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Porcine circovirus 2 (PCV2) causes porcine circovirus-associated disease, and co-infection with porcine reproductive and respiratory syndrome virus (PRRSV) severely affects the pig breeding industry. Both viruses target the macrophages in lymphoid tissues. Various porcine pathogens enter via the nasal cavity, and the nasopharynx-associated lymphoid tissue (NALT) acts as the mucosal immune system. However, the pathological analysis has not progressed. This study aimed to histologically examine the NALT of pigs with suspected PCV2 and PRRSV infections. Six pigs were subjected to necropsy, and their NALT, tonsils, and mesenteric lymph nodes were collected. Macrophages, lymphocytic depletion, multinucleated giant cells, intracytoplasmic inclusion bodies, and neutrophil infiltration increased in the NALT. In situ hybridization revealed positive signals for PCV2 in the NALT of all pigs and PRRSV in the NALT of three pigs. PCV2-positive macrophages were mainly identified in the follicles, whereas PRRSV-positive tissues were found primarily around the crypt and directly below the epithelium. Quantitative PCR revealed 10 8 -10 10 copies of PCV2 DNA/µL and 10 2 -10 4 copies of PRRSV DNA/µL in the NALT. Therefore, both PCV2 and PRRSV were detected in the NALT of pigs. In conclusion, the infection and replication of both viruses in the NALT and tonsils may suppress host immunity and promote co-infection with other pathogens.

In Japan, many pig producers buy antimicrobials using veterinarians' prescriptions. Even if veterinarians prescribe antimicrobials for treatments and metaphylaxis, producers may be using them for prophylaxis on farms with a high incidence of chronic diseases. To reduce the frequency of antimicrobial use, we applied the concept of medication adherence and launched an intervention in eight farrow-to-finish farms. For the intervention farms, we attempted to implement measures for preventing the repeated onset of chronic infections that interfered with adherence, confirmed the need to continue medications, and discontinued medications if possible. For each farm, these interventions began at different time points after 2017 and continued until 2020. We evaluated the effectiveness of this intervention based on the amount of active ingredients/population correction unit and the total number of defined daily doses for the weight group/slaughter pig. The ratio of antimicrobial use for both indicators in the year 2020 compared with the start of the intervention decreased on all farms. Among the eight non-intervention farms, six showed an increase in both indicators. These results suggest that interventions for chronic diseases administered by producers and veterinarians lead to high adherence to prescriptions and responsible and prudent use of antimicrobials on pig farms., (© 2022 Japanese Society of Animal Science.)

When a large-scale epidemic of porcine epidemic diarrhea (PED) occurred in 2013 in Japan, feedback feeding (feeding feces and gut tissues of infected piglets) was attempted to impart immunity to sows and immunize nursing piglets via breastfeeding. This study evaluated the effect of feedback feeding on PED control at 172 farms in Kagoshima and Miyazaki Prefectures. Univariable and multivariable generalized linear models were used to analyze the associations between conduct of feedback feeding and damage from the outbreak (outbreak period and the number of piglet deaths) at the farm level. The within-farm outbreak period shortened over time after the regional outbreak began on Kyushu Island (P=0.009) and was longer on large-scale farms (mean 66.0 days, P=0.003) than small-scale farms (29.4 days) and on farms that used feedback feeding (145.2 days, P=0.059) than those that did not (66.0 days). The number of dead piglets decreased over time since the first regional case (P<0.001) and was higher at farrow-to-finish farms (3.8 piglets/sow, P<0.001) than reproduction farms (0.7 piglets/sow). The effect of feedback feeding on the number of dead piglets was not significant, but its interaction term with farm style had a significant effect (5.0 more piglet deaths at reproduction farms than fallow-to-finish farms, P=0.001). These results suggest that feedback feeding made the damage from PED worse, though it was well established at a later stage of the regional PED epidemic.

In Japan, reducing the use of antimicrobials in pig production is a significant issue. However, there are no published reports concerning porcine disease treatment, as related to the age of the pigs and the indications (e.g., organ system) in Japan. In this study, we analyzed the prescription records of 17 farrow-to-finish farms from 2014 to 2018 in southern Kyushu, Japan. The farms' antimicrobial usage was calculated as the active ingredient per population correction unit (PCU) or the number of treated pigs per PCU using the defined daily dose. All data were analyzed according to the indications and production stages (sows, suckling pigs, post-weaning pigs, and fattening pigs). In terms of active ingredients/PCU, tetracyclines were the most commonly used (43.2-59.3%), and the largest amounts of antimicrobials administered through feeds were for treating the respiratory organs of fattening pigs. In terms of the number of treated pigs/PCU, tetracyclines were most frequently used (16.3-31.1%), and a high frequency of antimicrobials administered through feeds was used for the treatment of respiratory organs in post-weaning pigs. In this study, it was confirmed that tetracyclines were used frequently as a herd treatment for respiratory diseases in post-weaning and fattening pigs in southern Kyushu, Japan. The findings suggest that it is necessary to improve the treatment and prevention of respiratory diseases in post-weaning and fattening pigs in order to reduce the frequencies of antimicrobial treatments.

The objectives of this study were to assess abortion risk (AR) and the number of piglets that died during suckling periods per litter (DP) in farms infected with porcine epidemic diarrhea (PED) in relation to herd immunization procedures. Data were obtained from 91 farms in Japan that had PED infection during 2013 to 2014. The 91 PED-positive farms were asked the number of abortions that occurred and DP for 3 months (1 month before PED outbreak (previous month), 1 month after PED outbreak (the month of PED), and from 1 month after PED outbreak to 2 months after PED outbreak (following month)). AR in each month was calculated as the number of abortions divided by sow inventory. Both AR and DP in the month of PED were higher than those in the previous and following months (p < .05). Farms that performed a herd immunization procedure had higher AR and DP in the month of PED than those that did not perform the procedure (p < .05). In summary, PED occurrence increased AR and DP., (© 2020 Japanese Society of Animal Science.)

Porcine epidemic diarrhea (PED) is an emerging and/or re-emerging disease of pigs in several countries, with high morbidity and mortality in suckling piglets. Farms affected with PED perform various intervention practices to control and/or eliminate the PED virus. The objectives of the present study were to assess the effect of biosecurity measures and intervention practices to control PED on time to absence of clinical signs (TAC) and number of dead suckling piglets during TAC. A questionnaire was administered to 120-PED affected farms located across Japan between 2013, when the first case was reported in Japan, and 2014. Farms were asked to provide information on farm characteristics and internal or external biosecurity measures during PED outbreak, as well as on intervention practices to control PED. The TAC was defined as the number of days from the date that clinical PED signs appeared to the date that clinical PED signs disappeared. The number of dead piglets per sow (DP/S) was calculated as the number of dead suckling piglets during TAC divided by the sow inventory. Regarding the effect of biosecurity measures during PED outbreak on TAC and DP/S, longer TAC was observed in Actinobacillus pleuropneumoniae-positive farms and farms outsourcing pig transport to the slaughterhouse (p <  0.05). In addition, farms with divided truck entrances had lower DP/S than those without divided entrances (p <  0.05).Regarding the effect of intervention practices to control PED on TAC and DP/S, farms that performed feedback at 2 weeks or later after PED outbreak had longer TAC and higher DP/S than other farms (p <  0.05). Farms that fixed the hours staff worked in farrowing barn had lower DP/S than the other farms (p <  0.05). In conclusion, variables associated with long TAC were Actinobacillus pleuropneumoniae -positive farms, farms outsourcing pig transport to the slaughterhouse, and farms performing feedback at 2 week or later after PED outbreak. Additionally, those associated with high DP/S were farms without divided entrances, farms without a fixed hours worked in the barn, and farms that performed feedback at 2 week or later after PED outbreak., (Copyright © 2019 Elsevier B.V. All rights reserved.)

Porcine epidemic diarrhea (PED) occurred in Japan in 2013 after an interval of 7 years. The present study assessed individual productivity of sows exposed to PED virus at different periods of the production stage. The present study was performed at three commercial farms that had PED outbreak during the month of December 2013. Herd immunization was conducted for all gilts and sows. The production records were obtained for sows that were alive during the PED outbreak at each farm. The sows were categorized into six groups based on the period in which they were exposed to PED virus between days 0-30 (G1), 31-60 (G2), 61-90 (G3), or after 91 days of pregnancy (G4), during lactation (L), and after weaning (W). The control group was assigned based on the records before the period of PED outbreak (uninfected group). The number of sow's records obtained from farms A, B, and C were 1056, 1137, and 1035, respectively. Compared with the uninfected group, there was no reduction in the number of pigs born alive in the G1-G4 groups. Sows of the G4 and L groups, however, had 4-9 pigs fewer pigs weaned, and a 36%-77% greater pre-weaning mortality than the uninfected group (P < 0.05). There was no difference in farrowing rate and number of pigs born alive at subsequent parities among the sow groups. There were no interactions between sow groups and parity for sow productivity., (Copyright © 2018 Elsevier B.V. All rights reserved.)

Background: Since late 2013, porcine epidemic diarrhea virus (PEDV) has reemerged in Japan and caused severe economic losses to the swine industry. Although PEDV vaccines have been used widely, the disease has swept rapidly across the county, and is commonly observed in PED-vaccinated farms, and has recurred in domestic herds. To better understand PEDVs responsible for the reemerging outbreaks in Japan, full-length spike (S), membrane (M), and nucleocapsid (N) genes of 45 PEDVs collected in Japan during 2013-2016, were sequenced and analyzed., Results: Phylogenetic analysis based on S gene sequences revealed that all the recent field PEDVs were genetically distinct from the classical Japanese strains, and were classified into three genotypes: North American (NA), S INDEL, and Asian non-S INDEL. Our data suggested a possibility that multiple parental PEDV strains were introduced into Japan from abroad at the same time or similar times. The newly identified Japanese strains showed the closest relationship to the US strains. Two sublineages of Japanese strains circulating in Japan were similar to two sublineages identified in the US, suggesting common ancestors for these strains. In comparison with two vaccine strains used in Japan, the field strains had various changes in epitope regions, glycosylation sites, and phosphorylation sites. These substitutions, particularly observed in epitope regions of the S (521, 553, 568, and 570), M (5), and N (123, 252, and 255) proteins, may have affected antigenicity and vaccine efficacy, resulting in an unsuccessful PEDV control. Sequence comparisons between PEDVs collected from primary and secondary outbreaks in three herds revealed that the disease has developed to an endemic stage in which PEDV could persist for nearly two years in the herds or local regions, causing subsequent epidemics., Conclusions: These results elucidate the genetic characteristics, origin, and molecular epidemiology of PEDVs circulating in Japan, as well as the PEDV strains causing recurrent outbreaks. This study provides a better insight into the PEDVs responsible for recent outbreaks in Japan, and could potentially help to develop measures for controlling and preventing the disease.

Porcine epidemic diarrhoea virus (PEDV) is the aetiologic agent of porcine epidemic diarrhoea (PED), a highly contagious enteric disease that is threatening the swine industry globally. Since PED was first reported in Southern Vietnam in 2009, the disease has spread throughout the country and caused substantial economic losses. To identify PEDVs responsible for the recent outbreaks, the full-length spike (S) gene of 25 field PEDV strains collected from seven northern provinces of Vietnam was sequenced and analysed. The sequence analysis revealed that the S genes of Vietnamese PEDVs were heterogeneous and classified into four genotypes, namely North America and Asian non-S INDEL, Asian non-S INDEL, new S INDEL and classical S INDEL. This study reported the pre-existence of US-like PEDV strains in Vietnam. Thirteen Vietnamese variants had a truncated S protein that was 261 amino acids shorter than the normal protein. We also detected one novel variant with an 8-amino acid insertion located in the receptor-binding region for porcine aminopeptidase N. Compared to the commercial vaccine strains, the emerging Vietnamese strains were genetically distant and had various amino acid differences in epitope regions and N-glycosylation sites in the S protein. The development of novel vaccines based on the emerging Vietnamese strains may be contributive to the control of the current PED outbreaks., (© 2017 Blackwell Verlag GmbH.)

The objective of this study was to investigate factors that caused rapid spread during the early phase of the porcine epidemic diarrhea (PED) epidemic in Japan in 2013 and 2014. Anonymized datasets from all pig farms were provided by Kagoshima (709 farms) and Miyazaki Prefectures (506 farms). Semi-parametric survival analysis was conducted using the first 180 days from the first case on December 3, 2013 in Kagoshima Prefecture. To compare the hazard between different farm management types, univariable survival analysis was conducted. As farm sizes varied among different farm types, bivariable survival analysis was conducted for farm size categories and farm density per km 2 for each management type. A case-control study using a postal questionnaire survey was conducted in September 2014, and risk factor analysis was performed using generalized linear models with binomial errors. The hazard was significantly higher in farrow-to-finish farms than fattening farms [hazard ratio (HR) = 1.6, p < 0.01], but was not significantly different between reproduction and fattening farms (HR = 1.3, p = 0.16). In separate bivariable survival analyses for each farm type, large- and middle-scale farms had higher hazard than small-scale farms in fattening (HR = 5.8 and 2.6, respectively, both p < 0.01) and reproduction farms (HR = 4.0 and 3.6, respectively, both p < 0.01). In farrow-to-finish farms, large-scale farms had higher hazard than small-scale farms (HR = 2.8, p < 0.01), and higher farm density per km 2 was also a risk factor (HR = 7.6, p < 0.01). In the case-control study, questionnaires were returned from 78 PED virus-infected and 91 non-infected farms. The overall response rate was 34%. Risk factors of the final model were occurrence of porcine reproductive and respiratory syndrome in the past 5 years [odds ratio (OR) = 1.97, 95% confidence interval (CI): 0.97-4.00, p = 0.054], use of a common compost station (OR = 2.51, 95%CI: 1.08-5.83, p = 0.03), and use of a pig excrement disposal service (OR = 2.64, 95%CI: 1.05-6.63, p = 0.04). High hazard in farrow-to-finish farms suggested transmission from slaughterhouses to susceptible suckling piglets. Hazard associated with large-scale farms and high density might be due to frequent vehicle entrance and transmission by roads. Improvement of farm hygiene management and avoidance of risky practices associated with contact with pig excrement were keys in preventing invasion of PED virus to a farm., (Copyright © 2017 Elsevier B.V. All rights reserved.)

Porcine epidemic diarrhea (PED) was detected for the first time in seven years in Japan in October 2013 in Okinawa Prefecture. By December 2013, PED had spread into Miyazaki and Kagoshima Prefectures in the southern part of Kyushu, one of the regions with the highest farm density in the country. The objective of the study here was to assess the spatial dynamics of PED spread during the 8 months of the epidemic in the southern part of Kyushu between December 2013, the month observed first case in the studied region, and July 2014. Information on location and capacity of all farms in the prefectures (n=1269) was obtained from a government database containing demographic information for livestock producers. Additionally, data on PED detection (positive or negative) was obtained from the regional Livestock Hygiene Service Center. The Cuzick-Edwards (CE) test, the Knox test, the directional test, and the permutation model of the scan statistic were used to assess the spatio-temporal distribution of the epidemic. PED cumulative farm level incidence was 19.5% (248/1269) through the study period. The highest density of positive farms was observed in the most farm-populated areas of the prefecture. The CE test revealed an extensive degree of spatial clustering, with clustering of positive sites being significant (P<0.01) up to the 35th level of neighborhood (approximately 5km in the studied data). The observed-to-expected ratio of cases was maximized at short spatio-temporal distances, with values of the observed-to-expected ratio of cases maximized when the thresholds were set at 2km and 10days, respectively. A significant (P<0.01) direction of spread was detected towards the northeastern direction. The permutation model detected five significant (P<0.01) clusters occurring at different stages of the epidemic wave. The strong spatio-temporal clustering of PED-infected farms during the first 6 months of the epidemic in the southern part of Kyushu is consistent with results obtained elsewhere and demonstrates the rapid spread of the virus in naïve populations., (Copyright © 2017 Elsevier B.V. All rights reserved.)

Porcine epidemic diarrhea (PED) is an emerging disease of pigs in several countries. In the present study, individual sow productivity of Berkshire sows exposed to PED virus at different stages of production was compared. On a commercial farrow-to-finish farm in Kagoshima Prefecture, Japan, the clinical presence of PED was observed in the farrowing barn on January 6, 2014, and all gilts and sows were immunized on January 9, except those in the farrowing barn. The sows were categorized into six groups based on the period in which they were exposed to PED virus: between days 0-30 (G1), 31-60 (G2), 61-90 (G3), or after 91 days of pregnancy (G4), during lactation (L), and after weaning (W). The control group was not exposed to PED during the period of PED outbreak. The study was based on 574 production records. The sows of the G4 and L groups had the fewest piglets weaned (4.8±0.4, and 4.0±0.3 pigs, respectively; P<0.05) and the greatest pre-weaning mortality (33.1±4.8%, and 39.7±4.1%, respectively; P<0.05). The number of piglets weaned and pre-weaning mortality, however, did not differ among the G1, G2, G3, and uninfected groups. The G4 and W groups had slightly lesser farrowing rates than the uninfected group (P<0.05), however, similar subsequent piglet litter performance as the uninfected group. In conclusion, the effect of PED on individual sow productivity differed with the production stage in which sows were exposed to PED virus., (Copyright © 2017 Elsevier B.V. All rights reserved.)

Since late 2013, after an absence of seven years, outbreaks of porcine epidemic diarrhea virus (PEDV) infection have reemerged and swept rapidly across Japan, resulting in significant economic losses. In this study, we report the emergence, mixed infection, and genetic characterization of 15 novel field PEDV variants with large genomic deletions. The sizes of deletion varied between 582 nt (194 aa) and 648 nt (216 aa) at positions 28-714 (10-238) on the S gene (protein). Among 17 PEDV samples isolated from individual pigs, all of them contained at least two distinct genotypes with large genomic deletions, and 94.1% of them were found to consist of strains with an intact S gene. These variants were found in eight primary and nine recurrent outbreaks, and they might be associated with persistent PEDV infection in the farms. Full-length S and ORF3 genes of eight variants derived from 2 samples were characterized. This is the first report of mixed infections caused by various genotypes of PEDV and would be important for the studies of viral isolation, pathogenesis, and molecular epidemiology of the disease., Competing Interests: The authors have declared that no competing interests exist.

Porcine epidemic diarrhea (PED) is an emerging disease of pigs that has recently led to large numbers of piglet deaths in a number of countries of Eastern Asia and The Americas. The objective of the present study was to identify and compare risk factors associated with PED infection in locally and non-locally PED-exposed farms in Japan. A questionnaire was administered to a convenience selection of pig farms located throughout Japan. Questionnaires were administered between November 2013 (when the first case was reported in Japan) and August 2014. PED-positive farms (cases, n=124) were asked to provide information on their status (positive or negative) and select herd management practices for the two weeks prior to onset of PED clinical signs. Negative farms (controls, n=128) were given the same questionnaire and asked herd management practices for the two weeks prior to a given reference date. This date was assigned based on the date of PED occurrence in the town/prefecture in which the farm was located. Case and control farms were categorized as "locally exposed" if they were located within a 5km radius from a PED-infected farm and "non-locally exposed", otherwise. Logistic regression analysis was used to identify factors associated with PED infection. Two separate regressions were done for locally exposed and non-locally exposed farms using PED status (positive/negative) as the dependent variable. PED in locally-exposed farms was associated (P<0.05) with increased farm size (in 100 pig increments), shorter distances to the closest PED-positive farm (less than1,001m), and a disinfectant contact time of less than 20min. In non-locally exposed farms, PED was associated (P<0.05) with increased feed truck visits to the farm, no visit of the veterinarian, and again a disinfectant contact time of less than 20min. These findings suggest that the mechanisms of PED spread in Japan were different for farms closer to case-farms compared to farms that were further away from PED cases. These results will contribute to understanding the epidemiology of the disease in Japan and will ultimately aid in designing and implementing effective prevention and control strategies in Japan and other regions epidemically infected by the PED virus., (Copyright © 2015 Elsevier B.V. All rights reserved.)

We investigated the associations of hind limb claw lesions with claw measures and reproductive performance of sows on a commercial breeding farm. We used a five-point scale (score of 0-4) for each hind limb claw region to generate a claw lesion score (CLS). Of 110 sows, 1.8% had no hind claw lesions, and the remaining 98.2% had at least one. The proportions of sows with a CLS of 2 and 3 were 49.1% and 3.6%, respectively. The junction region had the highest frequency of a CLS of ≥ 2 (P < 0.05). The mean toe length was 45.0 ± 0.4 to 46.4 ± 0.4 mm. Sows with the highest CLS of ≥ 2 in the lateral claw had longer sole dimensions than did sows with the highest CLS of ≤ 1 (P < 0.05). There were no significant differences in claw width or height between sows with the highest CLS of ≥ 2 and ≤ 1. There was also no difference in reproductive performance between sows with the highest CLS of ≥ 2 and ≤ 1. In conclusion, most sows had hind claw lesions. The association between CLS and claw measures indicates that the sole length can be used as an indicator of claw lesions., (© 2014 Japanese Society of Animal Science.)

The antimicrobial susceptibilities of 72 Brachyspira hyodysenteriae isolates collected from clinical cases of swine dysentery (SD) in 11 prefectures in Japan between 1985 and 2009 were investigated by an agar dilution method using five antimicrobial agents. There is a tendency of Japanese field isolates of B. hyodysenteriae to acquire resistance to the main antimicrobials used in SD treatment such as tiamulin, valnemulin, and efrotomycin. A responsible approach for selection and use of antimicrobial agents is required for SD treatment.

Two 60-day-old pigs showing clinical signs of malgrowth and diarrhea were diagnosed as atypical porcine proliferative enteropathy (PPE). The intestinal mucosal lesions in the piglets were characterized by the adenomatous proliferation of the crypt epithelium together with growth of small curved bacteria within the enterocytes. The lesions could be seen in the ileum and other portions of the intestine histologically, although no significant thickening of the gut wall could be observed grossly in the present case. The macroscopic findings are extremely important for the diagnosis of PPE, however, this paper shows that the histopathological and/or immunohistochemical findings were also critical to identify the disease.

This paper describes the isolation of porcine epidemic diarrhea (PED) virus in Vero and porcine cell cultures, and the influence of age on disease in experimental infection. PED virus was isolated from the small intestine of piglets inoculated with PED samples and cultured in Vero, porcine bladder and kidney cells propagated in collagen-coated tissue culture plates in maintenance medium (MM) containing trypsin. In porcine bladder and kidney cell cultures inoculated with isolated PED virus, cytopathic effects (CPE) including cell fusion were detected. Specific brilliant fluorescence was observed in the cytoplasm of these cells. Two- and 7-day old, and 2-, 4-, 8- and 12-week old specific pathogen-free (SPF) pigs were orally inoculated with PED virus isolated from an outbreak. All 2- and 7-day old pigs inoculated developed severe watery diarrhea from post-inoculation day (PID) 1 and died between PID 3 and 4. Although three of five 2-week old pigs developed diarrhea on PID 1-4, they eventually recovered. In the 4-week old group, three of five pigs had mild diarrhea for 1-2 days. None of the 8- and 12-week old pigs showed any clinical signs. Antibodies against PED virus were detected in all surviving pigs by virus neutralization (VN) test and immunofluorescence assay (IFA). Therefore, there is an age-dependent resistance to pathogenic PED virus infection in pigs.

A comparative study on the adhesion of attaching and effacing Escherichia coli (AEEC) to the enterocytes between the colon of a calf and the jejunum of a piglet showed differences in the developmental process of attaching and effacing (AE) lesions. In the calf, pedestals consisted of fused microvilli, while in the piglet they developed from the apical epithelial cell membranes after effacing microvilli. Microvilli adjacent to the AEEC attachment site were atrophic in the calf, whereas they were elongated in the piglet. The production of AE lesions in the calf may be indicative of a novel developmental process with AEEC infection.

A sudden outbreak of epidemic diarrhoea of piglets occurred in Japan, the principal features being watery diarrhoea, dehydration and high mortality in newborn animals. The microscopical lesions were villous atrophy in the small intestine, the villous enterocytes being vacuolated and cuboidal in shape. The villus-crypt ratio was severely reduced, varying from 1:1 to 3:1. Transmission electron microscopy showed numerous coronaviruses within the cytoplasm of enterocytes and among microvilli. Specific antigens of porcine epidemic diarrhoea (PED) virus were detected in the cytoplasm of enterocytes by the streptavidin-biotin (SAB) technique. Infected cells, which were most abundant in the villous epithelia of the jejunum and ileum, were present in small numbers in the large intestine, the crypt epithelia, the lamina propria and Peyer's patches. The study suggests that the SAB technique is useful for the diagnosis of PED.

The experimental infection of 25 young chicks with Treponema hyodysenteriae was carried out. Treponemes were isolated from 21 of 25 chicks on day 21 after inoculation. The ceca of chicks infected with treponemes were atrophied. The lumen was filled with a white watery fluid instead of digested feed. In some infected chicks, a cecal core was observed with the fluid in the cecum. The cecal core was grayish, hard, and rod shaped. It consisted of eroded cells and debris of treponemes and resembled the pseudomembrane. Bloody mucus was also observed in one chick. The thickness of the mucosae in 17 of 25 chicks were markedly increased. The histological changes were classified into two types. In the case of regressive changes of epithelial cells which mean severe erosion, the laminae propriae were exposed. Hemorrhage, edema, and heterophil infiltration in the laminae propriae were also confirmed. Numerous treponemes were observed within the edematous area under the remaining epithelia and also invaded the epithelial cells and laminae propriae. In the other case, progressive changes, that is, hyperplasia of mucosal epithelial cells and elongation of the crypt, were observed. The epithelia consisted mainly of cuboidal basophilic cells, mitotic cells, and goblet cells. The mitotic cells increased in number and were also observed near the superficial luminal surface of the ceca. Mucous goblet cells were also considerably increased in number. The erosion of superficial luminal epithelial cells was not so severe, but edema in laminae propriae was frequently observed. Electron-microscopic observation demonstrated that the basophilic epithelial cells were polyribosome rich, mitochondria poor, and lipid droplet poor. Furthermore, tonofibril-like structures under the terminal web in cytoplasms were lost, and numerous membrane-bound vesicles at the terminal web with free ribosomes were observed. In places, a number of vesicles were observed between microvilli, and some vesicles were released from the apical cytoplasm of epithelial cells. Microvilli also became scarce and irregular. The junctional complexes between cuboidal cells became unclear, the interdigitations between cuboidal cells became loose, and the intercellular spaces were widened. In conclusion, the appearance of numerous membrane-bound vesicles at the terminal web and the dilated intercellular space indicates increased secretion of water and electrolytes, and the abnormal proliferation of such immature epithelial cells indicates impaired absorption. On the basis of our findings, we can say that diarrhea induced by T. hyodysenteriae can be developed by synergism between impaired absorption and increased secretion by intestinal mucosae with hyperplastic immature epithelial cells.(ABSTRACT TRUNCATED AT 400 WORDS)