Rahnella aquatilis is a relatively rare enteric gram-negative rod that has been found in fresh water, soil, snails, certain beetles, and isolated human clinical specimens. This bacterium is of importance because of its ability to cause disease in humans, particularly those who are immunocompromised, with bacteremia, sepsis, respiratory infection, urinary tract infection and endocarditis reported (4). Reports of infection in non-human species are extremely infrequent, and this report is believed to describe the first reported case of infection in a sheep.
The owner of a 95-head mob of two-tooth stud Merino ewes in Numbla Vale, NSW, had noticed a few trailing behind the main mob over the previous few weeks and some were found dead several days later. The owner only reportedly noticed signs of illness during mustering. Signs only began when the mob was moved into the current paddock; this paddock contained a creek while other paddocks do not. Cattle were also running in this paddock but were clinically normal. Two other mobs were located on the property in different paddocks with no apparent disease. All ewes were four months pregnant.
Pasture in the paddock was improved, with short green feed of phalaris and clover available (despite dry conditions), particularly around the creek. Few weeds were present and the flock was not being supplementary fed.
The sheep had received 5-in-1 vaccines at marking and weaning and an annual booster. They had also received ‘Permatrace’ 3 year selenium and cobalt bullets at 12 months. The sheep were drenched one week prior with ‘Triguard’ (abamectin 1g/L, oxfendazole 22.7g/L, levamisole 33.9g/L, selenium 0.5g/L, cobalt 2.2g/L). Targeted chemical fluke control had never been undertaken, but closantel (which would remove some liver fluke) has been used at times to treat nematodes. The owner reported there were no scours, weakness, anaemia or bottle jaw at drenching.
Four ewes had died with the latest death occuring the morning of the visit. Unfortunately foxes had gotten to the carcass and it was unsuitable for post-mortem examination. The fifth ewe died whilst being caught for examination and sample collection.
Mob examination revealed well-grown sheep in good body condition (BCS 3/5). When driven one ewe separated from the mob and went down after several hundred meters, lay in sternal recumbency, with open mouth breathing and then died.
On examination prior to death there was no evidence of pallor, scouring, staggers, muscle tremors or neurological signs and the ewe was in good body condition.
The post-mortem examination was largely unremarkable. The rumen was full with fresh grass and adequate abdominal fat was present. Well-developed twin fetuses were present in-utero. The only significant finding was a large section of the duodenum grossly purple with prominent vasculature; the lumen was dilated with thick pasty, white contents and on cut surface appeared to have thickened mucosa. Formed fecal pellets were present in the rectum and there was no gross evidence of internal parasites.
Samples were collected and sent to the lab for CBC and biochemistry of the blood, histology of fixed tissues and brain, and fresh tissue culture. A worm egg count on faeces was also requested.
Histopathology found micro-abcessation in the liver consistent with a bacteraemia / septicaemia with concurrent eosinophilic enteritis, typically associated with parasite exposure.
Culture of the liver tissue was requested, returning a pure growth of Rahnella aquatilis.
Rahnella aquatilis, of the family Enterobacteriaceae, is a gram-negative rod and is a facultative anaerobe.1 It is typically found in soil and fresh water but has also been isolated from ticks (Ixodes holocyclus and Haemaphysalis longicornis being relevant to the area)2 and had been identified as a food contaminant.3 It is known to cause disease in immunocompromised humans; various infections, such as bacteremia (from renal infection), sepsis, respiratory infection, and urinary tract infection can be the result.4 However, we were unable to identify any literature describing Rahnella aquatilis causing disease or death in sheep or other livestock. Literature reports that all Rahnella spp. are naturally sensitive to doxycycline, aminoglycosides, some penicillins and cephalosporins, carbapenems, aztreonam, quinolones, sulfamethoxazole, trimethoprim, cotrimoxazole, chloramphenicol and nitrofurantoin.5
It appears that infection in this case was probably opportunistic and possibly the result of pre-parturient immunosuppression in the pregnant ewe. Concurrent parasite exposure may have further compromised the immune system. We recommended the owner remove access to the creek and isolate and treat any sheep from the mob that developed clinical signs.