Rape blindness is one of a number of possible disease manifestations associated with grazing brassicas. It has not been diagnosed in recent years in the Central West but is an example of the problems that can be encountered when farmers use alternative feed sources in dry times.
One hundred and thirty six Angus steers were grazing self sown canola (Brassica napus) for a 5 week period near Trangie, NSW. The district had experienced a dry summer, and pasture availability was reduced. As a result of this, this mob of steers was put into a paddock in early March where canola had been stripped in summer and then reshot. The reshot canola was approximately 10cm high, and besides canola stubble from the previous crop, there was little other grass or weeds to graze in the paddock. Hence, the diet of the steers was almost exclusively canola. (See Figure 1).
There had been very little rain on this paddock during grazing, and the canola was moisture stressed. The water source of the paddock was bore water.
The producer contacted the CWLHPA on the 22nd of April 2013 to report that six of the steers were showing signs of lethargy, ataxia and blindness.
On clinical examination, animals were completely or partially blind. All affected animals were difficult to yard. The steers that appeared completely blind walked into fences. The ones that appeared to have some vision could avoid the fences but baulked at the fences, shadows and humans as though vision impaired to some degree. The completely blind animals had an absent menace reflex and constricted pupils that were non responsive to light. All affected animals were lethargic, hypersalivating and were ataxic with a hypermetric forelimb gait. There were no muscle tremors, nystagmus or strabismus observed. (See Figure 2)
A diagnosis of sulphur induced polioencephalomalacia was made based on clinical findings and the history of grazing the canola crop. The producer had already removed the mob from the crop when the sick animals were noticed. The six affected animals were given thiamine at the dose rate of 8mg/kg intramuscularly. Four of the six treated animals recovered within 24 hours, the remaining two (who were the most severely affected) recovered within 3 days. There were no deaths.
Rape blindness is one manifestation of brassica poisoning of livestock and there is little written in the literature about it. Most authors agree that the toxic principle is sulphur induced polioencephalomalacia (PEM), (McKenzie, 2012; Radostits et al., 2000; Parkinson et al., 2011) although some also consider the possibility that the syndrome is caused by an unidentified toxin (McKenzie, 2012).
The way in which sulphur causes PEM is poorly understood. One theory is that hydrogen sulphide is produced in the rumen, eructed and inhaled which interferes with the blood supply to the brain. Another is that the sulphur destroys thiamine in the rumen, causing signs of PEM (McKenzie, 2012).
Usually the clinical signs of PEM include central blindness, but a pupillary light reflex is still present (Parkinson et al., 2011, Radostits et al., 2000). In this case, the clinical examination showed that the animals had constricted pupils that were non-responsive to light, which is reported to occur in severe cases (Radostits et al., 2000). The producer commented that when he drove down to check the animals in the yards at night time, there was no tapetal reflection in response to the headlights of the car. It is unclear why this incidence of rape blindness was severe enough to cause pupillary constriction and absence of a pupillary light reflex, but not severe enough to cause recumbency or death.
The response to thiamine treatment of animals affected by sulphur associated PEM is variable (Parkinson et al., 2011). The limited reports in the literature say that complete recovery from sulphur induced PEM can take several weeks, even after administration of thiamine (Hungerford, 1990 & Radostits et al., 2000). In this case, most animals recovered within 24 hours, and all within 3 days.
Cattle are at risk of sulphur poisoning when their diet comprises more than 40% brassicas, or if their whole diet contains more than 0.5% sulphur (McKenzie, 2012). Canola is reported to contain 0.5-1.3% sulphur (Johnston, 2009). Sulphur levels in this crop were not tested. Some water sources may also contain a high concentration of sulphur, adding to total dietary sulphur intake (Radostits, 2000). The amount of sulphur in the water was not measured on this property.
Canola is different from forage brassicas in that a canola plant takes up more sulphur from the soil as it is more reproductive than the vegetative forage brassicas. Even when grazing forage brassicas, it is recommended that it not constitute 100% of the animal's diet. It is suggested that stock grazing forage brassicas are given access to alternative grazing or fed supplementary hay (McCormick, pers comm. 2013).
Brassica crops that have been fertilised with sulphur based fertilisers are higher risk, as are stressed crops. This crop had not been fertilised, and not experienced a frost, but it was moisture stressed which may have contributed to the poisoning in this case. The length of time that cattle were grazing canola also probably contributed to the development of toxicity.