Cellulitis: Its Microbiology

Introduction

Cellulitis (also called infectious process or necrotic dermatitis) refers to the inflammation of subcutaneous tissue and is typically seen in the lower abdomen and thigh of broilers. Escherichia coli is the predominant bacteria isolated from cellulitis lesions although other types of bacteria may be found. Cellulitis in turkeys appears to be a different syndrome and is caused by the anaerobic bacteria Clostridium perfringes.

Pathogensis of Cellulitis

Cellulitis is an infection that is usually localized in the tissues under the skin. Infection occurs following injury to the skin that allow penetration of the bacteria. Skin scratches alone are not sufficient to cause cellulitis. Other predisposing factors appear to be necessary.

Cellulitis lesions have a predilection for the abdominal area. Only a few of the lesions occur in the ventral area which suggests that the cellulitis lesions seen at the abattoir are unrelated to omphalitis.

The skin overlying the lesion is discoloured and the difference in colour makes detection of the lesion easy once the feathers have been removed. Cellulitis lesions vary in size from very small (2cm²) to large (greater than 8cm²) with the majority being moderate to large in size. The lesions may also vary in colour from cream to yellow-brown.

Under laboratory conditions cellulitis may be reproduced by rubbing E. coli directly on feather follicles or scratches that have been made through the skin or by the subcutaneous injection of bacterial culture. Cellulitis lesions develop quickly and can be detected as soon as 18 hours after the challenge is given. In these animal models some birds also develop systemic lesions such as pericarditis or perihepatitis.

Using animal models a dose response has been observed in the production of cellulitis lesions. The greater the number of bacteria given the greater the chance that a lesion will occur. This infers that the reduction of the number of pathogenic E. coli in the barn may help to limit the occurrence of the disease.

Relationship of cellulitis to Other diseases Caused by E. Coli in Broilers

A high frequency of concurrent airsacculitis (and other lesions that may be attributed to infection by E. coli) in some flocks has been observed. In some, but not all individual birds, both systemic lesions (airsacculitis, pericarditis, perihepatitis, arthritis) and cellulitis lesions are present. The occurrence of both of these types of disease caused byE.coli in individual birds and/or at the flock level suggests that common predisposing factors may exist for both types of disease.

E. coli isolated from cellulitis lesions are capable of causing systemic disease in broilers. We have also observed that an isolate ofE.coli from airsacculitis in a turkey is capable of causing cellulitis in broilers. Very limited testing in animal models has occurred but it appears that isolates ofE.coli that are virulent for poultry may be capable of causing a number of types of disease.

General Characteristics of Escherichia Coli

E. coli is an ubiquitous bacteria in the environment. The normal ecological niche for the bacteria is the intestinal tract. E.coli (including those pathogenic for poultry) can be found in litter, dust, water, feed, and the gut of healthy birds. E. coli have the ability to grow rapidly and use a wide variety of types of material for nutrients. Under laboratory conditions (at 30°C) the number of bacteria present double every 45 minutes. In 24 hours a single bacteria will give rise to over 3 billion progeny.

Not all E.coli are capable of causing cellulitis. It appears that the E.coli capable of causing cellulitis must have some special characteristics (virulence factors). Unfortunately the group of E. coli that can cause cellulitis has quite a diverse range of characteristics.

Characteristics of E. coli Isolated from Cellulitis Lesions

The E.coli isolated from cellulitis lesions belong to a wide variety of 0 serogroups with serogroups 01, 02 and 078 being most common. However a large number of strains (up to 40%) are not typeable. This makes grouping of the strains difficult using conventional methods.

Grouping of the bacteria using other types of genetic analysis has been done to a limited extent. Many of the strains belong to a few genetic groups. It is interesting that some of the genetic groups that contain cellulitis derived isolates ofE.coli also contain E.coli capable of causing other types of diseases in poultry.

Virulence appears to be multifactorial. A number of virulence factors have been described but not single factor is required for the E.coli strain to be virulent.

The virulence factors include:

1.	Aerobactin Production: Aerobactin, a specialized protein, is produced by the bacteria to allow growth under iron-limiting conditions. The amount of available iron in the body of a broiler is very low and the production of aerobactin allows the bacteria to acquire enough iron to grow and produce disease.
2.	Ability to persist in normal serum: By a number of changes in the surface of the bacteria, virulent E.coli are able to avoid being killed by nonimmune factors in the serum of birds.
3.	Ability to adhere to cells: By production of specific surface structures (pili) pathogenic E.coli is able to attach to cells and to grow on their surface.
4.	Production of capsule: Capsule is a carbohydrate layer on the surface of the bacteria that allows the bacteria to evade the immune system of the host.

.
In contrast to E.coli pathogenic for some other species those derive from cellulitis lesions and other types of poultry diseases are non-cytotoxic and non-haemolytic. E.coli isolated from cellulitis lesions, like those derived from other poultry lesions, are often resistant to a wide variety of antibiotics.

Conclusions

The E.coli that cause cellulitis have the same general characteristics as those that cause systemic diseases in poultry. In addition, a single isolate may cause both types of disease. This suggests that there is only one group ofE.coli that cause disease in broilers and that the type of disease that occurs depends on the situation.

Since, at least in some flocks, there is a positive association of cellulitis with other disease caused byE.coli there is a suggestion of common predisposing factors for the diseases.

Brenda Allan
Veterinary Infectious Disease Organization
University of Saskatchewan
Saskatoon, Saskatchewan