Bovine Ephemeral Fever: Understanding the “3-Day Sickness” in Cattle

Etiology

Bovine Ephemeral Fever (BEF) is triggered by a virus classified under the Ephemerovirus genus, part of the Rhabdoviridae family. This virus is primarily transmitted through arthropods and exists in several antigenically diverse strains. In areas such as Australia, related but harmless ephemeroviruses circulate in the same ecological zones.

Epidemiology

Geographic Distribution

 Outbreaks tend to occur seasonally, often lasting months, and follow wind patterns. Infection rates in affected herds range from 25% to 45%, though highly susceptible populations can see morbidity approach 100%. In stable endemic areas, only 5–10% may show signs. Mortality is generally low (1%), but may reach 10% in severe cases.

Transmission and Vectors

Infected cattle and blood-feeding insects are the main sources of infection. Transmission is largely vector-borne, with Culex annulirostris, Aedes, Anopheles, and Culicoides species implicated. In Australia, mosquitoes—not midges—are the primary vectors. No confirmed reservoir host exists, though changes in disease patterns suggest the virus has adapted to local insects. The disease does not spread via direct contact, secretions, semen, or in utero.

Experimental Infection

BEF can be experimentally transmitted via IV injection of infected blood. Viremia lasts up to 3 days, with no chronic carrier state.

Environmental Factors

Outbreaks peak during warm months when vector populations rise. Periods of low activity are followed by re-emergence when population immunity declines. Other arboviruses (e.g., Akabane, Aino) may offer short-term cross-protection.

Host Susceptibility

Only cattle are naturally affected, though antibodies are found in wild ruminants. All ages are susceptible, but calves under 3–6 months usually don’t show signs. High-yield dairy cows and bulls are more severely affected. Breed differences are minimal.

Immunity

Natural infection provides solid immunity for 1–2 years against the same strain, but less protection against different strains.

Economic Impact

BEF significantly affects dairy and beef productivity. Milk drops of 30–70% are common, especially in high-yielding cows. Recovery is slow, with lasting reductions in yield. Fertility issues, increased mastitis, and occasional deaths from complications also contribute to losses. In the draught cattle and export industries, the disease can cause serious economic disruption.

Pathogenesis of Bovine Ephemeral Fever (BEF)

• Entry & Replication:

  • Infection requires intravenous (IV) transmission.

  • Virus replicates primarily in the vascular system.

  • Alter host cell biology to enhance viral entry and replication.

• Cellular Mechanisms Activated:

  • Activates intracellular pathways to upregulate:

    • Clathrin and Dynamin 2 (promote endocytosis).

    • COX-2-mediated E-prostanoid receptors (EP2 & EP4).

  • These changes enhance clathrin-mediated endocytosis of the virus.

• Incubation & Clinical Signs:

  • Incubation period: 2–10 days.

  • Biphasic fever with peaks 12–24 hours apart.

  • The fever lasts about 2 days.

• Key Clinical Symptoms:

  • Increased respiratory rate (tachypnea), dyspnea.

  • Muscle tremors, stiffness in limbs.

  • Pain and difficulty in movement.

• Inflammatory Response:

  • Generalized vasculitis and thrombosis.

  • Serofibrinous inflammation in serous and synovial cavities.

  • Increased endothelial permeability.

• Virus Localization:

  • Found in neutrophils, plasma, and serosal/synovial fluids.

  • Also in mesothelial cells of synovial membranes and epicardium.

• Pathological Mechanism:

  • Clinical signs are due to:

    • Release of inflammatory mediators.

    • Development of secondary hypocalcemia.

Clinical Signs of Bovine Ephemeral Fever (BEF)

• Age Susceptibility:

  • Young calves (under 3–6 months) are generally unaffected and show no visible symptoms.

  • Adult cattle, especially bulls and high-producing dairy cows, are more prone to severe illness.

• Incubation Period:

  • Symptoms typically emerge 2 to 4 days post-infection, though in some cases, onset may be delayed up to 10 days.

• Early Clinical Symptoms:

  • Abrupt rise in body temperature (105°–106°F / 40.5°–41°C), often fluctuating or higher in the evening.

  • Noticeable drop in feed intake and sudden decline in milk production.

  • Digestive irregularities, such as constipation or diarrhea, may occur.

  • Elevated breathing and heart rates, accompanied by nasal mucus and watery eyes.

• Muscular and Postural Changes:

  • Muscle tremors, stiffness, and general fatigue usually develop on the second day.

  • Limb weakness is common, often leading to reluctance to move or stand.

  • Swelling may appear on the shoulders, neck, or back.

  • Cattle often adopt a hunched stance, resembling cases of acute laminitis.

• Recumbency Patterns:

  • Many affected animals lie down during peak illness, assuming a posture similar to milk fever (with hind limbs stretched and head turned back).

  • Some animals may lie on their sides and remain down for several days.

• Less Common Manifestations:

  • Subcutaneous or pulmonary air accumulation (emphysema), possibly related to nutritional imbalances such as selenium deficiency.

  • In rare cases, animals experience a second illness phase 2–3 weeks after recovery.

• Recovery Timeline:

  • While fever and appetite improve quickly, some animals may continue to show weakness or lameness for a few extra days.

• Possible Complications:

  • Risk of pneumonia due to inhalation of ruminal contents or drenches.

  • Prolonged recumbency in severe cases may lead to culling or death, especially if animals are exposed to cold or wet conditions.

  • Abortion

• Mild Infections:

  • Some cattle show only a mild fever and reduced appetite with no serious complications.

Differential diagnosis by

• Botulism
• Parturient paresis
• Pneumonia
• Traumatic reticulitis

Treatment of Bovine Ephemeral Fever

• Supportive Therapy:

  • Use of nonsteroidal anti-inflammatory drugs (NSAIDs) like:

    • Flunixin meglumine (2.2 mg/kg/day, IV or IM)

    • Ketoprofen (3 mg/kg/day, IM)

  • These reduce pain, stiffness, and appetite loss but do not affect viral progression.

  • Typically administered for 3 consecutive days.

• Limited Effect on Respiratory Signs:

  • NSAIDs offer minimal relief for respiratory symptoms associated with BEF.

• Phenylbutazone:

  • May provide good symptom relief, but is impractical due to frequent dosing and drug residue risks.

  • Prohibited in cattle in many countries.

• Calcium Therapy:

  • IV or SC calcium borogluconate may benefit cows with hypocalcemia-like signs.

  • Helps recumbent animals to stand and recover when used appropriately.

• Nursing Care

Control and Prevention of BEF

• Movement Control:

  • Restricting cattle movement from infected zones helps contain outbreaks.

• Vaccination – The Primary Tool:

  • Live attenuated vaccines

    • Derived from a virus grown in tissue culture or mouse brain.

    • Adjuvanted with Freund’s incomplete or Quil A.

    • Require two doses for up to 12 months of protection.

    • Credited with reducing outbreaks (e.g., in Japan).

• Challenges with Live Vaccines:

  • Costly to manufacture and short shelf-life.

  • Risk of mutation back to virulence due to the unstable nature of RNA viruses.

  • Possible contamination during production.

• Inactivated Vaccines:

  • Safer alternative (e.g., formalin-killed preparations).

  • Require at least 3 doses and frequent boosters for sustained immunity.

  • Provide a more stable and non-replicating option.

• Monitoring Immunity:

  • Immunity levels can be tracked via:

    • Blocking ELISA

    • Virus neutralization tests

  • Protection correlates with high antibody titers.