Inclusion Body Disease of Boid Snakes
History:
Inclusion body disease of boid snakes has been recognized since the mid 1970's. It is named for the characteristic intracytoplasmic inclusions that are seen in epidermal cells, oral mucosal epithelial cells, visceral epithelial cells, and neurons. In the 1970's, through the late 1980's, this disease was most commonly seen in Burmese pythons, Python molusus bivittatus. Starting in the late 1980's until present, it has been seen most commonly in boa constrictors, Boa constrictor.
Host:
Inclusion Body Disease (IBD) has been identified in boid snakes including multiple subspecies of the boa constrictor (Boa constrictor), green anaconda (Eunectes murinus), Haitian boa (Epicrates striatus), Burmese python (Python molurus bivittatus), Indian python (P. m. molurus), reticulated python (P. reticulatus), ball python (P. regius), carpet python (Morelia spilota variegata) and diamond pythons (M. s. spilota). In addition, a disease resembling IBD was diagnosed in an eastern king snake (Lampropeltis getulus) that was housed with boa constrictors and in palm vipers (Bothriechis marchi). Tissues to be used in the proposed study are already in the PI's tissue repository.
Distribution:
Worldwide in captive boid snakes. Its occurrence in the wild is unknown. Several cases have recently been seen in captive pythons in Australia, Canary Islands, and Italy. The transport of captive snakes in the pet trade and transport between different zoologic institutions probably account for the spread around the world.
Ages Affected:
IBD has been identified primarily in adult snakes. However, all age groups should be considered susceptible. There are anecdotal reports of infection in neonates.
Etiologic Agent:
A retrovirus has been isolated from boa constrictors with IBD and may be the causative agent (Figure 1). However, we currently do not know if the isolated virus is the cause of IBD. Research is ongoing to determine the relationship, if any, between this virus and the formation of inclusions in cytoplasm of cells in infected snakes.
Clinical Signs:
Clinical signs are quite variable. Regurgitation and signs of central nervous system disease (Figure 2; Figure 3; Figure 4A and Figure 4B) are commonly seen in boa constrictors. Stomatitis, pneumonia, undifferentiated cutaneous sarcomas (Figure 5), lymphoproliferative disorders, and leukemia has all been seen. Burmese pythons generally show signs of central nervous system disease without manifesting any other clinical signs; regurgitation is not seen in Burmese pythons.
Pathology:
By light microscopy, in hematoxylin and eosin stained tissues sections of a wide variety of epithelial and neuronal cells, characteristic intracytoplasmic inclusions are seen. Several snakes have been seen with proliferative pneumonia (Figure 6). While inclusions are commonly seen in the liver, kidney, and pancreas (Figure 7; Figure 8; Figure 9), we have seen cases where there are very few inclusions. Thus, inclusions may be missed by pathologists in cases having few inclusions in the brain or other tissues. While the presence of characteristic inclusions is diagnostic for the disease, the absence of inclusions does not necessarily mean the snake is disease or IBD virus free. While cells having inclusions may show mild degenerative changes, inflammation is rarely seen in visceral tissues. In the brain, mild to severe encephalitis, with lymphocytic perivascular cuffing may be seen. Several snakes with lymphoproliferative disorders have been identified with lymphoid infiltrates in multiple organs. Inclusions can also be seen in lymphocytes and other white blood cells in peripheral blood films stained with Wright-Giemsa or the following modified hematoxylin and eosin (H&E) technique:
- Fix 1 minute in 10% neutral buffered formalin
- Stain 3 minutes in Harris hematoxylin
- One dip into acid alcohol
- Briefly wash in running water
- Dip into 0.5%ammonia to blue the nuclei
- Wash in running water
- Counterstain in eosin for 40 seconds
- Dehydrate in a series of alcohols: 95%-100%-100%
- Place in xylene and mount as with a paraffin embedded section
I have found this staining far superior than using Wright-Giemsa staining. With Wright-Giemsa staining the inclusions stain basophilic and may not be readily recognizable. With
H&E staining, inclusions stain exactly the same as in tissues embedded in paraffin, sectioned
and stained with H&E. Blood films should first be examined before tissue biopsies are
obtained and submitted for histopathology. However, we do not know how often inclusions
are seen in a peripheral blood film of a snake with IBD. It must be remembered that absence
of inclusions in a blood film does not necessarily mean the snake is free of IBD.
Transmission:
Exact route of transmission has not been identified. Possibly by: 1) direct contact; 2) intrauterine transmission to developing embryos in viviparous species and eggs in oviparous species; 3) venereal transmission. The snake mite, Ophionyssus natricis has been implicated as a vector for the virus since mite infestations are commonly seen in epizootics of IBD.
Research Need:
Currently, a presumptive diagnosis is based on the light microscopic identification of
intracytoplasmic inclusions in tissue sections. Some snakes have numerous inclusions in tissues and others have few. A more sensitive and specific diagnostic test is needed such as a
Western blot or molecular based PCR test that can be used to screen individuals and colonies
of snakes at risk. Deducing the DNA sequence that codes for the IBD protein and sequencing
retroviruses that have been isolated from snakes with IBD will be a major step in determining
the cause of IBD and will allow us to develop molecular and/or immunodiagnostic tests of
greater sensitivity and specificity than currently used histopathology.
To date we have received approximately $30,000.00 in support of research on IBD. Past research support has come from several primate donors, the National Reptile Breeders’ and Mid-Atlantic Reptile Expos. While we have made some progress, a sensitive and specific molecular based test is needed in order to diagnose IBD. Graduate student and post-doctoral fellows who have a very focused project do most research in laboratories. A graduate student is now specifically working on IBD in my laboratory. However, additional money is needed for more rapid progress to be made. Given the number of snakes that have died of this disease over the last 30+ years, IBD should be considered the most important health problem of captive snakes in the world today. Hopefully as the significance of this disease is appreciated, more money will become available.
References:
- Schumacher J, Jacobson ER, Homer BL, Gaskin JM. 1994. Inclusion body disease in
boid snakes. J Zoo and Wildlife Med 25(4):511-524.
- Axthelm MK. 1985. Viral encephalitis of boid snakes. Int Colloq Pathol Reptiles Amphib 3:25. (Abstract)
- Carlisle-Nowak MS, Sullivan N, Carrigan M, Knight C, Ryan C, and Jacobson ER. 1998.
Inclusion body disease in two captive Australian pythons (Morelia spilota variegata and
Morelia spilota spilota). Aust Vet J 76:98-100.
- Jacobson ER, Klingenberg RJ, Homer BL, Mader DR. 1999. Inclusion body disease. Bull
of the Assoc Reptil Amphib Vet. 9:18-25.
- Jacobson ER, Oros J, Tucker S, Pollock D, Vaughn K, Munn RJ, Lock B, Mergia A,
Yamamoto JK. 2001. Isolation and characterization of retroviruses from boid snakes with
inclusion body disease. Am J Vet Res 62:217-224.
- Oros J, Tucker S, Jacobson ER. 1998. Inclusion body disease in two captive boas in the
Canary islands. Vet Rec 143:283-285.
- Raymond JT, Garner MM, Nordhausen RW, Jacobson ER. 2001. A disease resembling
includion body disease of boid snakes in captive palm vipers (Bothriechis marchi). J Vet
Diagn Investig 13:82-86.
- Wozniak E, McBride J, DeNardo D, et al. Isolation and characterization of an antigenically distinct 68-kd protein from nonviral intracytoplasmic inclusions in boa constrictors chronically infected with the inclusion body disease virus (IBD: retroviridae). Vet Pathol 37:449-459.
For More Information Contact:
Dr. Elliott Jacobson
PO Box 100126
Department of Small Animal Clinical Sciences
College of Veterinary Medicine
University of Florida
Gainesville, Florida 32610-0126
E-mail: JacobsonE@vetmed.ufl.edu