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Arthrogryposis in Calves

Congenital and inherited abnormalities can result in birth deformities of newborn animals. Congenital disorders can be due to viral infections of the foetus or to ingestion of toxic plants by the dam at certain stages of gestation. The musculoskeletal system can also be affected by certain congenital neurologic disorders.

Arthrogryposis (joints fixed in abnormal positions) is one such birth defect seen in cattle and sheep. Causes include infections of the dam with Akabane virus and Pestivirus (Mucosal disease in cattle and Border disease in sheep), as well as inherited defects.


Akabane disease   Top
Akabane disease is an arboviral disease of cattle, and less commonly sheep and goats.

The term 'arbovirus' refers to viral diseases that are spread by blood-sucking insects such as mosquitoes and midges. The major carrier is the biting midge, Culicoides brevitarsis. Infection is widespread in tropical Australia and extends through much of New South Wales. The southern limits vary according to the extent to which seasonal conditions favour the distribution of the insect vector. A wet spring and northerly winds favour spread of the midges.

In the more northern regions the infection occurs during all seasons of the year so that most of the cattle in these areas become infected at an early age and are therefore immune before they get in calf. Immunity will persist for long periods of time, which is thought likely to be several years, and possibly for life.

In southern areas of Australia where conditions are not always favourable for the midges, outbreaks occur when previously uninfected cattle are exposed to the virus for the first time during pregnancy. It will often be a problem in first calf heifers as older cows are more likely to be immune to the disease.

Outbreaks usually occur in late winter, indicating that the peak of virus activity and foetal infections occurs during the previous late summer and early autumn period.

The virus does not produce any signs of disease in young or adult animals, however, problems arise when infection occurs for the first time during pregnancy and the virus passes through the placenta to the foetus, causing severe disease of its central nervous system (CNS).

The foetus can be infected at almost any stage of gestation and the nature of the problems produced depends upon the stage of pregnancy when infection takes place.

Infections in very, very early gestation will result in abortion. Bear in mind that cases seen in the early stages of a prolonged outbreak are the result of viral infection in late gestation, and those seen toward the end of the outbreak are due to infection having occurred when the foetus was in an early stage of development. Thus, the periods during which the actual viral infection occurs is usually much shorter than the actual outbreak of congenital defects, which commonly last several months.

Calves born alive will show symptoms of one or two syndromes; arthrogryposis and hydranencephaly.

Arthrogryposis refers to the fixed flexion of one or more joints and wasting of muscles. Severely affected calves are usually born dead and often cause calving difficulties (dystocia), necessitating an embryotomy to deliver them. Arthrogryposis usually occurs at an early stage in an outbreak.

Hydranencephaly occurs when most or all of the cerebrum or forebrain is replaced by fluid. Calves affected as such are often refered to as 'dummy' calves as they are mostly blind, unable to suck, are slow and do not respond to stimuli such as noise. These calves usually appear to be normal at first glance, however, some may have a domed forehead, shortened muzzle or other abnormalities, such as an undershot or overshot jaw.

Hydranencephaly usually occurs towards the end of an outbreak. Calves with both defects occur in the middle of an outbreak.

The usual practice is to humanely destroy affected animals. Prevention of akabane disease could be achieved by ensuring that all females are infected before they are joined. There is no commercially available vaccine.

Arthrogryposis Multiplex (AM)   Top
Arthrogryposis Multiplex is a lethal genetic defect. Affected calves have a bent and twisted spine. They are small and appear thin due to limited muscle development. Legs are often rigid and may be hyperextended (common in rear limbs) or contracted. Affected calves are usually stillborn and calving difficulties due to the rigid limbs is common.

However, unlike with Akabane Disease, calves with the genetic defect AM do not have neurological defects like hydranencephaly.

AM has only recently been discovered in the Angus breed in Australia. The reported incidence is low (only 15 affected calves to date according to Angus Australia). There is now a simple DNA test available in Australia to identify cattle carrying the AM gene. Hair or semen smaples can be submitted to the University of Queensland or Pfizer Animal Genetics.

AM is a simply inherited recessive defect, meaning that two copies of the lethal gene (one from the sire and one from the dam) need to be present for a calf to be affected. Animals with only one copy of the lethal gene (and one copy of the normal form of the gene), that appear normal, are known as 'carriers'. Carriers, will on average, pass the lethal gene to a random half (50%) of their progeny.

The DNA test means that carriers can be identified and prevention of the disease is manageable. Only the most significant animals such as sires and embryo donor dams need to be tested. Stud breeders can identify AM carriers while commercial producers can avoid using AM-carrier or AM-suspect bulls. Angus Australia (AA) will be maintaining a database of animals carrying the AM gene.
 

Dr Julia Adams BVSc - Petalia Resident Vet


 
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