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Increase in Salmonella enterica subspecies diarizonae serovar 61:k:l,5,(7) in sheep

2001; Wiley; Volume: 149; Issue: 18 Linguagem: Inglês

10.1136/vr.149.18.555

2042-7670

R. H. Davies, S. J. Evans, S. Chappell, S. A. Kidd, Y. E. Jones, B. E. Preece,

Identification and Quantification in Food

THE arizona group of salmonellas (which includes the current subspecies arizonae and diarizonae) was initially isolated from reptiles (Edwards and Ewing 1986) and was first identified in sheep in 1952, from carcases of newborn lambs (Ryff and Brown 1952). Arizona was first identified in abortion material from sheep in England and Wales in 1976 (Sojka and others 1983). Although there are approximately 300 serovars in the diphasic arizona diarizonae subspecies (Old 1992), in the UK a single serovar, Salmonella enterica subspecies diarizonae serovar 61:k:1,5,(7) (Le Minor and Popoff 1988), is responsible for all the identified incidents in sheep, with only occasional isolates of this serovar being made from other species (Anon 1999). This short communication describes the progressive rise in S 61:k:1,5,(7) isolated from sheep between 1990 and 1998, such that it has become the most common serovar isolated from that species since 1998. Isolates were made from diagnostic material submitted to Veterinary Laboratories Agency (VLA) and Scottish Agricultural College regional laboratories throughout England, Scotland and Wales. Salmonella isolates are reportable under the Zoonoses Order 1989 and data are collated by the VLA — Weybridge. Standard isolation techniques involved direct plating onto McConkey's Agar (CM115; Oxoid) and selective enrichment in selenite cysteine broth (CM699; Oxoid), followed by plating on brilliant green agar (CM329; Oxoid) with added novobiocin (44206; Merck). Suspect non-lactose fermenting colonies are then confirmed serologically and biochemically. Although most isolates are normally associated with ovine abortions or stillbirths, a review of 119 incidents identified during 1998 and 1999 revealed that 29 (24.4 per cent) of those were associated with cases of parasitic gastroenteritis and other alimentary tract disturbances in both lambs and adult sheep. Where reproductive failure was reported there was frequent concurrent infection with other ovine abortifacients. In 35 incidents for which complete bacteriological details were available, S enterica subspecies diarizonae serovar 61:k:1,5,(7) was isolated on only 11 occasions (31.4 per cent) by direct culture. On the remaining 24 occasions (68.6 per cent) it was isolated only using enrichment media. A standard abortion protocol was introduced into the VLA regional laboratories (previously Veterinary Investigation Centres) throughout England and Wales in December 1997. Before that date there was some local variation in ovine abortion protocols and full screening for salmonellae may not necessarily have taken place where other abortifacients were identified. As a result of the introduction of the new protocol, all submissions were screened for salmonellae even when other causes of ovine abortion were identified. These changes may have had some influence on the increase in reports of S 61:k:1,5,(7) between 1997 and 1998, but the overall increasing trend remains. Fig 1 shows the fall and gradual rise in S 61:k:1,5,(7) in sheep, from 29 incidents in 1990 to 16 incidents in 1992 to 84 incidents in 1999. This means that, from 1998, S 61:k:1,5,(7) became the most common serovar isolated from sheep, and in 1999 represented 45.7 per cent of the total Salmonella incidents. Table 1 shows the relationship of numbers of S 61:k:1,5,(7) to the number of diagnostic samples received from sheep and the rise, and subsequent fall, since its peak in 1995 of Salmonella typhimurium DT104. Between 1995 and 1999 the number of reports of S 61:k:1,5,(7) in sheep have more than doubled. Figures for 2000 suggest that this rising trend is continuing, with 112 incidents being recorded. Incidents of Salmonella enterica subspecies diarizonae serovar 61 :k:1,5,(7), Salmonella typhimurium and all other salmonella serotypes in sheep in Great Britain from 1989 to 1999 Reports of S 61:k:1,5,(7) are highly seasonal (Fig 2) and correlated with the lambing season, as would be expected, as most isolates are the result of sample submissions for the investigation of abortions or stillbirths. Most incidents are reported in the first four months of the year; the peak month for reports is March which represents up to 40 per cent of the animal incident reports. The increase in reports has been greatest in England and has been seen across most regions, although the greatest number of reports have been from northern and south-east England. Incidents of Salmonella enterica subspecies diarizonae serovar 61 :k:1,5,(7) in sheep in Great Britain by month of submission, from January 1996 to December 1999 The rise in S 61:k:1,5,(7) in sheep is particularly interesting as it is occurring at a time of substantial reduction of Salmonella in all the livestock species (Anon 1999) and in human beings. A wide range of Salmonella serovars, notably S typhimurium, Salmonella dublin and Salmonella montevideo have been associated with enteritis, abortion and systemic infection in sheep (Saunders and Kinch 1966, Gitter and Sojka 1970, Linklater 1983). The increase has been made more noticeable by the decrease in reported cases of S typhimurium DT104, which increased dramatically during the period from 1990 to 1996, and was thought to result from the secondary spread of infection originating in cattle herds (Anon 1995). The role of arizona infection in clinical disease in sheep is unclear. The serovar 61:k:1,5,(7), like Salmonella abortusovis, is considered to be host adapted to sheep (Martin and others 1967, Sting and others 1997), although it does occasionally occur in other livestock, as well as monkeys and humans. The organism has frequently been isolated from aborted fetuses and placentae (Greenfield and Greenway 1973, Schweighardt 1991), in association with placentitis and fetal bronchopneumonia (Long and others 1978). There are, however, often other more prominent causes of abortion, including other salmonellas, identified at the same time (Linklater 1983, Sojka and others 1983) and the organism is often found in clinically normal sheep, only becoming invasive in stressed or debilitated animals (Greenfield and Greenway 1973, Harp and others 1981, Pritchard 1990). The organism has also been isolated from diarrhoeic faeces (Schweighardt 1991) although it may be difficult to isolate unless present in large numbers (Alvseike and Skjerve 2000). Experimental infections with ari-zona have often not resulted in clinical signs (Harp and others 1981, Sojka and others 1983), but in other studies bacterins produced from arizona have protected lambs against experimentally induced enteritis caused by entero-toxin elaborated by the arizona (Harp and others 1981, Mahurin 1982). The organisms may be carried for many months in the reproductive and intestinal tracts (Long and others 1978, Pritchard 1990). It may also act as a long-term coloniser of the nasal cavities (Hannam and others 1986) where it may be associated with chronic proliferative rhinitis (Meehan and others 1992, Brogden and others 1994). Any increase in Salmonella in a food animal species may raise concerns about possible risks to humans, but less than 0.5 per cent of human Salmonella infections are caused by subspecies other than enterica (Old 1992, Popoff and Le Minor 1997). Most of these cases have been associated with arizona from reptiles, Salmonella bongeri or unusual Salmonella strains acquired during foreign travel (Toeller and others 1978, Hall and Rowe 1992). Reports of S 61:k:1,5,(7) in man are uncommon outside the USA (Berkman 1981) but the organism has frequently been isolated from abattoirs (Hall and Rowe 1980, Jethon 1990) and has caused diarrhoea, fever and submandibular lymphadenopathy in a small number of children and adults in Turkey (Berkman 1981, Fatma and others 1998). S enterica subspecies diarizonae is differentiated from S enterica subspecies enterica (which includes the main mammalian and avian pathogenic salmonellas) by a series of different biochemical reactions. Seventy-five per cent of diarizonae ferment lactose (Le Minor and Popoff 1988) so will not be detected on standard indicator medium such as MacConkey's agar, brilliant green agar or Rambach, which rely on the detection of non-lactose fermentation (Hall and Rowe 1980). S 61:k:1,5,(7) is a late lactose fermenter so is usually still detected by standard culture (Long and others 1978). Other arizona strains may, however, be missed, so a combination of different indicator media to detect lactose-fermenting strains by hydrogen sulphide production and serological inhibition of motility in selective semi-solid medium such as DIASSALM or modified semi-solid Rappaport Vassilliadis medium is recommended. The reason for the increase in S 61:k:1,5,(7) in sheep is unknown, but it has coincided with a period of increased importation of breeding stock from other countries and a reduction in purebred flocks which breed their own replacements. Introduction of Salmonella by imported sheep has resulted in widespread disease caused by Salmonella brandenberg in New Zealand (Anon 1998) and other infections worldwide (Higgs and others 1993). S 61:k:1,5,(7) has, however, been recognised in the UK for over 20 years, so the increase in infection may possibly be more related to the exposure of immunologically naive animals placed on endemically infected farms or passing through livestock markets. There have been few visits by MAFF (now DEFRA) veterinarians to infected flocks and, therefore, little information has been collected on other epidemiological features of infection. To help resolve this question, more work is required to study the epidemiological features of the flock infections and to compare the molecular genetic relationships between current and historic isolates of S 61:k:1,5,(7) from various countries. It would also be advisable to carry out some survey work using a variety of culture methods, to identify whether other unusual salmonellas may be present which may have been missed by standard techniques. The data described in this paper were derived from MAFF-funded surveillance. The authors would also like to thank veterinary and laboratory staff within the VLA for technical assistance.