Bluetongue - Europe (06): BTV-8 origin

International Society for Infectious Diseases
<http://www.isid.org>

The circulation of a new BTV serotype in a new area raises the
question of its origin. Till now, BTV-8 seems correlated to the
African strains. But are we absolutely sure that its introduction is
so recent? Is it possible that BTV already circulated in limited way
without being clinically detected and that the wide diffusion has now
made the phenomenon evident?

To verify this hypothesis it appears interesting to compare the
evolution of the maximum temperatures in the months of July 2005 and
July 2006 (incubation/spreading period) in some of the infected
areas. The high environmental temperatures have shown to be pivotal
to determine the competence of the vectors for the Orbiviruses.

High temperatures positively affect the competence of the bluetongue
vectors for virus transmission. This is due to 2 factors: the
enhancement of the vector population and of the virogenesis in the
vector. (see references).

The temperatures recorded in the areas affected by bluetongue during
July 2006 were significantly higher than in 2005: (Source:
<http://www.weatheronline.co.uk/>)

AACHEN-Germany
July 2005 - 1/32 (1 day at 32 C of max. temperature)
July 2006 -2/30;  2/31;  2/32; 1/34; 1/36 1/37.

CHARLEVILLE-France
July 2005 - 1/30;
July 2006 -  1/30; 3/31; 1/32; 3/34; 1/35.

MAASTRICHT-Netherlands
July 2005 - 1/32
July 2006 - 3/30; 2/31;2/33; 2/34; 1/36.

DUREN-Germany
July 2005 - 1/31
July 2006 - 5/30; 3/33;2/34; 1/37.

LIEGE-Belgium
July 2005 - 1/30
July 2006 - 1/32; 1/30;2/31;2/33;2/34; 1/37.

The relatively lower temperatures in July 2005 might have been
accompanied by lower diffusion of the vectors, and reduced, but not
excluded viral transmission potential. In case BTV-8 was introduced
to Europe earlier than 2006, there could have been a low infection
rate, enabling undetected spread of the virus without (or with a low
number) of clinical cases in 2005 or earlier.

A low seroprevalence due to low infection rate could allow the
circulation of a specific strain for several years in the cattle
population without being clinically detected, and perhaps even during
adverse seasons. Furthermore high temperatures promote the rapid
reproduction of the vector's population. More vectors mean increased
spread. A low diffusion, if associated to low morbidity, i.e. the
ability to cause symptoms, can determine virus circulation without
contemporary clinical evidence and vice versa. Such limited virus
circulation could also determine a lower immunity in host population,
inducing the permanence of the same serotype of the virus.

References:
1. Mellor P.S., Boorman J., Baylis M. CULICOIDES BITING MIDGES: Their
Role as Arbovirus Vectors. 2000. Annu. Rev. Entomol. 45:307-340
2. Mellors P.S. Culicoides: vectors, climate change and disease risk
- Veterinary Bulletin, Vol. 66 (4), pp 301-306.
3. Mullens B.A., Tabachnick W.J, Holbrook F.R., Thompson L.H. Effects
of temperature on virogenesis of Bluetongue virus serotype 11 in
Culicoides variipennis sonorensis. 1995. Med. Vet.Entomol., 9:71-76
4. Purse B.V., Mellor P.S., Rogers D.J ., Samuel A.R., Mertens P.P.C.,
Baylis M. Climate change and the recent emergence of bluetongue in
Europe-Nature Reviews/Microbiology (2005)
5. Welby M., Baylis M., Rawlings P., Mellor. P.S. Effects of
temperature on survival and rate of virogenesis of African horse
sickness virus in _Culicoides variipennis sonorensis_ (Diptera:
Ceratopogonidae) and its significance in relation to the epidemiology
of the disease. 1996. Bull. Entomol. Res. 86:715-20
6.  Wittmann E.J. Baylis M. (2000) Climate Change: Effects on
Culicoides-Transmitted Viruses and Implications for the UK. -
Veterinanary Journal Vol.160, pp 107-117.
7.  Wittman E.J., Mellor P.S., Baylis M.-Effect of temperature on the
transmission of orbiviruses by the biting midges, _Culicoides
sonorensis_ -Medical and Veterinary Entomology (2002)

--
Prof. Manlio Fadda
Universita' di Sassari - Facolta' di Medicina Veterinaria
settore VET/05- Malattie Infettive degli Animali Domestici
07100 SASSARI - ITALY

Prof Fadda, who has been involved in the epidemiological
investigations and follow-up of the bluetongue outbreaks in Italy
(Sardinia) during recent years, is acknowledged for his views. The
final evaluation of the origin, history and scope of the BTV-8
infection in Europe might eventually be clarified when a
retrospective serosurveillance in wider realms within Europe is
undertaken.

The current epizootic in Europe, caused by BTV-8, might have peaked
by now, but the vectors seem to be still active in Belgium, northwest
Germany and southern Netherlands. During the last week, 33 new
outbreaks have been identified in these countries.

So far, 243 outbreaks of bluetongue have been reportedly confirmed in
the 4 affected European countries. The following data have been
derived from the 22 Sep 2006 update of table 11 (Animal disease
situation per country and per disease), provided by EU's Animal
Disease Notification System (ADNS), presenting the accumulated number
of confirmed outbreaks from the beginning of the epizootic until 22
Sep 2006. The table is
available at
<http://ec.europa.eu/food/animal/diseases/adns/table_11_2006/adns_220906_en.pdf >.

Country/outbreaks (total)/last date of confirmation/ outbreaks since 14 Sep

Belgium / 98 / 21 Sep 2006 / 10
Germany / 85 / 21 Sep 2006 / 14
France / 4 / 4 Sep 2006 / 0
Netherlands / 56 / 21 Sep 2006 / 9.

The most recent updates (5 Sep 2006) of country reports with maps,
provided to the EU, are available at the following URL's:
Belgium:< http://ec.europa.eu/food/committees/regulatory/scfcah/animal_health/bluetongue_scofcah_belgium09.pdf>
France:< http://ec.europa.eu/food/committees/regulatory/scfcah/animal_health/bluetongue_scofcah_france09.pdf>
Germany:< http://ec.europa.eu/food/committees/regulatory/scfcah/animal_health/bluetongue_scofcah_germany09.pdf>
Netherlands:< http://ec.europa.eu/food/committees/regulatory/scfcah/animal_health/bluetongue_scofcah_netherlands09.pdf>.