L a w s o n i a in t r a c e l l u l a r i s a n d E quine Pro l i f er at iv e E n t e ro p a t h y Allen E. Page, DVM, PhDa, Nathan M. Slovis, Dipl. ACVIM, CHT, CHT-Vb,*, David W. Horohov, MS, PhDa KEYWORDS Equine proliferative enteropathy Lawsonia intracellularis EPE KEY POINTS Lawsonia intracellularis is the etiologic agent for equine proliferative enteropathy (EPE), which typically affects weanling and yearling horses. In North America, EPE cases often occur between August and January, although cases outside of this time frame have been reported. Clinical signs of EPE are usually nonspecific and include lethargy, pyrexia, anorexia, peripheral edema, weight loss, colic, and diarrhea. Diagnosis is based on the presence of hypoproteinemia and hypoalbuminemia along with clinical signs and positive commercial serologic and/or molecular testing. Treatment of EPE requires the use of antimicrobials with good intracellular penetration as well as supportive care to prevent or decrease secondary complications.
INTRODUCTION
Lawsonia intracellularis is an obligate, intracellular, gram-negative bacterium that is associated with proliferative enteropathy (PE) in a variety of species. An economically devastating disease of commercial pig production worldwide, L. intracellularis has been shown to have a detrimental effect on average daily gain and market weight in pigs.1,2 L. intracellularis is also the etiologic agent for equine PE (EPE) which is now an emerged pathogen that has been reported worldwide and typically affects young horses, with those between 4 and 9 months of age particularly susceptible to disease.3–5 The pathogenesis of L. intracellularis–induced PE involves an initial colonization of the mitotically active enterocytes of the crypts resulting in crypt hyperplasia. This hyperplasia leads to adenomatous thickening of the mucosa (commonly noted
a Department of Veterinary Science, Maxwell H. Gluck Equine Research Center, University of Kentucky, Lexington, KY 40546, USA; b McGee Medicine Center, Hagyard Equine Medical Institute, 4250 Iron Works Pike, Lexington, KY 40511, USA * Corresponding author. E-mail address: [email protected]
Vet Clin Equine - (2014) -–http://dx.doi.org/10.1016/j.cveq.2014.08.001 vetequine.theclinics.com 0749-0739/14/$ – see front matter Ó 2014 Elsevier Inc. All rights reserved.
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in the ileum), with later involvement of other intestinal segments possible.6 In horses, EPE results in hypoproteinemia and hypoalbuminemia,3 among other signs, with some horses ultimately dying of the disease despite aggressive care.7 EPIDEMIOLOGY
The epidemiology of L. intracellularis remains poorly understood in the horse, although it is thought that transmission occurs through the ingestion of fecal material from wild or domestic animals.8 Because of the significant impact of L. intracellularis on the swine industry, progress in understanding porcine PE (PPE) epidemiology is much more evolved than it is with EPE. It has been hypothesized that PPE persists and is transmitted within swine operations via poor/inadequate disinfection techniques as well as subclinical shedders of the bacterium. Indeed, work has shown that subclinically infected pigs can efficiently spread the bacterium among cohorts.9 Although a study with bimonthly fecal polymerase chain reaction (PCR) testing did identify several weanling horses that shed the bacterium in the absence of clinical signs,10 the role, if any, subclinically infected horses play in transmission remains to be determined. In one study, clinically affected horses were shown to potentially play a role in the transmission of L. intracellularis.11 It is, therefore, currently recommended that horses with documented EPE not be allowed to comingle with the rest of the herd until after at least 1 week of antimicrobial therapy. Exposure to L. intracellularis is widespread, with reports of EPE cases occurring worldwide.12,13 In the United States, regional exposure varies greatly, with farmspecific seroprevalence ranging from 14% to 100%.5,10,14,15 Only 11% of exposed young horses will develop a form of EPE (5% clinical EPE and 6% subclinical EPE).5 With EPE, the most commonly affected age groups are weanling and young yearlings, with those between 4 and 9 months of age appearing to be most susceptible to infection.3,15 EPE has also, on occasion, been diagnosed in older horses up to 17 years of age at one of the author’s clinic (NMS); these older horses typically have an additional underlying disease process (Slovis, 2009). In North America, the disease is often detected between August and January,3,10,15 although cases outside of this time frame have been reported.5 It was once hypothesized that exposure to pig feces is a potential source of infection for horses. However, in most cases of EPE, no history or evidence of exposure to pigs or pig feces has been reported.16 Further, molecular typing of equine isolates using variable number tandem repeat (VNTR) sequencing has demonstrated a clear distinction between the isolates obtained from porcine and equine species.17 A recent study also revealed evidence of L. intracellularis host adaptation in horses and pigs using different isolates.18 Fecal shedding and serologic responses were higher and longer in foals infected with the equine isolate compared with foals infected with the porcine isolate. Similarly, reduced average daily weight gain and diarrhea were observed in pigs with the porcine isolate. This species specificity for equine and porcine isolates of L. intracellularis is also noted in other animal models of the disease. For example, hamsters challenged with an equine isolate of L. intracellularis do not develop infection, whereas hamsters challenged with a pig isolate can develop disease.19 Conversely, rabbits challenged with an equine isolate develop lesions, whereas they do not develop typical intestinal lesions when challenged with the pig isolate.19 Since lagomorphs may represent an effective reservoir/amplifier host due to their large population, close contact with horses, short reproductive cycle, and worldwide distribution, recent work has examined a possible role of rabbits in the epidemiology of EPE. When nasogastrically intubated with feces from rabbits that had been experimentally
Equine Proliferative Enteropathy
challenged with an equine strain of L. intracellularis, foals were noted to seroconvert and shed the bacterium in their feces, however, clinical signs of EPE were noted.20 A role for rodents, rats in particular, with respect to the transmission of L. intracellularis warrants additional investigation as work has shown a proportion of rats on PPE-endemic farms can shed up to 1010 L. intracellularis organisms per gram of feces.21 This finding is of particular importance as experimental challenge studies in pigs have induced PPE at bacterial doses as low as 105 per pig,22 whereas EPE has been induced in horses using doses of 1010 per horse.11,23 Further, it has been shown that the bacterium may be infective for up to 2 weeks in the environment at temperatures of 5 C to 15 C,24 raising the possibility of a prolonged period during which there is a risk of exposure. Raccoons can be particularly pervasive on horse farms; although L. intracellularis has been found previously in raccoon feces on an EPE-endemic farm in central Kentucky (unpublished data, Page, Slovis, and Dr. Nicola Pusterla, 2008), VNTR comparisons of the raccoon and equine-strains showed that they were genetically unrelated (unpublished data from Dr Connie Gebhart, University of Minnesota, 2008). In addition to the studies mentioned earlier, several have revealed the presence of L. intracellularis in a variety of other species, including black-tailed jackrabbits, cottontail rabbits, cats, striped skunks, Virginia opossums, coyotes, guinea pigs, mice, hamsters, hedgehogs, ferrets, rabbits, wild pigs, dogs, foxes, calves, wolves, deer, ostriches, emus, monkeys, and giraffes6,25–27; however, none have been successfully implicated in the induction of clinical EPE. Although there is evidence that L. intracellularis variants may have evolved to be adapted to more than one host,28 it seems likely that susceptibilities to the bacterium are driven by the origin of the isolate28; thus, the role that each carrier species plays, if any, in the epidemiology of EPE is unknown. It is important to note that L. intracellularis is not currently considered a zoonotic disease. CLINICAL SIGNS
As noted previously, EPE is generally seen in weaned horses less than 1 year of age. Clinical signs of EPE are usually nonspecific and include lethargy, pyrexia (>38.5 C), anorexia, peripheral edema (Fig. 1), weight loss (Fig. 2), colic, and diarrhea.3,4,16 Because of the nonspecific nature of the clinical signs, diagnostic testing may be
Fig. 1. A 5-month-old Thoroughbred with EPE displaying marked edema of the head and throatlatch region.
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Fig. 2. Severe weight loss in a 4-month-old Thoroughbred with EPE.
necessary to rule out protein loss in the urine as well as peritoneal and pleural cavities. For those horses that recover, the recovery period can take weeks to months before they regain the appearance of unaffected cohorts. DIAGNOSTIC TESTS
Although the definitive diagnosis of EPE requires direct observation of L. intracellularis within enterocytes of the hyperplastic small intestine at necropsy, a presumptive, antemortem diagnosis can generally be made based on the age of the affected animal and clinical signs as well as the presence of hypoproteinemia/hypoalbuminemia, thickened small intestinal loops on abdominal ultrasound, and concurrent positive commercially available diagnostic tests for L. intracellularis. Abdominal Ultrasonography
Proliferative enteropathy caused by L. intracellularis results in small intestinal mucosal hyperplasia; as a result, abdominal ultrasonography can be a useful diagnostic in horses. The use of a 2.5- to 5.0-mHz ultrasonographic probe is typically needed in order to obtain a depth of 8 to 20 cm. Although a linear rectal probe may provide sufficient depth to detect thickened small intestine, a negative result using such a probe should not rule out EPE. By itself, small intestinal wall thickness greater than 3 mm is not pathognomonic for EPE10; but increased thickness (Fig. 3) accompanied by compatible clinical and clinicopathologic signs is highly suspicious. Practitioners should avoid relying solely on abdominal ultrasonography for EPE diagnosis because EPE cases can have normal small intestinal wall thickness3; other pathologic conditions can cause small intestine and colonic serosal edema, including salmonellosis, Clostridium difficile, and peritonitis. Total Protein/Albumin/Clinicopathologic Changes
Hypoproteinemia and hypoalbuminemia, which cause the dependent edema seen in clinical EPE cases, are the most consistent laboratory finding associated with EPE.3–5,8 The pathophysiology of hypoproteinemia and hypoalbuminemia is still under debate. Replication of L. intracellularis results in the proliferation and hyperplasia of the rapidly dividing cells of the crypts, leading to an overpopulation of
Equine Proliferative Enteropathy
Fig. 3. Ultrasound image revealing thickened sections of small intestine serosa in a 6-monthold Thoroughbred filly. The wall thickness measured 6.78 mm (normal
INTRODUCTION
Lawsonia intracellularis is an obligate, intracellular, gram-negative bacterium that is associated with proliferative enteropathy (PE) in a variety of species. An economically devastating disease of commercial pig production worldwide, L. intracellularis has been shown to have a detrimental effect on average daily gain and market weight in pigs.1,2 L. intracellularis is also the etiologic agent for equine PE (EPE) which is now an emerged pathogen that has been reported worldwide and typically affects young horses, with those between 4 and 9 months of age particularly susceptible to disease.3–5 The pathogenesis of L. intracellularis–induced PE involves an initial colonization of the mitotically active enterocytes of the crypts resulting in crypt hyperplasia. This hyperplasia leads to adenomatous thickening of the mucosa (commonly noted
a Department of Veterinary Science, Maxwell H. Gluck Equine Research Center, University of Kentucky, Lexington, KY 40546, USA; b McGee Medicine Center, Hagyard Equine Medical Institute, 4250 Iron Works Pike, Lexington, KY 40511, USA * Corresponding author. E-mail address: [email protected]
Vet Clin Equine - (2014) -–http://dx.doi.org/10.1016/j.cveq.2014.08.001 vetequine.theclinics.com 0749-0739/14/$ – see front matter Ó 2014 Elsevier Inc. All rights reserved.
2
Page et al
in the ileum), with later involvement of other intestinal segments possible.6 In horses, EPE results in hypoproteinemia and hypoalbuminemia,3 among other signs, with some horses ultimately dying of the disease despite aggressive care.7 EPIDEMIOLOGY
The epidemiology of L. intracellularis remains poorly understood in the horse, although it is thought that transmission occurs through the ingestion of fecal material from wild or domestic animals.8 Because of the significant impact of L. intracellularis on the swine industry, progress in understanding porcine PE (PPE) epidemiology is much more evolved than it is with EPE. It has been hypothesized that PPE persists and is transmitted within swine operations via poor/inadequate disinfection techniques as well as subclinical shedders of the bacterium. Indeed, work has shown that subclinically infected pigs can efficiently spread the bacterium among cohorts.9 Although a study with bimonthly fecal polymerase chain reaction (PCR) testing did identify several weanling horses that shed the bacterium in the absence of clinical signs,10 the role, if any, subclinically infected horses play in transmission remains to be determined. In one study, clinically affected horses were shown to potentially play a role in the transmission of L. intracellularis.11 It is, therefore, currently recommended that horses with documented EPE not be allowed to comingle with the rest of the herd until after at least 1 week of antimicrobial therapy. Exposure to L. intracellularis is widespread, with reports of EPE cases occurring worldwide.12,13 In the United States, regional exposure varies greatly, with farmspecific seroprevalence ranging from 14% to 100%.5,10,14,15 Only 11% of exposed young horses will develop a form of EPE (5% clinical EPE and 6% subclinical EPE).5 With EPE, the most commonly affected age groups are weanling and young yearlings, with those between 4 and 9 months of age appearing to be most susceptible to infection.3,15 EPE has also, on occasion, been diagnosed in older horses up to 17 years of age at one of the author’s clinic (NMS); these older horses typically have an additional underlying disease process (Slovis, 2009). In North America, the disease is often detected between August and January,3,10,15 although cases outside of this time frame have been reported.5 It was once hypothesized that exposure to pig feces is a potential source of infection for horses. However, in most cases of EPE, no history or evidence of exposure to pigs or pig feces has been reported.16 Further, molecular typing of equine isolates using variable number tandem repeat (VNTR) sequencing has demonstrated a clear distinction between the isolates obtained from porcine and equine species.17 A recent study also revealed evidence of L. intracellularis host adaptation in horses and pigs using different isolates.18 Fecal shedding and serologic responses were higher and longer in foals infected with the equine isolate compared with foals infected with the porcine isolate. Similarly, reduced average daily weight gain and diarrhea were observed in pigs with the porcine isolate. This species specificity for equine and porcine isolates of L. intracellularis is also noted in other animal models of the disease. For example, hamsters challenged with an equine isolate of L. intracellularis do not develop infection, whereas hamsters challenged with a pig isolate can develop disease.19 Conversely, rabbits challenged with an equine isolate develop lesions, whereas they do not develop typical intestinal lesions when challenged with the pig isolate.19 Since lagomorphs may represent an effective reservoir/amplifier host due to their large population, close contact with horses, short reproductive cycle, and worldwide distribution, recent work has examined a possible role of rabbits in the epidemiology of EPE. When nasogastrically intubated with feces from rabbits that had been experimentally
Equine Proliferative Enteropathy
challenged with an equine strain of L. intracellularis, foals were noted to seroconvert and shed the bacterium in their feces, however, clinical signs of EPE were noted.20 A role for rodents, rats in particular, with respect to the transmission of L. intracellularis warrants additional investigation as work has shown a proportion of rats on PPE-endemic farms can shed up to 1010 L. intracellularis organisms per gram of feces.21 This finding is of particular importance as experimental challenge studies in pigs have induced PPE at bacterial doses as low as 105 per pig,22 whereas EPE has been induced in horses using doses of 1010 per horse.11,23 Further, it has been shown that the bacterium may be infective for up to 2 weeks in the environment at temperatures of 5 C to 15 C,24 raising the possibility of a prolonged period during which there is a risk of exposure. Raccoons can be particularly pervasive on horse farms; although L. intracellularis has been found previously in raccoon feces on an EPE-endemic farm in central Kentucky (unpublished data, Page, Slovis, and Dr. Nicola Pusterla, 2008), VNTR comparisons of the raccoon and equine-strains showed that they were genetically unrelated (unpublished data from Dr Connie Gebhart, University of Minnesota, 2008). In addition to the studies mentioned earlier, several have revealed the presence of L. intracellularis in a variety of other species, including black-tailed jackrabbits, cottontail rabbits, cats, striped skunks, Virginia opossums, coyotes, guinea pigs, mice, hamsters, hedgehogs, ferrets, rabbits, wild pigs, dogs, foxes, calves, wolves, deer, ostriches, emus, monkeys, and giraffes6,25–27; however, none have been successfully implicated in the induction of clinical EPE. Although there is evidence that L. intracellularis variants may have evolved to be adapted to more than one host,28 it seems likely that susceptibilities to the bacterium are driven by the origin of the isolate28; thus, the role that each carrier species plays, if any, in the epidemiology of EPE is unknown. It is important to note that L. intracellularis is not currently considered a zoonotic disease. CLINICAL SIGNS
As noted previously, EPE is generally seen in weaned horses less than 1 year of age. Clinical signs of EPE are usually nonspecific and include lethargy, pyrexia (>38.5 C), anorexia, peripheral edema (Fig. 1), weight loss (Fig. 2), colic, and diarrhea.3,4,16 Because of the nonspecific nature of the clinical signs, diagnostic testing may be
Fig. 1. A 5-month-old Thoroughbred with EPE displaying marked edema of the head and throatlatch region.
3
4
Page et al
Fig. 2. Severe weight loss in a 4-month-old Thoroughbred with EPE.
necessary to rule out protein loss in the urine as well as peritoneal and pleural cavities. For those horses that recover, the recovery period can take weeks to months before they regain the appearance of unaffected cohorts. DIAGNOSTIC TESTS
Although the definitive diagnosis of EPE requires direct observation of L. intracellularis within enterocytes of the hyperplastic small intestine at necropsy, a presumptive, antemortem diagnosis can generally be made based on the age of the affected animal and clinical signs as well as the presence of hypoproteinemia/hypoalbuminemia, thickened small intestinal loops on abdominal ultrasound, and concurrent positive commercially available diagnostic tests for L. intracellularis. Abdominal Ultrasonography
Proliferative enteropathy caused by L. intracellularis results in small intestinal mucosal hyperplasia; as a result, abdominal ultrasonography can be a useful diagnostic in horses. The use of a 2.5- to 5.0-mHz ultrasonographic probe is typically needed in order to obtain a depth of 8 to 20 cm. Although a linear rectal probe may provide sufficient depth to detect thickened small intestine, a negative result using such a probe should not rule out EPE. By itself, small intestinal wall thickness greater than 3 mm is not pathognomonic for EPE10; but increased thickness (Fig. 3) accompanied by compatible clinical and clinicopathologic signs is highly suspicious. Practitioners should avoid relying solely on abdominal ultrasonography for EPE diagnosis because EPE cases can have normal small intestinal wall thickness3; other pathologic conditions can cause small intestine and colonic serosal edema, including salmonellosis, Clostridium difficile, and peritonitis. Total Protein/Albumin/Clinicopathologic Changes
Hypoproteinemia and hypoalbuminemia, which cause the dependent edema seen in clinical EPE cases, are the most consistent laboratory finding associated with EPE.3–5,8 The pathophysiology of hypoproteinemia and hypoalbuminemia is still under debate. Replication of L. intracellularis results in the proliferation and hyperplasia of the rapidly dividing cells of the crypts, leading to an overpopulation of
Equine Proliferative Enteropathy
Fig. 3. Ultrasound image revealing thickened sections of small intestine serosa in a 6-monthold Thoroughbred filly. The wall thickness measured 6.78 mm (normal
