"In cattle and sheep, we've seen GD lesions develop from damage to the intestinal mucosa, so we're starting to take a closer look at that in poultry."
DR. STEPHEN COLLETT

In recent years, poultry growers have been seeing an increase in the incidence of gangrenous dermatitis (GD)—a subcutaneous infection in poultry that may be caused by clostridial bacteria of various species.

It's become a major health problem among broiler flocks in the United States, resulting in high mortality, carcass condemnations and trimmed parts. Economic losses have been estimated to be as much as $1.31 per affected bird.

Poultry researchers are not yet sure why gangrenous dermatitis is on the rise, but they are making headway in understanding this complex disease. One of the insights they have gained recently points increasingly to a possible link between GD and intestinal health.

Dr. Stephen Collett, professor of veterinary medicine at the University of Georgia, delivered a presentation on gangrenous dermatitis at the Orlando meeting and highlighted what has been learned about this intriguing disease.

GD requires portal of entry

Collett began his talk by explaining that three primary etiologic agents are responsible for breaks of gangrenous dermatitis: Clostridium perfringens, Clostridium septicum and Staphylococcus aureus.

None of these organisms, he said, is adept at infecting its host without a portal of entry. "They need a damaged area," Collett added. "Often it's damaged skin, a scratch. But in cattle and sheep, we've seen GD lesions develop from damage to the intestinal mucosa, so we're starting to take a closer look at that in poultry."

Immune suppression also plays a key role in setting up the host for infection. Infectious bursal disease (IBD), chick anemia virus (CAV) and other common poultry problems all can take a heavy toll on bird immunity and make conditions ripe for a GD break.

Environmental stressors also come into play. Extremes in temperature can, of course, sap bird immunity. In addition, such extremes—especially the combination of high heat and moisture—can foster conditions that make it easy for GD's causative organisms to grow and thrive in broiler houses. It is not surprising, then, that GD has the tendency to be a recurring problem on farms where favorable environmental conditions exist, Collett said.

So-called ’blue wing’ lesions are common in immunocompromised chicks that have been vertically infected with chick anemia virus.
(Photos courtesy of Dr. Collett.)

GD has an incubation period of 3 to 5 weeks and the course of the disease is 10 days to 2 weeks. Collett emphasized that one of the unfortunate characteristics of GD is that it tends to be a late-stage illness, with the disease often appearing in birds after considerable resources have already been spent growing them.

Few clinical signs of gangrenous dermatitis are observed in the field. "We find dead birds; we don't see many clinical signs," Collett said. "But one sign of GD you do see in the field are blue-wing lesions. That occurs especially in young chicks that have been infected vertically with CAV. Their immune systems are very compromised and they develop those typical kinds of lesions."

Postmortem examinations of birds with GD exhibit massive hemorrhaging under the skin, along the back, the thigh and the hips. GD-causing organisms characteristically are gas forming, so an accumulation of gas is often seen in the liver and sometimes in other anatomic structures. Postmortem studies may also show evidence of retained yolk sacs (associated with poor chick quality), anemia and pallor in the appearance of bone marrow.

Control of GD demands multifaceted effort

Collett said that treatment and prevention of GD is best coordinated on several fronts:

1. Providing good nutrition
2. Controlling moisture content in the litter (with frequent clean-outs)
3. Using vaccines to thwart immunosuppressive diseases such as IBD and CAV
4. Reducing risks of skin injury by limiting stocking density and providing adequate feed and water to reduce pecking
5. Administering antibiotics as needed

Interestingly, Collett said that experience in human medicine is helping poultry researchers better understand when to use the various types of therapeutic antibiotics.

"For example, what they find in humans is that penicillin is very effective at killing off an organism, but it's very ineffective at stopping an infection that has already begun," he said, adding that researchers think that may be because penicillin appears to have no effect on the chain of events that produce toxins in the organism.

"But if they instead treat an already infected person with tetracycline instead of penicillin, there's a much greater effect—not because tetracycline is more effective at killing the organism, but because it shuts off the genetic pathway for producing toxin."

Collett says that with an eye on those observations in human medicine, many poultry veterinarians are now leaning toward in-feed tetracycline as the drug of choice for treating GD breaks.

Published reports have suggested that coccidiosis vaccination may indirectly help reduce or eliminate the incidence of GD by greatly lowering the risk of coccidiosis breaks resulting from late coccidia cycling. (See CocciForum No. 12b)

Spring 2008

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