Presenting problem: “Red Leg” in Frogs

Understanding “red leg”

Red leg syndrome, also known as “pink belly disease” or bacterial dermatosepticemia, is one of the most common clinical conditions of captive frogs (Wright 2012, Densmore and Green 2007). Associated with peracute to acute bacterial septicemia, red leg is generally a disease of captive animals although the condition has also been implicated in rare mass mortalities of wild amphibians (Densmore and Green 2007, Wright and Whitaker 2001).

Clinical findings

The affected amphibian is often presented with pink or red skin lesions on pale areas of the body (Fig 1). Additional presenting complaints can include lethargy, anorexia, prolapse of the stomach or cloacal tissue, hemoptysis, twitching or convulsions, and sudden death (Densmore and Green 2007).

Red leg in a frog.

Figure 1. Red leg syndrome in an American toad (Bufo americanas). Photo credit: Dr. Ken Harkewicz

Erythema, caused by vasodilation, congestion, and hemorrhage, is a common physical examination finding and occurs most often on the hind limbs or ventrum (Densmore and Green 2007). Hemorrhagic ulceration of the skin on the tips of the toes sometimes progresses to exposed phalangeal bones or sloughed digits (Wright and Whitaker 2001). Additional findings can include ulcerated jaws, hemorrhagic vesicles, hydrocoelom, subcutaneous edema of the upper limbs and ventral abdomen, prolapse of the stomach or cloaca, tetany, and/or flaccid paralysis (Wright 2012). Ocular lesions may also be present and can include corneal edema, hypopyon, endophthalmitis, and exophthalmos (Wright and Whitaker 2001).

Gross necropsy findings may include fibrinocellular or sanguinous coelomic exudate, splenomegaly, nephritis, dilatative gastroenteritis, as well as petechia and ecchymoses in skeletal muscles, joints, coelomic mesothelial surfaces, and nearly any organ in which vascular thrombi and bacillary emboli can develop (Wright and Whitaker 2001).

Pathogenesis

Historically this syndrome was synonymous with Aeromonas hydrophila infection, however disease can also be associated with Flavobacterium as well as other Gram-negative bacilli including Pseudomonas spp., Citrobacter freundii, Proteus vulgaris, Klebsiella, and Salmonella (Wright 2012, Drake et al 2010, Densmore and Green 2007, Wright 2001). Gram-positive bacteria like Enterococcus, Streptococcus, and Staphylococcus aureus have also been linked to red leg syndrome (Drake et al 2010, Densmore and Green 2007).

Red leg syndrome (RLS) is often associated with poor husbandry, such as that due to poor maintenance of substrate, poor water quality, overfeeding, or lack of filtration. A detailed history  can identify deficits in diet and housing that can be corrected, possibly preventing spread of the condition within a collection.

Additional risk factors include mechanical damage to the skin, transport stress, recent acquisition, exposure to other infected animals, and/or low dietary vitamin A levels (Wright 2012, Drake et al 2010).

Signalment

All anuran ages are susceptible to RLS and there is no gender predisposition (Wright 2012). Susceptibility varies among species (Wright and Whitaker 2001). As the disease is more easily transmitted in water, aquatic species tend to be more prone to RLS. Terrestrial species like toads, other than during mating periods in water, are less prone to catching or spreading infection. Crowding frogs in small environments with poor water quality and filtration will also make transmission of RLS easier from one individual to the next.

Prognosis

The prognosis for RLS is often quite poor and the syndrome is associated with high mortality rates ranging from 81% to 100% (Wright 2012, Drake et al 2010). The chance of survival improves if the amphibian is still eating and shows only mild petechiation on physical examination (Wright 2012). If the disease is diagnosis early in its course and treatment stated early on, prognosis is better.

Key points of urgent care

Acute care of the suspect RLS patient is often aimed at preventing spread of infection to asymptomatic cage mates, and strict biosecurity practices are essential for success (Baitchman and Pessier 2013, Wright 2012). Use a new pair of gloves for handling each animal and change gloves before moving between enclosures (Baitchman and Pessier 2013).

Eliminate primary infection by treating suspected pathogens (Wright 2012). Empirical parenteral broad-spectrum antibiotic use is recommended for any amphibian with a suspected septicemia and a variety of regimens have been described for anurans suffering from RLS (Table 1) (Wright 2012, Densmore and Green 2007, Wright and Whitaker 2001). Include oxytetracycline if chlamydophilosis is suspected. Wright (2012). Injectable amikacin (Amikacin Sulfate Injection, Teva Pharmaceuticals) can be administered transdermally (2.5 mg/kg q48h) onto the dorsum. Fluoroquinolones are also a potential choice, however the alkaline pH of injectable enrofloxacin (Baytril 2.27%, Bayer Health Care) makes it extremely caustic to amphibian tissues. Dilute enrofloxacin in sterile saline at a 1:1 ratio and administer the antibiotic topically or subcutaneously at 10 mg/kg once daily (Baitchman and Pessier 2013). Dilution of medications with sterile water or saline may also be needed for dosing purposes. Dilution of medications with sterile water or saline, so that an easily measured volume like 0.1 ml per dose is possible, may also increase success of treatment.

Table 1. Antimicrobial regimens for the amphibian suffering from sepsis* (Carpenter 2013, Wright 2012)
RegimenDrugDose
#1Enrofloxacin**10 mg/kg topical or SC
#2Amikacin
 
2.5 mg/kg topical q48h for aquatic species and well hydrated terrestrial species.
#3Amikacin

3rd generation beta-lactam

Ex: Ceftazidime
2.5 mg/kg topical q48h
20 mg/kg SC, IM or topical q48-72h
#4Amikacin

Oxytetracycline
2.5 mg/kg topical q48h

50-100 mg/kg IM q48h
#5Amikacin

Metronidazole
2.5 mg/kg topical q48h

20 mg/kg PO q48h x 20 days OR

50 mg/kg PO q24h x 3 days
*  Dilution of medications with sterile water or saline is often required for dosing purposes

** The alkaline pH of injectable enrofloxacin makes it extremely caustic to amphibian tissues. Dilute enrofloxacin in sterile saline at a 1:1 ratio.

Batrachochytrium dendrobatidis infection is an important differential diagnosis for RLS. An itraconazole bath is the treatment of choice for B. dendrobatidis. Although itraconazole can have potentially toxic side effects in many species, chytridiomycosis can be rapidly fatal without treatment (Holden et al 2014, Wright 2012).

House the frog on clean, moistened paper toweling during treatment and provide supportive care, including nutritional support, fluid therapy, and thermal support for species such as waxy monkey tree frogs (Phyllomedusa sauvagii) or White’s tree frogs (Pelodryas caerulea), these species require temperatures above room temperature. The vast majority of frogs do well at room temperatures cooler than 72°F (22°C).

Maintain electrolyte balance with the use of amphibian Ringer’s solution (ARS) as a 24-hour bath (Wright 2012). To provide supplemental oxygen, maintain patients in clear plastic bags containing a small amount of ARS. The bag is then inflated with oxygen. Fluids and oxygen are changed periodically, often three times daily, depending on patient size and the volume of liquid and air in the bag. After an initial 8-14 hour period of cooling to slow bacterial growth, providing a basking spot to permit behavioral fever may also be indicated (Wright 2012).

Once urgent care is provided, refer the patient to an experienced herptile veterinarian whenever possible.

Differential diagnoses

There are a variety of pathogens that may present with clinical signs similar to RLS. Important differentials include chytridiomycosis, ranaviruses, and chlamydophilosis (Table 2) (Densmore and Green 2007).

Table 2. Differential diagnoses for red leg syndrome in amphibians* (Wright 2012, Densmore and Green 2007, Wright and Whitaker 2001)
DDegenerative
AAnomaly
MMetabolic
NNeoplasia
NutritionHypovitaminosis A
IInfectionChytridiomycosis
Mucormycosis
Iridovirus
Ranaviruses
Chlamydophilosis
Mycobacteriosis
Cutaneous nematodiasis, roundworms
InflammationChemical irritation (such as poor water quality)
Infarct
TTraumaThermal trauma (contact with a heated substrate)
Physical trauma  (including stress of being handled)
ToxinAmmonia toxicosis
*Important differentials are bolded

CHYTRIDIOMYCOSIS

The non-hyphal, zoosporic, chytrid fungus Batrachochytrium dendrobatidis is a recently emerged pathogen that causes the infectious disease chytridiomycosis (Table 3) (Stadler 2013, Stockwell et al 2012, Densmore and Green 2007). This pathogen has caused widespread and dramatic population declines in both wild and captive amphibians worldwide (Murphy et al 2015, Stadler 2013, Stockwell et al 2012, Densmore and Green 2007). Often a primary condition, the chytrid fungus can also serve as an opportunistic organism causing secondary fungal infection in immunocompromised individuals.

CHLAMYDOPHILOSIS

Infection caused by bacteria from genus Chlamydophila has been reported in wild and captive anurans (Densmore and Green 2007). The most common etiologic agents include Chlamydophila psittaci and C. pneumoniae (Densmore and Green 2007). Reported clinical signs include petechia, skin sloughing, hydrocoelom, accumulation of excess fluid in lymphatic sacs, lethargy, and cutaneous depigmentation (Densmore and Green 2007). The liver, spleen, and kidney may be gross enlarged and there may be histological evidence of marked histiocytic or granulomatous inflammation (Densmore and Green 2007). Recommended treatment regimes include oral tetracycline (150 mg/kg PO q24h x 5-7 days) (Carpenter 2013, Densmore and Green 2007).

RANAVIRUSES

The ranaviruses are the best described group of pathogenic amphibian viruses belonging to family Iridoviridae (Densmore and Green 2007). Ranaviruses are frequently associated with die-offs and acute population declines in wild amphibians and are rarely isolated from clinically normal individuals (Densmore and Green 2007). Clinical signs may include non-specific signs of illness such as lethargy and anorexia, neurologic deficits such as abnormal body posture or abnormal swimming behavior, raised, vesicular or erosive skin lesions, and erythema. Red skin is often associated with petechial or paintbrush hemorrhages, particularly around the mouth or base of the hindlimbs (Densmore and Green 2007). Ranavirus infection can be diagnosed through cell culture, PCR, and/or histopathology (Densmore and Green 2007). Johnson and Wellehan (2005) have postulated that acyclovir may have a potential clinical use in controlling disease.

Diagnostics

Red leg has been described as one of the “most overdiagnosed and misdiagnosed” diseases of amphibians (Densmore and Green 2007). Physical examination findings can include many of the clinical signs observed in many clinically ill amphibians such as hyperemia, hydrocoelom, subcutaneous edema, and/or prolapse (Wright 2012, Wright and Whitaker 2001). An important part of the exam in the aquatic frog should also include water quality testing. Ammonia and pH levels should be evaluated to rule out topical irritation to the skin as a source of the erythema.

Additional diagnostic tests include microbial culture and cytology.

  • Perform cytology, including wet mounts, on skin scrapes to look for fungi and protozoa, (Wright 2012). On rare instances, the presence of elementary bodies may be seen with cytologic staining suggesting chlamydophilosis (Wright 2012).
  • Perform aerobic and anaerobic culture and antibiotic sensitivity testing of cardiac blood or coelomic fluid from both affected and unaffected specimens (Wright 2012, Densmore 2007). Keep in mind that aeromonads, including A. hydrophila, can be cultured from clinically normal, aquatic and terrestrial amphibians (Drake et al 2010, Wright and Whitaker 2001).
  • Diagnosis of RLS is particularly unreliable when it is based solely on culture results from necropsy samples. Postmortem bacterial invasion of organs occurs rapidly in amphibians and culture should not be attempted from amphibians that died more than 1 to 3 hours beforehand (Densmore and Green 2007).

Advanced or confirmatory testing may include biopsy of lesions for histopathology and viral isolation if ranavirus or another virus is suspected (Wright 2012).

Treatment

Depending on the underlying cause, chronicity of disease, and efficacy of treatment, management of RLS can be impractical or unsuccessful in many cases (Densmore and Green 2007). Culture and sensitivity test results may suggest a change in antimicrobial therapy, although the regimen should not be changed if the patient appears to be responding (Wright 2012). Consider prophylactic antifungal therapy when the patient is placed on broad-spectrum antibiotics to prevent overgrowth of opportunistic fungal organisms. Remember that hygiene is essential; patients should be returned to a clean, disinfected container after each treatment (Stadler 2013).

Additional case management should focus on client education and improvement of species-specific husbandry. Consider stripping down and disinfecting the enclosure, replacing all cage furniture and removing all cage substrate as they can be sources of reinfection (Wright 2012).

Nebulization with quaternary ammonium and biguanidine disinfectant (F10SC Veterinary Disinfectant; Health and Hygiene) (1:250 diluted in water q24h) was recently described as an adjunctive treatment in tomato frogs (Dyscophus guineti). The mortality rate in this collection was approximately 14.3%, much lower than the average RLS outbreak and the authors recommended this treatment as a potentially less stressful route of drug administration for terrestrial species (Drake et al 2010).

Prevention and control

Appropriate quarantine practices are important. A minimum of 30 days is recommended in low-risk amphibians, however a 60-90 day quarantine is ideal in many species (Wright and DeVoe 2013). Proper caging and diet, including appropriate vitamin A levels, is also important to maintain healthy amphibian skin (Wright 2012).

Tadpoles

In tadpoles afflicted with RLS, erythema and petechia are noted on skin of the ventral and lateral body as well as the tail. Hemorrhages may also be present over the eyes in bullfrog tadpoles (Wright and Whitaker 2001). For the important differential chytridiomycosis, tadpoles are usually infected subclinically however the mouthparts can be affected. For diagnostics, swabs can be collected from the mouthparts of either live or dead tadpoles (Stadler 2013).

Summary

Red leg syndrome is one of the most common clinical conditions of captive frogs. The classic physical exam finding is hyperemia of pale areas of the body, however there are many other potential abnormalities ranging from hemorrhagic vesicles and ulceration to subcutaneous edema and coelomic effusion. Red leg syndrome is also known as bacterial dermatosepticemia and this condition is classically caused by Aeromonas spp. infection. Disease can also be associated with other Gram-negative or Gram-positive bacteria. Additional risk factors for RLS include poor husbandry, skin lesions, hypovitaminosis A, and transport stress. Cytology and microbial culture are important diagnostic tests, however culture samples should always be collected promptly after death. Perform water quality testing to rule out topical irritation in aquatic frogs. Other important differential diagnoses for RLS include chytridiomycosis, ranaviruses, and chlamydophilosis. The prognosis for RLS is very poor and treatment is generally less successful than preventing spread of disease in a group. Biosecurity is crucial and owner education should also focus on improving husbandry. Enclosures should also be stripped down and disinfected. All cage furniture should be removed and all cage substrate replaced as these items can be a source of reinfection. Affected anurans should be started on empirical broad-spectrum antibiotics. Affected animals are also usually started on prophylactic antifungal therapy to prevent overgrowth of opportunistic fungal organisms. Patients should be returned to a clean, disinfected container after each treatment.

References and further reading