Conclusiones a la aplicación de sistemas clasificadores

L. intracellularis has been found in swine herds all around the world. Serological data indicate that US (96%) and Northern Europe (70-90%) have a higher prevalence of L.

intracellularis than Southern Europe (50-70%) (34). Evaluation of L. intracellularis antibodies using specific blocking ELISA in intensive pig farms in China revealed that the true prevalence (i.e. prevalence corrected for the imperfect sensitivity and specificity of the testing method) was 77% (35) and the highest prevalence was detected in fattening pigs, breeding sows and boars and in Northern or Southern regions where intensive pig production is highest (35). These data correspond to a serological investigation in Australia and Korea, where high prevalence was detected among intensive production systems. In Australia, commercial blocking ELISA indicated 72% seroprevalence for Western Australia and 88% for Queensland, while the mean percentage of seropositive animals within the herds was 84% (36). A study from South Korea reported seroprevalence in growing and fattening pigs of 45% and 59%, respectively, while 100% of herds had seropositive animals (37). These results should be taken with caution because the Korean study

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used IFAT assay to detect L. intracellularis-specific antibodies, which has not been standardized and may lead to sizable variations across different laboratories.

When PCR was used to detect bacteria in five large pig intensive production farms in Denmark, all examined herds were seropositive for L. intracellularis and 75% of animals were infected (38). The results from PCR testing of nursery herds in Sweden revealed that 48% of herds were actively infected with L. intracellularis (39). These data confirm worldwide distribution of causative agent of PE. PCR can be used to detect L. intracellularis in feces, which can positively identify individual animals that actively shed bacteria and are a source of infection. Studies in the field and in controlled challenge exposure showed that the PCR method detected bacteria in pig feces 1-2 weeks before serum antibodies could be detected with IPMA (40, 19). These data indicate that adequate timing and application of diagnostic tests are very important for acquiring representative data. Positive animals identified by serology or PCR may not show the clinical signs of PE or may only be determined as positive by one diagnostic method. In a recent study where authors followed the progression of lesions associated with PE in experimentally infected pigs, they detected fecal shedding using PCR as early as 3 dpi and detection in feces remained positive up to 35 dpi (7). IgA secretion in intestinal lavage was detected at day 15 and was still detectable up to day 29 (7). Although this study did not evaluate serum antibodies, it indicates that PCR analysis may be a sufficient diagnostic method to detect early stages of infection prior to induction of humoral immunity. In the case where pigs tested positive via serological methods but negative with fecal PCR, animals did not actively shed bacteria but were previously exposed to bacteria (41). Data from different studies where seroprevalence and bacterial presence in feces were examined emphasize the need of applying both serology and PCR to generate representative data that could be used in the prevention and better management of PE.

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The high prevalence and global distribution of L. intracellularis reflect the superb capability of these bacteria to be transmitted among animals. Transmission may occur through three main sources: 1) via fecal material that persists due to poor sanitation measures and by housing of large numbers of susceptible animals of different ages in small spaces (42, 43, 2) by persistence of bacteria in sows that act as subclinical carriers (44); and 3) transmission by rodents present in and around pig farms (45, 10). The fecal-oral route is the mode of transmission that provides bacteria an efficient mode to spread among susceptible animals as the bacteria can survive outside the host and remain infectious for two weeks in feces at 5-15 ^oC (46). The infectious dose in feces is 10⁴ – 10⁶ L. intracellularis but because infected pigs can shed 7 x 10⁸ bacteria per g of feces, the fecal material is highly likely to be adequate for transmission (47). Subclinically infected animals can shed bacteria intermittently for extended periods of time (at least for 10-12 weeks after initial infection in asymptomatic experimentally infected pigs) (48, 49). The ease by which the bacteria can be transmitted via feces stresses the importance of proper hygiene, sanitary and disinfectant measures in pig farms. Slatted concrete flooring is a major factor aiding in the high prevalence of L. intracellularis seropositive pigs (42) and animals raised in modern intensive production systems tend to have higher seropositive prevalence than animals raised outdoor (42, 37, 36, 35). The modern pig production systems tend to intensify production by adding more animals in limited spaces thus providing ideal conditions for L. intracellularis to spread through animal to animal contact or via feces and that allowing adequate cleaning, disinfecting and drying time is necessary to promote proper sanitation (46). Biosecurity is important in preventing the introduction of L. intracellularis into farms and quarantine showed efficacy in keeping some herds free from PE (50, 39). The role of rodents in L. intracellularis transmission had been speculated due to bacteria capability to infect hamsters (51), mice and rats (52, 53, 54), and the prevalence of

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rodents in and around pig farms (45). A study showed as high as ≥70.6% of wild rats captured in pig farms were infected with L. intracellularis and some specimen shed a high number of bacteria per gram feces (10¹⁰ per g) (45). The duration of fecal shedding in experimentally infected mice and rats from the same study persisted for 14-21 days and humoral immunity in both species lasted for 40 days (45) suggesting that these animals could readily infect pigs through their contaminated feces. Mice fed L. intracellularis–infected pig feces became infected and actively shed bacteria in feces (10⁴ bacterial cells per day) (10). In turn, pigs that consumed the rodent feces were infected and shed an average 10⁴ bacteria per g of feces (10). These studies indicate that rodents may act as reservoirs for bacteria and they may play an important role in transmission of L. intracellularis.

Persistence of L. intracellularis in sows with subclinical disease are a major source of bacterial transmission within the herd (44). Sow group housing increases the transmission of bacteria between sows and piglets and this housing practice can increase the number of shed bacteria in feces within rooms (50). The age and parity of the sows are important factors that impact bacteria shedding (42). Older sows with 3-5 parity or more are associated with fewer numbers of seropositive offspring whereas offspring from gilts have increased risk of being seropositive after 5 to 26 weeks (55). These results suggest that higher parity sows have developed protective immunity to L. intracellularis which can benefit the offspring through passive immunity or that these multiparous sows may shed lower numbers of bacteria that are insufficient to cause infection. The role of passive immunity in protection of piglets is not clear although sows with anti-L. intracellularis serum antibodies that do not shed the bacteria have been shown to protect offspring suggesting that passive immunity may be protective (22, 44).

Transmission between herds occurs mainly through the introduction of infected animals into a naïve herd. Newly introduced subclinically infected replacement stock experience stress

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during transport and by being introduced to a new environment. This increased stress may result in them actively shedding L. intracellularis into the barn, which may lead to infection of co-housed animals.