig immune system, and the reason why diseases already under control are keeping recurring following PRRS.

4. Enzootic Pneumonia

Kurmit Liaw from Taiwan gave a lecture on the effect of EP infection on the morbidity and severity of PMWS. He did some study regarding the allegation that "mineral oil emulcified vaccine increases PCV-2 infection level and causes PMWS", which shows that emulcified vaccine will not exacerbate PMWS. Vaccines using different medias show little difference in either PMWS morbidity or lung lesion level. It is the appropriate timing, ie. vaccinating 2 weeks prior to PCV-2 infection, that matters.

May 2007