In March 2012 a stud/commercial Merino operation experienced scouring in a mob of commercial Merino lambs.
The lambs were marked in December and weaned early January. On the advice of a local rural reseller the lambs were drenched with Baycox (tolturazil) as a scour preventative to avoid a scouring episode similar to that which had occurred at the same time the previous year. The lambs also received a Vitamin B12 injection and a monepantel drench.
Over the course of the next few weeks the lambs were introduced to a special grain diet with supplements to enhance rumen development. The weaners were mustered daily from a paddock of improved pasture and confined in a small holding yard for two hours with the grain and supplements available in self feeders.
Scouring was seen only in one of the two mobs and began approximately two weeks after introduction to the supplementary feed. Suspecting the supplementary feed to be the cause of the scouring the producer withdrew this feed from the affected mob.
The scouring continued and after a further two weeks, at the time of investigation, approximately 30% of the mob was affected and there was a distinct tail developing in the mob (Figure 1).
The majority of lambs were a good weight with some of the scouring animals maintaining their weight. Scouring varied from quite runny faeces to firmer faeces.
Mucous membrane colour was pink, while many of the affected lambs were depressed and lethargic.
A lamb who collapsed at the tail of the mob from mustering was euthanased for post-mortem examination. This animal was severely underweight, its rectal temperature was 39.6 C, mucous membranes were pale, it was approximately 15% dehydrated and there was evidence of profuse watery faeces. A blood sample was taken and this appeared to be very 'watery'.
The carcass was pale. There was ascites and increased pleural fluid in the chest cavity. The small intestine was inflamed and the walls of the intestine felt thickened. There were small white foci evident in the intestinal wall approx 1-2 mm in diameter (see figure 2). Intestinal lymph nodes appeared swollen.
The small intestine exhibited acute enteritis with minimal to moderate multifocal neutrophil infiltration with intralesional coccidian. Lymph node was reactive and oedematous.
Samples were positive for Mycoplasma ovis by PCR and negative for Yersinia sp. by culture.
A pooled faecal sample returned a nematode egg count of 0 epg and + coccidian.
The weaners were prescribed sulphadimidine soluble and dosed as per label instructions.
The owner was advised to cease any further management activities for a few weeks to minimise stress. The weaners were also placed onto a clean paddock of improved pasture to stop exposure to high coccidia numbers in the contaminated paddock.
Coccidiosis in sheep is caused by the protozoa Emeria spp. and disease is seen in young sheep under intensive management (Glastonbury 1990). Merino weaners are a susceptible group due to the stressful nature of weaning and their lack of previous exposure to the organism. In addition this mob was also subjected to additional stress from both a change in diet (grain and supplements) and also daily mustering and forcing into a feedlot style yard.
Coccidial oocysts passed in faeces require cool, moist conditions to sporulate and are also able to survive in the environment for up to two years in favourable conditions (Radostits et al. 2007). The intensive management of the feedlot style yarding would have allowed rapid build up of large numbers of oocysts on the ground, uptake by the faecal-oral route and resultant expression of disease in individuals. The daily confinement would have allowed transmission of high numbers of coccidia to otherwise healthy animals.
Routine worm egg count monitoring often picks up on low levels of coccidia present. The presence of coccidia in the faeces does not confirm disease and the absence does not confirm freedom of disease. Diagnosis is often based on history, clinical signs and histopathology of the small intestine.
The role of Mycoplasma ovis in this case is unclear. Conditions that encourage the transmission of M. ovis are also suitable for that of coccidiosis. This includes management procedures that bring stressed and naive animals in close contact. It is likely M ovis infection spread at marking in December. It is possible that the M. ovis infection caused immunosuppression in individuals which may have assisted the disease progression of coccidiosis and a result M. ovis is likely to have exacerbated the 'tail effect' of the mob.
Generally a lamb with coccidiosis can be expected to make a spontaneous recovery after about 7 days, without treatment (Glastonbury 1990). Concurrent infection with M. ovis is likely to have caused prolonged recovery.
The use of Baycox (tolturazil) as a preventative for coccidiosis is not warranted, and is not in accordance with the label. There are currently no established Export Slaughter Intervals for tolturazil in sheep and therefore any recommendation for off label use needs to be made by a veterinarian and with caution. On a practical note, the use in this case did not prevent an outbreak of coccidiosis. Tolturazil would have been effective as a treatment (Bruer and West 1993) but given the recent prior use in the same animals it was not considered.