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

18. West Nile virus
18.1. HAZARD IDENTIFICATION
18.1.1. Aetiological agent
Family: Flaviviridae; Genus: Flavivirus: Species: West Nile virus (Thiel et al 2005).
18.1.2. OIE list
West Nile fever is a listed disease of multiple species.
18.1.3. New Zealand status
An exotic organism that is not listed as unwanted or notifiable by MAF (2009).
18.1.4. Epidemiology
West Nile virus was originally isolated in Uganda in 1937. It is found all over Africa and
has also been found in France (1962), Romania (1996), and Russia (1999) (Bunning et al
2004). The virus spread to the United States in 1999 and subsequently to adjoining
countries. Disease is seen mainly in humans and horses but the virus also causes deaths in
wild birds. Most cases in humans are asymptomatic but at the height of the US epidemic in
2003, 9,862 cases were reported and there were 264 deaths (CDC 2009a).
The virus is transmitted by mosquitoes and maintained in a mosquito/bird cycle (CDC
2009b). At least 43 species of mosquitoes have been suspected of acting as vectors of the
virus (Gingrich & Williams 2005). The virus can be transmitted from infected mosquitoes
to non-infected mosquitoes when they feed together on non-infected hosts (Higgs et al
2005). Animals other than birds may be infected. Clinical cases are seen predominantly in
horses in which the infection proves fatal in about 40 % of cases, but many horses are
subclinically infected and develop antibody. However, in viraemic horses the virus titre is
too low to infect mosquitoes.
According to CDC “ People, horses, and most other mammals are not known to develop
infectious-level viremias very often, and thus are probably ‘dead-end or incidental-hosts’
(CDC 2009b). Cattle are also dead-end hosts (Ilkal et al 1988). Particularly relevant is the
Code statement Article 8.16.2. that “Members should not impose trade restrictions on
dead-end hosts such as horses”. The Code chapter makes recommendations for other
susceptible species (birds) and specifically excludes horses from them (Articles 8.16.5.,
8.16.6. and 8.16.7.). The Code also states that a free country or zone will not lose its free
status through the importation of seropositive animals whether from natural infection, or
vaccination induced (OIE 2009).
Fatal cases of the disease have been seen in alpacas (Dunkel et al 2004; Kutzler et al
2004b; Yaeger et al 2004). Alpacas and llamas produced antibody to the virus after
vaccination. The vaccine proved to be safe but no challenge tests were done to prove
immunity (Kutzler et al 2004a). It is highly likely that camelids will be dead-end hosts like
horses, cattle and other mammalian species.
64 ● Import risk analysis: Llamas and alpacas from specified countries MAF Biosecurity New Zealand