Summer pneumonia is the term used to describe pneumonia and pleurisy in sheep. It has strong seasonality in Australia, most frequently seen in late summer and autumn, hence the term 'summer pneumonia'. Disease may occur as isolated cases but is more often recognised in outbreak form, and most commonly in young, weaned animals. Several outbreaks were seen across the Central West Local Land Services (CWLLS) in the 2020-21 summer. Antibiotic therapy was recommended to manage the effects on health, production and welfare in cases where it was evident that infection was active and affecting significant numbers of animals. This paper reports on three cases seen in the southern area of our region.
In early January 2021 a producer in the Grenfell area was concerned with sporadic deaths in a mob of 250 composite prime ewe lambs over the previous three weeks. The July-August drop homebred ewes were well grown, grazing good lucerne with supplementary hay and a loose lick supplying calcium, magnesium and salt on offer. Twelve lambs had died. Five of these had died in the last seven days, two overnight. A property visit was conducted to examine the recently deceased animals.
Both lambs were in lateral recumbency with a small amount of froth discharging from the mouth. Postmortem of one animal revealed dark purple lungs, wet, heavy and freshly adhered in part to the chest cavity on both sides. These regions of the lung fields were dark red and consolidated. The trachea contained blood-stained froth, and there were blood clots in the smaller airways.
Examination of the mob at rest in the paddock revealed a large number of coughing animals and it was decided to treat the whole group with injectable antibiotics. A script was provided for Alamycin LA at a dose rate of 1ml/10kg. The following day when walking the mob less than half a kilometre to the yards for treatment approximately 10% lagged behind and some were open mouth breathing. Response to treatment was excellent and no further losses were recorded.
Laboratory tests on samples collected at post-mortem confirmed a fibrinosuppurative and necrotising bronchopneumonia and fibrinous pleuritis. Mannheimia haemolytica and Pasteurella multocida were cultured. Mycoplasma spp were also grown with species type not identified.
In February 2021 a producer at Warraderry contacted our offices regarding sporadic losses in a mob of 750 mixed-sex composite lambs. These were September drop and purchased from Victoria in December. They had been weaned onto the truck on consignment. Losses occurred intermittently starting soon after arrival and would spike with stressors such as shearing or hot weather. Vaginal prolapse was also noted in a number of ewe lambs. After moving the mob a short distance to be yarded for assessment by an agent, a number of animals showed exercise intolerance and one died. A property visit was conducted.
Post-mortem of the deceased lamb revealed dark purple lungs, wet, heavy and adhered to the chest wall and other tissues by large amounts of yellow fibrin. The pericardium was firm and thickened and the pericardial sac contained 100ml bright yellow serous fluid. The surface of the heart was roughened with a layer of pale-yellow fibrin.
Coughing was evident amongst the yarded animals and it was decided to treat the mob with injectable antibiotics. A script was provided for Alamycin LA at a dose rate of 1ml/10kg.
Laboratory tests conducted on post-mortem samples confirmed a severe, fibrinosuppurative pleuritis and pericarditis. Pasteurella multocida was cultured from lung tissue and pericardial fluid.
A producer from Trundle contacted the CWLLS in February concerned with production performance in a portion of his 1100 August-September drop Dorper lambs. There had been deaths some months earlier, but the cause was not determined. Following these deaths the mob began to develop a significant tail. Worm egg counts were low, and even after the tail was drafted off and preferentially fed, weight gains remained low. The poor performing lambs were examined at a property visit.
The yarded lambs all appeared well despite poor body condition, although some did exhibit a moist cough. Two animals were examined – they were bright and alert, with pink mucous membranes and normal body temperatures. Both had nasal discharge, and one was coughing. This animal was selected for postmortem.
Postmortem revealed the lungs were normal in colour apart from a clearly demarcated area in the cranioventral lung field that was dark red and consolidated. There were also dry adhesions between the lung and chest wall, and the pericardial sac. These changes were consistent with an inactive pleuropneumonia and chronic tissue scarring.
As there were no deaths at the time and post-mortem lesions appeared no longer active, no treatment was advised. It was suggested that the outbreak had run its course with the earlier deaths, and now the survivors in the tail remained with chronic scarring and permanent limits to growth and production.
Summer pneumonia was clearly an important issue in our region last year. Costs of the disease stemmed from mortalities, reduction in weight gain in surviving affected animals, persistent ill-thrift related to chronic scarring in some, as well as monetary and labour costs associated with management and treatment. Condemnation or trimming costs at the abattoir are an additional cost in most circumstances borne by the broader meat industry.
It is widely accepted that summer pneumonia is underdiagnosed in sheep, probably because most cases are non-fatal. There are a large number of common viruses and bacteria that can circulate through a mob causing mild, primary respiratory disease with subtle production effects. More severe clinical disease and mortality is generally only seen in a small number of animals following opportunistic secondary bacterial invasion of compromised lung tissue.
Stress leading to weakened immunity and increased susceptibility is a significant factor in the development of an outbreak of respiratory disease, and can be precipitated by weaning, mixing, crowding, changes in diet, transport and extreme climatic conditions, including dust, temperature and humidity. Stress factors were quite clear in Case 2, with these lambs being weaned onto the truck, transported a reasonable distance from a very different environment, and then eventually being mixed with homebred lambs some time after arrival. However, stressors were less clear in Case 1 and 3. It was noted that the weather across summer was particularly humid for our district. Humidity can affect the ability of livestock to cool themselves, causing a different form of heat stress and potentially contributing to development of disease.
Prevention of summer pneumonia outbreaks relies almost completely on minimising the stressors as listed above. However, some of these are difficult to manage, particularly the climatic factors. The broad range of pathogens involved complicates vaccine development and research, although there has been some work looking at the use of cattle vaccines for Mannheima spp in sheep. As Mannheima haemolytica is just one of the secondary bacteria that can be involved in summer pneumonia, its inclusion in a vaccine will only be effective in animals specifically affected by this pathogen. Mycoplasma ovipneumoniae is considered to be one of the more important primary pathogens initiating respiratory disease in sheep, and there has been some research looking at test and cull programs to eradicate it from individual flocks. There are also early studies looking at the potential efficacy of a vaccine to prevent Mycoplasma ovipneumoniae infection in sheep. More research is required in this area. The involvement of mycoplasma as initiators may explain the success of blanket treatment with oxytetracycline.
Blanket treatment of all at-risk animals was used in both Case 1 and 2. This decision is never taken lightly in food producing animals, but summer pneumonia is considered to be one of the high impact conditions for which treatment with antimicrobials is justifiable for reasons of animal health, welfare and productivity. Often when only clinically affected animals are treated in these outbreaks, new cases continue to develop. Ongoing cases lead to prolonged suffering, increased production losses, and the increased likelihood of chronically affected animals, similar to those seen in the tail of Case 3. Parenteral treatment was chosen over in-feed medication as the best way to deliver antibiotic to the respiratory system, and oxytetracycline is the most frequently recommended antibiotic to treat pneumonia. Meat withhold periods, export slaughter intervals and Safemeat guidelines were clearly outlined on the antibiotic prescriptions provided and discussed at length with the producers to manage the risk of antibiotic residues.