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

It has been suggested that as many as 7 % of foetal deaths in Swiss dairy cattle may be
caused by infection with BVDV (Rufenacht et al 2001). BVDV2 strains that cause a more
severe form of the disease have been described in the USA (Pellerin et al 1994). In these
cases the mortality rate was up to 10 % (Potgieter 2004) and the disease was characterised
by severe leucopenia and haemorrhagic disease (Brownlie 2005).
Immunotolerant persistently infected cattle may be clinically normal or may not thrive and
die within a year. They are always infected with non-cytopathic strains of the virus
(Brownlie 2005). Superinfection of persistently infected animals with a cytopathic BVDV
strain results in the development of mucosal disease (MD) (Potgieter 2004; Brownlie
2005). The cytopathic strain that re-infects the persistent carrier animals may result from a
mutation of the persistent non-cytopathic strain or from infection with a new extrinsic
cytopathic virus (Potgieter 2004; Brownlie 2005). Mucosal disease is invariably fatal. In
acute cases death occurs within 2-21 days while in chronic cases the animal may survive
for up to 18 months (Potgieter 2004).
An ELISA is available to detect BVDV antibody (Drew 2008). Despite the fact that
serologically positive animals are usually no longer infected, exceptions are known to
occur and a minority of persistently infected animals are also serologically positive. Some
bulls that develop antibody titres continued to excrete infectious virus in semen for at least
5 months after experimental infection (Givens et al 2003). Further, a single case of a bull
that was serologically positive and had no detectable virus in its blood but consistently
excreted virus in its semen (Voges et al 1998). This led to a change in the
recommendations made in the Code. It is now necessary for bulls that are antibody positive
when they enter an AI station to have their semen tested for virus and for bulls that
seroconvert to have every batch of semen that they have produced since their last negative
serological test, tested for BVDV.
It is assumed that male camelids could similarly be persistently infected although
seropositive when imported. The antibody ELISA will not detect persistently infected
animals that are immunotolerant. Antigen detection ELISAs are available but less sensitive
than the RT-PCR. Although there is no Code chapter for BVDV, the Manual lists virus
isolation or antigen-detection ELISA as the prescribed tests for international trade. An RT-
PCR is available to detect viral RNA in blood (Rufenacht et al 2001; Stokstad et al 2003).
Kapil et al (2009) state that the commercial antigen ELISA can give false positive results
when testing camelid serum. He notes “screening new world camelids by BVD viralspecific
PCR on whole blood will detect active infection/viraemia”, although PCR testing
is not validated for international trade.
BVD and MD are primarily diseases of cattle. Historical reports describe several
serological studies that confirm camelids are susceptible to infection with BVDV. In a
serological survey conducted in Peru on 117 alpacas that grazed with cattle, the prevalence
of antibodies to BVDV was 11 %. In 270 llamas from Oregon, USA, the seroprevalence
was reported to be 4.4 % (Wernery & Kaaden 2002). A survey of crias (unweaned baby
camelids) in the USA found that 25 % of the herds studied were seropositive to BVDV
(Topliff et al 2009). Historically, disease caused by BVDV in camelids had been described
only once, in 1994. The affected llamas suffered excessive nasal discharge and diarrhoea
before death. Experimental infection of four pregnant llamas during gestation did not result
in clinical signs of disease or foetal infection or persistent BVDV infection of crias (Wentz
et al 2003).
MAF Biosecurity New Zealand Import risk analysis: Llamas and alpacas from specified countries ● 25