Histophilus somni caused significant mortalities in a pasture-based cattle enterprise on the Southern Tablelands during winter 2012. The same property experienced further losses in summer 2012-2013. Potential risk factors associated with the two outbreaks were compared in an attempt to prevent future losses.
Worldwide prevalence of infection of cattle with the bacterium Histophilus somni is reported to be high, whilst incidence of disease is low (Radostits et al, 2007). Cattle affected by Histophilus somni complex have one or a combination of septicaemia, meningitis, pneumonia, pleuritis, myocarditis, arthritis, opthalmitis, conjunctivitis, otitis and reproductive failure including abortion (McGavin and Zachary, 2007). Prior to the first investigation on this property in June 2012, no case of Histophilosis in cattle had been recorded in the Goulburn district.
The affected herd is arguably the most progressive cattle enterprise in the district. Through emphases on pasture improvement and fodder cropping, advanced genetics, thorough on-farm and feedback recording, and innovative marketing, more than three tonnes of home-bred prime grass-fed beef is supplied every fortnight direct to high-end retailers. Product from the farm is highly sought after, selling for up to $60 per kilogram. The system requires two calf drops per year, autumn and spring.
Outbreak one. Deaths occurred in weaned heifers in June 2012 when they were about 9 months of age. These spring 2011 calves had been weaned in March 2012, with both heifers and steers initially run together after weaning. All are booster-vaccinated for clostridial diseases and drenched at weaning. In May, the weaners were weighed and drafted, with heifers and steers subsequently run in separate mobs. Of one hundred heifers, seven had died and a further two were sick when the property was first visited in mid June. There had been no deaths in the steer portion at that time. On reflection, the manager recalled unexplained but similar deaths in the previous batch of weaner heifers in the same paddock.
Most affected animals were found dead without any prior sign of illness. One early case was noticed to be listless and standing away from the mob. She was treated by the owner with oxytetracycline and NSAID injections, and made only marginal improvement. Another heifer was also found to be listless and weak enough to be caught in the paddock. This heifer was febrile (41.5oC), had intermittent unilateral mucopurulent nasal discharge, drooping ears, increased respiratory rate without coughing, was very reluctant to rise, stood with hunched back, and had three grossly swollen fetlock joints. Subsequent post mortem of this heifer revealed copious stable foam in the trachea, extensive dark red-purple discoloration of ventral lung lobes, extensive haemorrhagic pleurisy, spectacular fibrinous pericarditis (figure 1), and distended joint capsules of distal limb joints.
NSW DPI State Diagnostic Veterinary Laboratory at Menangle isolated Histophilus somni and reported severe fibrinous pericarditis and suppurative meningitis from samples submitted. These findings and those of three other autopsies supported a diagnosis of Histophilus somni complex. Despite no recent close contact between the mobs, three steers also subsequently died. The heifers were run on improved pasture, and the steers on an oat crop.
Outbreak two. Autumn-drop calves were weaned as one mob in November 2012. In early December, they were drafted and weighed, and heifers and steers run in separate mobs. The steers were initially run on an oat crop, then a ryegrass pasture where the first deaths occurred in January, seven weeks after weaning. Six steers died, but no heifers. Autopsy of the steer in Figure two showed predominantly joint involvement, with multiple swollen joints and fibrinous synovitis. It was noted that three of the first four steers to die were the last three steers to be weighed at least three weeks earlier, although none appeared sick at that time.
Assessment of risk factors
Many apparently healthy cattle carry the organism, particularly in the reproductive tract (Parkinson et al, 2010), and introducing Histophilus somni to naive cattle does not always produce disease (McGavin and Zachary, 2007). Histophilus somni disease complex is more commonly associated with cattle in feedlots. Whilst it has been observed in unweaned beef calves and young cows on pasture (Radostits et al, 2007), the pathogenesis of the complex in cattle at pasture is not understood.
On the property investigated, no consistent association was established between occurrence of disease and observed risk factors. Some of the factors considered include:
Sire/dam. The age and breed of dam and sire bears no relationship to which cattle died in each outbreak. Both cross-bred and pure-bred cattle were affected (as seen in Figure three).
Age of cattle affected. Cattle affected in winter 2012 were 9 months of age; those affected in summer 12-13 were 10 months old. Each outbreak occurred six to seven weeks after weaning, and began about three weeks after they were last yarded. Affected cattle were described as either weaners (three of four mobs) or yearlings in the series of cases reported by Read and Slattery (2011). Outbreaks in feedlots are usually in calves between six and twelve months of age (Radostits et al, 2007, Parkinson et al, 2010), and within the first five weeks on feed (Harris and Zanzen, 1989).
Inter-species infection. The possibility of transfer of infection between cattle and sheep has not been ruled out (Radostits et al, 2007). However, this property only runs cattle, as do most neighbouring farms. There was no contact between affected cattle mobs and sheep.
Source of cattle. Comingling of cattle from different sources is regarded as a major risk factor for Histophilosis in Canadian feedlots (Radostits et al, 2007). Transport or comingling occurred in three cases in pasture-fed cattle in northern New South Wales (Read and Slattery 2011). All cattle in the two Goulburn outbreaks were home-bred, albeit in up to five different mobs on the one property. There were no introductions, no transport, and the calves were not yard-weaned.
Stress. Although Radostits el al (2007) conclude there is no information available on environmental and management risk factors, 'stress', environmental stress factors, a preceding viral infection, or concurrent bacterial infection such as Mannheimia haemolytica or Pasteurella multocida are suggested predisposing factors according to other authors (McGavin and Zachary, 2007; Parkinson et al, 2010).
Stress: environmental. One of the two outbreaks at Goulburn occurred in mid winter. The winter of 2012 had about average rainfall and temperatures, although a wet autumn meant the ground was saturated. The property is undulating to steep, providing good shelter from winter winds. The second outbreak occurred midsummer. Summer temperatures were above average on several days, following on from unseasonable late snowfalls in the preceding October. Green timber in most paddocks provides adequate shade.
Consecutive batches of weaners have died in the same paddock, although deaths in outbreak two commenced prior to cattle entering that paddock. Wide variations are reported for survival of Histophilus somni in the environment. Harris and Janzen (1989) describe survival for up to 70 days at ambient temperature (23.5oC) in blood and nasal discharges, while most authors regard the organism as fragile. Refrigeration of laboratory samples is recommended, for example, to extend survival of the organism from one to three days (Casademunt, 2013).
Effective stocking rate in the two Goulburn outbreaks is above district average. While the overall number of cattle grazed over the entire area is similar, these weaners are rotationally grazed through saved improved pasture paddocks or crops at up to about 16 dse/ha. Feed availability is not compromised, but the grazing system arguably leads to closer animal contact than the district norm. Of interest, the three pasture-based outbreaks reported by Read and Slattery (2011) all grazed cereal crops (oats and/or wheat), presumably at higher than average stocking rates. Fodder crops especially cereals are frequently grown for short-term grazing by other weaner cattle in the Goulburn area, so this closer contact alone is not the major determinant of these outbreaks.
Stress: parasitological. Internal parasite numbers frequently rise in cattle after weaning. Investigation by Smeal (1983) in a nearby herd showed that helminth numbers in June in this district are relatively low in calves drenched at weaning in the autumn. He also demonstrated that pasture larval numbers decline rapidly from mid spring, so that the spring-weaned and drenched calves which died in summer would likely have had low worm counts. In addition, affected calves had gained considerable bodyweight since weaning, and were in good condition when they died. So stress from worms is not considered a significant contributing factor.
Stress: nutritional. Provision of adequate pasture for weaned cattle is a priority of this enterprise. At the time of the deaths, these young cattle were grazing pastures or crops capable of promoting weight gains, as assessed by regular weighing. They had variously grazed both improved mixed pastures (phalaris-dominant, ryegrass-dominant) and grazing oats. Pasture-based cases reported from northern NSW involved grazing oats or wheat (Read and Slattery, 2011), but the underlying indication in all cases is that the animals were not under nutritional stress preceding each outbreak.
Stress: concurrent disease. Bacterial culture of samples from Goulburn isolated a pure growth of Histophilus somni, but only from joints. No bacteria were cultured from lung and pericardium. Persistent infection with pestivirus was ruled out in one of the outbreaks in northern NSW (Read and Slattery, 2011). Concurrent infection with pestivirus was implicated in the development of bovine respiratory disease in Australian feedlot cattle (Dunn et al, 2000), and is noted as a possible predisposing factor in Histophilosis (Radostits et al, 2007). Samples have been collected from cohorts of the summer cases to assess pestivirus activity around the time of the outbreak. Results are not yet available.
Both the owner and the manager of this herd made astute observations and valuable contributions. The paper by Libby Read and Shaun Slattery, in particular their excellent photographs, was of great assistance in establishing the initial diagnosis.