Herd Health: What the Krakow Symposium tells us about PCV2 transmission via spray-dried plasma protein
Monday, February 4, 2008
Two papers presented at last summer's Krakow Emerging Diseases Symposium presented seemingly contradictory results. But caution is needed in interpreting their findings
by S. ERNEST SANFORD
Spray-dried plasma protein (SDPP) has become an essential component of piglet feeds over the last two decades and is widely used in piglet feeds across North America and elsewhere. Very soon after the circovirus epidemic exploded in Quebec and Ontario in 2004/2005, speculation was rife about the possibility of SDPP being involved in the spread of the deadly PCV type 2 (PCV2) virus.
At the Krakow Emerging Diseases Symposium last summer, two papers with contradictory results were devoted to examination of the potential of PCV2 transmission via SDPP. Here are brief summaries of the findings reported by the researchers in those two papers.
Lack of transmission of PCV2 via spray-dried plasma
(Polo et al., p. 75)
Specific pathogen free pigs, three to four weeks old, from four different sows were fed either zero per cent SDPP or eight per cent SDPP for 45 days. The SDPP selected for this study was the production batch, of five batches, that had the highest amount of PCV2 DNA as determined by qPCR, an amount one to four times greater than that typically found in SDPP in commercial feed.
Blood samples taken on days 0, 10, 35 and 45 were analyzed for PCV2 antibodies (Abs) using the Ingezim Circovirus IgG/IgM Elisa serology test (Ingenasa, Spain). PCV2 DNA was determined by PCR.
No clinical signs of PCVD were observed during the entire 45 days of the feeding trial. No viremia was identified by PCR and no pigs developed IgG or IgM Elisa Ab titres. Thus, this study showed that oral administration to SPF pigs of SDPP containing high levels of PCV2 DNA in feed did not result in transmission or seroconversion to PCV2.
PCV2 transmission via spray-dried plasma
(Changxu Sung et al. p. 92).
For this second study, the researchers analyzed SDPP sourced from China and the United States and commercial piglet feeds collected randomly from pig farms in Southern China for presence of PCV2 DNA.
They further fed commercial piglet feeds containing different amounts of SDPP to one group of mice, experimentally, and challenged two groups of mice with live cloned PCV2 virus and inactivated PCV2 virus, respectively. Additional groups of mice were inoculated intramuscularly and intraperitoneally with PCV2. Appropriate controls consisting of mice fed feed without SDPP and mice inoculated with NaCl were maintained.
Serum samples, collected before treatment and weekly for three weeks after the treatment, then twice more monthly thereafter, were monitored by qPCR for PCV2 DNA and by ELISA for PCV2 antibodies (Abs). Hams and other pig meats purchased from local markets in China were also analyzed for PCV2 DNA.
All SDPP samples and 62.5 per cent of the commercial piglet feeds were positive for PCV2 DNA. Of much greater significance, however, PCV2 DNA was identified in blood of mice one week after feeding on feeds spiked with SDPP. Mice also seroconverted to PCV2 on ELISA testing, with Abs developing some time after the PCV2 DNA detection. It's noteworthy that mice inoculated with "inactivated" PCV2 virus also developed PCV2 viremia and seroconverted to PCV2, clearly indicating that the "inactivation" process had failed to inactivate PCV2.
PCV2 DNA was never identified in the controls throughout the study and none of the control mice seroconverted to PCV2. Between 18 and 35 per cent of hams and the other pig meats purchased locally in China were positive for PCV2 DNA by qPCR.
Thus, in contrast with the previous study cited above, and all other studies conducted to infect pigs with PCV2 via SDPP, these researchers clearly showed that infection after consumption of SDPP spiked piglet feeds. Controls fed non-spiked feeds remained negative.
A few caveats are in order when considering the results of this second paper. First, since mice, and not pigs, were used in this study, one has to exercise caution in making the leap across species. PCV2 has been previously shown to infect mice, but more studies on SDPP need to be done directly on pigs.
Secondly, and more importantly, we were not told if the SDPP used to spike the feed in the experiment came from local Chinese sources or from the U.S. manufacturer of SDPP. If the former, their preparation techniques might not have inactivated the PCV2 in the SDPP and hence the different results from the first experiment.
We do know for a fact, from the results reported in their own experiments above, that whatever "inactivation" process the researchers used did not inactivate the PCV2.
Isolation of Chinese virulent PRRS virus strain
(Changxu Sung et al. p. 180).
Another presentation in Krakow dealt with the identification of a highly virulent PRRS virus purported to be the cause of the "High Fever Disease Syndrome" killing millions of pigs in China since 2006 and through to late 2007.
An epidemic outbreak of "Mystery Fever Disease Syndrome" causing high mortality started in South China in late 2006 and quickly spread throughout China. Clinical signs included high fever (up to 42 C), red discolouration of skin and extremities, anorexia, vomiting, diarrhea and central nervous system signs. Growing and finishing pigs and pregnant sows were more frequently and severely affected. Sows aborted. Morbidity and mortality were both very high.
Tissues and blood samples collected from diseased pigs from 14 herds in five provinces were submitted to the researchers for analysis. Several PRRS viruses were isolated from the submitted tissues. Four representative isolates were sequenced in ORF 5 and analyzed. Four different PRRS virus isolates were then used to experimentally challenge pigs.
PRRS viruses were isolated from a "high percentage" (exact figures were not reported) of the sampled tissues. Sequences analysis showed that all four viruses were very closely related, being 98.3-99.8 per cent homologous at the amino acid level, but only 87.8- 89.4 per cent to ATTC 2332 PRRS reference virus.
This indicates that the PRRS virus isolates from the different provinces were closely related. Furthermore, all four pigs experimentally infected with the field isolates died six to 17 days after challenge. Three sentinels also died in nine to 17 days post-challenge. All pigs, experimentally challenged and sentinels, developed fevers up to 42 C.
This study suggests that a new, highly virulent strain of PRRS virus might be the main cause of the "Mystery Fever Disease Syndrome" (also called "Blue Ear Disease") sweeping through China.
I would add that this pig disease outbreak, characterized by high fevers and marked mortality in China, is of great concern to most of us in countries with major pig production. Specific information from China is sparse and the mortality figures seem much too high to be explained solely by even a new, highly virulent PRRS virus strain. Information gathered from other sources indicates that, in at least some of the cases, other agents like Classical Swine Fever (hog cholera) virus and PCV2 were also involved. BP
S. Ernest Sanford, DVM, Dip. Path., Diplomate ACVP, is a swine specialist with Boehringer Ingelheim Vetmedica (Canada) in Burlington.