Disease Description

Enzootic Bovine Leucosis (EBL) is a malignant neoplastic disease of the lymphatic system. It is caused by a Type C oncovirus and results in leucosis and sarcomas. Cattle are the only animals known to be naturally infected. Leucosis in sheep caused by a Type C oncovirus has been reported in Germany, but not in Australia.  There is no effective treatment for the disease.

EBL was first reported in Queensland in 1966. EBL is present in the Northern Territory in beef cattle but has not been reported in these in other states. There have been reports of sporadic lymphosarcoma but the first verified case in South Australia was in December 1993. It was an Australian Illawarra dairy cow and further testing in this breed indicated a high prevalence.

There is only one serological type of bovine leucosis virus and all European, American and Australian strains have been found to be the same. The virus isolated from sheep is closely related to or identical to the virus isolated from cattle.

EBL is subject to a test and slaughter program in some EU countries. Queensland adopted a voluntary test and slaughter program in known infected dairy herds in conjunction with an EBL accreditation scheme. The other states have now followed this and there is an agreed national policy with regard to protocols and herd status classifications.

Clinical Picture

The juvenile or calf form of lymphosarcoma is seen mostly in animals up to six months of age, but occasionally calves are congenitally infected. The most frequent abnormalities observed are swollen superficial lymph nodes. The swellings are generally symmetrical, with the nodes being firm, smooth, mobile, and painless. There is also enlargement of lymph nodes which are not normally palpable, such as the parotid, deep cervical and popliteal nodes. Enlargement of the medial iliac node (detected by rectal palpation) is relatively common. Ulceration of the skin over the lymph nodes is not seen. Heart and respiratory rates are increased. Coughing and harsh respiratory sounds are common. Occasionally animals may have a fever, diarrhoea, ruminal tympany or liver enlargement.

The thymic form is mostly seen in animals 6 months to two years of age. Loss of condition and/or the development of a swelling in the neck (pre-sternal area) is usually seen. Palpation reveals a firm mass that may or may not be surrounded by oedema. Distension of the jugular veins is usually present but pulsation is rare. Increased heart and respiratory rates are occasionally seen, as well as coughing, straining and a temperature rise.

The skin form is relatively rare and occurs in cattle 1.5 to 3 years old. There are numerous tumours in various parts of the skin, about 5-10 cm in diameter. Some lymph nodes and internal organs may also be involved.

Occasionally, juvenile and cutaneous forms regress and disappear but they are usually fatal. Cases of juvenile and thymic forms of lymphosarcoma are usually sporadic lymphosarcoma and not EBL.

The adult form of lymphosarcoma occurs in animals over 3 years of age. Animals suddenly become ill, lose condition over a period of weeks or months before death occurs. Occasionally animals die suddenly due to rupture of a tumorous spleen or abomasum.

The clinical signs vary enormously, depending on which organ system is most affected. Clinical syndromes seen include paresis due to a tumour in the spinal cord, heart disease due to a tumour in the heart, respiratory distress due to a tumour in the retropharyngeal nodes. There is usually an enlargement of various superficial and internal lymph nodes in addition to the main presenting clinical signs.

Generally there is a persistent lymphocytosis prior to the development of lymphosarcoma. It usually commences when the animal is at least 2 years of age and persists until death. Provided only persistent lymphocytosis is present, production performance is not affected.

Diagnosis

The definitive diagnosis of EBL is based on laboratory tests. The Agar Gel Immunodiffusion Test (AGDT) has generally been replaced by an anitbody ELISA test which can be performed on serum or milk. Negative animals usually are not infected with the virus. A positive test result is usually followed up by a tumour biopsy and histological examination. In addition, virus isolation by the cultivation of lymphocytes from serologically positive animals can be undertaken.

Transmission

The virus can be transmitted in the uterus so that the calf is born congenitally infected. The virus can also be transmitted to young calves via milk.

Infection can be spread by contact, although the mechanism is not fully understood. Calves and sheep have been experimentally infected via the oral route.  Ear-tagging or immunising needles are another means of mechanical spread.

The virus can be spread mechanically by inoculation with shared needles during routine vaccination or treatment and not disinfecting dehorning equipment between animals. The virus has also been spread by the inoculation of infected blood products during vaccination against babesiosis. The possibility of mechanical transfer during pregnancy testing is still being debated.

PIRSA's Role in Prevention and Control

Primary Industries and Resources South Australia (PIRSA) in conjunction with the South Australian Dairy Industry and with funds from the Cattle Industry Fund, is currently undertaking the South Australian EBL eradication program, in South Australian dairy herd based on a nationally agreed testing regime. Screening is done using milk collected as part of the herd testing program. This is a bulk sample and gives a general idea of a herd’s status. If it is positive, then each animal over 6 months of age in the herd is tested to identify the infected animal(s).

Large dairy herds which contain greater than 250 cows at any stage of the year are now subjected to a more intensive bulk milk testing program.  This testing also occurs in association with the herd testing program.  At the 30 June 2003 there were no dairy herds that were under investigation for EBL.

Abattoir traces from beef and dairy herds with gymphosarcomas have been investigated by complete testing of the herds.

Importation of dairy cattle from interstate will need to be accompanied by a completed Form 1 Health Certificate.  A requirement of the Form 1 Health Certificate is that all dairy cattle must be tested negative within 30 days prior to entry and not from a herd whose EBL status is Nonassessed (NA), Infected (IN), Suspect (SU) or Provisionally Clear (PC).

Treatment

There is no effective treatment.

Contacts

The majority of rural veterinary practices have an accredited veterinarian who will be able to provide information on Enzootic Bovine Leucosis.  The PIRSA EBL Project Manager is Dr Peter Nosworthy, SE Headquarters, Struan.

Telephone: 08 8762 9140 or email: nosworthy.peter@saugov.sa.gov.au