Questions and answers about bluetongue

1.  Q:  Is infection transmitted from wildlife to sheep?

     A:  No - infection does NOT go directly from animal-to-animal.  The virus is transmitted by insects - biting midges (Culicoides spp.).  Wildlife and sheep are exposed at the same time.

2.   Q.  Do we know where and how the virus survives over the winter?

      A:  Not at this time.  This is an important scientific question that several research groups are currently trying to answer.

3.    Q.  How commonly does abortion and deformation occur in sheep and cattle following bluetongue virus infection?

       A:   Rarely, if ever, following natural infection.  It does occur following the use of cell-adapted strains of virus, such as modified live vaccines.  This is one reason the use of modified live virus vaccines is controlled in the US.   They can cause disease.

4.    Q:  Can bluetongue be transmitted vertically, such as across the placenta or in semen?

       A:   Not that we know.  Some older scientific papers claimed that this occurs.  These claims have been discredited by a series of studies that attempted to reproduce those findings.  References to that work are available on request.

5.    Q:  Why doesn't the state do aerial spraying to control Culicoides midges in an attempt to stop the disease?

       A:  The scientific consensus is that insect spraying is useless.  The current large outbreak of bluetongue in southern and northern Europe, including the United Kingdom, has been difficult to control.  Insect spraying has not been essayed there since it is considered unlikely to work.  For a useful discussion of the current state of play in using insecticides to try to control the current BTV outbreak in the UK, see The Veterinary Record volume 161 page 633 (2007).  The bottom line at present, to quote from that correspondence, is that it is unclear whether treatment of stock with insecticide will have a significant effect in slowing or stopping the transmission of BTV.  Any benefits that might accrue from widespread use of effective insecticides, such as synthetic pyrethroids, must be balanced against the environmental impacts of these compounds.

6.    Q.  How hard a freeze does it take to kill the carrier midges so that outbreaks will end?

        A.  It is necessary for there to be MULTIPLE frosts in a row to kill insects.  Even then, some adults survive and re-emerge in low numbers if there is a subsequent warm spell.  So several frosts, followed by mild weather, such as we are having in Wyoming at present, can prolong an outbreak.

7.    Q.  What are the medium and long-term effects in a flock that experienced bluetongue?

       A.   Sheep lose condition for several weeks due to oral infection and lesions in the lungs.  Secondary bacterial infections can be a problem, particularly pneumonia.  Sheep do NOT become persistently infected and (as noted above) tend not to abort, since wildtype virus does not cross the placenta.  Sheep may however abort due to fever and terminal illness.  There are no long-term consequences.

8.    Q.  How can my veterinarian confirm a clinical suspicion of bluetongue?

       A.  Since clinical signs are suggestive but not diagnostic, it is important to confirm the disease is due to bluetongue and not another diseases - an important rule out, when BT is new to an area, is foot and mouth disease, since both cause lesions in the mouth and digits.   Confirmation requires a blood sample (purple top for virus isolation; red top for serology) from live sheep, and tissues collected from the carcass.  They can be submitted to the Wyoming State Veterinary Laboratory through a veterinarian.

9.    Q.  Is there a vaccine that protects a flock against bluetongue?

       A.  Yes - and no.  There is a vaccine that is licensed for use in the state of California that has been used to reduce the incidence of disease due to BTV serotypes 10, 11 and 17.   The vaccine contains live virus, and it can cross the placenta to cause fetal abnormalities.   An experimental canarypox vaccine has been developed by a team that includes researchers at the University of California in Davis, and the Pfizer company.  The market for this product is small and it is not yet licensed for use in the US.  If producers, or their organization, are interested in the product, contact either Pfizer (Dr. Robert Nordgren, Merial Limited, 115 Transtech Drive, Athens, GA 30601, USA) or Dr. Jim MacLachlan, Department of Medicine and Epidemiology, School of Veterinary Medicine, University of California, Davis, CA 95616, USA.

10.  Q.  What did Montana do to control the 2007 outbreak?

      A.  Montana, which had a more widespread BT episode than Wyoming this year, quarantined counties where the disease was known or suspected to be present (16 counties total).  This was done in an attempt to eliminate the movement of infected sheep to other parts of Montana, and to other states.  No state-wide control was attempted and the state did not try extensive use of insecticide.

Dr. Donal O'Toole

11/09/2007