Bluetongue virus (BTV) is a non-communicable, insect-borne disease that is seasonally driven by heat, moisture and frost. Biting mosquitoes and midges transmit the virus to ruminants such as sheep and, to a lesser extent, to goats and cattle.
BTV usually emerges in September and reaches its peak from January to early March. The vectors die with the first frost and in the colder winter months.
Transmission of BTV
Dr Steve Cockcroft, a veterinarian from Adelaide in the Eastern Cape, believes prevention is always better than cure. He says the virus is transmitted by vectors that feed mainly on the exposed areas of sheep, such as the ears, face, around the vulva and udder, and the front axilla.
It is transmitted to other animals when insects that feed on sheep move on to cattle and goats, turning these animals into carriers or hosts.
The virus attacks the blood vessel walls, and the damage causes plasma to leak. “Fluid starts accumulating in different organs, such as the lungs, or cause swelling of the face. A noticeable symptom is lesions on the feet, which can make walking very painful for sheep, while the bowels may also be affected to a lesser extent.”
The virus is particularly harmful to foetuses, especially in the first two months of gestation. Since the brain of a foetus develops in the first ten weeks of gestation, a ewe that is infected with bluetongue during early gestation will either abort or give birth to lambs with brain damage and large skulls.
Dr Dave Waterman, a veterinarian from Stutterheim in the Eastern Cape, says vectors prefer wet, low-lying areas after the late summer rains. They are usually active in the evenings. He warns that post-drought conditions tend to increase animals’ vulnerability to insect-borne viral diseases. Hence it is essential that all necessary vaccinations are administered.
Symptoms and diagnosis
BTV tends to be a problem in lambs that have not been weaned, newly weaned lambs that are sensitive to the virus or young sheep that come from an area where BTV is not a major factor.
Sheep tend to appear drowsy and can become isolated from the herd. The following symptoms may also be indicative of bluetongue infection:
- A temperature between 39,5 and 41°C.
- Cyanosis of the tongue (giving it a blue appearance), swollen mouth or lips, open sores on the gums where the front teeth incise, nasal discharge and swollen eyes and ears.
- Coronary bands (the part between the hoof and wool) have a red to bluish colour. This indicates bleeding within the hooves and can cause lameness.
- Infected animals are quickly fatigued due to a lack of oxygen when the herd is driven. It may also be indicative of secondary infection such as pasteurella.
According to Dr Cockcroft, aortic bleeding just above the heart is a specific post-mortem sign of bluetongue infection.
Precautions and control measures
There are 24 different, very specific viral strains. Onderstepoort Biological Products has an attenuated vaccine available for BTV. The vaccine consists of the following fractions:
- A: Contains the most important strains.
- B: Carries moderately important strains.
- C: Has fewer common strains.
All three vaccine fractions must be administered three weeks apart; vaccinations must be done in July and August and completed by September.
“Vaccines administered at the wrong time, such as during the mating period or gestation, can cause problems. We advise farmers not to vaccinate ewes within the first two months of gestation. This means no vaccinations in the first week or two prior to mating until the tenth week (or day 70) of gestation.”
As it takes nearly two months to produce semen, rams must also not be vaccinated in the two months before the breeding season, or during mating, he explains. The best time is after mating.
Lambs must be vaccinated with vaccine A, B and C, with three weeks between each. Although the label stipulates that lambs must be vaccinated between five and six months of age, it has been found that they can contract BTV from the age of four to six weeks. “Early vaccination helps increase lambs’ survival rate at weaning. They can then be vaccinated again along with the rest of the herd six or seven months later,” says Dr Cockcroft.
According to Dr Waterman, the BTV vaccine is very effective, provided no shortcuts are taken. He also recommends not to vaccinate ewes during early gestation or in the mating season. “It is, however, safe to vaccinate ewes that have been pregnant for more than three months.”
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For enquiries, contact Dr Steve Cockcroft on 046 684 0484 or 083 431 3258 and Dr Dave Waterman on 043 683 2223 or email@example.com. – Carin Venter, Stockfarm