Bluetongue

Also known as

Blue tongue and bluetongue virus (BTV)

Description

Bluetongue is a disease of ruminant livestock caused by a virus. Bluetongue is generally more common (and more serious) in sheep than in cattle. The virus is transmitted to livestock by several species of blood-sucking gnats (genus Culicoides). These small insects are also called biting midges, no-see-ums, sand flies or punkies.

The bluetongue virus was one of the first recognized insect-borne viruses; the disease was first described in South Africa in 1902. Bluetongue virus exists throughout tropical, subtropical and temperate regions of the world wherever these species of Culicoides occur and are capable of transmitting it to animals. The various virus serotypes occur in Africa, Asia, South America, North America, the Middle East, India, and Australia.

Several serotypes have been found in the southern and western part of the U.S. where they are transmitted by Culicoides sonorensis. Northern states and Canada do not support Culicoides sonorensis populations, and are considered free of the virus.

Although cattle can serve as reservoirs for BTV, clinical disease (with signs of illness) in cattle is rare. When encountered in cattle, clinical disease is usually seen in fewer than 5% of the infected animals, and there is rarely any mortality.

Cattle develop a prolonged viremia (presence of virus in the blood) lasting several weeks. This enables the virus to survive in cattle during cold winters in the absence of insects, or at other times when insect vector populations are small or absent. Cattle can therefore introduce the virus into different areas.

If a cow is infected in early pregnancy, this may lead to embryonic death. Reproductive effects include abortion, infertility, mummification of the fetus (it dies but is not aborted, remaining in the uterus for an indefinite period), or birth of a full-term dead calf.

Congenital defects occur if the virus damages but doesn’t kill the fetus; the fetus is most vulnerable if the virus is encountered at 60 to 140 days’ gestation, resulting in weak or dummy calves, deformed legs/feet, blindness, white eyes, overshot lower jaw (protruding farther than the upper jaw) and persistent gum covering over the front teeth.

If the cow is infected in mid-gestation the fetus may end up persistently infected (similar to what happens with BVD virus). The fetus is infected with that strain of the virus (which can occur when the fetus is past 140 days gestation), but shows no evidence of disease until re-infected with that same strain or another strain. Resulting illness may be moderately to extremely serious.

Fetuses infected later in gestation usually survive and are not persistently infected, and develop antibodies in utero to the specific infecting virus serotype.

Bluetongue outbreaks can result in millions of dollars in losses due to the impact on livestock health (especially in sheep) and the loss of markets for livestock due to regulatory restrictions on animal movement to limit spread of the disease.

The greatest impact on cattle industries is due to trade barriers. Bluetongue-positive cattle cannot be moved from outbreak areas (even if the disease is only occurring in sheep and not being seen in cattle) because of the prolonged viremia. Cattle can infect the gnats that bite them and spread the virus for up to a month after infection.

Southern regions of the U. S. have been endemic for certain BTV serotypes for many years though the disease itself has been rare in cattle. The biggest impact is market losses due to restrictions on international cattle trade in livestock and semen/embryos to regions considered bluetongue-free like some countries in the European Union.

Many countries, including Canada, require a negative blood test for bluetongue and some require a quarantine period in conjunction with the test to make sure an animal isn’t developing the disease before it can enter. Some countries won’t allow animals from areas or countries where bluetongue has been diagnosed, even if the animal has a negative test--since some may still carry the virus.

Bluetongue can be diagnosed based on characteristic lesions, but the diagnosis should always be confirmed by virus isolation and identification using any of the available methods for detecting BTV in blood. A rise in BTV antibody may also be used to confirm a diagnosis. A blood sample taken from a newborn calf before it nurses can determine if the calf was affected in utero.

Signs of the disease include ulcers in the mouth, nose, trachea, esophagus and rumen. Capillaries are affected, resulting reduced blood supply to tissues. Hoof horn may slough off. Skin inflammation, hair loss, sloughing skin, oozing crusts on skin or muzzle, or ulcers on udder or teats may occur.

The sick animal may have fast shallow respiration, excessive salivation (drooling long, stringy strands of saliva) and a swollen, protruding tongue--bluish from lack of oxygen in the tissues. Fever may be as high as 106 degrees F.

Signs

• Ulcers in mouth, nose, trachea, esophagus and rumen
• Lameness
• Muscle stiffness
• Hoof cracks
• Inflammation at coronary band
• Loss of hoof horn
• Skin inflammation and sloughing skin
• Hair loss
• Oozing crust on skin or muzzle
• Ulcers on udder or teats
• Fast, shallow respiration
• Drooling
• Swollen protruding bluish tongue
• High fever.

Causes

There are at least 25 strains of the virus, but only 6 have been identified in the U.S. Bluetongue is usually seen from mid-summer until frost but in areas with mild winters where gnats are active year-round the disease can appear any time.

The female gnat requires a blood meal before she can lay eggs. Different Culicoides species prefer different habitats to lay eggs. Culicoides sonorensis prefers water with high manure content for subsequent larval development. Some African Culicoides vectors lay eggs directly in the feces of ruminants. The eggs are always laid near ruminant animals.

Gnats become infected while feeding on infected cattle, then transfer the disease to other cattle or sheep as they fly from animal to animal to suck blood. Bluetongue can also be transmitted through infected semen, and spread by bloodsucking lice or certain types of soft tick. There may be mechanical spread via biting flies and hypodermic needles used on multiple animals.

Prevention

There is a vaccine for sheep and in some parts of the world for cattle. Inactivated BTV vaccines are efficacious and safe but relatively expensive. The modified live vaccines are efficacious and cheap but often unsafe. They also sometimes spread the disease. Both types of vaccine interfere with testing because the antibodies from vaccine cannot be differentiated from the antibodies present in infected animals. Protection is also limited to the serotype in the vaccine.

To be effective the vaccine must be specific for the serotype being transmitted to the animals. Modified live (attenuated) vaccines available for several different serotypes have been widely used in South Africa.

Unfortunately, though effective and cheap to produce these vaccines produce viremia in vaccinated animals that can infect Culicoides, and there is evidence that the modified virus can revert to virulence. These vaccines can also cause reproductive problems in sheep. Killed vaccines (consisting of completely inactivated virus) are available.

Several experimental BTV vaccines show promising improvements, but none of these next generation vaccines have been licensed yet. Some of the difficulties in creating a safe vaccine may be overcome in the future with use of DNA vaccines.

Management to prevent this disease includes trying to control gnats (draining stagnant ponds and standing water, use of insecticides, periodic spraying of animals) or housing cattle in early morning and evening when gnats are active, or using repellents to protect animals from attack, but none of these methods have been very effective in halting outbreaks of bluetongue.

One thing a person can do, however, is never use the same hypodermic needles on multiple animals in regions where bluetongue exists, and keep equipment clean between animals when castrating, dehorning, tagging, etc. to avoid blood contact.

Treatment

As with the other arthropod-borne viruses, there is no effective treatment for bluetongue disease other than treating the clinical signs in an attempt to reduce the severity.

Good nursing care (shelter and good feed, if the animal will eat) and washing the affected areas with mild disinfectant solution may provide some relief from discomfort, but there is no effective treatment to halt the virus.