Lice infestations are an
inevitable consequence of rearing high densities of fish in aquaculture. As salmon
lice, Lepeophtheirus salmonis, graze on the skin they create a route for
opportunistic pathogens to enter such as the bacterium Pasteurella,
which causes pasteurelliosis.
In addition, the lice have been shown to harbour aquatic pathogens themselves,
for example Aeromonas salmonicida. However, until recently their role as
vectors had never been proven. Jakobs et al. (2011) provided the first evidence that lice act as vectors, in this case for Infectious Haematopoietic
Necrosis Virus (IHNV). This virus has been previously blamed for several deadly
epidemics in North American aquaculture of Atlantic salmon, Salmo salar.
IHNV naïve adult lice from farmed
salmon and salmon parr (a juvenile
stage) were collected for use in the experiment. The IHNV strain used was from
an epidemic that occurred in the 90s. Infection was confirmed throughout the
experiment via RT-PCR and tissue culture methods. Firstly, it was confirmed
that lice were able to acquire IHNV from exposure to the virus present in a
water bath. The virus remained associated with the lice for 24hrs after the
exposure. Secondly, it was found that lice were able to obtain IHNV from
exposure to fish injected with the virus. However, in this case the virus was
only associated with lice for 12hrs after removal. Eventual loss of the virus
from the lice indicates that the virus is unable to replicate within the
crustacean. This is unlike other diseases such as malaria where the trypanosome
replicates within the mosquito. It was then shown that exposure of lice to
naive salmon led to high mortalities and infection with the virus. Exposure to lice,
which had gained the virus through water-bath exposure, led to 70.6% mortality
and 13 out of 17 fish testing positive for IHNV. Similarly, exposure to lice that
had gained IHNV from infected salmon lead to comparable levels of infection and
mortality in naïve salmon. An important question was also, are lice able to
transmit the virus to fish via direct contact or through the water? In short, the
ability of IHNV to infect salmon without attachment of the lice was zero. Interestingly,
in this case exposure to IHNV infected lice led to lower mortalities than the
first set of experiments. Which could be due to differences in host susceptibility
or virus load.
Overall, this study provides some evidence that salmon lice are able to transmit the virus IHNV to naive salmon although this affect was highly variable in the study. Also, given the short time of persistence
of the virus within the lice, it is unlikely that lice are important vectors of
IHNV between farms and wild populations over large distances. Instead, lice may
help to transport the virus between pens along with water bourn dispersal of
virions during an outbreak. However, if other pathogens are able to persist
within the lice they could act as vectors. Further research should focus on determining
whether lice can act as a vector for other diseases such as Aeromonas salmonicida.
Ref: Jakob, E., Barker, D.E. and
Garver, K.A (2011). Vector potential of the salmon louse Lepeophtheirus salmonis in the transmission of infectious
haematopoietic necrosis virus (IHNV). Diseases
of Aquatic Organisms, 97, 155-165.
Hi Tom,
ReplyDeleteReally nice study! Did the study just focus on cultured salmon? It would be interesting to look at the difference in the microbial communities associated with wild salmon and those in farms.
I was also wondering if they standardised the time the lice were attached for, it would be good to see if there would a be any correlation between the time the virus was associated with the lice depending and how long the lice spent attached to their host.
Thanks for the post,
Freya
Hi Freya, yes the salmon were from farms. I would not be surprised if caged Salmon show a difference in their microflora because of differences in diet along with all the things like probiotics which we feed to them.
DeleteWith regards to the length of exposure. For the first set of treatments, i.e. lice with IHNV from either water bath or infected salmon, they went for 44 day treatments and 39 day treatments respectively. During the second set of experiments, i.e. do lice have to be attached to transmits the disease, it looks like all of the treatments were run for 30 days. The first set of experiments showed much higher mortality and infection with IHNV via the lice than the second set but this set in quite early and there appeared to be subsidence in the number that died.
Also (please see my amendment to the post) the second set of experiments showed much lower mortality so there appeared to be a difference in the Salmons response, perhaps environmental etc. So although the lice definitely can transmit the virus infection does appear highly variable.
Hi Tom,
ReplyDeleteGreat post - am i right in thinking that the viruses were only able to replicate within the fish? The viruses just hitched a ride with lice, once the lice were in contact with the salmon it gave the the viruses an opportunity to get in (supposedly through the lice bites)?
Yes you are correct, the authors concluded that the lice were able to transmit the virus mechanically i.e. it was not able to replicate in the lice. However, the authors could not ascertain if the virus were externally or internally carried by the lice. On attachment the lice 'graze' and lead to scale loss and breaks in the skin. This presumably could provide the virus with a route in, and also provides other pathogens with a window of entry. Or it could be that just the very close proximity of the lice on attachment allow the virus to hop across into the body of the salmon, instead of definitely requiring holes in the skin. I think this is most likely as the main transmission method for the virus is through water. But the I doubt the grazing of the lice makes it any better, it also raises cortisol levels which leads to immuno-supression. But please see my addition to the blog post.
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