Final Risk Analysis - Biosecurity New Zealand

7.2.2. Exposure assessment
BTV is transmitted by Culicoides vectors. A Culicoides surveillance programme has been
operating in New Zealand since 1991 (Ryan et al 1991), under which around 15,000 insects
collected from light traps are examined annually (Motha et al 1997) and sentinel cattle are
monitored for seroconversion to viruses transmitted by Culicoides spp. (bluetongue, epizootic
haemorrhagic disease, Akabane and Palyam viruses). To date, seroconversion to arboviruses
has not been detected in sentinel cattle and no Culicoides have been trapped.
Bluetongue virus can be excreted in bull’s semen (Parsonson et al 1981) but only while
animals are viraemic (Bowen et al 1983; Howard et al 1985). Infected cattle may remain
viraemic for about 50 days (Verwoerd and Erasmus 2004). Therefore it would be possible for
an imported infected bull to excrete the virus in its semen for a period of around two months
after infection. The likelihood of exposure of females with which the bull has mated over that
time is non-negligible.
Although no reference could be found for iatrogenic transmission of BTV, mechanical
transmission of this disease is thought unlikely to be of major significance in disease
epizootics (Radostits et al 2007).
7.2.3. Consequence assessment
Female cattle that have mated with an infected imported bull or inseminated with his semen
could become infected (Bowen et al 1985; Schlafer et al 1990; Bowen and Howard 1984) and
could remain viraemic for up to 50 days. However these animals are unlikely to show clinical
signs and would not be infectious for other cattle. The virus could only be transmitted by
Culicoides vectors and these are not present in New Zealand.
The OIE Terrestrial Animal Health Code states that countries that are south of 34° S and are
not adjacent to a country not having a bluetongue virus free status may be considered free
from bluetongue. Furthermore, the OIE Terrestrial Animal Health Code states that “A BTV
free country or zone in which surveillance has found no evidence that Culicoides likely to be
competent BTV vectors are present will not lose its free status through the importation of
vaccinated, seropositive or infective animals, or semen or embryos/ova from infected
countries or infected zones” (OIE 2008).
Bluetongue is not a zoonotic disease and the virus does not constitute a threat to human
health.
It is a disease of ruminants and there is no threat to indigenous animals or birds. Some species
of deer are susceptible to the infection. The effect the virus might have on thar is not known.
However since vectors for the virus do not occur in New Zealand, the consequences of
introducing the virus would be negligible.
The likelihood that the virus could establish in New Zealand is negligible, so the consequence
assessment is negligible.
7.2.4. Risk estimation
There is a very low likelihood that, if a viraemic bull were imported, it would be used for
natural service or semen collection during the period of viraemia. If it were so used, there is a
very low likelihood of transmission of BTV to female cattle by this route. Infection of female
MAF Biosecurity New Zealand Import Risk Analysis: Cattle from Australia, Canada, the EU & the USA ● 21