More take-homes from the Cancun IPVS conference

Saturday, January 31, 2015

In part two of his report on the topics discussed at the 23rd IPVS conference in June, the author reviews papers presented on porcine epidemic diarrhea and porcine circovirus type 2

by ERNEST SANFORD

In the December issue of Better Pork, I reported on the 23rd International Pig Veterinary Society (IPVS) Congress held in Cancun, Mexico, and the discourse by Dr. John Harding, Canadian swine veterinarian and professor at the Western College of Veterinary Medicine, University of Saskatchewan, Saskatoon, on transboundary and emerging diseases.

In this follow-up report, I will give brief accounts of other papers presented during the congress that I found particularly interesting. Porcine epidemic diarrhea (PEDV) will take the lead position, with several researchers reporting on characteristics of the PEDV, including features of the immune response generated by direct exposure to the virus or via feedback as practiced in the field.

Kinetics of immune humoral response (IgM, IgA, IgG) to porcine epidemic diarrhea virus (PEDV) in experimentally infected pigs
– Giménez-Lirola LG, et al. Vol 1, Pg 253
Background. Fifty-six three-week-old pigs, negative to PEDV, TGEV and PRRSV, were inoculated with PEDV. Fecal and serum samples were tested for ELISA antibodies (Abs) on Day 0 and every seven days thereafter.
Results. Clinical diarrhea occurred three to four days post-inoculation (dpi). Diarrhea stopped by 10 dpi. Fecal shedding occurred in 100 per cent, 85 per cent, 42 per cent, and zero per cent of the pigs on seven, 14, 21 and 28 dpi. Anti-PEDV IgM, IgA and IgG developed. IgA and IgG started gradual decline after three to four weeks post-inoculation.
Take homes. All major Ab isotypes developed post-PEDV inoculation. IgA might be best for monitoring Ab response. Anti-PEDV Abs may not last long.

Oral/nasal inoculation of four-week-old pigs with PEDV: Tissue tropism, shedding, carriage, antibody response and serosal transmission
 – Hesse D, et al. Vol 1, Pg 251
Background. Four-week-old pigs challenged with gut content from "feedback" material were studied to determine the characteristics of PEDV.
Results. There were mild clinical signs (diarrhea) at two dpi, which were resolved by eight dpi. Fecal and nasal swabs were PCR-positive for PEDV by eight dpi. Peak fecal shedding occurred five to six dpi (significantly higher than nasal swabs). Most pigs were negative on fecal and nasal swabs at 21 dpi. Some pigs shed PEDV up to 35 dpi. There was no aerosol transmission. PEDV nucleic acid was abundant in the environment – walls, pens, feed bins, etc. All challenged/inoculated pigs seroconverted to PEDV. Only the gastrointestinal tract contained PEDV on IHC-stained histology sections. Turbinates, trachea, lungs, bronchial lymph nodes, spleens and other visceral tissues were all negative for PEDV on IHC-stained histology sections.
Take homes. Aerosol transmission did not occur in this study. Only the GI tract was infected with PEDV. All pigs seroconverted to PEDV post-inoculation.

PEDV: Immunity following feedback
– Murtaugh M, et al. Vol 1, Pg 254
Background. Feedback material was given to sows once at 80 days of gestation. Or feedback was repeated to some sows after three weeks (i.e. feedback twice).
Results. There was no effect from the second feedback. Abs returned to baseline levels at six to eight weeks post-feedback. Litters from these sows had no detectable Abs. (N.B. In another study, serum Ab levels were negative in some litters, but positive in others.) Half of the litters contained immunoglobulins with anti-PEDV Abs. Anti-PEDV IgG or IgA Abs were never detected in milk (colostrum was not checked) or feces of sows or piglets.
Take homes. Effective PEDV immunity is hard to induce in sows. It is also hard to transfer PEDV immunity from sows to piglets. Feedback produces effective anti-PEDV immune response in adult sows. These maternal Abs are not, however, transferred to piglets. Secretary Abs (IgA) was not present in feces.
Overall, PEDV infection induces short-lived immune responses, but may not provide solid protection to sows or piglets.

Understanding PEDV timeline of exposure based on clinical findings
– Greiner L, et al. Vol 1, Pg 252
Background. This study aimed to determine how PEDV is introduced into a herd. Swiffer brushes were used to collect samples for PCR for PEDV from six areas in each of 12 swine farms: the shoe change area, lunch table, lunch bag change table and UV window, the dirty side of the shower, inside the fridge and the fridge door handle.
Results. Five of the 12 herds broke with PED during the study period (30 days). Two of the five herds that broke were filtered. Four of the five herds had positive or suspect PCR-positive samples three days prior to the onset of clinical signs. The fifth farm had a positive sample only on the day clinical signs started.
Take homes. Food and fomites of the caretakers were not the source of PEDV infection. PEDV was present in multiple farm locations ~48 hours prior to the clinical break. The virus appears to be moving throughout a facility prior to reaching a threshold for clinical signs. There also appears to be a 24-48 hour post-introduction delay before clinical signs of the outbreak are seen.

Experimental inoculation with PEDV in sparrows and mice
– Lee JH, et al. Vol 2, Pg 68
Background. A question frequently asked is: "Can wild animals or birds transmit PEDV between and within herds?" Since sparrows and mice are commonly found around swine farms, they were experimentally inoculated with PEDV to see if they could become infected and transmit the virus. Sixteen sparrows and 16 laboratory mice were experimentally inoculated with PEDV. Their intestines were analyzed by PCR and IHC for PEDV. ELISA tests were used to determine if seroconversion had occurred post-inoculation.
Results. Intestinal samples were negative on both PCR and IHC for PEDV. There was no seroconversion to PEDV.
Take homes. Sparrows and mice did not get infected with PEDV and are therefore not a likely source of infection or transmission to swine herds.

Evaluation and comparison of efficacy between killed and live vaccines against PEDV in field study
– Jang H, et al. Vol 1, Pg 291
Background. This study compared the efficacy of live and killed PED vaccines, both of which are available in Asia. The study contained four groups: killed vaccine first followed by killed vaccine booster (K/K); live vaccine first followed by live vaccine booster (L/L); live vaccine first followed by killed vaccine booster (L/K); and negative unvaccinated controls.
Results. Serum samples from the K/K group had the highest IgG (two times higher than L/L or L/K groups). IgA levels of all three groups were equal (no significant differences). IgG titres of L/L and L/K groups were similar. K/K group had the highest SN titre. The SN titre in colostrum was highest in K/K group. The SN titre in piglet serum was highest in K/K group
Take homes. The SN titre is most critical for immunity to PEDV. The highest SN titres in colostrum, sow and piglet were in the K/K group. The question still needs to be asked: "Will SN titres transfer to the piglets"?

Porcine Circovirus type 2 (PCV-2)

PCV-2 preferentially infects fetal thymus
– Sydler T, et al. Vol 1, Pg 304
Background. The PCV-2-associated reproductive disease results in late-term abortions and stillbirths with necrotizing and fibrosing fetal myocarditis. This study examined organ tropism of subclinical PCV-2 field infections. Tissues from different ages of fetuses were examined from 66 sows with reproductive failures in 38 herds.
Results. One hundred per cent of thymuses were infected with PCV-2 (confirmed by immunohistochemistry) Spleen, mesenteric lymph nodes, tonsils, heart, liver and placenta were also infected (confirmed by IHC), but no histological lesions were seen in any tissues. Most significantly, there was no myocarditis, an essential feature for confirmation of PCV-2-associated fetal death.
Take homes. Most PCV2 infections are latent (silent). PCV2 infects piglets early in fetal life. A similar phenomenon is seen in infectious anemia virus infections (another circoviral infection) in chickens.

Humoral immune response to PCV-2 in vaccinated and non-vaccinated animals
– Solis C, et al. Vol 1, Pg 305
Background. Ab responses were checked in different stages of production of PCV-2-vaccinated and PCV-2 non-vaccinated pigs. They were also checked for Abs to a "decoy" epitope. The host develops useless Abs to the "decoy" epitope.
The study contained four groups of pigs: Group 1, 0-21 days old; group 2, 21-84 days old; group 3, 84-180 days old; group 4, more than 180 days old.
Results. The vaccinated pigs had neutralizing Ab (NA) titres in group 2 (21-84 days old). The highest NA titres were in groups 3 (84-180 days old) and 4 (>180 days old). The highest NA titres in vaccinated pigs were in group 3 (84-180 days old). This is the age when pigs are most likely to get clinical PCVAD. There was no difference in this age group in Ab titres against the "decoy" epitope. (N.B. Abs to a "decoy" epitope is non-protective and believed to predispose the host to PMWS.)
Take Homes. Vaccination provided positive protection against PCVAD. There was no difference in Ab titres against the "decoy" epitope. PCV-2 load in serum was not significantly lower in vaccinated animals, hence no vaccine effect was seen in lowering serum PCV-2 levels.

Investigation to better understand the clinical importance of Porcine Astrovirus (PAstV) infection in pigs
– Opriessnig T, et al. Vol 1, Pg 242
Background. Astroviruses (AStVs) are associated with enteric disease in many hosts, including humans. Most cruise ship diarrhea outbreaks are caused by AstVs. Five genotypes of AstVs occur in pigs.
Some 509 fecal samples from 255 herds in 19 U.S. states were examined for PAstVs by PCR.
Results. All 509 fecals were positive for PAstV. All five genotypes of PAstV were identified in the samples. PAstV4 was most commonly identified (317/325 – 97.5 per cent). PAstV3 showed 50.5-55.3 per cent identity with mink AstV. PAstV1, 2 & 3 are more commonly related to human AstV.
Take homes. The clinical relevance of PAstV infections is still unclear. Healthy pigs can harbour PAstVs. Diarrheic pigs with PAstV can also be infected with other common enteric pathogens (rotavirus, coronavirus, calicivirus). BP

S. Ernest Sanford, DVM, Dip Path, Diplomate ACVP, is a Swine Veterinary Consultant based in London.