Ephemeral fever in animals

EPHEMERAL FEVER OR THREE DAY FEVER
  • Ephemeral fever is an insect-transmitted, noncontagious, viral disease of cattle and buffalo. Inapparent infections can develop in goats also.

Etiology

  • Ephemeral fever virus is classified as a Rhabdovirus (single-stranded, negative sense RNA).
  • It is best isolated from infected cattle by inoculation of mosquito ( Aedes albopictus ) cell cultures with defibrinated blood, followed by transfer to baby hamster kidney (BHK-21) or monkey kidney (Vero) cell cultures after 15 days.
  • The virus can be transmitted from infected to susceptible cattle by IV inoculation; as little as 0.005 mL of blood collected during the febrile stage is infective.
  • Although the virus has been recovered from several Culicoides species and from Anopheline and Culicine mosquito species collected in the field, the identity of the major vectors has not been proved. Transmission by contact or fomites does not occur, and the virus does not appear to persist in recovered cattle.
  • Most recovered cattle have a lifelong immunity.
  • The prevalence, geographic range, and severity of the disease vary from year to year, and epidemics occur periodically.
  • During epidemics, onset is rapid; many animals are affected within days or 2-3 wk.
  • Ephemeral fever is most prevalent in the wet season in the tropics and in summer to early autumn in the subtropics or temperate regions (when conditions favor multiplication of biting insects); it disappears abruptly in winter.
  • Morbidity may be as high as 80%; overall mortality is usually 1-2%, although it can be higher in lactating cows, bulls in good condition, and fat steers (10-30%).

Clinical Findings

  • Signs, which occur suddenly and vary in severity, include biphasic to polyphasic fever, shivering, inappetence, lacrimation, serous nasal discharge, drooling, dyspnea, atony of forestomachs, depression, stiffness and lameness, and a sudden decrease in milk yield.
  • Affected cattle may become recumbent and paralyzed for 8 hr to >1 wk.
  • After recovery, milk production often fails to return to normal levels until the next lactation. Abortion, with total loss of the season’s lactation, occurs in ~5% of cows pregnant for 8-9 mo.
  • The virus does not appear to cross the placenta or affect the fertility of the cow.
  • Bulls, heavy cattle, and high-lactating dairy cows are the most severely affected, but spontaneous recovery usually occurs within a few days.
  • More insidious losses may result from decreased muscle mass and lowered fertility in bulls.

Diagnosis

  • Diagnosis is based almost entirely on clinical signs in an epidemic.
  • All clinical cases have a neutrophilia with the presence of many immature forms, although this is not pathognomonic.
  • Laboratory confirmation is by serology, rarely by virus isolation.
  • Whole blood should be collected from sick and apparently healthy cattle in affected herds.
  • Samples must be sufficient to provide 2 air-dried blood smears, 5 mL in anticoagulant (not EDTA), and 20 mL for serum.
  • Isolated viruses are identified by neutralization tests using specific ephemeral fever virus antisera and by ELISA using specific monoclonal antibodies.
  • The neutralization test and the blocking ELISA are recommended for antibody detection and give similar results.
  • A 4-fold rise in antibody titer between paired sera collected 2-3 wk apart confirms infection.

Treatment and Control

  • Complete rest is the most effective treatment, and recovering animals should not be stressed or worked because relapse is likely.
  • Anti-inflammatory drugs given early and in repeated doses for 2-3 days are effective.
  • Oral dosing should be avoided unless the swallowing reflex is functional. Signs of hypocalcemia are treated as for milk fever .
  • Antibiotic treatment to control secondary infection and rehydration with isotonic fluids may be warranted.
Last modified: Wednesday, 22 February 2012, 4:49 AM