RESPIRATORY DISEASES

Many of the major respiratory diseases have been described elsewhere in this book. Some affect calves, and others can affect all age groups. They include:

• calf pneumonia (Chapter 3)

• IBR (Chapter 4).

• lungworm (Chapter 4). ‘Reinfection husk’ is the name given to the syndrome where partially immune dairy cows are subjected to a high larval challenge from the pasture. Although the cows cough (perhaps causing the milking units to fall off) the partial immunity of the cow may prevent any larvae from being seen in the faeces. This makes the syndrome more difficult to diagnose.

The respiratory conditions described in this chapter include fog fever, pulmonary haemorrhage, allergic respiratory diseases and bovine influenza A (described on page 426).

Fog Fever

Fog fever is the name given to a syndrome of severe respiratory distress in cattle. It is mainly seen in the autumn, especially in September and October, and affects cattle which are two years old or more. Suckler cows are particularly prone, although I have also seen outbreaks in milking cows. Disease typically occurs zero to two weeks after the cattle have been moved onto a lush autumn aftermath and this is especially so if their previous grazing was a very sparse and dry pasture. Many theories were sug­gested as to a possible cause, for example an allergy to lungworm larvae or to fungal toxins on pasture, but it is now known that the syndrome is an anaphylaxis, sometimes called a hypersensitivity reaction. This is the name given to an overactivity of the normal immune defences of the animal, as described in Chapter 1. Lush autumn grazing, particularly if it has a high nitrogen content, contains increased levels of the amino-acid L-tryptophan.

Clinical signs

The syndrome is very sudden in onset. One or more cattle may be seen standing listlessly in the field, not grazing, and characteristically their breathing is accompanied by forced grunts. The toxin 3-methyl indole leads to congestion of the lungs and many of the small alveoli burst, leaving the animal ‘broken winded’, a condition known technically as pulmonary alveolar emphysema. (The alveoli are shown in Figure 4.7.) Although the cow can breathe in without any problems, the loss of elasticity in the broken alveoli means that she has great difficulty in breathing out, and if you stand and watch her carefully you will see that she grunts as her flanks and chest move inwards, trying to expel air from the lungs. In this respect fog fever resembles human asthma but it differs from cows with severe pneumonia which show difficulty both when breathing in (inspiration) and when breathing out (expiration). The burst alveoli allow air to infiltrate between the lung tissues and to pass deeper into the body, and in long-standing cases I have even seen cows with air crackling under the skin of their backs (subcutaneous emphy­sema). Although this looks peculiar, it is no cause for alarm and provided the animal recovers, the air will slowly disperse. Badly affected animals stand with their necks stretched forwards, mouths open and froth around their lips. They cannot eat or drink and eventually they die, simply because they can­not get sufficient air.

Prevention and treatment

Making a more gradual change from bare summer grazing to lush autumn aftermaths, for example by strip-grazing, is considered to help reduce the severity of outbreaks, and feeding hay or straw at this time may also be worthwhile. Treatment is quite complex, and the drugs used will depend on how bad the animal is and how long it has been affected. Non-steroidal anti-inflammatories, antihistamines, corticosteroids and other anti-inflammatory drugs may be used to try to reduce the toxic effects of 3- methyl indole, and antibiotics will help to prevent a secondary bacterial pneumonia developing in the congested lungs.

Even if all these treatments are given there will still be a proportion of animals in which the lung changes are so severe that death is inevitable. Affected cattle may have great difficulty in breathing, so it is important not to walk them too far or too quickly. It may even be necessary to pen them into the corner of the offending field and carry hay and water rather than risk moving them.

Pulmonary Haemorrhage

Occasionally you may see a cow bleeding from the nose, as in Plate 11.11. Although it may not look par­ticularly serious, in most cases the blood is coming from a ruptured pulmonary (= lung) artery or vein. The animal may have had pneumonia as a calf, and although it may have appeared to recover, a small abscess was left in the lung. Over time the abscess may slowly erode into a blood vessel, which then ‘leaks’, and the blood passes up the trachea and back down into the mouth and nose. In some cows the haemorrhage is so severe that the animal is simply found dead, sometimes with the coughed-up blood splashed all around its pen.

Pulmonary haemorrhage may also result from thromboembolism. An abscess anywhere within the body may start to ‘leak’ and allow clumps of blood and bacteria to float around in the blood vessels. These clumps are known as emboli. Some may localise in the lungs, or even in the major blood vessels within the lungs, and grow until they eventually lead to blood vessel rupture and haemorrhage.

Allergic Respiratory Diseases

Cattle can become allergic to mouldy hay or straw and develop a syndrome which is effectively bovine

farmer’s lung, sometimes technically called inter­stitial pneumonia. The fungus Micropolyspora faeni is often involved and, as you would expect, disease is more common -

in the winter, when cattle are housed, perhaps on damp and mouldy straw, or fed on mouldy hay

in wetter areas of the country, which are the areas most likely to have damp hay and straw

The primary clinical sign is coughing. In severe cases, growth is affected and weight loss may occur. All ages of cattle can become affected, including milking cows. Dusty feed in itself can also cause coughing and/or sneezing, and as described in Chapter 3, dust will make cattle more susceptible to pneumonia.

There is no specific treatment. Anti-inflamma­tory drugs will help, but the most important thing is to remove the mouldy or dusty food and bedding.