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Arquivo Brasileiro de Medicina Veterinária e Zootecnia

Print version ISSN 0102-0935On-line version ISSN 1678-4162

Arq. Bras. Med. Vet. Zootec. vol.54 no.2 Belo Horizonte Apr. 2002 

Factors associated with fecal-shedding of Salmonella spp by horses on US operations

[Fatores associados com Salmonella spp em fezes de cavalos em criatórios norte-americanos]


W.C. Losinger1, J.L. Traub-Dargatz2, L.P. Garber3, P.J. Fedorka-Cray3, S. Ladely3, K.E. Ferris3, K. Morgan3

1United States Department of Energy, New Brunswick Laboratory
9800 South Cass Avenue, Bldg 350, Argonne
Illinois 60439 USA
2Colorado State University, College of Veterinary Medicine and Biomedical Sciences
3United States Department of Agriculture


Recebido para publicação em 7 de novembro de 2001.




In a cross-sectional national study that included 972 operations with > 3 horses on 1/1/98 in 28 states in the USA, 8,417 fecal specimens were collected from horses and cultured to test for the presence of Salmonella spp. Operations were characterized as Salmonella spp-positive if at least one fecal specimen tested positive for Salmonella spp. Percentages of Salmonella spp-positive operations were computed by management and other factors (collected from operation-level questionnaires) that were hypothesized to be related to fecal shedding of Salmonella spp. A logistic-regression model was constructed to identify factors associated with horses’ shedding Salmonella spp in feces on an operation. The odds of an operation being Salmonella spp positive increased as the number of resident horses increased. In addition, the following factors were found to be associated with increased odds of an operation being Salmonella spp positive: horses were used primarily for breeding; operation cleanliness was characterized as poor by the data collector; and new resident equids had been added to the operation without routine quarantine.

Keywords: Equine operations, national survey, risk factor analysis, logistic regression



No levantamento que envolveu 972 criatórios de cavalos, com número igual ou maior que três cavalos por criatório em 28 estados norte-americanos, foram coletadas 8.417 amostras de fezes de cavalos e submetidas à cultura para testar a presença de Salmonella spp. Os criatórios foram classificados como positivos se pelo menos uma amostra testada fosse positiva para Salmonella spp. As percentagens de criatórios positivos foram calculadas de acordo com o manejo e outros fatores (coletados por meio de questionários) que hipoteticamente estavam relacionados com Salmonella spp em fezes de cavalos. Um modelo de regressão logística foi ajustado às observações de forma a identificar fatores associados com Salmonella spp nas fezes de cavalos nos criatórios. A probabilidade de o criatório ser positivo para Salmonella spp aumenta com o número de cavalos do criatório. Além disso, os seguintes fatores foram identificados como associados ao aumento da probabilidade do criatório ser positivo para Salmonella spp: cavalos utilizados essencialmente para reprodução, deficiência na higiene do criatório e adição de novos cavalos sem respeitar as normas de quarentena.

Palavras-chave: Criatório de cavalo, levantamento nacional, análise de risco, regressão logística




Analyses of fecal specimens collected from 972 operations with > 3 horses in 28 states participating in the Equine ’98 Study of the United States Department of Agriculture (USDA), Animal and Plant Health Inspection Service, National Animal Health Monitoring System (NAHMS) indicated that at least one resident horse was shedding Salmonella spp on 1.8% (SE, 0.7) of operations (Traub-Dargatz et al., 2000).

Clinical signs associated with equine salmonellosis range from no abnormal clinical signs to peracute death (Smith, 1981a; Smith, 1981b; McCain & Powell, 1990; van Duijkeren et al., 1995). Diarrhea (often preceded by anorexia, fever, colic, and leukopenia) is the most common clinical sign of equine salmonellosis (Smith, 1981a; Smith, 1981b; van Duijkeren et al., 1995). Apparently-healthy carriers of Salmonella spp include some horses that intermittently shed Salmonella spp in the feces, and horses that harbor Salmonella spp in the mesenteric lymph nodes or in the mucosa of the large intestine, and shed Salmonella spp in the feces when stressed (Smith, 1981a; Smith, 1981b; McCain & Powell, 1990; van Duijkeren et al., 1995). These carriers may then serve as a source of infection for other horses (Smith, 1981a; Smith, 1981b). Mature horses are more likely to show clinical signs of equine salmonellosis if stressed by factors such as parasitic infection, transportation, antimicrobial drug treatment, surgery, or colic (Smith, 1981a; Smith, 1981b; van Duijkeren et al., 1995, Owen et al., 1983; Palmer et al., 1985). Foals are considerably more susceptible to salmonellosis than adult horses (Smith, 1981a; Smith, 1981b; Walker et al., 1991; Walker et al., 1995). Salmonella spp shed in feces from horses pose a potential health threat to humans and to other animals (Murry and Hinckley, 1992).

Numerous reports have described cases and outbreaks of equine salmonellosis in veterinary hospitals (Morse et al., 1976; Smith & Reina-Guerra, 1978; Roberts & O’Boyle, 1981; Palmer et al., 1985; Carter et al., 1986; Begg et al., 1988; Cohen &Honnas, 1996; Hartman et al., 1996). Horses admitted to hospitals are frequently ill and often under stress (due, for example, to transport, injury, colic, surgery, antimicrobial drug treatment, or a change in diet), and may therefore be at increased risk of showing clinical signs of salmonellosis.

Relatively few studies have examined Salmonella spp shedding by horses on equine operations (Roberts & O’Boyle, 1981; Begg et al., 1988; Walker et al., 1991; Walker et al., 1995; Traub-Dargatz et al., 2000). No previous national study of operation-level factors associated with shedding of Salmonella spp by horses has been conducted. The purpose of this investigation was to use data collected during the NAHMS Equine ’98 Study to identify operation-level factors associated with detection of Salmonella in horse fecal specimens on operations in the United States.



The methods followed for collecting data for the Equine ’98 study and for collecting and analyzing horse-fecal specimens from operations in the United States have been previously described (Traub-Dargatz et al., 2000). Briefly: personnel from the USDA, National Agricultural Statistics Service collected data (through personal interviews) from 2,904 operations with equids in 28 states during the spring of 1998. Operations with > 3 horses were eligible to participate in the next phase of the study, which involved on-site visits by a federal or state veterinary medical officer (VMO) or animal health technician (AHT) to administer more detailed questionnaires on horse health and management, and to collect biologic specimens for laboratory evaluation. In each state, biologic specimens were collected on approximately one-half of operations during the summer (June 15 through September 11, 1998) and on the other operations during the winter (November 2, 1998 through March 3, 1999). The number of horses from which fecal samples were collected per operation followed a scale depending on the number of resident horses: if an operation had<10 resident horses, all were sampled; if an operation had between 10 and 19 resident horses, 10 were sampled; if an operation had between 20 and 49 resident horses, 15 were sampled; 20 resident horses were sampled from operations with > 50 resident horses. This procedure assured > 90% confidence of detecting > 1 horse fecally shedding Salmonella spp if the true prevalence of fecal shedding was > 10% of horses on the operation (assuming uniform test sensitivity and specificity, and concentration of the organism in the sample) (Cannon & Roe, 1982). The sample collection and culture methods have been previously described (Traub-Dargatz et al., 2000). Briefly, for each horse, the VMO or AHT used a latex glove to collect two fresh fecal balls from the ground. The feces were placed in a plastic bag, labeled, and shipped overnight (in insulated boxes containing ice packs) to the Agricultural Research Service laboratory in Athens, GA. The specimens were cultured using an enrichment process and multiple media in order to optimize recovery of Salmonella (Traub-Dargatz et al., 2000).

Sample weights were created in order to produce statistically-valid estimates related to the target population, which was all operations with > 3 horses in the states included in the study. Operations where at least one fecal specimen tested positive for Salmonella spp were considered “Salmonella spp-positive.” The percent of Salmonella spp-positive operations was computed for operation-level questionnaire variables thought potentially to be related to Salmonella fecal shedding, using SUDAAN (Shah et al., 1996) to generate estimates for the target population. Correlations between independent variables were examined. Because SUDAAN does not adjust log-likelihood ratio statistics for stratification and clustering, the SAS (version 6.12, SAS Institute, Inc., Cary, NC, USA) LOGISTIC procedure (with P<0.25 for an independent variable to enter and P<0.15 to remain in the model) was used to apply stepwise variable-selection to develop a provisional logistic-regression model. The number of resident horses, region, and season were forced into the model, as these variables were reflective of the study design, and to prevent other variables from entering the model merely because of regional or operation-size differences in management. A final, weighted logistic-regression model was constructed with SUDAAN by starting with the provisional SAS logistic-regression model and removing variables with P<0.10 in a backward-selection procedure.



A total of 8,417 fecal samples from 972 operations were cultured for Salmonella spp (Traub-Dargatz et al., 2000). One to four fecal samples were collected from 276 operations; 425 operations each provided 5 - 10 fecal samples; 202 operations furnished 11 - 15 fecal samples; and 69 operations yielded 16 - 20 fecal samples each. Serotypes identified have been listed (Traub-Dargatz et al., 2000). Table 1 provides the percent of operations that were Salmonella spp-positive for study variables hypothesized to influence shedding of Salmonella spp.



The logistic-regression model is presented in Table 2 (932 operations provided all data needed to be included in the model). The model indicates that the odds of an operation being Salmonella spp-positive increased as the number of resident horses increased. Operations where the primary use of equids was breeding were more likely to be Salmonella spp-positive when compared to other operations. Salmonella spp had a higher odds of being detected on operations where the data collector (VMO or AHT) considered the cleanliness of the equine facility to be poor. In addition, operations where new resident equids had been added during 1997 without routine quarantine from the resident equids tended to be Salmonella-positive more than operations that either did not add new residents or routinely quarantined new residents.




The NAHMS Equine ’98 Study (which was the first NAHMS equine study) was designed to permit inferences to be drawn for the population of equids in 28 states that accounted for about 78% of the farms with horses and ponies (and approximately 78% of the horses and ponies on farms) (Traub-Dargatz et al., 2000). The NAHMS Equine ’98 data used in this study reflect operations with > 3 horses on January 1, 1998 in the 28 states included in the study.

The Equine ’98 Study was not designed to determine the number of horses infected with Salmonella spp, but rather to measure the percentage of horses in whose feces Salmonella spp was being shed and was detectable using the described culture methods (Traub-Dargatz et al., 2000). A horse may have been infected with Salmonella spp without shedding the organism in the single fecal sample that was analyzed. Thus, multiple tests of feces from a given horse would have been required to determine whether the horse was infected with Salmonella spp (van Duijkeren et al., 1995). Salmonella spp can be intermittently shed in feces, especially of clinically normal horses, and thus may have been undetected. Also, it is possible that, despite using optimal sampling and culture methods, Salmonella spp was present but not detected in some samples.

One limitation of this study is that the questionnaire data were collected during sequential visits, and some recall bias and other errors inherent in this approach may have occurred (Sukhatme & Sukhatme, 1970). Using a questionnaire from a prior NAHMS survey of the dairy industry, Erb et al. (1996) found that respondents gave discrepant answers on about 8.5% of the questions when the questionnaire administration was repeated on a different date. Although every effort was made to verify that the data were collected accurately and consistently, we did not perform a repeatability study nor require written verification of veterinary care.

Management information regarding equids was generally gathered at the operation-level, not at the individual horse-level. Therefore, we cannot conclude from this study that a specific management practice was applied to the horse that shed Salmonella spp, but rather that operations which employed a particular practice had a different odds of having > 1 horse shedding Salmonella spp than operations which did not employ that particular practice. Because Salmonella spp was detected on very few operations, the precision of estimates of coefficients and of odds ratios from the logistic-regression model (Table 2) was low. The purpose of this investigation was not to construct a predictive model that could be used to pinpoint the probability of finding a horse shedding Salmonella spp on a particular operation, but rather to identify management factors potentially associated with an operation having >1 resident horse shedding Salmonella spp. The results of this study could be used by investigators to identify areas of future research, and by persons associated with the equine industry to uncover possible areas on which to focus attention to reduce Salmonella-shedding in horses.

Traub-Dargatz et al. (2000) reported that the operation-level shedding of Salmonella spp was higher among operations with >20 horses versus operations with fewer horses, but that the prevalence of fecal shedding did not differ significantly between resident horses on operations with >20 horses versus operations with fewer horses. The logistic-regression model (Table 2) similarly indicated that the number of resident horses on the operation was a significant predictor of an operation having one or more horses shedding Salmonella spp in their feces, implying that one is more likely to detect Salmonella spp on an operation when a larger number of horses are tested. More horses were tested for Salmonella spp as the inventory of resident horses increased on the operation. Other factors related to Salmonella spp fecal-shedding on operations with more horses are also possible, such as more equine births, more temporary visits by non-resident equids, and more movement by resident equids on and off the operation (USDA…, 1998a, USDA…, 1998b).

Higher levels of fecal shedding of Salmonella spp and of equine salmonellosis during the late summer (compared to the late winter or early spring) have been previously reported (Morse et al., 1976; Roberts & O’Boyle, 1981; Smith, 1981a; Smith, 1981b; Carter et al., 1986). In our study, odds ratios for regional and seasonal comparisons were high, but not statistically significant (Table 2). Since Salmonella spp multiply optimally at 37ºC in moist environments (Carter and Chengappa, 1990), regional and seasonal differences may be the result of optimal survival and multiplication of Salmonella spp in the horse’s environment. Moreover, hot, humid weather may create stress in horses, and lead to fecal shedding of Salmonella spp in carriers (Morse et al., 1976; Smith & Reina-Guerra, 1978).

There have been several reports of outbreaks of salmonellosis on equine breeding operations (Smith, 1981a; Walker et al., 1991; Walker et al., 1995). Begg et al. (1988) detected no shedding of Salmonella spp among 75 mares on a breeding operation in Australia; however, 39 of the mares had been vaccinated with a killed Salmonella typhimurium vaccine during late pregnancy. In the United States, equine breeding operations tend to be larger than other types of operations (USDA…, 1998a). Even with operation size (i.e., the number of resident horses) forced into the model, primary use of resident horses (dichotomized as breeding versus other) entered and remained in the logistic-regression model as a predictor of detecting fecal shedding of Salmonella spp in horse feces (Table 2). Compared to some other types of operations, those that use their horses primarily for breeding may be more likely to transport horses on and off the premises, keep horses in somewhat crowded conditions, and have intimate contact between horses (USDA…, 1998a; USDA…, 1998b). Foaling may be stressful for mare: Walker et al. (1995) reported that some mares had shed Salmonella spp at or shortly after parturition, even though they had numerous negative fecal cultures for Salmonella spp prior to foaling. Foals are particularly susceptible to peracute equine salmonellosis (Walker et al., 1991: Walker et al., 1995).

The data collector’s assessment of the cleanliness of the equine facility (as poor, average or good) was subjective. Enumerator variability is normally larger with judgment questions than with other types of questions (Sukhatme & Sukhatme, 1970). What one person considers poor cleanliness, another might judge as average cleanliness. However, we have no basis to suspect bias (on the part of the VMO or AHT) in any particular direction. The results of this study do indicate that poor cleanliness (based on the VMO or AHT assessment) was associated with an increased odds of detecting Salmonella spp fecal-shedding on the operation. Poor equine-facility cleanliness was also found to be associated with higher levels of strongyle egg shedding by horses in the NAHMS Equine ’98 Study (King et al., 2000). Smith (1981a) recommends maintaining clean, uncrowded conditions, and isolating affected animals as potential methods for controlling the spread of Salmonella spp.

Biosecurity practices (such the use of quarantine for new arrivals) are important to reduce the risk of morbidity and mortality due to infectious diseases (Smith, 1981a; Smith, 1981b; Doherr et al., 1998). Contact with other animals can result in the introduction and/or spread of infectious agents which cause disease (Smith, 1981a; Smith, 1981b; Walker et al., 1995; Doherr et al., 1998). In addition, stress associated with transportation may cause increased fecal-shedding of Salmonella spp by infected equids (Owen et al., 1983). While the practice of adding new resident equids without routine quarantine from the resident equids during 1997 was associated with increased Salmonella spp fecal-positivity, no information was collected on whether particular resident equids had contact with the new unquarantined equids, nor whether any particular horse tested was a recent acquisition. The results of this investigation suggest that operations could reduce their odds of having one or more horses shedding Salmonella spp in feces by not permitting unquarantined equids to join the herd.

The outcome of this study indicates a need (especially for operations with a large number of horses and for operations involved primarily in horse-breeding) to be aware of the factors that may lead to salmonellosis. Operations should use optimal hygiene whenever possible, and take care to isolate or quarantine new acquisitions. Further research is necessary to acquire a better understanding of the mechanisms of transmission and shedding of Salmonella spp on equine operations, and the impact of Salmonella spp on the equine industry.



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