Wesselsbron Virus

Also known as

Wesselsbron disease

Description

Wesselsbron (WSL) virus is an arthropod-borne virus that causes Wesselsbron disease in cattle, sheep, goats, camels, pigs, donkeys, horses, ostriches, and wild ruminants--with occasional incidental infection in humans (a nonfatal, influenza-like disease in humans).

Wesselsbron disease is an acute, flavivirus infection of mainly sheep, cattle, and goats in sub-Saharan Africa, including the island of Madagascar. The virus was found in certain mosquitoes in Thailand in 1966, but there is no recent evidence for its presence outside Africa.

Infection is common, but clinical disease is infrequent—and possibly under-reported. Newborn lambs and goat kids are most susceptible, with some mortalities. Infection in adult sheep, cattle, and goats is usually subclinical, but may be severe in sheep with preexisting liver damage.

Occasional abortion occurs in ewes, together with congenital malformation involving the central nervous system. There may be malformation of joints in the fetuses of sheep and cattle and hydrops amnii (too much fluid surrounding the fetus) in ewes.

This virus seems to be widespread in Africa, where it can cause reproductive losses and mortalities in newborns in small ruminants. The effects of Wesselsbron virus on other animal species are still unknown; however, occasional reproductive losses have been seen in experimentally-infected cattle. A few clinical cases have been described in humans, often associated with exposure in the laboratory or during necropsies.

Overall effect in animals and humans is hard to evaluate since Wesselsbron disease resembles other illnesses such as Rift Valley fever. Wesselsbron virus might have the potential to become established outside Africa, similarly to other flaviviruses such as West Nile virus and Zika virus.

After an incubation period of 1–3 days in newborn lambs, nonspecific signs of illness including fever, lack of appetite, listlessness, weakness, and increased respiration become evident. Death may occur within 72 hours. In calves and adult sheep, goats, and cattle, nonfatal fever or infection without signs may occur.

In newborn and young animals, a moderate to severe jaundice and enlargement of the liver are common with Wesselsbron disease; the liver is yellowish to orange brown.

The clinical signs and high mortality in lambs can be an indication of Wesselsbron disease. It must be distinguished from Rift Valley fever, though the two diseases may occur together. The virus can be isolated from almost all organs of lambs that die during the clinical stage of the disease.

Signs

• Abortion
• Abnormalities in newborns
• Fever
• Lack of appetite
• Listlessness
• Weakness
• iIncreased respiration

Cause

Wesselsbron disease is caused by Wesselsbron virus, a member of the yellow fever group of the genus Flavivirus, family Flaviviridae. Wesselsbron disease has been described mainly in sheep and goats, but experimental infections suggest that this virus can also affect cattle. Experimentally infected horses and pigs had few or no clinical signs.

Antibodies to Wesselsbron virus have been found in a number of domesticated animals, including horses, donkeys, pigs, camels and dogs, and in wild zebras, wild ruminants and lemurs.

Wesselsbron virus is transmitted by several kinds of mosquitoes but there is also at least one report of virus isolation from an ixodid tick.

There is no evidence that Wesselsbron virus is transmitted between ruminants or other animals.

Humans have been infected by handling contaminated material and virus cultures in the laboratory, and body tissues during necropsies. Transmission in mosquito bites may account for some cases. There is no evidence for person-to-person transmission. The Wesselsbron virus is relatively fragile, and unlikely to remain infectious for more than a few days in carcasses or animal products.

Laboratory studies suggest that Wesselsbron virus can cause occasional reproductive losses in pregnant cattle. Abortions, congenital defects of the brain, and weak, infected calves, which died soon after birth, have been described. However, most calves born to experimentally infected pregnant cattle seem to be healthy and uninfected.

Some pregnant and non-pregnant cattle, including newborn calves, developed a nonspecific, mild febrile illness after being inoculated with Wesselsbron virus. Other non-pregnant cattle only had a fever, with no clinical signs.

Ruminants in some parts of Africa are commonly infected with Wesselsbron virus, with some surveys reporting exposure rates as high as 50% in cattle, sheep and goats. However, some of these studies might have also measured cross-reactive antibodies to other flaviviruses.

Infections seem to occur year-round in some regions. In others, outbreaks are seen mainly when heavy rains favor the replication of mosquitoes that breed in floodwaters.

Most infections in non-pregnant adult ruminants are thought to be subclinical or mild. A limited number of field studies and experiments in cattle suggest that, while reproductive losses are possible in this species, they are probably uncommon.

Use of the attenuated vaccine in pregnant ewes may result in early embryonic death, severe congenital abnormalities, hydrops amnii, abortion, or fetal mummification.

Based on the distribution of certain mosquitoes associated with Wesselsbron disease, the incidence of infection is likely greater than people realize.

The high prevalence of antibodies in warmer and moister coastal areas of southern and eastern Africa suggests that domestic herbivores may play a significant role in maintenance of the virus, and virus activity appears to occur year round.

In drier areas, however, prevalence is generally lower, with irregular disease outbreaks, usually in conjunction with Rift Valley fever when abnormally heavy rains lead to an abundance of floodwater-breeding mosquitoes. People may become infected by mosquitoes or by handling organs from infected animals.

Prevention

Any cases of this disease should be reported. Veterinarians who encounter or suspect Wesselsbron disease should follow national and/or local guidelines for disease reporting. In the U.S., state or federal veterinary authorities should be informed immediately of any disease outbreaks caused by pathogens not already present in the U.S.

Avoid moving pregnant and newborn small ruminants into endemic areas, especially if they have not been exposed previously to Wesselsbron virus. In the past, an attenuated vaccine for sheep was available in some areas.

If given to pregnant ewes, this vaccine could cause reproductive losses similar to field strains of Wesselsbron virus. This vaccine has been discontinued.

Although vector control is theoretically possible, it is not considered practical for protecting livestock.

Production of an attenuated vaccine was discontinued shortly before 2000. Injudicious use of the vaccine in pregnant ewes resulted in severe economic losses due to abortion and fetal malformations.

Treatment

There is no treatment for Wesselsbron disease, except supportive care.