Investigation into the role of potentially contaminated feed as a source of the first-detected outbreaks of porcine epidemic diarrhea in Canada

Abstract

In January 2014, approximately 9 months following the initial detection of porcine epidemic diarrhea (PED) in the USA, the first case of PED was confirmed in a swine herd in south-western Ontario. A follow-up epidemiological investigation carried out on the initial and 10 subsequent Ontario PED cases pointed to feed as a common risk factor. As a result, several lots of feed and spray-dried porcine plasma (SDPP) used as a feed supplement were tested for the presence of PEDV genome by real-time RT-PCR assay. Several of these tested positive, supporting the notion that contaminated feed may have been responsible for the introduction of PEDV into Canada. These findings led us to conduct a bioassay experiment in which three PEDV-positive SDPP samples (from a single lot) and two PEDV-positive feed samples supplemented with this SDPP were used to orally inoculate 3-week-old piglets. Although the feed-inoculated piglets did not show any significant excretion of PEDV, the SDPP-inoculated piglets shed PEDV at a relatively high level for ≥9 days. Despite the fact that the tested PEDV genome positive feed did not result in obvious piglet infection in our bioassay experiment, contaminated feed cannot be ruled out as a likely source of this introduction in the field where many other variables may play a contributing role.

Keywords: porcine epidemic diarrhea; spray-dried porcine plasma; transmission.

Figure 1

Dot plots of PEDV shedding of individual pigs as determined by N gene rRT‐PCR assay conducted on rectal swab samples. Swab samples that did not produce a C _t after 45 cycles were given a value of 45. All C _t values were subtracted from 45 and then plotted. (a) Positive control group inoculated with PEDV positive colon suspension. (b) Spray‐dried plasma group. (c) Negative control pigs placed in contact with positive control pigs beginning at 7 dpi. (d) Negative control pigs placed in contact with spray‐dried plasma‐inoculated pigs beginning at 7 dpi.

Figure 2

Comparison of S protein gene sequences obtained from bioassay piglets versus those of field cases. One‐hundred and eighty‐eight nucleotides of the ~224‐bp‐small S gene conventional RT‐PCR amplicon (primers excluded) obtained from rectal swab samples of bioassay piglets and Canadian and USA field cases were aligned. NCFAD 2014‐022 is the identical sequence of the 5 SDPP samples sequenced directly. Plasma #21‐22, #25‐26, #29‐30 and #32 are from SDPP‐inoculated bioassay piglets. ONTindex is the Ontario index case. NCFAD2014‐18 #1‐10 are the 10 Ontario field cases that followed the index case. Pellet # 35 is from a rectal swab specimen taken at 3 dpi from piglet # 35 of the feed group. PEI is from a field case from Prince Edward Island. NCFAD 2014‐35 is from a field case from Quebec. MAFRI is from a field case from Manitoba and NCFAD 2014‐41 is from an environmental sample from Manitoba. Colorado (KF272920) and Indiana (KF452323) are USA PEDV isolates.

Figure 3

Histopathology and IHC findings in ileum. (a–d) Plasma‐inoculated pig. (a) There is evidence of villus atrophy, submucosal edema (*) and dilation of lymphatics (arrow) HE stain, Bar = 100 μm. (b) There are increased mononuclear inflammatory cells within the lamina propria (*). Note the presence of cuboidal surface epithelial cells that are sometimes vacuolated (arrow). HE stain, Bar = 20 μm. (c) There is positive immunostaining of surface epithelial cells (arrows). Bar = 50 μm. (d) Immuno‐staining is primarily visualized within the brush border. Bar = 10 μm. (e–f) Positive control pig 5 days post‐contact. (e) Villi are shortened and there is abundant necrotic debris within the lumen. HE stain, Bar = 100 μm. (f) There is extensive positive immunostaining within surface enterocytes. Bar=100 μm. (g–h) Negative control pig Ileum. (g) In some areas, there was mild inflammation and shortening of villi. HE stain, Bar = 100 μm. (h) PEDV antigen was not detected by IHC. Bar = 100 μm.