As a bit of self-therapy, I have decided to deliberately sift through some of the most prevalent pathogens that effect the frog population of our world. I have (temporarily) set aside my severe repugnance (*croak*) towards amphibians, in general, to take some time to learn about the very thing I sometimes will to become extinct (I’m telling myself that I know better, though). I will not allow my nasty attitude about an important organism within our delicately balanced ecosystem to overshadow my belief that each creature has immense importance in this world, which is why I am subjecting myself to this study. Apparently, frogs deserve some respect in the circle of life considering they do serve as a central component in many ecological systems. They play an important role in the food chain by consuming insects and serving as a food source for larger animals. Due to their permeable skin and need for water and land habitats for survival, frogs also serve as a great indicator of the health of various environments. Likewise, if you have ever dissected a frog in a science class, you will understand how they serve as a great model for us to better understand anatomy, histology, reproduction, etc. of not only amphibians, but birds, mammals, and reptiles as well.
Frogs have somehow even managed to have a date devoted to their salvation – it appears that on April 29th frog lovers unite and celebrate the world’s various slimy, jumpy, unpredictable creatures.
The first pathogen that is of particular concern for the frog population of the world is a chytrid fungus, Batrachochytrium dendrobatidis. This fungus causes the disease known as chytridiomycosis and is a generalized amphibian pathogen that has been attributed to a global, dramatic amphibian decline in recent years. The disease usually proves to be fatal, with the fungi dysregulating the amphibian’s osmoregulatory and ionic transport systems, eventually causing death. The lethality of the pathogen can be attributed to its apparent ability to infect most of the world’s amphibian species (around 6,000), with 100% mortality rates in many of the populations. The disease affects the outer skin layers of frogs that are largely comprised of keratin. Hyperplasia and hyperkeratosis causes the skin of the frogs to dramatically thicken, therefore starving the animal of water and electrolytes from lack of absorption through the skin. In turn, this imbalance can cause the heart of the infected animal to cease.
Fig. 1 – Before and after of a poor little frog suffering from chytridiomycosis. Note the thickening of the skin – environment.nsw.gov.au
Scientists first noticed a decline in frog populations, and even extinctions in some species, in the 1970s and 80s. The fungus was first identified and characterized in the late 1990s and is thought to have spread throughout the world to countless species by the sale and trade of pet amphibians. Currently, about 500 amphibian species are believed to be diminished by this fatal fungus and about 90 species have been presumed as extinct. The fungus thrives in cool, moist environments, which reflects in the frog decline of species that reside in this type of habitat.
Several species from the Ranavirus genus have also been found to have caused a mass mortality in the wild frog population (though not nearly as severe as the fungi described above). The ranavirus is highly virulent and may cause 90-100% mortality in adult frogs and tadpoles. Fish and reptiles, especially turtles, are the reservoir for the ranavirus, with transmission to frogs occurring through contact with an infected animal, ingestion of infected tissues, and/or contact with contaminated soil or water. The virus has the capability of persisting in aquatic environments for weeks without a reservoir, contributing to its infective capability. Larvae undergoing metamorphosis and juvenile frogs appear to be at the highest risk for contracting the ranavirus due to their vulnerable state. Often thin and lethargic before death, many dead frogs found together can be an indicator of the presence of this disease. Other symptoms include redness of the skin, ulcers or limb necrosis, eye problems, and generalized bleeding; however, much about this virus and how it interacts between animals and within the environment is yet to be understood.
Aeromonas hydrophila is a Gram-negative, rod-shaped, bacterium that is widely distributed throughout nature, especially in warm, fresh and brackish waters. It is also cited as causing widespread mortality in captive populations of frogs. However, due to the bacterium’s ubituitous nature and for reasons below, specific data on mortality and disease prevalence caused by A. hydrophila is difficult and still under research. A. hydrophila is thought to infect immunocompromised frogs (stressed, parasitized, during breeding periods or cold temperatures), as it is a ubiquitous opportunistic bacterium. However, researchers have established that opportunistic pathogens can have drastic, sudden effects on host populations, sometimes even altering the ecology of the impacted community. Although viruses and fungi are also capable of causing what we know as “red leg” syndrome in frogs, Aeromonas hydrophila is generally the causative agent, although this disease can be caused by a multitude of different causative agents. Red leg syndrome (technically referred to as bacterial dermatosepticemia) presents itself as reddening on the frog’s abdomen and underside of the legs. This disease is due to the bacteria’s production of an endotoxin and hemolysin. Together along with stress factors, the activity of the hemolysin along with the endotoxin-infected cells causes the signature “red leg” sign of infection.
Fig. 2 – Ventral, cutaneous hyperemia and erythema (“red leg”) in a juvenile American tree frog. This little guy was treated with 0.3% gentamicin sulfate ophthalmic solution (a broad spectrum antibiotic with its mechanism of action being the inhibition of bacterial protein synthesis). After 2 weeks of treatment, the frog made a full recovery and “now actively tried to avoid handling, and was able to jump and climb appropriately” – nature.com
In honor of my One Health studies, I would also like to remind everyone that frogs are carriers of the bacterium Salmonella typhimurium which can infect humans (triggering the occasional outbreak) and cause its victims to suffer from some pretty miserable symptoms including diarrhea, fever, and abdominal cramps ~12-72 hours after infection. These symptoms usually last anywhere from 4-7 days…A frog is no prince. So, I can’t believe I’m typing this, but no kissing or snuggling, and wash your hands after touching, if you must.
Now every recommended article on my computer and phone is about frogs and I couldn’t eat breakfast. Still get the willies at every frog picture but I guess exposure is the best treatment. Save the frogs!
Sources and Supplemental Information
https://phys.org/news/2017-05-frogs.html
https://www.environment.nsw.gov.au/
http://www.amphibianark.org/the-crisis/chytrid-fungus/
Cádiz, A., Reytor, M.L., Díaz, L.M. et al. The Chytrid Fungus, Batrachochytrium dendrobatidis, is Widespread Among Cuban Amphibians. EcoHealth 16, 128–140 (2019). https://doi.org/10.1007/s10393-018-1383-9
DEAN MCCURDY, PH.D. AND KRISTEN KRUM PREVALENCE OF AEROMONAS HYDROPHILA IN FROGS AND TOADS AT THE PIERCE CEDAR CREEK INSTITUTE, BIOLOGY DEPARTMENT, ALBION COLLEGE, SEPTEMBER 22, 2005
Rigney, M.M., Zilinsky, J.W. & Rouf, M.A. Pathogenicity ofAeromonas hydrophila in red leg disease in frogs. Current Microbiology 1, 175–179 (1978). https://doi.org/10.1007/BF02601673
Diagnosis: Redleg in a Tree Frog. Lab Anim 34, 26–27 (2005). https://doi.org/10.1038/laban0305-26
