Herd Health: New, advanced tools for diagnosing erysipelas
Wednesday, December 2, 2009
Erysipelas diagnostics are catching up to the advances in molecular diagnostics, resulting in delivery of markedly more positive and quicker results
by S. ERNEST SANFORD
Erysipelas is a bacterial disease, primarily found in pigs, caused by Erysipelas rhusiopathiae. The disease occurs wherever pigs are raised worldwide and causes acute systemic and chronic infections. Severe outbreaks can induce substantial morbidity and mortality within days in a susceptible pig population.
Although erysipelas outbreaks are less common today than in past years, its red, diamond-shaped skin lesions are still well known and recognizable to almost everyone associated with the swine industry. Other manifestations of erysipelas include arthritis, endocarditis and abortion.
Historically, valvular endocarditis was caused almost exclusively by the erysipelas organism. It should be noted, however, that in recent decades the vast majority of valvular endocarditis cases are caused by Strep suis. Erysipelas was much more common in growing pigs several decades ago, but dropped off significantly as we moved to totally indoor, confinement rearing operations.
However, there has been a resurgence of erysipelas outbreaks in many regions of North America in recent years, suspected to be due to the emergence of more virulent forms of the organism or new strains to which pigs have no immunity and against which vaccines do not provide full protection. Investigations have not supported any of these theories thus far.
My intent in this article is to highlight several of the new diagnostic tools that are now available for erysipelas cases. But before I do that, I'll go over aspects of the epidemiology, as well as treatment and control measures for erysipelas.
The erysipelas bacterium, E. rhusiopathiae, is commonly cultured from the tonsils of normal healthy pigs. These carriers can shed the organism in their feces and oronasal secretions, thus contaminating the environment and providing a fertile source of infection. The organism lives in places habituated by pigs and can be cultured from the soil, manure, pen floors, slats, bedding, feed troughs and drinking water, especially where clinical disease has occurred previously.
Pigs that have recovered from a recent infection can shed the organism in feces and urine and it can survive in the ground for more than a month. Birds, rodents and other animals may be infected and can pass the organism to pigs. Outbreaks are still most common in outdoor pigs, pigs raised under poor hygiene conditions, newly purchased replacement gilts and during periods of hot weather.
Although many new high powered antibiotics are effective against Erysipelothrix spp., penicillin remains the drug of choice for treatment. Response is rapid. Repeated treatments might be necessary over the next two to three days to prevent relapses or persistence of the organism leading to chronic infection.
Diagnosis of erysipelas had become stagnated, limited to culture of blood and tissue samples from acutely infected, untreated pigs incubated on blood agar plates. These culture attempts very often resulted in no growth or overgrowth by other more aggressive agents on the laboratory plates. Today, however, erysipelas diagnostics are catching up to the advances in molecular diagnostics, including DNA analytical methods, resulting in delivery of markedly more positive and quicker results.
The practicing veterinarian can now diagnose more complicated cases of erysipelas by submitting blood and/or tissue samples for immunohistochemistry (IHC) and polymerase chain reaction (PCR) examinations at the diagnostic laboratory. IHC and PCR are done on either tissue samples taken at post mortem or from blood samples taken from acutely infected pigs.
One other new test is the pulse field gel electrophoresis (PFGE) test that is not available at most diagnostic laboratories. As a result, this test is seldom used but, when available, it serves to answer very critical questions. PFGE is also useful for epidemiological investigations. These new tests can also be used for trace-backs from slaughterhouse condemnations.
Submission of samples to the diagnostic laboratory for confirmation of the diagnosis of erysipelas must start with choosing an untreated animal, then making a choice of the following tissues to be submitted: Skin lesions, kidney, liver, spleen; blood from an acutely infected pig.
Joint fluid is often submitted for culture because of obviously distended joints and increased joint fluid seen grossly at post mortem. Joint lesions are usually chronic manifestations of the disease and the lesions seen grossly are usually the result of chronic immune reaction and not direct inflammation to the erysipelas organism. The end result is negative growth on culture.
Here are some of the tests currently available in some diagnostic laboratories:
Routine bacteriological culture
This is the standard test, but it has several drawbacks often resulting in no growth or overgrowth of the organism.
Enrichment bacteriological medium
An enhanced bacterial method that improves ability to grow the organism. Reported by D. Richard Wood in Iowa more than 30 years ago, it is seldom utilized anywhere for routine bacteriological work. Iowa State seems to have recently rediscovered this technique and is reporting markedly increased isolations of the organism. The test is seldom used elsewhere.
IHC
The IHC test is highly sensitive and specific in detecting Erysipelothrix spp. organisms in formalin-fixed tissues submitted to the laboratory for histological examination. This test is especially useful in cases where the pigs have been treated with antibiotics, meaning that it is virtually impossible to culture the organism via routine or enhanced bacteriological methods. The test is fast and inexpensive.
PCR
A PCR test has been developed that differentiates among different serotypes of Erysipelothrix spp. Results can be obtained in as little as five hours, but the test is expensive compared with other methods.
PFGE
This test can also differentiate genotypes and serotypes of Erysipelothrix spp. It does not depend on antibody or antiserum, but it has very limited availability and is used by very few diagnostic laboratories. It is also very time-consuming, taking a minimum of four days to get an answer.
To sum up, Erysipelas caused by Erysipelothrix rhusiopathiae is a very old and well recognized disease, especially when the signature diamond-shaped lesions are seen on the skin. Acute erysipelas outbreaks can result in very high morbidity and mortality. Even after many decades, penicillin is still the antibiotic of choice in treating outbreaks of erysipelas. BP
S. Ernest Sanford, DVM, Dip. Path., Diplomate ACVP, is a swine specialist with Boehringer Ingelheim Vetmedica (Canada) in Burlington. Email: ernest.sanford@boehringer-ingelheim.com