Bovine ephemeral fever

Etiology

• Spread is largely independent of cattle movement, and transmission doesn’t occur through contact with infected animals or their saliva or ocular discharge. This is in contrast to similar diseases.
• Various species of culicoides midge and mosquitos have been demonstrated to be infected with the virus and outbreaks can often be mapped in a wave like front, consistent with a disease with a wind borne vector spread.
• However, recent studies have shown the need for intravenous inoculation to instigate disease in experimentally infected cattle suggesting the vector to be a capillary feeder (mosquito) rather than a pool feeder (midge).

Predisposing factors

General

• Naïve animals and young growing animals in infected areas will be most susceptible. Acquired immunity is not life-long, so previously infected animals may be susceptible dependent upon when the last disease incursion occurred.

Specific

• Appropriate climatic conditions including rainfall to promote vector multiplication and weather fronts ensuring vector translocation.
• Naïve recently purchased animals or young, unvaccinated or naïve animals six months of age or older, in an infected area.

Pathophysiology

• The virus causes a widespread vasculitis affecting the endothelium of small vessels of the synovial membranes, tendon sheaths, muscles, fascia, and skin resulting in widespread fibrinous effusions in the joints , pleura, pericardium and peritoneal cavities.
• Hypocalcemia is a common finding and is believed to be the reason for recumbency in many cases Hypocalcemia and hypophosphatemia: overview.

Timecourse

• Clinical signs usually manifest within 24 hours of infection with a rapid clinical course to apparent recovery in most cases, within 72 hours.
• Production performance in terms of milk production and in some cases, lameness may take a number of weeks to resolve fully.

Epidemiology

• As infected vectors pass through an area, all naïve animals are susceptible.
• In more arid regions of the world, epizootics have been associated with attendance at oasis or watering holes, where conditions are suited to vector multiplication and survival and large numbers of animals are found to congregate.
• It has been reported that previously infected animals are resistant for at least 422 days so depending upon future challenge, some previously infected animals may succumb to clinical disease.