A commensal of the nasopharynx, Mannheimia haemolytica is an opportunist, gaining access to the lungs when host defences are compromised by stress or infection with respiratory viruses or mycoplasma1. It often occurs as part of the bovine respiratory disease complex in young animals between weaning and 12 months of age2,3. Although calves as young as 2 weeks of age can be affected, this is not common2.
This is a report of an unusual outbreak of pneumonic pasteurellosis (M. haemolytica) in extensively-managed, unweaned beef calves on a drought-affected property near Burren Junction NSW in early September 2013.
All cattle on the property had been boxed together due to extended dry conditions. The mob consisted of approximately 60 yearlings, 48 homebred shorthorn cows and 77 shorthorn heifers introduced from Guyra NSW in mid-June. The heifers had begun calving in mid-July and their calves ranged from 3 to 8 weeks of age. The homebred cows had just commenced calving.
The mob had been grazing drought-failed winter cereal crops for the previous 6 weeks before being moved to the current paddock. The current 900 Ha paddock contained a failed barley crop sown into stubble. The barley was 10cm high and still reasonably green.
The paddock movement required mustering from a large paddock and walking 3km in dusty conditions. The cattle were checked daily for the first 6 days after being moved with no illness detected. On the 8th day post-movement, the owners found 4 dead calves. Another 2 calves were sick and one weaner looked depressed and hollow. The following morning, one of the calves (Calf 1) and the weaner were found dead. The second sick calf (Calf 2) was attempting to suckle but was very weak. All affected calves were progeny of the heifers.
The paddock was watered by 4 troughs (cap and pipe) near an old dam which was nearly empty. Five of the calves died within 50m of the dam. The weaner and the sixth calf died approx 500m from the dam.
Weather conditions over the previous two weeks had been unseasonably hot during the day (approx 30°C) and cool at night (approx 10°C). In the week prior to the paddock movement, there had been three days of sub 20°C maximums with night minimums approaching 0°C. There had been no rainfall for more than 2 months.
The herd was examined 9 days after the movement to the current paddock. All cattle were in store or forward store condition. Calves appeared active and strong. There was no evidence of toxic plants in the paddock. The water in the dam was greenish and cloudy. No algae was seen, although it was a hot, windy day.
The calf seen to be sick but trying to suckle earlier that morning was found moribund and with agonal breathing (Calf 2). It died shortly after.
Calf 2: Approximately 4 weeks old. The liver was 20% enlarged. The abomasum was gas-filled with scant, watery contents including some grass (but no milk). There was a marked pleuritis with extensive fibrinous adhesions. The ventral half of the lungs was consolidated with a moist exudate. The heart was flabby.
Calf 2: Approximately 8 weeks old. This calf had died overnight and although it was necropsied, the organs were considered too decomposed to be assessed.
A closer examination of the herd was undertaken after the calves were necropsied. One calf was found to be depressed with a soft cough and nasal discharge. It was recommended that the herd be monitored closely and any calves found sick should be treated with Engemycin (oxytetracycline). Moving the herd to the yards for treating was not recommended due to the 2-3km journey required.
In the week following the property visit approximately 12 calves were treated with oxytetracycline. A further 5 calves and 1 yearling died. It was difficult to determine if all deaths occurred in treated calves. The sick calves were progeny of both the introduced heifers and the homebred cows. Deaths ceased 8 days after the initial dead calves were found.
Lung samples submitted for histopathology revealed severe, subacute, fibrinosuppurative and necrotising pneumonia, consistent with bacterial infection.
A profuse, pure growth of Mannheimia haemolytica was cultured from the lung tissue. Antibiotic sensitivity was conducted and it was found that the bacteria were sensitive to tetracyclines.
The dam water was negative for blue-green algae.
Stress is the most significant risk factor for the occurrence of pneumonic pasteurellosis. In beef cattle, the disease commonly occurs in outbreaks 7 - 10 days after transportation1. Classically this occurs after transportation to a feedlot. In the north west of NSW, pneumonic pasteurellosis is usually seen as outbreaks approximately 2 weeks after weaners are purchased and transported from saleyards or another property in winter. The weaners are most often grazing oats or native pasture.
Parkinson et al. (2010) comment that marked changes in the weather are a significant risk factor for the disease in dairy calves.
This case is unusual for the north west of NSW in that it occurred in unweaned calves. A check of property records in the North West LHPA did not find any diagnosed cases in that age group. It is proposed that the stressful risk factors included mustering and moving paddocks under extremely dusty conditions, following a period of marked weather changes. It is possible that maternal immunity to M. haemolytica was compromised due to nutritional stress on the heifer mothers. One study has found that maternally derived antibodies against M. haemolytica reached lowest levels at 4 to 12 weeks after birth4. In that study, calves began production of antibodies against M. haemolytica between 8 and 13 weeks of age4.
The role of viruses was not investigated in this case. It is possible that viral agents contributed, although there was no known contact with unfamiliar cattle in the 8 weeks prior to the outbreak.
A distinct challenge of this case was treating sick calves under extensive conditions. Movement of the herd to yards for treatment was likely to precipitate further cases. During daily monitoring, sick calves were caught in the paddock and treated with a long-acting dose of oxytetracycline.