Wesselsbron disease is an acute infection of mainly sheep, cattle, and goats in sub-Saharan Africa due to a flavivirus, Wesselsbron virus, that is transmitted by mosquitoes of the genus Aedes. It is therefore associated with heavy rainfall and flooding, and cases may be concurrent with and resemble Rift Valley fever. Infection is common, but clinical signs are infrequent (though probably underreported). Newborn lambs and goat kids are the most susceptible, and mortality may occur. Infection in adult sheep, cattle, and goats is usually subclinical; however, disease may be severe in sheep with preexisting liver pathology. Abortion occasionally occurs in ewes, together with congenital malformation of the CNS with arthrogryposis of the ovine (and also the bovine) fetus and polyhydramnios in ewes. Incidental spillover occurs to humans, causing a nonfatal, influenza-like disease.
Etiology and Epidemiology of Wesselsbron Disease in Animals
Wesselsbron disease is caused by a flavivirus (family Flaviviridae, genus Flavivirus), an enveloped, positive-sense RNA virus in the yellow fever virus complex. Flavivirus has not been well characterized; however, it has properties typical of a hemagglutinating flavivirus. It has been isolated from vertebrates and arthropods from many sub-Saharan African countries. Evidence of infection has been reported in cattle, sheep, goats, camels, pigs, donkeys, horses, ostriches, and wild ruminants.
The main mosquito vectors of Wesselsbron disease are from the genus Aedes, although Wesselsbron virus has been isolated from other genera, including Culex, Anopheles, and Mansonia. The incidence of infection is likely greater than is generally realized. The high prevalence of antibodies in warmer and moister coastal areas of southern and eastern Africa suggests that domestic herbivores play an important role in maintaining the virus, and activity appears to occur year-round. In drier areas, however, seroprevalence is generally lower, with irregular disease outbreaks occurring, usually in conjunction with Rift Valley fever Rift Valley Fever , when abnormally heavy rains lead to an abundance of floodwater-breeding Aedes mosquitoes.
Humans may become infected with Wesselsbron disease by mosquitoes or by exposure to tissues or fluids from infected animals—for example, when performing necropsies, or in the laboratory.
Clinical Findings of Wesselsbron Disease in Animals
After an incubation period of 1–3 days in newborn lambs, nonspecific clinical signs of illness, including fever, anorexia, listlessness, weakness, and increased respiration, become evident. Death may occur within 72 hours. In calves and adult sheep, goats, and cattle, nonfatal febrile or inapparent infection occurs. Occasional abortion, congenital CNS malformations with arthrogryposis, and polyhydramnios occur in ewes. Neurologic signs secondary to Wesselsbron disease have also been described in horses, and death has been reported in a dog.
Wesselsbron disease and Rift Valley fever share many clinical and pathologic features, and they may occur concurrently in a given area. However, Wesselsbron disease is usually milder, producing much lower mortality, fewer abortions, and less destructive liver lesions. The causative virus appears to be more neurotropic than the Rift Valley fever virus, and severe fetal teratology of the CNS occurs after experimental infection. Use of the live, attenuated vaccine in pregnant ewes may result in early embryonic death, severe teratology of the CNS, arthrogryposis, polyhydramnios, abortion, or fetal mummification. Infection in humans produces mild to severe, nonfatal influenza-like clinical signs.
In newborn and young animals, a moderate to severe icterus and hepatomegaly occur with Wesselsbron disease; the liver is yellowish to orange brown. Petechiae and ecchymoses are commonly found in the mucosa of the abomasum, the contents of which are chocolate-brown in color. Histopathology reveals mild to extensive necrosis of the liver. Lesions in adult animals are usually much milder.
Diagnosis of Wesselsbron Disease in Animals
Outbreaks of abortion and neonatal mortality associated with heavy rainfall and flooding
Virus isolation to distinguish from Rift Valley fever
Clinical signs and epidemiology, together with a relatively high mortality in lambs, are an indication of Wesselsbron disease. However, it should be distinguished from Rift Valley fever. The virus can be isolated from almost all organs of lambs that have died during the clinical stage of the disease. Intracerebral inoculation of newborn mice is the best method of isolation. The virus can be distinguished from the Rift Valley fever virus by intraperitoneal inoculation of weaned mice: Wesselsbron disease virus will not kill the mice; Rift Valley fever virus will. Virus neutralization testing can confirm its identity.
Serodiagnosis has been based on hemagglutination inhibition, complement fixation, ELISA, virus neutralization and RT-PCR. 1 References Wesselsbron disease is an acute, mosquito-borne viral infection of mainly sheep, cattle, and goats in sub-Saharan Africa that is associated with rainfall and flooding. Infection is common, but... read more Hemagglutination inhibition testing is limited by its cross-reactivity with other flaviviruses.
Control of Wesselsbron Disease in Animals
Vaccination of non-pregnant animals
A live, attenuated Wesselsbron virus vaccine may be administered to nonpregnant animals, and it confers lifelong immunity. In the past, however, injudicious use of the vaccine in pregnant ewes resulted in severe economic losses due to abortion and fetal malformations. Attempts to control mosquito vectors are of little value as a preventive measure; however, housing animals indoors at night and moving them away from low-lying wetland areas may decrease exposure to vectors.
In domestic ruminants, particularly sheep, abortions and neonatal mortality due to Wesselsbron disease are associated with heavy rainfall and flooding.
Wesselsbron disease is easily confused with, and may occur together with, Rift Valley fever.
Wesselsbron disease is a zoonotic infection, so care should be taken to prevent exposure to tissues and fluids from affected animals.
For More Information
Swanepoel R, Coetzer JAW. Rift Valley fever. Anipedia.
Smith DR. Waiting in the wings: the potential of mosquito transmitted flaviviruses to emerge. Crit Rev Microbiol. 2017;43(4):405–422.