Could spray-dried plasma be a source of PCV2?

Monday, December 6, 2010

U.S. research suggests that porcine SDP could represent a biosecurity risk for the pork industry and, as a result, bears further scrutiny

by JANICE MURPHY

Incorporating spray-dried plasma (SDP) into the diet of weaned pigs has been proven to increase feed intake and growth performance. However, limited research has been carried out to confirm that the spray-drying process eliminates viral contaminants, such as porcine circovirus type 2 (PCV2). Fear of spreading the virus has resulted in some producers removing plasma proteins from their rations completely.

PCV2 is a small circular DNA virus that has been proven to be extremely resistant to inactivation. PCV2 is the primary component of PCV associated disease (PCVAD), a systemic infection that can result in 70-80 per cent mortality in clinically affected pigs.

The virus was first associated with disease in the late 1990s, but by 2005 the number and severity of PCVAD cases had increased in Canada, Kansas, Iowa and North Carolina. Subsequently, PCV2b was identified, unveiling a new strain of PCV2 that had not previously been isolated in North America.

The source and route of transmission of PCV2b in North America is unknown. Potential routes of PCV2 transmission include direct contact with infected animals and contaminated objects, or vertical transmission. Based on previous research, direct contact with infected animals is likely the more efficient route compared to other sources of transmission. One possibility is the horizontal transmission of PCV2 through SDP products.

In order to investigate the potential for infectivity of PCV2 after spray-drying, researchers at Iowa State University obtained SDP derived from a pig experimentally infected with a PCV2b isolate to test in a swine bioassay using PCV2 naïve animals.

Twelve colostrum-fed, specific-pathogen-free (SPF) pigs were obtained from a herd that is routinely tested for major swine pathogens and known to be free of PCV2, porcine reproductive and respiratory syndrome virus (PRRSV), porcine parvovirus (PPV) and swine influenza virus (SIV).

The pigs, from two litters, were weaned at three weeks of age, randomly divided into groups of three, and housed in four separate rooms. All groups were fed a balanced, pelleted, medicated, complete ration that was devoid of animal proteins, with the exception of whey.

In the four experimental treatments, pigs were:
1)    inoculated intraperitoneally (i.e. in the abdominal cavity) with PCV2-contaminated SDP (SDP-IP),
2)    inoculated by oral gavage or stomach tube (SDPOG),
3)    inoculated intraperitoneally with PCV2-positive plasma (POS), or
4)    left un-inoculated (NEG).

After inoculation, blood samples were collected weekly for seven weeks and tested for the presence of anti-PCV2-IgG antibodies and PCV2 DNA.

The source of inoculated plasma was blood from a colostrum-fed, SPF pig experimentally infected with PCV2b at three weeks of age. At 35 days post-inoculation, the pig was euthanized, having exhibited clinical signs consistent with PCVAD, including severe respiratory distress, diarrhea, and loss of condition. Two litres of blood were collected and the plasma subsequently derived.

The diagnosis of PCV2 was confirmed using quantitative PCV2 PCR and immunohistochemistry in the lungs, intestine and lymphoid tissues. Further tests for other diseases, including transmissible gastroenteritis virus, rotavirus, Lawsonia intracellularis, PRRSV, SIV and Mycoplasma hyopneumoniae, were negative.

The collected plasma was spray-dried in a bench-top spray dryer. As this obviously was not a commercial scale dryer, there were some differences in the final product. Some of the differences between the experimentally-produced SDP used in this study and a typical commercially-produced SDP might include the source animal(s)', disease status, pooling effects, processing temperatures, product retention time and post drying conditions. Table 1 outlines some of these differences in detail.

The experimental results showed that there were no signs of clinical disease or gross lesions in any of the pigs for the duration of the study. Individual pigs in the POS, SDP-IP and SDP-OG groups did have slightly enlarged lymph nodes in the chest compared to the NEG group.

Pigs within the NEG group remained seronegative throughout the duration of the trial. By 21 days post-inoculation, two out of three pigs in the POS group seroconverted, or developed specific detectable antibodies to PCV2 in the serum as a result of infection or immunization; by 28 days all three pigs had seroconverted.

In the SDP-IP group, two out of three pigs seroconverted at 35 days post-inoculation; by 49 days all pigs had seroconverted. In the SDP-OG group, two out of three pigs seroconverted by 35 days post-inoculation and one animal remained negative right through day 49.

Based on the development of viremia and timing of seroconversion, the researchers surmised that intra-pen transmission occurred within the SDP-IP and SDP-OG group.

Previous investigations have shown that the average time for a newly infected animal to infect a susceptible, naïve animal is approximately 18 days when animals are placed in the same pen together.

Ideally, the researchers would have liked to house each pig individually to perform replicates of the experiment, but the trial conditions did answer the question of whether a group of animals can be infected with PCV2 through exposure to an experimentally generated SDP product containing PCV2, which was their goal. However, the outcomes of this experiment cannot be extrapolated to assess the significance of this source of infection under typical field conditions.

This work provides clear evidence that experimentally-produced, spray-dried plasma collected from a pig experimentally infected with PCV2b is infectious to naïve animals when administered both intraperitoneally and orally.

However, commercially produced SDP products use pooled plasma from clinically healthy pigs and it is, therefore, not appropriate to directly translate results from this experimentally- produced SDP process to the commercially-produced spray-dried porcine plasma process and end-product currently used in the swine industry. This work does suggest that porcine SDP could represent a biosecurity risk for the pork industry and, as a result, bears further scrutiny. BP

Janice Murphy is a former swine nutritionist with the Ontario agriculture ministry who now lives and works in Prince Edward Island.