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Import risk analysis: Llamas (Lama glama) and alpacas (Vicugna ...

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18. West Nile virus<br />

18.1. HAZARD IDENTIFICATION<br />

18.1.1. Aetiological agent<br />

Family: Flaviviridae; Genus: Flavivirus: Species: West Nile virus (Thiel et al 2005).<br />

18.1.2. OIE list<br />

West Nile fever is a listed disease of multiple species.<br />

18.1.3. New Zeal<strong>and</strong> status<br />

An exotic organism that is not listed as unwanted or notifiable by MAF (2009).<br />

18.1.4. Epidemiology<br />

West Nile virus was originally isolated in Ug<strong>and</strong>a in 1937. It is found all over Africa <strong>and</strong><br />

has also been found in France (1962), Romania (1996), <strong>and</strong> Russia (1999) (Bunning et al<br />

2004). The virus spread to the United States in 1999 <strong>and</strong> subsequently to adjoining<br />

countries. Disease is seen mainly in humans <strong>and</strong> horses but the virus also causes deaths in<br />

wild birds. Most cases in humans are asymptomatic but at the height of the US epidemic in<br />

2003, 9,862 cases were reported <strong>and</strong> there were 264 deaths (CDC 2009a).<br />

The virus is transmitted by mosquitoes <strong>and</strong> maintained in a mosquito/bird cycle (CDC<br />

2009b). At least 43 species of mosquitoes have been suspected of acting as vectors of the<br />

virus (Gingrich & Williams 2005). The virus can be transmitted from infected mosquitoes<br />

to non-infected mosquitoes when they feed together on non-infected hosts (Higgs et al<br />

2005). Animals other than birds may be infected. Clinical cases are seen predominantly in<br />

horses in which the infection proves fatal in about 40 % of cases, but many horses are<br />

subclinically infected <strong>and</strong> develop antibody. However, in viraemic horses the virus titre is<br />

too low to infect mosquitoes.<br />

According to CDC “ People, horses, <strong>and</strong> most other mammals are not known to develop<br />

infectious-level viremias very often, <strong>and</strong> thus are probably ‘dead-end or incidental-hosts’<br />

(CDC 2009b). Cattle are also dead-end hosts (Ilkal et al 1988). Particularly relevant is the<br />

Code statement Article 8.16.2. that “Members should not impose trade restrictions on<br />

dead-end hosts such as horses”. The Code chapter makes recommendations for other<br />

susceptible species (birds) <strong>and</strong> specifically excludes horses from them (Articles 8.16.5.,<br />

8.16.6. <strong>and</strong> 8.16.7.). The Code also states that a free country or zone will not lose its free<br />

status through the importation of seropositive animals whether from natural infection, or<br />

vaccination induced (OIE 2009).<br />

Fatal cases of the disease have been seen in <strong>alpacas</strong> (Dunkel et al 2004; Kutzler et al<br />

2004b; Yaeger et al 2004). Alpacas <strong>and</strong> llamas produced antibody to the virus after<br />

vaccination. The vaccine proved to be safe but no challenge tests were done to prove<br />

immunity (Kutzler et al 2004a). It is highly likely that camelids will be dead-end hosts like<br />

horses, cattle <strong>and</strong> other mammalian species.<br />

64 ● <strong>Import</strong> <strong>risk</strong> <strong>analysis</strong>: <strong>Llamas</strong> <strong>and</strong> <strong>alpacas</strong> from specified countries MAF Biosecurity New Zeal<strong>and</strong>

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