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Iceberg diseases impact on productivity and profitability

Ewe deaths and culling of ewes due to poor body condition are often accepted as normal within sheep farming businesses. Veterinary surgeon, Ed Hill from Thrums Veterinary Group, explains why this should not be the case.

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If ewe deaths exceed 2-4 per cent per year or if poor condition is responsible for a high proportion of ewes being selected for culling, then it could be the case that there is a disease process dragging down the productivity of the flock.


Diseases such as OPA (Jaagsiekte), Maedi Visna and Johne’s are increasingly being identified as the cause of ewe deaths and poor body condition. They are often referred to as ’iceberg diseases’ as deaths and culls only represent a small part of the problem, while the bigger picture of increased feeding costs, poor lamb growth rate, higher lamb mortality and an increase in other diseases may be lurking unseen beneath the surface.

 

Cull ewe screen


Mr Hill says: “Screening of thin cull ewes provides an excellent opportunity to establish the presence of these diseases in the flock. Post-mortem examination (PME) is the most effective way of picking up these diseases, as well as potentially identifying other issues such as a fluke or worm problem and trace element deficiencies.


“Ideally four to six representative thin ewes should be submitted to your local veterinary investigation centre or private veterinary surgeon. Another option is to get your vet to take samples from 12 thin ewes – lungs should be scanned for OPA, blood should be collected for MV and Johne’s and dung should be collected for Johne’s. Additional tests such as those for fluke, worms and trace elements can also be run.


“Despite popular belief, often sheep do not die for no good reason. Therefore, do not waste the opportunity to have fallen stock post-mortemed. Speak to your vet who will be best placed to advice on surveillance and control of these crippling diseases.”

Ovine Pulmonary Adenocarcinoma (OPA)

OPA often referred to as Jaagsiekte disease is a caused by a virus that leads to development of tumours in the lungs. As the tumours grow ewes lose condition, develop a soft cough and do not cope well with exercise.

 

The tumours leave the sheep more susceptible to developing pneumonia, which is usually fatal. Infection is passed from one sheep to another via respiratory secretions such as saliva as well as in milk and colostrum. Therefore, spread from a ewe to her lambs is common.


OPA can be diagnosed by PME. Alternatively, ultrasound scanning of lungs can pick up tumours down to the size of a sugar cube in live animals. Control is best aimed at reducing risk of infection entering a flock by lung scanning purchased animals during quarantine and buying from as few, well trusted sources as possible.


For infected flocks, control should be aimed at reducing the opportunity for virus to spread to uninfected members of the flock. Identification of diseased ewes through scanning allows their removal before they die.

Johne’s disease

Johne’s is caused by bacteria which is spread in the dung of infected animals and can survive for months to years in the environment. Sheep tend to be infected in the first few months of life but it is not until they are adult sheep they develop the chronic weight loss associated with the disease. Unlike in cattle, diarrhoea is not a consistent feature, but sheep may develop soft, pasty faeces in advanced disease.


Diagnosis is best made on PME, but blood and dung samples can also be useful for diagnosis in the live animal. A number of control methods exist. Blood testing and removal of positive animals may help and is most cost effectively done as a member of a health scheme.

 

Adopting a heaving culling strategy and keeping a young flock will reduce the opportunity for infected animals to spread the bacteria and develop clinical disease. There is a vaccine available for Johne’s disease in sheep which is very effective at reducing bacterial shedding and stopping infected sheep losing weight and dying.

Maedi visna (MV)

MV is caused by a virus. The main route of spread is via respiratory secretions but spread through colostrum and milk from a mother to its lamb is also possible. Disease does not usually become evident until sheep are about four years or older (though it is possible to see it in younger sheep).


Affected sheep gradually lose condition, breathing may become more laboured, and higher rates of mastitis may also be seen. This may not be detectable clinically but will result in poorer growth rate of lambs. More rarely, nervous signs may be seen including dragging of a limb, incoordination and collapse. Again, PME is the most reliable method of diagnosis, but very good blood tests also exist for the disease.


For uninfected flocks prevention should be aimed at buying only MV accredited replacements. Where this is not possible then sheep should be blood tested during quarantine. Control in infected flocks should be aimed at a test and cull policy, membership of a health scheme supports these actions.


Other options include heavy culling as for Johne’s, creation of ’clean’ and ’dirty’ flocks until the ’dirty’ flock has been removed. Total destocking and repopulation with MV accredited stock may be an option in severely infected flocks.

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