Erysipelas is an infectious disease caused by Erysipelothrix rhusiopathiae seen mainly in growing pigs and characterized clinically by sudden death, fever, arthritis, and skin lesions. The disease may be acute, subacute, or chronic. Although acute septicemic swine erysipelas can result in a high mortality rate, the greatest economic loss probably occurs from the chronic, nonfatal forms of the disease.
E rhusiopathiae is a gram-positive bacillus. Colonies on agar media are grayish translucent and nonhemolytic with smooth or rough morphology. At least 28 different serovars exist. Serovars 1 and 2 and strains forming smooth colonies are most commonly isolated from pigs with the septicemic form of the disease. E rhusiopathiae can survive for several months in animal tissue, eg, frozen or chilled meat, cured and smoked ham, and dry blood. It can survive in swine feces for up to 6 mo at temperatures below 54°F (12 °C).
On farms where the organism is endemic, pigs are exposed naturally to E rhusiopathiae when they are young; their maternal antibodies provide a degree of active immunity without visible disease. The organism is excreted by infected pigs in feces and/or oronasal secretions and survives for short periods in most soils. Recovered pigs and those chronically infected may be carriers of the organism, possibly for life. The mode of entry is by ingestion and through skin abrasions. Following ingestion, the organism most likely enters the body via the tonsils or lymphoid tissue of the GI tract.
The acute, subacute, and chronic forms of swine erysipelas may occur in sequence or separately. Pigs with the acute septicemic form may die suddenly without previous signs. This occurs most frequently in finishing pigs (100-200 lb [45-90 kg]). Acutely infected pigs are febrile (104-108°F [40-42°C]), walk stiffly on their toes, lie on their sternums separately rather than piling in groups, and are reluctant to move. They squeal plaintively when handled and may shift weight from foot to foot when standing. Anorexia and thirst are common. Skin discoloration may vary from widespread erythema and purplish discoloration of the ears, snout, and abdomen, to diamond-shaped skin lesions almost anywhere on the body, but particularly the lateral and dorsal parts. The lesions may occur as pink or light-purple areas of varying size that become raised and firm to the touch within 2-3 days of illness. They may disappear or progress to a more chronic type of lesion such as diamond-skin disease. If untreated, necrosis and separation of large areas of skin can occur, but more commonly, the tips of the ears and tail may become necrotic and slough.
Clinical disease is usually sporadic, and affects individuals or small groups, but sometimes larger outbreaks occur. Mortality is 0-100%, and death may occur up to 6 days after the first signs of illness. Acutely affected pregnant sows may abort, probably due to the fever, and suckling sows may show agalactia. Untreated pigs may develop the chronic form, usually characterized by chronic arthritis, vegetative valvular endocarditis, or both; such lesions may also be seen in pigs with no previous signs of septicemia. Valvular endocarditis is most common in mature or young adult pigs and is frequently manifest by death, usually from embolism or cardiac insufficiency. Chronic arthritis, the most common form of chronic infection, produces mild to severe lameness; the affected joints may be difficult to detect but tend to become hot and painful to touch and later visibly enlarged and firm, resulting in lameness. Dark purple, necrotic skin lesions that commonly slough may be seen. Mortality in chronic cases is low, but growth rate is retarded.
In acute infection, in addition to skin lesions, lymph nodes are usually enlarged and congested, the spleen is swollen, and the lungs are edematous and congested. Petechiae may be found in the kidneys, heart, and occasionally elsewhere. In chronic erysipelas, valvular endocarditis is seen as proliferative, granular growths on the heart valves, and embolisms and infarctions may develop. Arthritis may involve joints of one or more legs or the intervertebral articulations; the joint enlargement is proliferative but nonsuppurative, and tags of granulation tissue form in the articular cavity. In chronic cases, there may be proliferation and erosion of the articular cartilage; this may be followed by fibrosis and ankylosis of the joint.
Acute erysipelas is difficult to diagnose in individual pigs showing only fever, poor appetite, and listlessness; however, in outbreaks involving several animals, the presence of skin lesions and lameness is likely to be seen in at least some cases and would support a clinical diagnosis. Erysipelas responds extremely well to penicillin—a marked improvement within 24 hr also supports the diagnosis. The typical diamond-shaped skin lesions are diagnostic. Arthritis and endocarditis are difficult to diagnose in live animals because other agents can cause similar syndromes. Serology can prove unreliable, although a rising titer in an agglutination test (with controls) is helpful, as is the complement fixation test. An ELISA has been developed and is considered reliable for chronic infections on a herd basis.
At necropsy, demonstration of the organism in stained smears or cultures confirms the diagnosis, although in chronic arthritis cases, organisms may not be cultured. E rhusiopathiae can be isolated readily on blood agar plates from spleen, kidney, and long bones of acutely sick pigs (and from the tonsils and other lymph nodes of many apparently healthy ones).
Penicillin is the drug of choice for the treatment of acutely affected pigs, and it has been used concurrently with antiserum. Penicillin should be given daily for 2-3 days; alternatively, a long-acting form may be used. Improvement is usually seen in 24 hr. Treatment of chronic infection is usually ineffective or not cost effective, and such pigs should be culled. If acute cases develop suddenly in an unvaccinated herd, antiserum, if available, may be administered to in-contact pigs. It may be more cost effective to use a long-acting form of penicillin or tetracyclines before implementing a vaccine program.
Prevention is best achieved by regular vaccination using killed bacterins or, in some countries, attenuated vaccines prepared by serial passage or strains of low virulence for pigs. The formalin-killed, aluminum-hydroxide-adsorbed bacterin confers an immunity that, in most instances, protects growing pigs from acute disease until they reach market age. An oral vaccine of low virulence is also used. Young breeding stock, including boars, should be vaccinated twice at intervals of 3-5 wk at selection or before entering the herd, and then revaccinated every 6 mo or after each litter. Piglets born to vaccinated sows will be protected for 10-12 wk. Further protection will require vaccination at 12 wk. A booster vaccination 3-5 wk later is recommended. Vaccination of heavily pregnant sows is not advisable.
Vaccination raises the level of immunity but does not provide complete protection. Acute cases may develop after stress, and protection may not be provided against the arthritic or cardiac forms of the disease. Antigenic variation exists between bacterial strains, so a vaccine may not be equally effective against all wild strains.
Good sanitation, efficient disposal of feces, and regular disinfection of pens is also important in the prevention of erysipelas.