Blackleg is an emphysematous myonecrosis caused by the bacteria Clostridium chauvoei. C. chauvoei is a sporulated anaerobic rod that is ubiquitous in most areas of cattle production of the world (Uzal, 2011). The spores are highly resistant to environmental change and can persist in the soil for many years (Radostits et al, 1994). The exact pathogenesis is unknown, but it has been suggested that cattle ingest spores from the soil and sporolation of the bacteria within the intestines leads to seeding within several tissues including muscle. Spores remain latent in tissues until damage to the muscle from trauma, over-exercise or biting insects cause local anoxia that provides suitable conditions for spore germination. Proliferation of the bacteria and release of its exotoxins within the muscle mass causes myonecrosis with associated oedema, gas production and haemorrhage (Parkinson et al. 2010).
Blackleg most commonly occurs in young, well-fed cattle less than 2 years of age (Radostits, 1994). The hind limb muscle mass is most frequently affected but has also been reported in myocardium, diaphragm and the tongue. Clinical signs may include fever, anorexia, depression and lameness but sudden death without observed clinical signs is common (Parkinson et al. 2010). Crepitant swelling over the affected muscle mass is seen and the affected muscle is darkened and hemorrhagic, hence the name 'blackleg.'
Clostridial diseases are one of the highest causes of death in cattle on the Tweed-Lismore region of the north coast of NSW (Ball et al, 2011). Vaccination against the common clostridial diseases of cattle, including C. chauvoei, has been available for many decades in Australia.
Hart (2011) undertook a retrospective review of blackleg outbreaks in his district. This review aims to undertake a similar task for the Tweed-Lismore district. To enable meaningful comparison between Hart's study and this review, it was essential that the same case definition be utilized. Confirmed cases are defined as those cases where a diagnosis of blackleg was made by histopathology, culture and/or fluorescent antibody test for Clostridium chauvoei. Cases diagnosed based on clinical signs or gross postmortem findings were excluded from this review.
Eighty seven laboratory reports were found in archives covering the period between 1968 and 2012. Forty seven of these reports were consistent with the case definition for blackleg. This compares to a dataset of forty five laboratory confirmed cases between 1973 and 2002 by Hart (2011). There were a number of periods where laboratory reports were not found. For example, no diagnosis prior to 1968, between 1969 and 1973 and between 1993 and 1999 could be located in office records. There was variation over time as to the number of cases and whether or not confirmatory testing to a standard of the case definition of this review was achieved (Table 1).
|Year range||Cases listed on file as blackleg (a)||Confirmed cases for this review(b)||b/a*100|
|1968 - 1978||16||11||68.7%|
|1979 - 1989||24||19||79.2%|
|1990 - 2000||22||10||45.5%|
|2001 - 2011||25||8||32.0%|
Based on the results from this review, confirmed cases of blackleg occur widespread across the Tweed-Lismore region where cattle are run (Figure 1).
Blackleg can occur at any time of the year in the Tweed-Lismore district, but is most prevalent during the winter months and the lowest risk time is in summer (Figure 2). Hart (2011) found that summer had the highest number of cases and the lowest risk period was autumn.
In the Tweed-Lismore region, blackleg occurs in cattle aged 2 to 24 months of age and most commonly reported in calves aged 4 to 6 months of age (Figure 3). The highest number of cases identified by Hart (2011) was in the 6 to 8 month age group.
Fluorescent antibody testing was the most common laboratory test used to confirm blackleg cases during this time period in the Tweed-Lismore region (Table 2). Histopathology and culture were the least commonly used confirmatory laboratory tests (Table 2).
|Type of test||Number||Percentage|
|Fluorescent antibody test||24||50%|
|Bacteriology (stained muscle impression smear)||14||29.2%|
The eighty seven laboratory reports of blackleg located for the Tweed-Lismore region do not represent the true incidence for the district. North coast district veterinarians believe that the incidence of blackleg is very high and many cases are not documented in LHPA records. This may be because farmers don't always report suspect blackleg cases, experienced veterinarians may make a field diagnoses without confirmatory laboratory testing (in part because of the cost of laboratory testing) and record keeping by the LHPA was sometimes inadequate.
The variation over time of the number of cases reported suggests fewer cases are being reported and/or recorded in the LHPA database now compared to the 1970s and 1980s. Similarly, the number of cases confirmed with laboratory testing has decreased over time suggesting diagnoses are being made based on clinical signs or post mortem findings alone. This is likely due to the aforementioned reasons related to cost of testing and suitability and willingness of farmers to perform laboratory testing.
Areas in the Tweed-Lismore region where blackleg has not occurred can largely be explained by the land not being used for cattle production. This includes national parks, sugar cane plantations or other land use not suitable for cattle production. The following areas had multiple cases; Alstonville, Corndale, Eureka and Nimbin. The significance of this is likely to be minimal and does not allow conclusions to be drawn that these areas are more prone to disease. This is more likely to be a coincidental finding.
Literature on the environmental risk factors for typical blackleg of cattle suggests the disease is common in areas subject to flooding or areas following excavation of soil. This suggests that disturbances in soil may expose latent spores that are highly resistant to environmental changes and can persist in soil for many years (Radostits et al. 1994). The Tweed-Lismore region is an area subject to high rainfall, which could explain the widespread distribution of blackleg (Bureau of Meteorology, 2012).
Current literature, presumably from temperate areas, states that typical blackleg occurs in the warm months of the year (Radostits et al, 1994). However, this review found that confirmed blackleg cases were more common during winter months. One possible suggestion for this is the short feed available during winter requires stock to be in closer proximity to the soil during feeding. As C. chauvoei spores persist in soil, it is possible that greater exposure to spores occurs during this time. Seasonal difference between Hart's 2011 review and this review cannot be explained by rainfall patterns as average rainfall is highest during the summer months in both Camden and Lismore regions (Bureau of Meteorology, 2012).
Cattle aged 2 months to 2 years have traditionally been considered most susceptible to blackleg (Useh et al. 2003). This review demonstrated that blackleg was diagnosed in cattle in this age group. The majority of confirmed blackleg cases in the Tweed-Lismore region occurred in calves described as from 4 to 6 months of age. However, calving in the Tweed-Lismore region occurs between May and October. This review reveals an inconsistency as most calves reach 4-6 months of age from October to March and yet most cases occurred from June to August. It is likely that the age of the affected calves has not been accurately known or recorded and that calves greater than 6 months of age are actually most affected.
Hart (2011) commented that the records did not specify the age of the affected animals or was presented in a broad range, making analysis and comparison difficult. However, Hart's records indicate that most deaths occurred between 3-8 months of age. This is reasonably consistent with our data given our caution about the true age of affected calves on the north coast.
Fluorescent antibody testing was the most common laboratory test used to confirm blackleg and histopathology and culture the least common.
The limitations of this study relate to the size of and limitations of the data available. As the LHPA online record keeping system only began in 2012, almost all case records are hardcopies and it is likely past records have been misplaced making the complete dataset unavailable.
The vaccination status was not recorded for almost all confirmed cases in this review. Future surveys on the rate of blackleg vaccine usage and reasons as to why producers may elect not to vaccinate would be beneficial. It is recommended that despite the lack of confirmatory laboratory testing performed, district veterinarians should attempt as often as possible to obtain a positive laboratory finding before blackleg is diagnosed. District veterinarians should also carefully consider which cases are coded as blackleg in the LHPA online record keeping system especially cases without confirmatory laboratory testing.
This review provides support for the view that blackleg is widespread and common in the Tweed-Lismore region. In part this may be because of the high number of new and part-time farmers on the North Coast region who lack knowledge of the disease and its prevention. The north coast LHPA's recommendations is that all calves should be vaccinated at 8 and 12 weeks of age and dams should receive a booster vaccination during late pregnancy (Kemsley, 2012).