After the winter period of quiescence, 2001 witnessed a dramatic resurgence in bluetongue incidence, especially in August and September, with epidemics focused on the western Mediterranean, and the eastern Mediterranean and the Balkans.
In July 2001 clinical bluetongue disease was again detected in sheep on the French island of Corsica in the western Mediterranean, suggesting that the virus had been able to overwinter there (see EMPRES Bulletin No. 16/1 - 2001). Virus characterization studies indicate that the virus is identical to that responsible for outbreaks in 2000 (BTV serotype 2). Culicoides imicola midges were identified at the site.
In the case of Italy, outbreaks were reported in August and September 2001 from the island of Sardinia and from the region of Calabria on the mainland of southern Italy. Bluetongue was also detected near the border between Lazio and Tuscany, north of Rome on the west side of Italy at the same latitude as Corsica, reappearing in 2001 after its first detection in 2000. Again, these events could be taken to suggest that the virus had overwintered. A new development is the fact that the serotype reported in March 2001 in the south of Italy was serotype 9, not serotype 2 as reported in 2000 and elsewhere in 2001.
In contrast, there have been no new reports of bluetongue from Spain's Balearic Islands, nor from North Africa or Turkey.
Eastern Mediterranean and the Balkans
Outbreaks of bluetongue in sheep were reported in the northwestern part of Greece in August 2001, again on the mainland (the serotype was not stated this year but in the last three years serotypes 4, 9 and possibly 16 have been identified). In September, Bulgaria signalled a resurgence of disease in the west of the country close to The Former Yugoslav Republic of Macedonia, which also reported the disease in September.
At the same time there were reports of bluetongue occurring in Kosovo and Serbia in Yugoslavia close to the borders of The Former Yugoslav Republic of Macedonia and Bulgaria, indicating the size of the area affected between Greece, Bulgaria and Serbia. In Yugoslavia outbreaks have been confirmed as far north as latitude 45o 30' N.
An attenuated (egg-passaged) serotype 2 vaccine imported from South Africa (Onderstepoort Veterinary Institute) was used in Corsica and in the Balearic Islands in 2000 and a locally produced live serotype 4 vaccine was used in Turkey in 1999 and possibly later. Bulgaria also resorted to vaccination in 1999/2000, using a pentavalent (serotypes 3, 8, 9, 10 and 11) imported from South Africa, but did not intend to vaccinate in 2001. Italy announced its intention to carry out vaccination campaigns against bluetongue in spring 2001 using South African monovalent serotype 2 vaccine in Sardinia, Sicily and Basilicata and bivalent serotype 2 and 9 vaccines in eastern Calabria. Details of what was actually undertaken are not yet available. All other countries involved in outbreaks since 1999 chose not to include vaccination in their control options.
In both Greece and France (Corsica) control measures imposed in 2001 include quarantine of infected premises identified by serological testing, slaughter of the sick animals (both sick and suspected of being infected in the case of Greece), a vector control campaign and a ban on the movement of ruminants within a 20 km radius of the outbreaks. Bans on the movement of ruminants and germplasm from the territories of the islands of Corsica, Sardinia and Sicily to the mainland territories of France and Italy are also being enforced. In 2001, a European Commission directive designated the entire territory of Greece as a single surveillance zone for bluetongue; live susceptible animals may not, therefore, be exported to other member countries.
In assessing the efficacy of bluetongue vaccines in controlling the disease it is interesting to note that the bluetongue virus was not detected in the Balearic Islands in 2001 following the large-scale use of vaccine in 2000. Similarly, the disease has not recurred in the area of Bulgaria where vaccination was practised. However, it should also be noted that there was no recurrence in northeastern Greece or in Sardinia in 2001 even though vaccination was not practised there in 2000.
Neither the International Office of Epizootics (OIE) nor the World Health Organization (WHO) have published any reports of West Nile virus infection or disease in animals or humans from the Mediterranean Basin in 2001. It was learned that West Nile virus had been detected in 1999 in Tuscany in Italy - one year before the outbreak in the Camargue in France.
One issue is that, although high bluetongue virus seroprevalence rates were demonstrated in goats in Greek outbreak areas in 2000, all reports of clinical disease were restricted to sheep.
Bluetongue outbreaks on mainland Greece were detected for the first time in 1999-2000 and were clustered around the northeast of the country in association with outbreaks in neighbouring Turkey and Bulgaria, the bordering areas of the three countries forming what was probably one focus of infection established by an influx of infected vectors. Somewhat suprisingly, the outbreaks reported in 2001 are in two prefectures in the northwest of the country close to the border with Albania and The Former Yugoslav Republic of Macedonia.
Vector studies conducted in Greece during the bluetongue incursion in 1999 failed to identify vectors of the species Culicoides imicola, but detected large numbers of C. obsoletus. The latter species is not a prime vector of bluetongue. Studies from late August to mid-October did, however, find large numbers of C. imicola among collections of 19 identified species.
There have been no reports of activity of other orbivirus infections in the region. Epizootic haemorrhagic disease (EHD) viruses and the Simbu serogroup (including Akabane virus) are both known from eastern parts of the Mediterranean Basin and are transmitted by Culicoides midges. To these should, perhaps, be added other orbiviruses such as African horse sickness (AHS), equine encephalosis and the Palyam serogroup.
Like bluetongue virus, the EHD virus group comprises numerous serotypes causing bluetongue-like disease in deer and cattle. Akabane manifests as periodic, seasonal epidemics of abortions, stillbirths, premature births and deformed foetuses or neonates in cattle, sheep and goats. The pregnant dam manifests no clinical symptoms at the time of infection, yet infection in the first trimester produces foetal damage in utero. Akabane disease tends to occur when climatic factors are favourable for the insect vector and virus to spread beyond their normal distribution into areas where there are susceptible animal populations, including large numbers of pregnant animals. Generally, this occurs only near the extremities of the northern and southern limits of its distribution where vectors make only periodic incursions into susceptible populations, or in susceptible stock introduced to the endemic areas. Clinical disease is rarely observed in endemic areas because most susceptible animals are infected while young and are solidly immune by the time they reach breeding age.
A strong case can be made for monitoring such arbovirus infections for which the extent of disease occurrence is determined primarily by the distribution of permissive vectors, which in turn is dependent on temperature and the amount of rain. For its own protection, Europe needs to understand what is happening in terms of the evolution of vector-borne diseases in the Mediterranean Basin and in related ecosystems such as those of the Tigris and Euphrates river systems.
Should vector-borne diseases, in fact, be moving northwards and westwards, then a very important sector of livestock production - that of milk and meat production from small ruminants - is at risk (see map showing sheep density).
It might be of significance to note that Brazil reported an outbreak of bluetongue affecting sheep and goats in Paraná State (26o S) in August. Serological evidence of bluetongue virus activity was also noted in Misiones and Corrientes provinces of Argentina in October as a result of a routine surveillance programme.
Sources of information
International Office of Epizootics (OIE): www.oie.int
World Health Organization (WHO): www.who.int
European Commission: http://europa.eu.int/eur-lex/en/lif/dat/2001/en_301D0433.html
Istituto Zooprofilattico Sperimentale dell' Abruzzo e del Molise "G. Caporale": www.izs.it
COURTESY OF JAN SLINGENBERGH, AGAH PARASITIC GROUP