In the sheep population of the federal state of Thuringia, only the serovar 61: k: 1, 5, (7) was detected in more than 80% of the examined flocks. The dominant appearance of SASd in sheep confirms study results from other countries. However, the detected prevalence of 82% in this study is high compared to data from other countries, such as Norway (20%) [9], Switzerland (11%) [17], USA (72%) [15] or Sweden (12–40%) [6]. Real differences probably exist between countries, but the variation will also be heavily influenced by different study designs, sampling methods or sampling material. In view of the moderate to high prevalence of SASd in different countries, the occurrence of this serovar seems to be due to a long lasting persistence in sheep flocks because of its host associated characteristics. Other Salmonella serovars that are not confined to sheep are obviously rare and do not establish an epidemic pattern, and their occurrence appears to be related to pastures contaminated with wild bird faeces [2].
Despite the considerable detection rate of SASd in sheep flocks, it might be possible that the real incidence in sheep is even higher because of the biochemical features of this serovar [14]. The lactose positive SASd organisms might constitute a special diagnostic problem, therefore, serovars of Salmonella subspecies diarizonae may slip through undetected [19]. Hence, in addition to method ISO 6579-1 [18], it is recommended to use further indicator media when SASd strains are suspected [23]. This is supported by this study as the variation also in the production of hydrogen sulphide between SASd strains observed hampers the detection when xylose lysine deoxycholate agar is used.
Antimicrobial testing revealed that SASd organisms from 74 sheep flocks in Thuringia were sensitive to 13 of 14 antimicrobials. All strains were resistant to only sulfamethoxazole, confirming results from earlier studies [6, 22] that did not find resistant SASd strains. Therefore, the risk of transferring antimicrobial resistances via SASd strains from sheep to other animal hosts or humans might be considered as negligible.
Although a wide distribution of SASd was found in this study, clinical symptoms of a disease exclusively due to SASd have not been detected in that period. Despite signs of rhinitis, nasal inflammation or aborted foetuses which were noted occasionally in different studies [4, 10, 11], it cannot be ruled out that factors other than SASd were involved in producing these clinical signs. This occasional occurrence of clinical symptoms is also supported by the observation that experimental infections of sheep with SASd indeed resulted in intestinal or nasal colonisation but not in the induction of clinical signs of a disease [24, 25]. The lack of clinical symptoms in sheep flocks harbouring SASd [6, 9] has been regularly observed.
In this study, SASd was detected in nearly all flocks with more than 100 sheep and even in 80 -86% of sheep flocks with only 30–100 animals. A positive correlation was found between the increasing flock size and the increasing probability of detecting SASd. A higher SASd prevalence in larger sheep flocks was also found in earlier studies [26] which might be due to the more successful and long-lasting persistence of SASd in such flocks. Others [9] found a low within-flock prevalence regardless of the flock size, indicating that the transmission rate of the organism is limited. However, comprehensive information on the infection routes for both within the flock and from herd to herd are still missing. The role of the “ram circle”, the exchange of rams between different farms, in the transmission of SASd between flocks has been discussed [9, 26], though, clear evidence is not available. Even the mode of spreading of the organism between single animals is not completely known, and detailed studies on infection routes are still needed.
To obtain information on a possible epidemiological connection and on the distribution of SASd in the Thuringian sheep population macrorestriction analysis of a representative number of isolates originating from different regions and flocks was carried out. Because of the high degree of similarity in antimicrobial resistance pattern of SASd and the missing correlation between biochemical index and macrorestriction pattern, only results of the macrorestriction analysis were used to generate macrorestriction groups [21] for discrimination among SASd strains. The high number of genotypes revealed after digestion with both XbaI and SpeI resulted in a high number of macrorestriction groups, indicating low clonality of SASd. As most macrorestriction groups were dispersed throughout the federal state and because of the lack of epidemiological data on risk factors for transmission routes it was not possible to make conclusions on an epidemiological context of SASd organisms in Thuringia. Despite the more frequent occurrence of macrorestriction group C in related counties, reasons for an exchange of this SASd group between the sheep herds could not be identified. Comparable data from macrorestriction studies of SASd are rare [17, 22, 27] and cannot be compared directly with results of this study.
The wide distribution of SASd in sheep flocks in Thuringia also raises the question on its zoonotic potential. How to deal with findings of SASd in sheep flocks? In contrast to poultry, cattle and pigs, there is no regulation in place on the control of Salmonella infections in sheep in Germany. Despite the likely moderate to high prevalence of SASd in the German sheep population, meat, meat products and cheese from sheep are very rarely contaminated with Salmonella organisms, SASd has not been isolated from these foods [28]. Human infections caused by serovar 61: k: 1, 5, (7) have not been notified in Germany since 2000, so that the significance of SASd for public health is negligible compared to that of Salmonella enterica subspecies enterica serovars [29].
Therefore, SASd seemed merely to be a commensal and colonising inhabitant of the intestine and the upper respiratory tract in healthy sheep, which may occasionally become invasive only in debilitated animals. The pathogenic significance of SASd as monocausal agent is considered as low since clinical signs in sheep flocks harbouring SASd are, consistent to results of this study, not regularly observed [6, 9]. It is also concluded that control measures applied upon findings of SASd in sheep have very little impact on reducing risks to human health and that measures to eradicate this serovar from sheep herds will probably not reduce its prevalence in the sheep population [6]. For these reasons, Sweden was the first country to make an exception for serovar SASd in Salmonella control [6], thus reducing activities from combating to monitoring serovar SASd in sheep. Despite the limited but confirming data on the disease-causing and zoonotic potential of SASd, the Swedish strategy could also be a guideline for Germany, which is supported also by the results of this study. Nevertheless, given the special and interesting characteristics of SASd and the persisting lack of knowledge on the infection, further studies on the pathogenicity and transmission routes of this organism in sheep will be most valuable.