Technical Bulletin Pfizer Animal Health New Studies Associate Equine Tapeworm Infection with Colic

EMX08048 Pfizer Animal Health Technical Bulletin April 2009 New Studies Associate Equine Tapeworm Infection with Colic

Bobby Cowles, DVM, MS, MBA John M. Donecker, VMD, MS, Dipl ABVP (equine) Robert E. Holland, Jr., DVM, PhD Pfizer Animal Health New York, NY 10017

Key Points

• Recent studies reported at the 2007 International Conference of the World Association for the Advancement of Veterinary Parasitology provide new information on the clinical relevance of equine tapeworm infection, including enteric pathology that potentially results in colic. • In a European study, ileocecal lesions including necrotizing enteritis and neuronal degeneration were identified in 35 horses with active tapeworm infection. More than 70% of the ileocecal lesions were classified as either moderate (grade 2) or severe (grade 3), indicating that advanced tapeworm-induced pathology existed in most infected horses. • All 6 horses with heavy tapeworm burdens (n >100) had severe, grade 3 ileocecal lesions. Seven horses (20%) with light (n <30) or moderate (n = 30-100) tapeworm burdens also had grade 3 lesions, indicating that even horses with low-intensity infections are at risk of severe necrotic enteritis. • Neuronal degeneration was most severe in grade 3 cases, affecting ganglions in the myoenteric and submucosal plexi of the gut wall and providing neuropathologic evidence of the association between equine tapeworm infection and increased incidence of colic. • In a second study, prevalence of tapeworm infection in 157 horses was randomly evaluated at necropsy during a 12-month period. Anoplocephala perfoliata was identified in 27% of the horses, exceeding what fecal testing would indicate in many herds. Horses <2 years of age, perhaps due to immunologic susceptibility, were at greater risk of tapeworm infection than adult horses. The geometric mean A. perfoliata burden was 15, although some horses were infected with as many as 476 tapeworms. • There was no significant difference in seasonal prevalence of tapeworm infection in either juvenile or adult horses, indicating that a year-round treatment program for tapeworms would have a benefit in endemic herds. Until recently, diagnosis and control of tapeworm Diagnosis and its clinical infection in horses never acquired the urgency associated with that for cyathostomins and other implications nematodes. At one time, equine tapeworm infection was considered all but inconsequential Active tapeworm infection is difficult to due to the absence of clinical signs, inability to diagnose. The significance of this limitation is induce experimental infection, and the difficulty that it contributes to underestimating the true in diagnosing infection in the living animal. prevalence of tapeworm infection in a herd, However, beginning in 1990s, it became apparent which in turn encourages negligence in tapeworm that A. perfoliata, the most common equine tape- control. In most cases, the clinician or horse worm, is not the innocent bystander it was once owner will simply resort to routine deworming thought to be, in the words of one expert.1 instead of performing a diagnostic test that can confirm active infection. Some horse owners Evidence clearly shows that equine tapeworms: mistakenly assume that broad-spectrum deworm- ers such as macrocyclic lactones (avermectins) or • Have a high prevalence worldwide and in many benzimidazoles will control tapeworms, unaware areas of the U. S., including >95% that neither of these anthelmintic classes has a seroprevalence in the upper Midwest in a recent cesticidal (anti-tapeworm) effect. In tapeworm survey,2-4 creating the potential for increased endemic areas, a positive diagnosis is a useful risk of clinical disease. adjunct to empirical treatment that can help guide • Induce demonstrable equine gastrointestinal a control strategy. (GI) pathology centered on the critical nexus at the ileocecal junction.5-7 There are two approaches to tapeworm diagnosis, • Are correlated with risk of ileal impaction, (1) coprologic (fecal) testing using a centrifuga- spasmodic colic, and cecal intussusceptions and tion/flotation technique, and (2) serologic testing rupture.8-10 using an ELISA format to detect tapeworm antibodies as evidence of prior exposure. There has been a renewed interest in equine Although coprologic diagnosis has the advantage tapeworm infection, driven by the association of not requiring specialized equipment, it is labor between the tapeworm burden and colic, and by intensive and is generally conceded to be a colic’s role as the single most important cause of hit-or-miss affair. This is partly due to the irregu- equine mortality.11 This report describes results lar shedding of tapeworm proglottids (segments) of recent studies that shed light on the pathology in the feces and frequent failure of the proglottid and clinical relevance of tapeworm infection in to rupture and release its eggs. Tapeworm eggs horses, and establish tapeworms as an important have a characteristic morphology and can be equine parasite. readily visualized microscopically, but they are smaller than cyathostomin eggs and tend to sink Topics include: rather than float when placed in solution.

• Identification of necrotizing enteritis and Sedimentation/flotation methods tend to capture submucosal neuronal degeneration in the large only a small sample that often has few if any intestine as components of tapeworm pathology eggs. All of these factors conspire to make fecal that can affect enteric function. analysis an unreliable diagnostic method that • The relationship between worm burden and allows most tapeworm eggs to go undetected. the lesions mentioned above, including the Not surprisingly, sensitivity of coprologic adverse effects of even low levels of tapeworm methods has been found to be low, variously 12 infection. determined to be from 22.5 to 37.5 percent or 13 • How seasonality and the horse’s age affect 61% depending on the study and methodology. infection intensity. Furthermore, although the probability of a positive diagnosis increases with infection intensity, fecal testing is a poor indicator of the actual tapeworm burden in the individual horse and in the

2 herd overall since only a representative minority attachment. of horses in larger groups are generally tested.12,13 • Grade 2 (moderate) – Characterized by edema, inflammation, and focal pseudomembranous A serologic test using ELISA methods was devel- enteritis with fibrin membranes at the tapeworm oped in Europe at the University of Liverpool to attachment site. Histopathology consisted of detect tapeworm antibodies,14 but this test is not a non-specific chronic enteritis with infiltration commercially available in the U. S. An equine of eosinophils affecting the lamina propria and tapeworm ELISA test has been developed at the submucosa, and neuronal degeneration in the University of Tennessee and is available from the submucosa and myoenteric plexi. College of Veterinary Medicine Clinical Immu- • Grade 3 (severe) – Characterized by regional nology Service.15,16 Although more expensive necrotizing enteritis at the site of tapeworm than fecal testing, serologic diagnosis is more attachment, and reduced distensibility of the convenient and suitable for evaluating large num- ileocecal valve. Histopathology consisted of a bers of samples. The U. K. test has been shown fibro-necrotic enteritis affecting all mucosal to have a 68% sensitivity (ability to identify layers, and neuronal degeneration in the positive individuals) and a 95% specificity submucosa and myoenteric plexi. (ability to exclude negative individuals). ELISA identifies prior exposure, not active infection, a disadvantage compared to fecal testing. How- ever, because antibody levels decay in a linear fashion, ELISA optical density values have been correlated with tapeworm infection intensity.14 More recently, an experimental polymerase chain reaction (PCR) test developed in Europe for detection of A. perfiolata in fecal samples has GRADE 1 been shown to be a valid moleclar alternative to coprological and serological methods.17,18

Necrotizing enteritis and neuronal degeneration are features of tapeworm pathology GRADE 2

A university research group in Europe has developed a scoring system for assessing severity of ileocecal gross pathology and histopathology in horses with confirmedA. perfoliata infection.19 The investigators correlated lesions severity with tapeworm burdens in studies of horses necrop- GRADE 3 19,20 sied at an abbatoir. Using this approach in a Figure 1 – Gross ileocecal lesions (left-hand views) and corresponding recent study of 35 horses, tapeworm burdens at histopathology (right-hand views) in cases of equine tapeworm infection the ileocecal junction were determined as light are classified according to the severity grade, which correlated in a non- linear manner with the parasite burden.20 A Grade 1 lesion shows edema, (n <30), medium (n = 30-100), or heavy (n >100) hyperplasia, and scattered hemorrhagic foci at the site of attachment. and correlated with an ileocecal lesion grade and Histopathology reveals a well-defined area of mucosal inflammation. A associated histopathology.20 As illustrated in Grade 2 lesion shows more extensive focal inflammation and edema combined with pseudomembranous enteritis with fibrin membranes Figure 1, ileocecal lesions were scored according around the tapeworm attachment points. Histopathology shows a to the severity of the gross pathology as follows: nonspecific chronic enteritis with infiltration of eosinophils affecting the lamina propria and submucosa. A Grade 3 lesion has a more extensive regional necrotic enteritis in the area of tapeworm attachment and much • Grade 1 (mild) – Characterized by mild edema, reduced distensibility of the ileocecal valve. Histolopathology shows a hyperplasia, hemorrhagic foci and mucosal fibro-necrotic enteritis affecting all layers of the mucosal sample. (Photo used with permission of D. Marciat, Virbac SA). inflammation at the site of tapeworm

3 Table 1 – Correlation between ileocecal lesion grade and number of horses with light, medium or high tapeworm burdens20 ______

No. (% of each lesion grade) horses and respective tapeworm burdens at necropsy ______Ileocecal lesion No. /total Medium grade (% of total) Light (<30) (30-100) Heavy (>100) ______

1 – Mild 14/35 (40%) 14 (100%) 0 0

2 – Moderate 8/35 (23%) 6 (75%) 2 (25%) 0

3 – Severe 13/35 (37%) 4 (31%) 3 (23%) 6 (46%) ______

Table 1 shows the distribution of ileocecal lesion of tapeworms as well as their effect on gangli- grades among the 35 horses evaluated and the ons at the ileocecal junction may be significant percentage of each grade with light, medium, factors contributing to neural impulse deficit or or heavy tapeworm burdens. All 6 horses with other enteric neuromuscular dysfunction. These heavy tapeworm burdens (>100) had severe, findings provide neuropathologic evidence of the grade 3 ileocecal lesions, a statistically association between equine tapeworm infection significant (p<0.01) correlation that conformed and increased incidence of colic.1,9,23 to earlier findings by the same research group as well as other investigators.5,6 However, 7 horses (20%) with light (<30) or moderate (30-100) tapeworm burdens also had grade 3 lesions, indicating a non-linear relationship between the number of tapeworms at the ileocecal junction and occurrence of severe pathology at that site. In other words, even horses with low-intensity tapeworm infections are at risk of severe, regional necrotic enteritis.

Grade 2 and 3 ileocecal lesions included neuronal degeneration (Figure 2), which tended to be more severe in grade 3 cases. Ganglions Figure 2 – An example of neuronal degeneration in a ganglion of the in the myoenteric (Auerbach) and submucosal enteric nervous system in a horse with equine tapeworm infection.20 Ganglions in the submucosal (Meissner) and myoenteric (Auerbach) plexi (Meissner) plexi of the gut wall were affected. of the gut wall were involved, potentially affecting gastrointestinal move- Neuronal degradation involving both plexi could ment and secretion. Neuronal degeneration was a feature of grade 2 be expected to compromise excitatory or and grade 3 ileocecal lesions. (Photo used with permission of D. Marciat, Virbac SA). inhibitory enteric motor activity at the highly innervated ileocecal junction. In addition, A. perfoliata as well as other cestodes contain acetylcholinesterase,21,22 an enzyme involved in the synthesis of neurotransmitters that terminate neural transmission. Thus, the histochemistry

4 Tapeworm prevalence is affected However, some individuals had much higher numbers of tapeworms, up to 476 for A. by the host’s age but not by perfoliata and 137 for A. magna. seasonality • There was no appreciable difference in seasonal prevalence of tapeworm infection in either To assess prevalence of tapeworm infection and juvenile or adult horses. age predisposition, the same European research group performed necropsies on a sizeable and The study indicated that prevalence of tape- varied population (n = 157) of juvenile and adult worm infection in a random equine population horses.24 Necropsies were evenly distributed is approximately one in four animals in the case among the four seasons of the year, which of A. perfoliata, exceeding what coprological enabled a determination of seasonal fluctuations diagnosis would indicate in many herds. Data in incidence of equine tapeworm infection. also indicated that the juvenile horse, perhaps due to immunologic susceptibility, is at greater The prevalence of tapeworm infection by risk of tapeworm infection than the adult, a result Anoplocephala species and age of the horse is corroborated in serologic studies by other inves- 26 shown in Table 2. Several findings were note- tigators. Additionally, A. magna should not be worthy: ignored as a potential equine parasite. Perhaps counter-intuitively, tapeworm infection is not • Although A. perfoliata is by far the restricted to warmer seasons, but appears to be predominant equine tapeworm in horses world prevalent throughout the year. This outcome wide, A. magna was identified in 14% of the was consistent with a U. S. seroprevalence study population evaluated versus 27% for showing the high exposure rates in states that ex- A. perfoliata. (In contrast, in the U. S. perience the full range of seasonal climate varia- A. perfoliata is diagnosed almost exclusively3,25) tion (for example, a 95.8% prevalence in states in • Juvenile horses <2 years of age had a higher the upper Midwest and a 62.3% seroprevalence 4 prevalence of tapeworm infection than adult in New England states). These data suggest that horses >2 years. In the case of A. magna, the a year-round treatment program for tapeworms difference was statistically significant (p<0.05), would have a benefit in endemic herds. 17% in juvenile horses versus 4% in adults. • The geometric mean tapeworm burden was 15 for A. perfoliata and 4.2 for A. magna.

Table 2 – Prevalence of tapeworm infection in juvenile and adult horses at necropsy24 ______

Age group and no. positive horses ______

Tapeworm GM tapeworm Juvenile (<2 yrs, Adult (>2 yrs, All horses* species burden (range) n = 69) n = 53 (n = 157) ______

A. perfoliata 15 (1-476) 21 (30%) 11 (21%) 43 (27%)

A. magna 4.2 (1-137) 12 (17%)a 2 (4%)b 22 (14%) ______GM = geometric mean. *35 horses were of unknown age. a,bColumns with different superscripts were significantly different (p<0.05).

5 Clinical perspective References

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