Calf pneumonia is second only to neonatal diarrhoea in its impact on the health and welfare of growing cattle.
It is costly in terms of the drugs used to treat and prevent the disease.
Growth rates are reduced and calves may die.
There is also the hidden cost of increased labour demands generated by an outbreak of pneumonia.
Independent studies suggest the average loss per calf at risk lies between 21 and 30.
In herds where calf mortality is high and some survivors have such badly damaged lungs that repair is prolonged and growth rates impaired for life, losses can be up to four times higher.
The bacteria and most of the viruses that cause the calf pneumonia complex are present in most calf groups and only cause significant disease when natural defences to disease are lowered.
The principal factor is the quality of air calves breathe and the amount of dust it contains from feed, bedding and – importantly – the animals themselves.
Dust overloads the natural defences on the lining of the airways to the lungs.
Bacteria, which would otherwise be cleared up by the body, can then penetrate and survive in the lower airways and pneumonia results.
The important bacteria in this respect are Pasteurella multocida, Mannhiemia haemolytica, Histophilus somni and Mycoplasma bovis.
But that is not the whole story, because the viruses that affect the respiratory tract can also damage the surface defences and favour bacterial growth.
The important viruses are respiratory syncytial virus (RSV), parainfluenza 3 (Pi3) and infectious bovine rhinotracheitis (IBR).
These also suppress the animal’s immunity and a fourth virus, bovine viral diarrhoea (BVD), is a further important cause of immune suppression, but does not damage the airways.
Nutritional factors also exist, most importantly acidosis at weaning, which also reduces resistance to disease.
But a deficiency of selenium, copper and vitamins A and E also feature.
To finish off the complex there is stress, which, again, reduces the immune defences, typically at weaning, castration, dehorning, housing and when mixing groups.
This model explains most of the pneumonia seen on farms, but occasionally outbreaks of RSV or IBR infections can be so severe that calves die in the viral phase of the disease complex before bacterial pneumonia kicks in.
The only other complicating issue is lungworm.
Treatment with a white wormer may serve to further lower the animals’ ability to resist bacterial pneumonia.
Put the whole picture together and what you will see to begin with is a group of calves that look healthy.
But food intake may be reduced in a few and a clear discharge may be present at the nose or eyes.
Some calves may have raised temperatures of 39.5C and above, allowing a viral infection to worsen rapidly.
In the course of one to two days calves stop feeding completely and show increased effort when breathing and run high fevers.
The clear nasal discharge will change to a creamy yellow one.
As infection spreads, up to half will be clearly affected in the course of 10 to 20 days.
Where the age range of calves is wide, as on dairy units, such outbreaks may appear insignificant, although over a calf group’s lifespan that is rarely the case.
Early antibiotic treatment is extremely effective in allowing the calf to clear up the bacterial infection and return to an even keel, but deciding at what point antibiotic treatment is required can be difficult.