Red leg disease in amphibians

Red leg disease is a form of septicaemia that affects amphibians, particularly those in captivity. This disease may have a multifactorial aetiology, but the bacterium considered principally responsible is Aeromonas hydrophila.

The infection is transmitted via contaminated water in which subjects are kept or following the introduction of sick animals or asymptomatic carriers into an aquarium or paludarium. The presence of the bacteria may pass unobserved, particularly if the bacterial load is fairly low, but predisposing factors, such as overcrowding, stress, high temperature, malnutrition and defective or poorly serviced water treatment systems can lead to the development of a clinically overt form of the disease.

The infection is able to cause septicaemia and in acute cases, the affected animals may die suddenly. In subacute cases, non-specific signs, such as loss of appetite, weight loss (Fig. 1) and apathy, appear first and are then followed by reddish colouring of the skin, particularly of the belly (Fig. 2) and the distal part of the limbs (Fig. 3), due to diffuse oedema and bleeding. Subsequently this colouring can extend to the animal’s whole body before the disease causes the frog’s death. This phenomenon is, for obvious reasons, more evident in pale-skinned animals than in those with pigmented skin (Figs. 4 and 5).

In chronic cases, the disease may be present for several months and oedema and bleeding may be found in the oral cavity accompanied by ulcers of varying depth. Histological examination reveals subcutaneous, muscular and gastro-oesophageal haemorrhages while the fluid within the lymph sacs is bloody.

The diagnosis is based on the clinical signs, which are basically pathognomonic, and culture studies (Fig. 6); a subsequent antibiogram should be carried out using fluid collected by a sterile technique from the lymph sacs.

Animals with overt symptoms should be isolated immediately; the animals’ container or aquarium should be disinfected and the water changed. When possible the temperature of the water in which the animals are kept should then be gradually reduced (18-19°C) in order to inhibit replication of the pathogen.

The prognosis of animals with overt symptoms is almost always dismal.

Various antibacterial chemotherapeutic agents can be used for the treatment, as reported in Table 1. It is best to use a gastric tube for drugs to be given orally.

Table 1. Antibiotics used in the treatment of red leg disease (From: Prontuario Terapeutico SCIVAC, 5^th edition, 2009 and from: Gabrisch and Zwart, Utet, 2001).