In a series of wetter years in the late 1980s, post-mulesing arthritis became a major problem and threat to Merino flockowners in northwest NSW.
About a week after mulesing, flockowners saw lambs with large swollen joints, were lame and found it difficult to move, feed and water. Some lambs died shortly after mulesing before signs of infectious arthritis appeared.
Department of Agriculture veterinarians were approached to help flockowners deal with the disease.
We needed to develop a method to address a widespread problem that was causing major losses. The problem was clearly related to mulesing, the environment, and management and was complex. There was no 'silver bullet' in sight.
Infected joints could be found at various locations around the body, but were mostly seen in the limbs. Which joints were affected varied between sheep.
Joints were usually distended and soft, with or without palpable heat. When opened, the joints were filled with soft, white or cream or very light green pus, of even consistency. The joint surfaces were usually eroded and inflamed. Inflammation could extend to tissues about the joint, but pathology was essentially within the joint. Sheep with affected joints did not recover, and either became unthrifty or died.
Laboratory examinations of tissue from joints from different flocks consistently found Fusobacterium necrophorum. It was decided to focus on identifying the risk factors that allowed this organism to become pathogenic. If this was achieved, those risks could be managed to prevent the disease.
F. necrophorum (FN), a gastro-intestinal tract dweller, was known to be pathogenic in certain situations (usually a combination of moisture, mud, often with faecal material), and needs to be potentiated by other common faecal organisms, like E. coli. FN is a component of a number of polymicrobial diseases. FN was part of syndromes like footrot in sheep, footrot in cattle, foot abscess in sheep, thrush in horses, calf diphtheria, necrobacillosis in various animals, sore throats and abscesses in humans. FN can cause septicaemia, as well as localised abscesses and necrotic lesions.
There were a host of factors that may have been important.
Flockowners were particularly concerned about steps they could take immediately: hygiene of marking and mulesing; disinfecting implements; different marking and mulesing techniques; what wound dressings were best; when to mules.
Mulesing contractors knew places with differing levels of PMA, and put forward factors that might explain these differences. The presence of small black flies was identified.
Given the nature of the organism, it was important to consider a broad range of factors that could alter the level of contamination of mulesing and marking wounds, and their susceptibility to infection by FN and its potentiators.
It was decided to ask flockowners across Western Division about PMA, using a postal survey. The level of concern and debate was high, suggesting that a postal survey would be acceptable, and people would respond to a properly designed questionnaire.
A series of case studies looked in detail at flocks with severe losses, documenting prevalence, clinical presentation, marking and mulesing technique, dressings, rainfall, time of events, management and handling during marking and mulesing, and general environment.
The survey design was discussed with case study flockowners, mulesing contractors, Rural Lands Protection Board and Dept of Agriculture staff, including Dr Idris Barchia, a biometrist, who ensured the data could be analysed.
The survey considered:
The survey was sent to 637 flockowners, either case studies, selected from Rural Lands Protection Board lists, or from people who had attend PMA field days
Survey response could be anonymous but had a property identity code included. There were 299 responses, or 47%. Response rates varied between Boards and field days from 31% to 70%. Lambs reported marked or mulesed totalled 1,005,314.
Post-mulesing arthritis in the Western Division of NSW in 1988/89
Of 273 respondents, 149 (56%) said that they did not see PMA after mulesing. Respondents reported seeing an estimated 11,083 sheep with infectious arthritis (ranging from 1 to 2000, mean 40.8)
Brewarrina, Wilcannia and Bourke had the highest attack rates. See table 1. Northern areas had generally higher risk
The risk of PMA varied considerably between months. See Graph 1.
2. Mulesing and marking technique
The method of castration, detailing and mulesing influenced PMA.
Castration using a ring reduced risk of PMA compared with using a knife [Odds ratios: knife 1.0; ring 0.51 (lower risk)]. A knife leaves a wound open to infection; a ring does not.
Detailing using a ring reduced PMA considerably compared with using a gas detailer, which had a lower risk than using a knife or mulesing shear [Odds ratios: knife 1.0; mulesing shear 0.82; gas detailer 0.49; ring 0.21]. A knife leaves a wound open to infection, as does a mulesing shear, but using the mulesing shear reduces risk slightly, so would be preferred if considering the two options. A gas detailer greatly reduced risk, probably because the method partially seals the wound. A ring reduced risk of PMA more than a gas detailer, again pointing to the advantage of not leaving any wound.
Mulesing technique using the modified technique reduced risk of PMA more than the radical method [Odds ratios: Radical 1.0 Modified 0.66]. A radical mules leaves a much larger wound than the modified technique. This was consistent with the concept that not leaving a wound by using rings reduced risk from castration and marking
Using a contractor carried a larger risk of PMA than when the operator or employee mulesed; but a lesser risk when marking
The type of wound dressing made a considerable difference, with powder dressing carrying a much lower risk than either no wound dressing; liquid dressings; or other types of wound dressings [Odds ratios: Powder 1.0; No wound dressing 2.69; liquid dressing 4.36; other dressings 3.43]. This indicated that using a liquid increased risk of PMA, which would be expected in terms of the preferences of FN. These liquids include oils. The other types of dressing usually are a combination of liquids and powder. The lower risk of powders compared with liquid dressings could be explained by their drying effect or their promotion of healing. If so, the delays in healing or providing moist conditions suited to FN by liquid dressings may be the basis of increased PMA
3. Management of marking and mulesing
It was likely that exposure of wounds to FN would increase with the time sheep were held in yards. So a general question was did PMA increase when sheep spent longer periods in yards?
Analyses were set up to give answers for each time factor, while taking other time factors into account, by using regression analysis. The results are given in terms of regression coefficients for each factor that was found to be related to PMA. See Table 2. Positive coefficients indicate increasing levels of PMA with increase in the factor considered. Negative coefficients indicate PMA decreased with increase in the factor.
For example, 'Hours lambs in yards' has a regression coefficient of +0.132, indicating that PMA increased the longer lambs spent in yards. The size of the coefficient relates how marked the increase or decrease in risk was.
This analysis showed that time that the longer lambs or ewes spent in yards, the greater the risk of PMA. This is consistent with the observation that sheep or wounds become increasingly contaminated by mud, faecal matter and urine with time in yards.
The time of release of final mob had largest reduction in risk. Sheep let out earlier in the day have usually been brought in the night before; have camped in yards over night, been mulesed in the morning then released. In general, terms, sheep spending time in yards overnight will have more mud, faeces and urine on them than sheep brought in during the day. Sheep let out late in the day are more likely to have been brought in during the day, been mulesed then released. Time of release of the final mob can be related to the size of mob being handled and the rate at which sheep can be marked and mulesed.
Bringing in larger numbers at a time was associated with reduced risk of PMA. The basis of this is not clear, but is believed to reflect the influence of other factors operating at the same time, such as time sheep were brought into yards and released from yards, and the hours spent in yards.
Slightly more PMA was seen when people used more sites. Using more sites might reflect either (A) previous problems when marking/mulesing in a single yards or small number of yards, with people attempting to reduce risk by using more yards to reduce risk from contaminated or problem yards. If people had no history of PMA, this decreased the risk of PMA (odds ratio of 0.52). (B) wanting to reduce the stress on lambs and ewes by hang more yards, and reducing the time they spent off feed and away from their paddocks. This should reduce the risk (C) the small increased risk might reflect the nett effect of both management objectives
Taking these results together shows that there is a complex interplay between factors that increase and decrease risk. There is no simple single answer. Management of mulesing can make a difference and reduce PMA, but this will depend on making decisions about the best combination of what number of sheep are coming in, when to yard them, when to mules, and when to let sheep out.
Other management options altering PMA risk were jetting and watering yards. Jetting before or at mulesing is used to reduce risk of flystrike. The jetting fluid is often directed away from the mules wound, but will increase the fluid about the wound slightly. Jetting slightly increased risk of PMA [Odds ratios: Jetted 1.0; Not jetted 0.9}. Watering yards is often a sign an owner is particularly careful of their sheep and quality of their wool (and their own personal comfort) by reducing dust. In a wet year, this may not be needed but was still done. Watering yards increased risk of PMA [Odds ratios: All yards watered 1.0. Some yards watered 0.61. No yards watered 0.37].
FN is favoured by wet and muddy conditions. Watering yards probably creates a more favourable environment for the bacteria, and increase contamination of wounds
4. Other environmental risk factors
Small black flies were a particular hazard. If black flies were greater than normal, the odds ratio of PMA was 2.94, compared to 1.0 for normal numbers, and 0.79 of numbers were less than normal. The risk of PMA was 3 times higher than expected with higher blackfly numbers, given all other factors. Black flies are known to irritate wounds, and prevent good scab formation, which would increase contamination, especially out in the paddock.
Seeing PMA before mulesing was a possible warning sign of PMA (Odds ratio of 1.30) but was not common beforehand
Seeing scabby mouth was associated with a reduced risk of PMA (Odds ratio of 0.52). Scabby mouth is more common in sheep on feed with burrs or prickles or dry twiggy feed, rather than lush soft feed from good rains. Scabby mouth was suspected as predisposing to PMA by some. The analysis suggested scabby mouth was more likely to be associated with drier conditions, and hence lower PMA prevalence
PMA varied by type of soil in paddocks with mulesed sheep. It is not proposed that soils have a direct role in the pathogenesis of PMA, as no clear relationships are known. Certain soils are seen more frequently in certain locations and types of country, which in turn are managed in certain ways, or have certain levels of other important risk factors. Soils may have some effect on the chance of bacteria surviving, or contaminating wounds when wet, but there is no plausible evidence of this. However black soils were associated with a slightly higher risk of PMA: Odds ratios for presence of each soil type were: soft red 0.82 (reduced risk); hard red 1.13 (low risk); black 1.80 (moderate risk); other soil type 1.40.
The type of water in paddocks running mulesed sheep altered risk in various ways, sometimes considerably. See Table 3.
The risk of PMA was reduced when sheep were on surface waters (rivers, creeks, ephemeral waters in paddocks). This would be consistent with the observation that sheep dispersed widely across paddocks, as happens when sheep are on surface waters in wetter years, which would reduce risk of contamination of wounds with faecal matter, from established sheep camps.
Channels are bore drains carrying water across paddocks, or occasionally irrigation channels. Both are more common in areas with denser stocking, and black or river soils (the latter associated with increased risk). The risk associated with channels is more likely to reflect the risk from the environments and management approaches for sheep in country that has channels.
Dams and troughs had a small increase in risk of PMA, in comparison with the reduction seen on surface waters and the markedly higher risk in sheep on channels. Dams and troughs are points where sheep congregate more compared with surface waters, which would be consistent with higher risk of wound contamination or other factors that predispose to mixed infection of mulesing wounds.
This was explored further by considering combinations of types of waters in paddocks in relation to risk of PMA. Particular combinations of types of waters used together are often associated with particular areas and types of country, so this analysis can offer broader perspective to risk by type of water and type of country. The type of water alone is considered unlikely to create any simple direct risk of PMA, apart from channel water being more alkaline, although no clear pathogenic path is known.
Flocks on channels alone had the highest risk of PMA; if channels were combined with dams and troughs, this risk was lower. Troughs or surface waters by themselves were associated with a very low risk of PMA, while dams alone were associated with moderate risk. This analysis clearly showed sheep running on channels had a higher risk, but relationships between type of water and PMA was otherwise difficult to interpret and apply.
Flockowners in Western Queensland also had severe losses in the late 1980s. Dr David Rossi and others in Queensland Department of Primary Industry used community gatherings to pull together a picture of factors that increased or decreased risk. These meetings developed a remarkably similar set of risk factors.
It was important to bring our assessment of these factors together into an understandable narrative of the pathogenesis of PMA for flockowners and their advisers. The PMA story that developed was:
PMA began with mulesing, marking and castration wounds being contaminated with sheep faeces in mud, moist soil or from flies.
Faeces (sheep dung) carries the bacterium that causes PMA (Fusobacterium necrophorum FN), as well as those other bacteria that FN needs to help it penetrate through wounds into the blood stream. Once in the blood stream, FN can kill some lambs, and could settle in joints, causing an infectious arthritis, with the distinctive creamy pus of PMA.
Having faecal matter in human or animal wounds is dangerous. Having open wounds, and the size of the wound/s, is very important. Contamination from dirty knives or mulesing shears is minor, by comparison. Wounds can be infected in yards and later in the paddock. More faecal matter is likely to get into wounds when mulesed lambs sit down in yards or in paddocks, as is seen in many after being mulesed. Contamination from dirty knives or mulesing shears is minor, by comparison
The chance of wound contamination with faecal matter increases with the time sheep spend in yards, the wetness of yards (from urine or watering), whether sheep are yarded overnight, if sheep lie down in sheep camps where dung has collected. Factors that prevent healing and wet wounds increase risk of PMA.
Although PMA was relatively widespread across Western Division in 1988/89, conditions best suited to its development occurred in August, September and October, in Brewarrina, Wilcannia, and Bourke districts. Places that had PMA in previous years, or had cases before mulesing, had a greater chance of seeing PMA in 1988/89. Places with black soil and channels were more frequently affected than places with other combinations of soil and water in 1988/89. PMA was prevalent because of the long wetter period leading up to and including 1988/89, and the timing of rains.
It was concluded that there was no simple way to prevent PMA in wetter years if mulesing in certain months.
Preventing PMA depends on managing known risks that increase or prevent PMA.
Assessing risk means a flockowner or manager would ask themselves questions that help work out the chance of PMA. If there is a chance of PMA, they can then work out how to avoid the risks. The questions that need to be asked include:
If there is enough concern about PMA, avoid it by: