Experimental natural transmission (seeder pig) models for reproduction of swine dysentery

Abstract

Swine dysentery is causally associated with Brachyspira hampsonii and B. hyodysenteriae infection. Given the importance of transmission models in understanding re-emergent diseases and developing control strategies such as vaccines, the objective of this experiment was to evaluate two experimental natural transmission (seeder pig) models in grower pigs, each with 24 animals. Seeder pigs were intragastrically inoculated using broth cultures of either B. hampsonii strain 30446 (genomovar II) or B. hyodysenteriae strain G44. In trial 1, three seeder pigs were placed into two pens containing nine susceptible contact pigs creating a 1:3 seeder:contact ratio. This was sufficient to achieve natural B. hampsonii infection of 13/18 (72%) contact pigs, however, the incidence of mucoid or mucohemorrhagic diarrhea (MMHD) in contact pigs differed significantly between pens (4/9 versus 9/9; P = 0.03). In trial 2, eight seeder pigs inoculated intragastrically with B. hampsonii did not develop MMHD but when re-inoculated with B. hyodysenteriae 14 days later, all developed mucohemorrhagic diarrhea within 13 days of re-inoculation. Two seeder pigs were placed into each of 4 contact pens each containing 4 pigs. This 1:2 seeder:contact ratio resulted in natural infection of 14/16 (87%) contact pigs with incubation period ranging from 9–15 days. There were no significant differences among pens in incubation period, duration, clinical period or severity of diarrhea. These trials demonstrated that a 1:2 seeder:contact ratio with groups of six grower pigs per pen sustained natural transmission of B. hyodysenteriae G44 with greater consistency in the incidence of MMHD among pens compared to a B. hampsonii 30446 transmission model using 1:3 seeder:contact ratio in pens of 12. Understanding why B. hampsonii intragastric inoculation failed in one experiment warrants additional research.

Improving the consistency of experimental swine dysentery inoculation strategies

“…The source of B. hampsonii (ST7) and B. hyodysenteriae G44 (ST 177) have been reported previously [ 11 , 23 , 24 ]. B. hyodysenteriae D19 is a unique field strain that does not match any reported sequence type in the PubMLST database [ 25 ] (Hill et al, unpublished).…”

“…The inocula were prepared as described previously [ 11 ]. Briefly, the frozen purified isolates of B. hampsonii, B. hyodysenteriae G44 or D19 were cultured in JBS [ 12 ] or BHIS (trial A only) broth, anaerobically incubated, and scaled up every 24-h using a 1:9 dilution of broth culture (v/v) in JBS or BHIS until the required volume for inoculation was achieved.…”

“…The purity of each inoculum aliquot was assessed by PCR amplification and sequencing of the nox gene [ 28 ]. The concentration of B. hyodysenteriae D19, G44, and B. hampsonii in the inocula was determined by quantitative PCR [ 11 ]. For trial A that used frozen-thawed inocula, the complete volume of purified inocula was first produced, then frozen and subsequently thawed to room temperature in daily aliquots 15 min before each inoculation.…”

“…In experimental settings, multiple strategies have been used to induce SD in naïve pigs including direct contact with infected pigs (seeder pig model) [ 11 ], intragastric inoculation with homogenates of large intestine and its contents from affected pigs [ 12 ], or inoculation with bacterial cultures grown on solid or liquid media [ 13 – 15 ]. However, success of these experimental models in our lab and others has been variable in terms of inducing SD, which could be related to the additive effect of multiple animal factors including but not limited to pig age, weight, housing, diet [ 6 , 7 ], genetics [ 16 ], events such as environmental and social stresses, dietary changes, or prior exposure to pathogenic Brachyspira spp .…”

“…The most common inoculation technique to induce SD through experimental challenge is intragastric inoculation of B. hyodysenteriae propagated in solid or liquid media for one or multiple consecutive days. While there are minor laboratory specific modifications, this technique has shown an incidence of MMHD ranging from 20 to 100% [ 11 , 17 – 21 ]. In our lab, intragastric inoculation is the gold standard technique to induce SD in challenge experiments.…”