Enteritis caused by Yersinia pseudotuberculosis has been reported in Australian sheep, most commonly as a cause of diarrhoea in young sheep in winter and spring. While Yersinia enteritis is known to occur in adult sheep, most reports focus on the disease in younger animals, and recent work has sought to clarify the role of Y. pseudotuberculosis and Y. enterocolitica in a weaner scours syndrome in Merino sheep in southeast Australia (McGregor et al., 2015). This paper reports a case of weight loss and diarrhoea caused by infection with Y. pseudotuberculosis in adult Dorper ewes. Epidemiological factors such as season, weather, and concurrent stress factors were consistent with the disease as reported in younger sheep.
In August 2015 the owner of a flock of 650 Dorper ewes observed weight loss, lethargy, and diarrhoea in around 10 adult sheep over a period of several weeks. The flock was grazing mixed native pastures with moderate feed availability. Rams were run with the ewes year-round, but most ewes lambed in late winter or early spring. The mob comprised sheep of various ages, both home-bred and introduced from the owner's previous property in South Australia.
In addition to the ewes observed with weight loss and diarrhoea, several adult sheep had also been found dead - at the time, these losses were attributed by the owner to internal parasites or pregnancy toxaemia. The mob was drenched with an anthelmintic and moved to a paddock with more feed available. A faecal worm egg count (samples collected on the day of drenching) revealed 567 eggs per gram (71% Haemonchus, 18% Oesophagostomum, 6% Teladorsagia, 5% Trichostrongylus).
The owner reported that all cases appeared to coincide with cold and wet weather. After some sheep failed to respond to drenching, 2 animals were submitted for necropsy.
Prior to euthanasia, the 2 animals were observed briefly in the paddock. Both were lactating ewes in reasonably poor condition (Body Condition Score 2/5) and had recently shed their wool. They appeared to be mildly lethargic. One was separated from the mob, and was ambulatory but collapsed when driven. The other appeared more alert and responsive. Both animals had moderate dark-green faecal staining of the perineum. The collapsed animal was euthanased by captive bolt, the ambulatory animal was euthanased by firearm.
Differential diagnoses included endoparasitism, Ovine Johne's Disease, malnutrition, bacterial enteritis, or a combination of these.
Necropsy findings were very similar for both ewes. Moderate ascites (clear, colourless fluid) and a mildly increased volume of pericardial fluid (also clear and colourless) were present.Serous atrophy of pericardial fat was present in one case. The mesentery was diffusely oedematous, with some areas of thickened, corded lymphatic vessels. The serosal surface of the intestines was also diffusely oedematous, with regionally extensive areas of moderate erythema. Within some parts of the jejunum and ilium, regionally extensive areas of a mild white, adherent mucosal exudate were present. Mesenteric lymph nodes were mildly to moderately enlarged and oedematous. Multifocal 2-3mm white, nodular lesions were present throughout the mucosal surface of the abomasum. Faeces were loose (but not watery) and dark green, but contained no visible blood, exudate, or mucus.
A faecal worm egg count on one necropsied animal revealed 40 Strongyle eggs per gram, and low numbers of coccidian oocysts.
Histopathology of multiple small intestine sections (including ileum, jejunum, and ileocaecal valve) revealed changes suggestive of yersiniosis. In some small intestine sections “scattered, superficial, focal, erosive microabscesses comprised of bacterial colonies surrounded by focal infiltrates of neutrophils” were present (Staples, 2015). The intestinal lamina propria was “expanded by moderate infiltrates of lymphocytes, plasma cells and fewer eosinophils” (Staples, 2015). The serosa was oedematous, and low numbers of coccidian oocysts were present. Histopathological findings in the abomasum were consistent with mild endoparasitism. Liver sections were normal. Ovine Johne's Disease as a primary or underlying disease was excluded in both animals.
A selective culture for Yersinia species on intestinal contents from each ewe resulted in a profuse growth of Y. pseudotuberculosis in both cases. The isolate was sensitive in vitro to all antibiotics tested (Ampicillin, Sulphafurazole, Trimethoprim, Tetracycline, Neomycin, Cefuroxime).
Yersinia enteritis caused by either Y. enterocolitica or Y. pseudotuberculosis is a described condition of sheep in Australia. Most studies have found infection and/or clinical disease to be most common in younger sheep, and in this respect, the case reported here represents a less typical presentation of Y. pseudotuberculosis enteritis. However, most other epidemiological factors are consistent with those described elsewhere.
Y. pseudotuberculosis is a common inhabitant of the intestines, and sheep may carry and shed the organism without developing clinical enteritis (Radostits et al., 2007). In general, it appears that enteritis develops in sheep when concurrent stress factors are present (Radostits et al., 2007). Infection, with or without associated enteritis, is reported to be most common in young sheep in winter and spring. A survey of sheep flocks in the Gippsland region of Victoria (along with an experimental flock at a regional laboratory) found Y. pseudotuberculosis infection (as defined by a positive faecal culture) in 2% (3/162) of flocks of <1 year old sheep, 19% (15/81) of flocks of 1 year old sheep, and 7% (3/46) of flocks of 2 year old sheep (Slee and Skilbeck, 1992). No Y. pseudotuberculosis was detected in the 140 flocks tested comprised of sheep ≥3 years old, leading the authors to conclude that “infection with either Y. pseudotuberculosis or Y. enterocolitica was rare in aged sheep” (Slee and Skilbeck, 1992). An earlier study in the same region also suggested that “field-acquired Y. pseudotuberculosis infection was restricted to…younger animals” (Slee and Button, 1990).
The mean age of sheep infected with Y. pseudotuberculosis was found to be 20 months in a survey of outbreaks of yersiniosis in southern New South Wales (Philbey et al., 1991). In this report, however, some cases were also identified in flocks of adult sheep from 2 to 5 years of age. Nevertheless, the majority of cases occurred in sheep less than 15 months old. It has been suggested that the rarity of Y. pseudotuberculosis infection in adult sheep is the result of immunity acquired following infection earlier in life (Slee and Skilbeck, 1992). If this is the case, it is perhaps unlikely that flocks regularly experiencing a weaner scours syndrome will see clinical yersiniosis in adult sheep. In the case described here, weaner scours in winter/spring had never been observed, possibly suggesting that initial exposure to Y. pseudotuberculosis was relatively recent for the sheep affected in this outbreak.
More recent work has also examined the role of Y. pseudotuberculosis in the 'weaner scours' syndrome observed in self-replacing Merino flocks in south-eastern Australia (McGregor et al., 2015). The syndrome is defined by diarrhoea, ill thrift, and mortality in 3-16 month old Merino weaners, unresponsive to anthelmintic treatment but often responsive to antibiotic treatment. Interim data suggests a “clear association between the weaner scour syndrome and the presence and amount of faecal shedding of Y. pseudotuberculosis” (McGregor et al., 2015).
A correlation is known to exist between shedding of Y. pseudotuberculosis and seasonal/environmental conditions. Shedding of the organism and clinical yersiniosis appear to occur exclusively in the cooler months; studies from Victoria and southern New South Wales isolated Y. pseudotuberculosis between May and October and June and September respectively (Slee and Button, 1990; Philbey et al., 1991). Sheep in four sentinel flocks in a study in south-eastern New South Wales shed Y. pseudotuberculosis predominantly in August and September (McGregor et al., 2015). Another survey detected Y. pseudotuberculosis in flocks between June and October, while Y. enterocolitica could be isolated throughout the year, with no apparent seasonal trend (Slee and Skilbeck, 1992). The year-round shedding of Y. enterocolitica in contrast to the seasonal shedding of Y. pseudotuberculosis has been reported elsewhere (Thomson et al., 2015).
A correlation between the onset of yersiniosis and general stress factors is often suggested - a link that exists in the case described here. Reported associations include a change of diet, starvation, trace element deficiency, shearing, weaning, lambing, and the onset of cold, wet, windy weather (Radostits et al., 2007; Philbey et al., 1991; McGregor et al., 2015). It is possible that these stress factors decrease host immunity to Yersinia spp. already established within the intestinal flora (Philbey et al., 1991). In the case reported here, concurrent stress factors were suboptimal nutrition, periods of cold/wet/windy weather, a moderate internal parasite burden, lambing, and the onset of lactation. The producer observed that all cases occurred in sheep which had shed their fleece, perhaps due to their increased susceptibility to cold exposure.
One report of an outbreak of Y. pseudotuberculosis enteritis in goats suggested that lactation may have been the most important stress factor (Seimiya et al., 2005). The outbreak affected around 29/100 lactating goats, but no non-lactating females, kids, or mature male goats developed clinical signs (Seimiya et al., 2005). The contribution of lactation stress in this case is unclear, as some ewes were lactating while others were yet to lamb.
Definitive diagnosis of enteritis caused by Y. pseudotuberculosis in sheep should include both histopathology and bacteriology. A consistent clinical history and presentation combined with either a positive culture or suggestive histopathology is probably sufficient evidence for diagnosis. Gross findings can include liquid intestinal contents, thickening of the mucosa of the small intestine, caecum, and colon, or enlarged, oedematous mesenteric lymph nodes, but many cases have no gross necropsy findings (Radostits et al., 2007).
The histological lesion suggestive of Y. pseudotuberculosis infection is acute, segmental, erosive enterocolitis with formation of microabscesses around colonies of bacteria (Philbey et al., 1991). These lesions were, in one study, most common in the jejunum and ileum, and were not detected in the duodenum (Philbey et al., 1991). Although microabscesses are generally confined to the intestinal tract, they may occur in other organs, such as the liver (Slee and Button, 1990). Histologically, mesenteric lymph nodes may appear reactive and oedematous (Philbey et al., 1991).
A variety of sample types are suitable for bacteriology. Y. pseudotuberculosis has been cultured from faeces, jejunum, ileum, caecum, colon, mesenteric lymph node, gall bladder, and rectal swabs (Philbey et al., 1991; Slee and Skilbeck, 1992). In two studies Y. pseudotuberculosis was not grown from duodenal contents (Slee and Button, 1990; Philbey et al., 1991). In the case reported here, samples of intestinal contents from sections of jejunum and ilium containing grossly visible mucosal pathology were cultured - in each case a profuse growth of Y. pseudotuberculosis was obtained.
There is some published data available on the antibiotic sensitivity of Y. pseudotuberculosis. The sensitivity of 25 isolates from sheep in southern New South Wales was: Tetracycline - 92% sensitive; Streptomycin - 100% sensitive; Neomycin - 100% sensitive; Lincospectin - 96% sensitive; Ampicillin - 80% sensitive; Sulphonamide - 16% sensitive; Trimethoprim-sulphamethoxazole - 32% sensitive; Furazolidone - 0% sensitive (Philbey et al., 1991). More recently, testing of 96 Y. pseudotuberculosis isolates from farms which have experienced a weaner scours syndrome in winter/spring found evidence of emerging resistance (McGregor et al., 2015). In particular, relatively high levels of resistance were found to sulphafurazole - the oral antibiotic commonly used to treat weaner scours outbreaks. In some outbreaks, oxytetracycline is also used to treat affected sheep. On one farm, 10% of isolates tested resistant to Tetracyclines; overall, 1% of Y. pseudotuberculosis isolates were resistant to this antibiotic class (McGregor et al., 2015).
In this case study, the Y. pseudotuberculosis isolate was sensitive to Ampicillin, Sulphafurazole, Trimethoprim, Tetracycline, Neomycin, and Cefuroxime. This could be reflective of the fact that no weaner scours syndrome had been observed on this property in previous years, and therefore no antibiotic treatment had been administered to the flock.
Treatment was aimed at reducing stress factors and providing antibiotics to the worst affected animals. The mob was moved to a paddock with better feed availability; maintenance of good nutrition is thought to help prevent the onset of disease (Radostits et al., 2007). Several clinically affected animals were treated with a long-acting oxytetracycline at the label dose rate. The producer had already administered an anthelmintic drench to reduce internal parasite burdens. These management interventions, along with milder conditions in spring, removed some of the stress factors likely to be contributing to the development of clinical cases. Some of the more severely affected ewes failed to respond to treatment, but several recovered completely, and no additional cases were reported.
Y. pseudotuberculosis should be included in the differential diagnosis list for adult sheep presenting with weight loss and diarrhoea, particularly when outbreaks occur in winter/spring and concurrent stress factors are present. Gastrointestinal tract samples for histopathology and bacteriology can confirm the diagnosis. An antibiotic sensitivity test is likely to be helpful in guiding treatment, and limiting antibiotic treatment to clinically affected animals may decrease the rate of the development of antibiotic resistance. Addressing stress factors is probably an important component of treating and preventing outbreaks of Y. pseudotuberculosis enteritis in adult sheep.
Thanks to the producer, for his provision of a detailed history for the case study, and Patrick Staples, for the histopathology report and diagnostic comments.