Uzročnik je bakterija po imenu Erysipelothrix rhusiopathiae, koja se i inače nalazi u organizmu zdravih svinja i to u tonzilama i u jednom dijelu crijeva, a da bi bolest nastala mora pasti opća otpornost organizma.
Za to su potrebi tzv. pogodovni čimbenici kao što su velike vrućine, nagle temperaturne promjene, nagle promjene tlaka zraka, svaka promjena hrane, stres tijekom transporta itd. Iako vrbanac ubrajamo u zarazne bolesti, ne radi se o kontagioznoj zarazi, što znači da ukoliko oboli jedna svinja u uzgoju, ostale svinje mogu i nemoraju oboliti.
Vrbanac je jedna od najpoznatijih zaraznih bolesti i gotovo da nema seoskog dvorišta gdje barem jednom nije neka svinja oboljela od ove bolesti.

Za ovu bolest je karakteristično da nastupa naglo, što znači da je životinja normalno jela ujutro, a uvečer već odbija hranu. Ubrzo se na površini kože pojave očarice (u narodu zvane fleke), koje su crveno ljubičaste boje, nepravilnog kvadratičastog ili rombodoidnog oblika. Ako takve očarice pipamo prstima osjeti se da su one za 1-2 mm uzdginute od okolnog tkiva.

Description
Erysipelothrix rhusiopathiae was first isolated by Koch in 1876.  In the genus Erysipelothrix - E. rhusiopathiae used to be the only named species, but now a non-pathogenic species E.tonsillarum has been identified based upon biochemical and genetic analyses.
Erysipelothrix rhusiopathiae is a pathogenic micro-organism which can infect a wide variety (over 50 species) of animals including vertebrates and non-vertebrates, from house flies to wild bears. It is especially common in domesticated pigs which can carry the organism and excrete it in their faeces without showing any external signs of infection, and a source of infection to humans are fish and marine animals.

Cause
Erysipelothrix rhusiopathiae is a non-motile, gram positive, nonsporulating rod and it is a facultative anaerobe.  Growth of this organism is improved by 5-10% CO2.  This micro-organism occurs in a variety of configurations including short chains, in pairs, in a "V" configuration and in random groups.  Virulence varies between different strains of this organisms, so-called "smooth" strains are pathogenic but "rough" forms are not.
Erysipelothrix rhusiopathiae usually enters its host through scratches or puncture wounds on the surface of the skin. The organism is resistant in the environment, and it is likely to be found in faecally contaminated environments, especially sewage and soil.

Breed Occurrence
Over 50 species of animals may be infected with this organism, but it is especially common in domesticated pigs. Adult pigs, and especially nursing sows, are more susceptible than others.

Signs

Four forms of the clinical effects of this organism (a disease called Swine Erysipelas)  have been identified in pigs, and they occur worldwide :
An acute form - septicaemia, fever, anorexia, diarrhoea, cyanosis and death
A subacute uriticarial form - diamond-shaped skin lesions, alopecia, sloughing of tail tip and ear tips, hyperkeratosis
A chronic  non-suppurative arthritic form 
A chronic cardiac form.- vegetative endocarditis

In sheep and lambs, and cattle  Erysipelothrix rhusiopathiae causes joint infection (called Joint Ill).
In birds (Ducks and Turkeys) the disease causes sudden deaths due to septicaemia
In fish this organism does not cause disease BUT it grows and persists for long periods on the body surface which  puts those who handle fish at high risk of contracting this organism.
Human infections are primarily found as a result of occupational hazards such as those who handle fish or pigs. In humans there are three clinical categories for the disease caused by this organism:
A localized cutaneous form (most common). Signs are usually a throbbing itching pain and swelling of the finger or part of hand
A generalized cutaneous form, and 
A septicemic form (associated with the heart disease endocarditis).


Diagnosis
Diagnosis is confirmed based upon isolation of the organisms from tissue biopsies or blood 

Treatment
A vaccine is available against Erysipelothrix rhusiopathiae infection in pigs, sheep and turkeys
The organism is sensitive to the following antibiotics :
Ampicillin
Penicillin

It is resistant to :
Aminoglycosides
Sulphonamides

Trihineloza