Canine Mast Cell Tumors

Serpulinapilosicoli:  What We Know and What We Do Not

            The organism - There are 5 distinct Serpulina sp. known to infect swine.  Two species are pathogenic.  Serpulinahyodysenteriae (formerly Treponemahyodysenteriae) causes swine dysentery.  Serpulinapilosicoli (formerly Anguillina coli) causes intestinal spirochetosis.  Three additional species are nonpathogenic i.e. Serpulinainnocens (formerly Treponemainnocens), intermedia and murdochii.  Some strains of S. intermedia have been associated with diarrhea; however, inoculation studies in pigs have not consistently reproduced disease.

            Serpulinapilosicoli can be differentiated from other Serpulina sp. by PCR or by culture (weak b-hemolysis) followed by biochemical testing, i.e. indole negative, hippurate hydrolysis positive and lack of b-glucosidase activity in the API-ZYM profile.  BJ medium that is most commonly used to culture S. hyodysenteriae in diagnostic laboratories is slightly inhibitory when used for isolation of S. pilosicoli due to the moderate sensitivity of S. pilosicoli to 2 of the included antibiotics, i.e. rifampicin and spiramycin.  Culture of S. pilosicoli is most sensitive with a modified BJ media that does not contain rifampicin or spiramycin.  Currently, the most sensitive diagnostic protocol involves culture on modified BJ media, followed by screening of suspect colonies using the hippuratehydroysis test and confirmation of positives by PCR testing.

            In addition to swine, S. pilosicoli also infects humans, non-human primates, dogs and several species of birds.  Strains of S. pilosicoli can colonize laboratory mice with fecal shedding for up to 30 days, suggesting the potential for rodents to act as resevoirs of infection for swine.  Likewise, birds, dogs and humans are also potential resevoirs for swine.  The pathogenic potential of swine strains of S.pilosicoli for humans is unknown, but zoonotic potential exists.

            The disease - Intestinal spirochetosis is a non-fatal large intestinal disease caused by S.pilosicoli that has been described in field studies of affected swine herds and in inoculation studies in which disease was reproduced.

            Clinical disease occurs in weaned pigs primarily 8-16 weeks of age, usually commencing 7-14 days after moving and mixing of pigs.  This is consistent with the reported incubation period in inoculation studies of 3-16 days.  Typically, 5-15% of pigs are affected and affected individuals exhibit diarrhea and poor growth for 2-3 weeks.  Some pigs may develop chronic diarrhea and exhibit poor growth for longer periods of time.  Clinical signs typically are present in a group of pigs for 3-6 weeks.  Affected individuals may require up to 28 additional days to reach a slaughter weight of 210 pounds.  Mortality rarely exceeds 1-2%.  Economic loss is primarily due to poor feed conversions and effects of uneven growth rates on pig-flows and market uniformity.  The diarrheic feces are usually first soft and wet ("wet cement" consistency) and then later change to a watery consistency and are gray to brown with a small amount of mucous ("oily" sheen).  During recovery or in chronic cases, feces may contain thick tags of mucous.  Occasionally, flecks of blood may also be present.  Affected pigs generally remain alert and active, but appetite is depressed and pigs may show abdominal discomfort and/or may appear gaunt and develop rough hair coats.

            Gross lesions of intestinal spirochetosis are usually subtle.  Pigs are variably gaunt and have rough hair coats.  The spiral colon is flaccid, enlarged and contains abundant watery content with variable amounts of mucous and occasionally some blood.  The colonic mesentery and serosa may be thickened by edema in acute cases and the serosa may be thickened by fibrin or fibrous connective tissue in chronic cases (serositis).  Colonic lymph nodes are sometimes enlarged.  Mucosal lesions are most common and severe in the mid-spiral regions of the spiral colon followed by the proximal spiral colon.  The cecal mucosa is either not involved or has mild lesions.  The colonic and cecal mucosa in affected areas may be congested (reddened) and thickened by edema fluid forming prominent ridges.  Mucosal erosions occur in variable numbers.  With few erosions, the mucosa appears relatively normal (glistening) with a few scattered adherent feed particles.  With many erosions, the mucosa appears granular and fibrin exudation admixed with necrotic cellular debris may result in multifocalfibrinonecrotic tags or plaques.  Variable amounts of mucous, and occasionally blood, may be in contents and on the mucosal surface.  Mucosal lesions are mild compared to classic lesions of swine dysentery or salmonellosis.  To see small erosions, i.e. adherent feed particles or small areas of fibrinonecrotic debris, the mucosa should be gently washed free of contents with flowing water.  Scraping contents from the mucosa with a knife should be avoided since this will often destroy many of the mucosal lesions and alter some microscopic lesions.

            Microscopically, there is a mild to moderately severe multifocal to diffuse superficial erosive colitis.  A variable amount of fibrinonecrotic debris is on the luminal surface in areas of erosion.  The mucosa is variably thickened by an increased depth of crypts (crypt hyperplasia), edema of the lamina propria and increased numbers of lymphocytes and plasma cells in the lamina propria and, to a lesser degree, the submucosa.  Goblet cell hyperplasia is common and may cause distention of crypts with mucous.  A lesions unique to S. pilosicoli is end-on attachment of the bacterial cells to the apical margin of mature epithelial cells on the colonic luminal surface creating a "false brush border" appearance.  Unfortunately, this lesion is present only in the early stages of infection and is thus present in a minority of diagnostic cases and cannot be used as a reliable diagnostic tool.  Large serpentine spirochetes typical of Serpulina sp. are more commonly present admixed with other bacteria in adherent fibrinonecrotic debris, in the superficial lamina propria and in the crypts.  Unfortunately, S.pilosicoli cannot be differentiated from other Serpulina sp. based on morphology at a light microscopic level.  Apart from the unique, but inconsistent, lesion of end-on attachment by S. pilosicoli, the microscopic lesions of intestinal spirochetosis are relatively nonspecific and can be mimicked by lesions of salmonellosis, mild swine dysentery or allergy to certain types of pelleted diets.

            Knowledge of the epidemiology/ economics of intestinal spirochetosisis limited, based on few inoculation studies and field epidemiologic studies.  Infection with S. pilosicoli has been reported in swine in nearly every country with a significant swine industry.  The proportion of infected swine herds in the U.S. is unknown.  In a limited study of diarrheic pigs on 10 grower sites in a single U.S. swine production company, S. pilosicoli was isolated in 50%.  In a study of 85 swine herds with a history of colitis in the U.K., S. pilosicoli was detected in 52% and was the sole pathogen detected in 33%.  In Finland, one study of 894 farms demonstrated S. pilosicoli in 18% and in another study of 50 finishing sites that were stocked from "LSO 2000 quality chain" health-status farrowing sites, S. pilosicoli was detected in 28%.  Transmission of S. pilosicoli is considered to be exclusively fecal-oral.  The greatest risk factor for infection of negative pigs is exposure to fresh feces from shedding carrier pigs.  Serpulinapilosicoli, like S.hyodysteriae, survives in anaerobic lagoons and in moist fecal matter; hence open flush gutters, inadequate cleaning of contaminated pens/facilities, contaminated truck/trailers, etc. are all significant risk factors for infection.  Other species known to sometimes carry S. pilosicoli, including humans, dogs, birds, and possibly mice may pose some biosecurity risk to negative herds.  Bird-proofing of buildings and rodent control are recommended as prudent preventative measures.

            Not all pigs that are infected with S.pilosicoli develop diarrhea.  In oral inoculation studies, nearly all pigs become colonized by S.pilosicoli, but only 1/3-2/3 develop diarrhea.  Pigs remain colonized and shed S. pilosicoli in feces for up to 6 weeks.  Pelleted feed increases the risk of diarrhea in S. pilosicoli infected pigs.  When the ration is changed from pellets to meal, the proportion of diarrheic pigs decreases.

            Diagnosis - A definitive diagnosis of intestinal spirochetosis requires the demonstration of typical colonic lesions, confirmation of infection by S. pilosicoli and elimination of other causes of colitis in swine.  Other diseases that should be excluded by testing are salmonellosis, swine dysentery, proliferative enteritis, whipworm infestation, and possibly yersiniosis caused by Yersiniapseudotuberculosis.  Tests available for detection of infection with S. pilosicoli include culture and/or PCR.  Fecal samples may be used to confirm infection and predict prevalence in populations; however, sensitivity for both tests is not as high in fecal samples as when testing is on colonic mucosal samples.  Duhamel estimated that the sensitivity of fecal culture for S. pilosicoli is approximately 80% under ideal conditions, i.e. shedding large numbers of organisms with no antibiotics in the feed.  Under most field conditions, the sensitivity would probably be lower.  Fecal samples should be collected on swabs and immersed in Ames transport medium with activated charcoal, chilled and shipped overnight to the laboratory for testing.  In-situ hybridization testing for S. pilosicoli done on formalin-fixed sections of colon has been described experimentally as a sensitive method for the diagnosis of intestinal spirochetosis, but is not yet available in diagnostic laboratories in the U.S.

            Treatment - Serpulinapilosicoli is generally sensitive to the same antibiotics as is S. hyodysteriae and variable clinical response of intestinal spirochetosis to treatment is described.  For 19 U.S. strains, all were susceptible to carbadox and tiamulin, 47% were susceptible to gentamycin and 42% were susceptible to lincomycin.  Most schemes for control of intestinal spirochetosis combine therapeutic levels of antimicrobials during the first few weeks in grower buildings with sanitation, i.e. cleaning and disinfecting pens/buildings between groups of pigs.  It is assumed, but not proven, that schemes combining treatment and sanitation for the elimination of S. hyodysenteriae would also be effective against S. pilosicoli.  It is unknown whether elimination of S. pilosicoli would be cost-effective.

            As is true of many diseases, especially recently recognized or emerging diseases, some of the most practical and important questions remain unanswered.

1. What is the prevalence of S. pilosicoli infection in the U.S. swine herd?

2. In an infected herd, what proportion of pigs become infected when?

3. What is the cost of subclinical infection?

4. What is the cost of clinical disease?

5. What are the best methods of treatment, control or elimination and are they cost-effective?

6. Is infection with other enteric agents additive or synergistic?

7. In diarrheal disease with multiple concurrent pathogenic agents, what is the relative contribution of each?

- by Greg Stevenson, DVM, PhD