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Other Diseases – Viral Diseases of Amphibianss

Lee Berger and Rick Speare

School of Public Health and Tropical Medicine

James Cook University

Townsville 4811

Australia.

Viruses

Ranaviruses

Ranaviruses are a genus of the Iridoviridae family of viruses. Ranaviruses are capable of causing diseases with formidable mortality in amphibians (Speare et al 2001). In 2001 Ranaviral disease of amphibians was placed by OIE on the Wildlife Diseases list.

1. Ranaviral disease in amphibians is caused by multiple "species" of closely related viruses placed in the genus Ranavirus.

2. Ranaviruses are icosohedryl, enveloped viruses containing double stranded DNA, and ranging in diameter (vertex to vertex) from 152 to 157 nm (Hyatt et al 2000).

3. Ranaviral disease is an emerging infectious disease of amphibians globally since it is being detected over an increasing geographic range and in more species (Table 1).

4. Ranaviruses have only recently been reported from Asia (Table 1) in association with amphibian mortality.

5. Ranaviral disease was placed by OIE on the Wildlife List for amphibians in 2001.

Biology and Pathogenicity of Ranaviruses

6. Some ranaviruses, e.g., Bohle iridovirus, can infect 3 classes of vertebrate (amphibia, reptilia and pisces).

7. Most ranaviruses produce systemic necrosis of haematopoietic tissues except RUK#13 from UK which may be associated with skin ulcers only (Hyatt et al 2000).

8. Ranaviruses have low host specificity in general (i.e., most can infect many species of host (Moody and Owens 1994; Hyatt et al 2000), but some species may have high host specificity (Jancovich et al 2001).

9. Ranaviruses are highly infectious since inoculating doses can be very low.

10. Ranaviruses are robust viruses capable of surviving for extended periods of time even in dried material (Landon 1989).

11. Aclinical carrier states with ranaviruses occur, and are probably the most common state in wild amphibians.

12. Movement of ranaviruses into an area will most probably be by movement of infected amphibians, fish or reptiles and infected equipment and other inanimate objects that have been contaminated by ranaviruses.

13. Once detected in an area, ranaviruses are not consistently detected thereafter.

14. Ranaviruses may be able to survive in the environment without a host, but will not multiply.

15. Ranaviruses are capable of causing a high incidence of morbidity and mortality in amphibians in captivity and experimentally.

16. Ranaviruses can cause a high incidence of morbidity and mortality in some species of amphibians in the wild.

17. In Australia there have been no epidemics of ranaviral disease detected in wild amphibians.

18. The pathological outcome of infection of amphibians with ranaviruses is variable and difficult to predict.

19. Some factors which determine this outcome are known (age of host, viral characteristics), but the environmental factors (e.g., pollution, UV, climate) that determine the outcome are unknown.

20. Chronic ranaviral disease in amphibians can occur experimentally and in the wild.

21. The significance of chronic ranaviral disease on wild amphibian populations is unknown.

22. The potential for amphibian carriers of ranaviruses to release viral particles into the environment is unknown.

Table 1: Ranaviruses reported from amphibians.

Location	Virus	Species	Reference
Asia
China	Tiger frog virus (TFV)	Tiger frog Rana tigrina	He at al 2002
	Rana grylio virus (RGV)	Rana grylio	Zhang QiYa et al 2001
Thailand	RTV	Tiger frog Rana tigrina	Ahne & Essbauer 2001
Oceania
Australia	Bohle Iridovirus (BIV)	Ornate burrowing frog Limnodynastes ornatus	Speare & Smith 1992
Europe
UK	RUK	Common frog Rana temporaria	Drury et al 1995; Hyatt et al 2000
	Rana esculenta iridovirus (REIR)	Edible frog Rana esculenta	Ahne et al 1998
	BUK	Common toad Bufo bufo	Ahne & Essbauer 2001
North America
Canada	Regina ranavirus (RRV)	Tiger salamanda Ambystoma tigrinum diaboli	Bollinger et al 1999
USA	Tadpole edema virus (TEV)	North American bullfrog Rana catesbiana	Wolf et al 1968
	Frog virus 3 (FV3), (FV1, 2, 9-23), LT1-LT4	Leopard frog Rana pipiens	Hyatt et al 2000
	Ambystoma tigrinum virus (ATV)	Tiger salamanda Ambystoma tigrinum stebbinsi	Jancovich et al 2001
	T6-20	Red eft Diemictylus viridescens	Ahne & Essbauer 2001
	NVT	Notophthalmus viridescens	Ahne & Essbauer 2001
	TEV, Redwoodvrius	Red legged frog Rana aurora	Ahne & Essbauer 2001
	FV1-3, FV9-23	Leopard frog Rana pipiens	Ahne & Essbauer 2001
	XV	African clawed toad Xenopus laevis	Ahne & Essbauer 2001
South America
Venezuela	Guatopo virus	Cane toad Bufo marinus	Hyatt et al 2000
?	LSV	Tiger frog Rana tigrina	Ahne & Essbauer 2001

Epidemiology

23. The epidemiology of ranaviruses is best understood in North America and UK where local and general epidemics with high mortality have been reported (Wolf et al 1968; Drury et al 1995; Jancovich et al 2001).

24. In Australia the epidemiology of ranaviruses in wild amphibians is not understood since although ranaviruses occur there have been no outbreaks or disease detected in wild amphibians although field investigations have been limited.

25. Serological studies on Bufo marinus show that ranaviruses are present in New South Wales, Queensland and Northern Territory (Zupanovic et al 1998). Fresh water tortoises in North Queensland also have antibodies against ranaviruses (Ariel 1997).

26. Serological studies have not been done on other amphibians in Australia since suitable techniques have not been developed for any species other than B. marinus.

27. Of the two endemic ranaviruses in Australia, Bohle Iridovirus (BIV) and Epizootic Haematopoeitic Necrosis Virus (EHNV), only BIV appears capable of infecting amphibians.

28. BIV can also experimentally infected a number of native and introduced freshwater fish, freshwater turtles, and snakes (Moody and Owens 1994; Ariel 1997).

29. Some other ranaviruses found outside Australia can cause experimental disease in native Australian amphibians (Zupanovich et al 1998). Guatapo virus killed the green tree frog Litoria caerulea.

30. The potential of foreign ranaviruses and those intercepted in imported fish and reptiles to cause disease in Australian amphibians is unknown.

31. From experimental trials and the epidemiology of ranaviruses overseas, the most likely outcome of a new ranavirus in Australia would be epidemic disease of an unpredictable extent.

32. This scenario means that ranaviruses may be highly significant to amphibians that have small populations confined to small geographic areas.

Other viruses of amphibians

At least 6 groups of viruses other than ranaviruses have been reported from amphibians (Table 2). These viruses can cause diseases in amphibians, but their impact on wild populations has not been well studied apart from Frog Erythrocytic Virus (FEV) in Canada.

Table 2: Viruses other than ranaviruses reported from amphibians. Amphibians were free-ranging unless “captive” indicated.

Virus / virus group	Amphibian host	Location	Effect on amphibians	Reference
Guatapo virus 6 (GV6)	Cane toad Bufo marinus	Venezuela	Necrosis of haematopoeitic tissue	Hyatt et al 2000
Amphibian erythrocytic viruses	North American bullfrog R. catesbiana Green frog R. clamitans R. septentrionalis	Canada	Anaemia; reduced survival	Gruia-Grey et al 1992
	Northern leopard frog R. pipiens	USA	unknown	Bernard et al 1969
	Leptodactylus ocellatus	Brazil	unknown	de Sousa & Weigl 1976
	Phrynohyas venulosa	Brazil	unknown	de Matos et al 1995
	Cane toad Bufo marinus	Costa Rica	unknown	Speare et al 1991
	Ptychadena anchieta	South Africa	unknown	Alves de Matos & Paperna 1993
	Bufo gargarizans	China	unknown	Werner 1993
	R. boulengeri	China	unknown	Werner 1993
	R. nigromaculata	China	unknown	Werner 1993
Amphibian leucocyte virus	R. catesbiana	Europe (captive source Mexico)	Lethargy, skin ulceration	Briggs & Burton 1973
Lucké tumor herpesvirus	Rana pipiens	USA	Lucké renal adenocarcinoma	Lucké 1938; McKinnell & Carlson 1979
Calicivirus	Ceratophrys orata	USA (captive)	Pneumonia, death	Smith et al 1986
Herpesvirus-like particles in skin	Rana dalmatina	Europe	Epidermal vesicles	Bennati et al 1994
Flaviviruses
Sindbis virus	Rana ridibunda	Europe	unknown	Kozuch et al 1978
West nile virus	Rana ridibunda	Tadzhikistan	unknown	Kostiukov et al 1985; 1986

Frog Erythrocytic Virus

FEV was discovered in wild populations of Rana spp. in Algonquin Park, Ontario Canada (Gruia-Grey et al 1989; Gruia-Grey and Desser 1992). Key details are:

1. FEV is a member of the viral family Iridoviridae.

2. FEV is a large (diameter up to 450 nm in diameter), enveloped, double strand DNA containing iridovirus of uncertain classification within the Iridovidae.

3. FEV is present in red blood cells.

4. FEV is transmitted between frogs by mosquitoes or midges, and is not transmitted by water, orally or by leeches.

5. Infection with FEV results in red blood cells changing shape from oval to spheroidal, and infected frogs can become anaemic.

6. Infection is more common in juveniles than adults.

7. Infection appears to contributes to mortality of juvenile frogs with more infected juveniles than FEV-free frogs disappearing from the population structure.

8. FEV has been reported only in Canada although similar large viruses have been discovered in red blood cells of amphibians in Costa Rica, Brazil, South Africa and USA (Table 2).

Lucké tumor herpesvirus

Lucké tumour herpesvirus (LTHV) has been reported only from the northern leopard frog, Rana pipiens, in USA (McKinnell and Carlson 1997). Recently LTHV has been referred to as Rana herpesvirus 1 (RaHV-1) (Davison et al 1999).

RaHV-1 is a member of the viral family Herpesviridae.
Genomic studies indicated that RaHV-1 belongs to the fish virus lineage of the herpesvirus family rather than to the lineage populated by mammalian and avian viruses (Davison et al 1999).
RaHV-1 induces renal adenocarcinoma in R. pipiens in USA.
The disease was described in 1934 (Lucké 1934) and its transmissible nature recognised in 1938 (Lucké 1938).
Clinical signs are bloating, lethargy and death, which occur when the tumour is large or has metastasised (Anver and Pond 1984). Single or multiple white nodules occur in the kidneys and grow into large masses. The tumour is an infiltrating and destructive adenocarcinoma, or less often it is more orderly and adenomatous (Lucké 1934).
Although the gross appearance of the tumour remains relatively unchanged, there are significant seasonal differences in the microscopic appearance. Winter tumours display cytopathic characteristics associated with the presence of virus (enlarged nuclei with eosinophilic inclusions) whereas summer tumours lack virus (McKinnell 1973).
Metastasis of the cancer depends on temperature with metastasis more common with higher temperatures (29°C vs 4°C).
Studies have shown that above 22°C virus replication does not occur and viral particles are not present in the tumour (Anver and Pond 1984).
Surveys of wild R. pipiens for the Lucké tumour have found prevalences up to 12.5% (McKinnell 1969). However, since 1977 the prevalence of Lucke renal adenocarcinoma appears to have declined in Minessota (McKinnell et al 1979). In retrospect this may have coincided with the arrival of the amphibian chytrid, Batrachochytrium dendrobatidis, a serious fungal pathogen of amphibians.
Neither RaHV-1 nor the disease it causes has been found in any species other than R. pipiens in USA.

Other Viruses Pathogenic to Amphibians

Other viruses found associated with disease or pathologic changes in amphibians have been reported in single papers with no experimental work. Hence, their significance as pathogens of amphibians is largely unknown.

Herpes-like virus of skin: In Italy, up to 80% of a wild population of R. dalmatina had epidermal vesicles associated with a herpes-like virus, but dead frogs were not found (Bennati et al 1994).

Calicivirus: Calicivirus was isolated from two captive Ceratophrys orata found dead. Both had pneumonia, while one also had oedema and the other had lymphoid hyperplasia (Smith et al 1986).

Leucocyte viruses: Polyhedral cytoplasmic DNA virus was found in the cytoplasm white blood cells of a Mexican R. catesbiana that was lethargic and had small exudative ulcers (Briggs and Burton 1973). The large iridovirus found in red blood cells of B. marinus in Costa Rica also was found in the cytoplasm of reticular cells in the spleen (Speare et al 1991).

Viruses that Can Use Amphibians as Reservoir Hosts

At least 2 arboviruses, West Nile virus and Sindbis virus, can infect amphibians and produce viraemias. West Nile virus in Rana ribidunda caused a viraemia capable of infecting mosquitoes (Kostiukov et al 1986; 1985). West Nile virus causes a serious disease in humans, birds and horses, has appeared in USA in 1999 and spread extensively (Petersen and Roehrig 2001) Antibodies against other arboviruses including Japanese encephalitis virus have been found in sera of amphibians indicating infection (Doi et al 1983), but whether amphibians can develop viraemias capable of infecting mosquito vectors is unknown.

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Updated 9 March, 2003
Rick Speare