Akabane Orthobunyavirus

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Also known as

Enzootic bovine arthrogryposis, hydranencaphaly

Description

Akabane orthobunyavirus is an insect-transmitted virus that can cause deformities in newborn calves. Infection with this virus does not affect the cow, but if she is exposed to the infection during pregnancy it can result in congenital (present at birth) abnormalities of the central nervous system, limbs and spinal column in the fetus.

The virus is found in Australia, Israel, Japan, Korea and Kenya where it is most commonly spread by biting midges (Culicoides species) and possibly other species of biting insects. It has never been reported in North America.

Infection mainly causes congenital arthrogryposis (deformities of the limbs and vertebral column, which include fused and malformed joints) and/or hydranencephaly (condition in which the brain’s cerebral hemispheres are absent to varying degrees and the resulting empty cranial cavity is filled with cerebrospinal fluid).

Outbreaks (multiple cases of deformities in newborns) are often associated with a high population of midges during a specific stage in gestation in the cow herd. A distant viral cousin, Cache Valley Virus, is present in North American sheep flocks and causes similar disease. The virus can sometimes be found in bison and goats.

Signs

  • Malformation of joints, brain, spine and jaw
  • Abortion - if damage to the fetus is severe
  • Premature birth
  • Stillbirth
  • Incoordination in the newborn
  • Encephalitis in the newborn

Causes

Spread by biting midges, the disease occurs when cattle are pregnant during the time these insects are plentiful. Infections often cause abortion and premature birth in autumn and congenital defects like malformed/crooked joints and hydranencephaly in mid-winter after the fetus is closer to full term.

Calves with arthrogryposis were generally infected as older fetuses (5 to 6 months’ gestation), while fetuses with just the brain affected were usually infected earlier (3 to 4 months’ gestation). Those in the middle stage of gestation may have both conditions.

Even though the calves at birth may be small and underweight, they often require assistance to be born because of the malformed, immobile joints that hinder straightening of the legs to enter the birth canal, creating a very difficult birth. Though these calves are small, they may be fully mature (full term, with fully-erupted teeth, full hair coat and complete hoof development). Because of the rigid joints and wasted muscles, these calves (if born alive) are usually unable to stand and walk.

The calves with hydranencephaly may also have trouble getting up and walking, and they lack intelligence and may be blind. They may suckle if a teat or bottle is put into their mouth, but have no ability to seek the dam.

Herds may experience outbreaks with multiple cases and then the disease might disappear for 5 to 10 years until there is another combination of susceptible cattle population and large insect population.

There are several strains of the virus and some closely related viruses that cause similar disease. A European outbreak caused by a new Orthobunyavirus began in 2011. That virus was first isolated near Schmallenberg in Germany.

Virus isolation from tissue samples sent to a diagnostic laboratory can be attempted with immunofluorescence (a method that uses antibodies chemically labeled with fluorescent dyes to visualize molecules under a light microscope) or PCR (polymerase chain reaction) tests that detect the genetic material of a virus. Diagnosis is generally made by simply examining the calf or fetus, however.

At necropsy there is severe muscle atrophy (wasting), and contracted tendons hold the joints firmly immobile. The joint fixation can be released by cutting the surrounding tendons. In calves with hydranencephaly, the cerebral hemispheres of the brain are missing and the vacant space is filled with fluid.

Prevention

Vaccination can be used to control the spread of this disease. A killed vaccine has been produced and proven to be effective. Control of the insect vectors is advisable if possible, but often difficult.

Treatment

No treatment is recommended because these calves cannot survive. There is no treatment for affected calves. Humane euthanasia is advised if the calf is still alive at birth.

About the Author

EquiMed Staff

EquiMed staff writers team up to provide articles that require periodic updates based on evolving methods of equine healthcare. Compendia articles, core healthcare topics and more are written and updated as a group effort. Our review process includes an important veterinarian review, helping to assure the content is consistent with the latest understanding from a medical professional.

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