After the discovery of the initial outbreak of ranavirus in the Netherlands in the Dwingerveld National Park in 2010 subsequent outbreaks have been closely monitored in order to better characterize the emergence of the disease and its potential ecological impact.
During this four-year period 155 dead amphibians were recovered from 52 locations and submitted to the DWHC for post-mortem investigation. In 18 of these locations the cause of death was shown to be ranavirus.
This virus is a member of a group of viruses which includes the Common Midwife Toad Virus-like ranavirus that has also been reported on in Spain.
The Dutch outbreaks in 2011 and 2012 occurred predominantly within a 20km radius of the location of the initial outbreak (in Drenthe and Noord-Overijssel) whilst the outbreaks in 2013 and 2014 occurred in more distant parts of the Netherlands (Gelderland, Noord-Brabant and Limburg).
In order to determine whether these outbreaks are linked, the strain of ranavirus from each outbreak has been characterized and CMTV-like viruses were found in all cases. The Dutch ranavirus outbreaks have been caused by viruses that resemble the CMTV, first reported in association with amphibian mortality in Spain. However, further characterization revealed that the precise strain of CMTV-like virus causing the initial outbreak in the north of The Netherlands (CMTV NL-group I, Noord-Nederland) was different to that causing the later outbreaks in the southern provinces of Noord‑Brabant and Limburg (CMTV NL-groep II, Zuid-Nederland). The two virus types also appear to differ in the extent to which they cause mortality, with the northern virus apparently causing a higher rate of death. However, this observation is based on a small number of cases and must be further investigated.
The fact that the virus has continued to cause amphibian deaths in and around the site of the first outbreak in 2010 suggests that there is an epidemic of ranavirus in The Netherlands.
Signs of infection
Previous studies have identified a classic range of signs associated with ranavirus infection. As part of this study researchers investigated whether the three previously described forms of tissue damage are also typical of the viruses involved in the Dutch epidemic. These signs include 1) the presence of virus in the cell which is visible through the microscope as the presence of so-called ‘viral inclusion bodies’ in cells; 2) tissue death (necrosis); and, 3) damage to blood vessels seen as red specks (mini hemorrhages) on the skin. Only these latter are visible to the naked eye whilst the presence of virus in cells and cellular death can only be diagnosed microscopically. Interestingly, this work shows that blood vessel damage in the skin was seen as often in non-infected as infected animals suggesting that this finding is not unique to ranavirus infection. In contrast, both tissue death and, as could be expected, the presence of virus in the cells was seen more often in animals submitted from sites where ranavirus was detected than from ranavirus-negative sites implying a role for ranavirus in these forms of tissue damage. It is noteworthy that tissue damage appeared to be more severe in some members of the pelophylax genus (true frogs) than in other amphibian species.
There are 16 native amphibian species in The Netherlands. Between 2011 and 2014 ranavirus has been found in seven of these: the common water, or green frog; the marsh frog; the pool frog; the common, or common brown, frog; the common toad; the common spadefoot, or garlic, toad; and, the smooth newt. This list contains both commonly occurring and endangered species indicating that ranavirus could pose a threat for already-threatened populations such as the common spadefoot toad whose distribution is already limited.
Investigation of Amphibian Mortality Events in Wildlife Reveals an On-going Ranavirus Epidemic in the North of the Netherlands, (2016). Jolianne M. Rijks, Bernardo Saucedo, Annemarieke Spitzen-van der Sluijs, Gavin S. Wilkie, Alphons J.A.M. van Asten, Jan van den Broek, Roschong Boonyarittichaikij, Marisca Stege, Fleur van der Sterren, An Martel Frank Pasmans, Joseph Hughes, Andrea Gröne, Steven J. van Beurden, Marja J.L. Kik (2016). PLoS ONE 11(6): e0157473. doi:10.1371/journal.pone.0157473