Introduction

Obligate anaerobic bacteria are usually found as mucosal commensals in the gastrointestinal, respiratory and urogenital tracts. They can also be found in moist areas of the skin. The survival of obligate anaerobes depends on a local reduction in or elimination of oxygen radicals. This can be created when there is a reduced blood supply of oxygen to tissues caused by localised tissue destruction or arteriole obstruction.

Alternatively, aerobic or facultative anaerobic microorganisms (bacteria, fungi, protozoa) use up in available oxygen allowing the growth of the obligate anaerobes. Therefore most infections where gram-negative anaerobes are involved tend to be polymicrobial.

It must be noted that samples that are collected for the growth of obligate anaerobes should be maintained in a oxygen radical reducing moist environment and reach the laboratory ASAP.  Most gram-negative anaerobic bacteria are susceptible to the antibiotic metronidazole.  This fact is used in the laboratory to distinguish obligate anaerobes that are susceptible to metronidazole from facultative anaerobes that are resistant to metronidazole.

Diseases caused by the obligate anaerobic bacteria

A Table listing the bacterium, its microscopic morphology, disease and host in shown in the Table below. Of these Two disease syndromes will be discussed, namely:

1. Infectious foot disease
2. Necrobacillosis including calf diphtheria and abscessation associated with rumenitis

infectious foot disease in sheep

The ruminant foot consists of two hooves connected by the interdigital skin. This skin is able to trap moisture and is subject to damage by stones etc. Even though only infectious foot disease of sheep will be discussed, cattle are also prone to similar diseases.

Primarily, the sheep hoof is the ecological niche of a non-sporing, gram-negative club-shaped anaerobe known as Dichelobacter nodosus.

A  number of infectious diseases of the sheep foot are recognised. They can be grouped:

• Infections associated with damage to the interdigital skin:
  • ovine interdigital dermatitis (OID)
  • ovine footrot
  • heel (foot) abscess
• Infections associated with direct damage to the hoof lamellae either through the toe or coronary band:
  • toe abscess
  • shelly hoof/white line disease
  • contagious digital dermatitis (not in Australia)

In this chapter the obligate anaerobic bacterial causes of infectious foot disease and lameness of sheep is dealt with. Other infectious causes of lameness include:

• strawberry footrot caused by Dermatophilus congolensis – only the skin above the coronary band is affected resulting in a proliferative inflammation with crusts.
• post-dipping lameness caused by Erysipelothrix rhusiopathiae – the joints of the feet and legs are affected
• viruses causing vesicles, erosions and crusts on the coronary band i.e. FMDV (absent from Australia)

Ovine interdigital dermatitis (OID)

When the interdigital skin of the foot is damaged i.e. skin maceration by constantly standing in water and mud and when the environmental temperature is above 10°C, bacteria in the soil such as the faecal obligate anaerobes Fusobacterium necrophorum and Prevotella melaninogenica and skin inhabitants obligate anaerobe Dichelobacter nodosus and facultative anaerobe Trueperella pyogenes are able to invade this tissue and cause tissue necrosis. Mud balls clinging to the skin between the feet increase skin abrasion and irritation. This is known as interdigital dermatitis, which in sheep is called “ovine interdigital dermatitis (OID)” and is characterised by skin redness and exudates in the interdigital area. Unless severe, lameness is not present. This condition is considered to be a precursor to ovine footrot which is primarily caused by Dichelobacter nodosus.

The pictures below show on the left severe OID and on the right mild OID

Heel (Foot) abscess

Heel or foot abscess is a necrotising or purulent infection involving the distal interphalangeal tissues and sometimes the joint. The incidence is usually sporadic, but up to 15% of the flock may be affected. It is often caused by Fusobacterium necrophorum and Trueperella pyogenes. Foot abscess occurs when there is OID or by damage to the dorsal synovial pouch. The infection spreads upwards infecting the joints.

This is the most common infectious disease of the foot in sheep and tends to affect heavier sheep more often.

Ovine Footrot (Important to learn)

An excellent overview of ovine footrot in Australia was provided by The Department of Agriculture in 2014.

Ovine footrot is a contagious bacterial disease of the sensitive hoof lamellae. It occurs in the temperate areas of Australia with only sporadic cases in Queensland. Although several different bacterial species (including Peptostreptococcus, Corynebacterium, Staphylococcus, Trueperella, spirochaetes, mycoplasmas and Fusobacterium necrophorum) may be involved in the generation of lesions, only the presence of one of 10 different serogroups of Dichelobacter nodosus will result in footrot using the Australian classification system.  The serogroups are based on the presence of different Type IV pili. These pili are involved in adhesion of the bacteria to the skin. The bacteria usually invade the interdigital area and spread from there into the sensitive hoof lamellae. When a thermolabile protease producing strain of Dichelobacter nodosus infects, a self-limiting form of footrot known as “benign” footrot results.  Lameness is usually mild and animals often recover when moved to dry ground. The disease is easily controlled by the use of footbaths that contain either 10% zinc sulphate or 5% formaldehyde.

In sheep, when a thermostable protease producing D. nodosus is present, lameness can be very severe and progressive. This is known as “virulent” footrot (malignant footrot; contagious footrot). This footrot is highly contagious and will spread in a flock with up to 100% of the flock being affected. The genetic basis for resistance to this disease has been established. The disease is first recognised in a flock by the presence of lameness and reluctance of the sheep to stand and walk. There may be drop in fertility when the rams cannot mate and a drop in body condition and wool production. After thoroughly cleaning of the hoof, dark malodorous fluid will seep from the foot and bits of the hoof may be torn off. This can be severe enough for the whole hoof to slough off, the so-called “slipper foot”. Flies are attracted to these wounds, lay their eggs in them, which hatch to feeding maggots =myiasis. Thus this is a disease affecting the welfare of animals.

Diagnosis of footrot

Footrot is usually distinguished from other diseases of the foot by damage the loss of hard hoof horn and the presence of gram-negative rods with terminal swellings in exudates. It is important to distinguish ovine footrot from heel abscess and toe abscess (described in the next section). Refer to the Table below.

Foot scoring (refer to chart below) determines the severity of lesions and can give an indication of whether benign or virulent footrot is present. Note that foot scoring should be done at the end of a wet season and all animals with a 3 or more score are removed from the flock. You will get the opportunity to practice foot scoring in the quiz.

Confirmation of virulent footrot should always be the detection of the thermostable serine proteases. A qPCR detecting thermostable protease positive strains of D. nodosus or gelatin gel thermostability test on cultured D. nodosus can be used. Deep foot scrapings (scraped until fresh blood seeps from the wound) are placed in anaerobic transport medium and submitted to the laboratory ASAP (on the same day). It is usually grown on hoof agar which is made from ground hooves.  (For the gelatin gel thermostability test, broth cultures of D. nodosus are heated for 16 minutes at 66.8C and then incubated on gelatin agar. Liquefaction of the gelatin is indicative of the presence of thermostable proteases).

​​​​​​​Purified cultures of Dichelobacter nodosus from affected farms will also be required for the production of autogenous vaccines.

Control of ovine footrot

Footrot occurs in all the temperate zones of Australia, including Tasmania, Victoria, South  Australia, NSW and Western Australia. It occurs sporadically in Queensland. It is a notifiable disease in Victoria, South Australia, NSW and Western Australia, not Tasmania or Queensland. Since 2009, the prevalence of footrot in NSW  is <1%, making this State a footrot protected zone and any control of footrot in NSW will be aimed at eradication of the agent. Whereas other States aim for disease control. Since this disease not only affects production, but also the welfare of the animal, treatment should be initiated as soon as possible.

• Clean and pare the hooves and separate sheep with lesions from those without. It is usually not necessary to pare the feet of unaffected or mildly affected sheep.
• Footbath the “clean” sheep and move them to a dry paddock.
• The affected sheep are treated by foot spraying with bacteriocidal sprays such as 20% cetrimide or 1.3% oxytetracycline; by footbathing ; or by foot-soaking (stand for 15 minutes in 10% ZnSO₄ and 0.02% laundry detergent or the surfactant sodium lauryl sulphate or use Radicate – a commercial copper salt footbath). This should be repeated for 3 treatments every 5 to 10 days.
• Additional therapy includes systemic antibiotic therapy with penicillin, erythromycin or oxytetracycline.
• Move affected sheep to a dry area.
• Severely affected sheep and those that don’t respond to treatment should be culled
• In an eradication programme, all sheep showing lesions in the dry season should be culled. Destocking of infected mobs is done in small flocks or where there is a segregated infected flock.
• Since D. nodosus can survive for up to 2 weeks in pastures under ideal conditions, it is recommended that pastures are spelled for 1 to 2 weeks dependent on the local environmental conditions.
• Natural immunity and Immunity to vaccines is short lived. No vaccine is used in NSW, Victoria and South Australia. A serogroup specific vaccine can be used in Tasmania and under an emergency permit on positive farms in WA. Autogenous vaccines containing the serogroups on the farm is likely to be more successful. Since there is serogroup competition it is often better to use only mono- or bivalent vaccines at one time. In farms where multiple serogroups are present a series of vaccines containing the different serogroups will have to be used.

How farms are eradicating ovine footrot

1. Regular footbaths
2. Footrot vaccine (Tasmania)
3. Culling of severely affected sheep
4. Allow paddocks to dry out in the dry season and inspect feet twice, one month apart.
5. All sheep on the dry paddocks with foot lesions are culled.

Toe abscess

This affects individual animals and individual digits. Damage to the sensitive hoof lamellae, due to e.g. grain laminitis of the hoof, breaking of an overgrown hoof or external injury can allow the invasion of mainly anaerobic bacteria that will result in abscessation and lameness. This is usually observed under the horn of the affected toe. These abscesses occasionally erupt and drain from the coronary band.

Shelly Hoof or white line disease in sheep

This disease is more common in sheep with pale hooves i.e. Merinos. It is associated with allowing hooves to overgrow (poor foot maintenance). Damage to the hoof lamellae results in an areas of the hoof wall detaching and forming a pocket. It becomes clogged with mud. This allows the growth of a variety of obligate and facultative anaerobes leading to infection.

Contagious digital dermatitis in sheep – absent from Australia

Digital dermatitis caused by a spirochaete (see later chapter), Treponema species, is a highly contagious, erosive, and proliferative infection of the epidermis proximal to the skin-horn junction in the flexor region of the interdigital space. In UK and Europe, it usually occurs in the colder months when animals are housed indoors. Either the lesions are erosive, proliferative and wart-like. The disease is not known to be present in Australia.

diseases with the involvement of fusobacterium (Necrobacillosis) 

Necrobacillosis refers to necrotic lesions that are caused by the non-sporing, gram-negative, filamentous, fusiform-shaped, obligate anaerobe known as Fusobacterium necrophorum. This intestinal inhabitant is shed in faeces and can survive for a short period of time in muddy soils that lack oxygen. It is an opportunistic pathogen where it will gain entrance to the body via wounds. Once in the body, Fusobacterium produces a necrotoxin or leukotoxin encoded by the lktA gene. It also produces potent endotoxins that elicit a strong inflammatory response.

Well known diseases in animals caused by Fusobacterium necrophorum  are shown in the Table below. Note that the list is not exhaustive and other Fusobacterium species can cause opportunistic infections associated with the digestive tract in all mammals.

Necrotic laryngitis “Calf diptheria”

Necrotic laryngitis occurs predominantly in young animals of 3 to 18 months of age. It is more common where animals are kept in overcrowded conditions in pens that have a build-up of faeces and mud. The disease also occurs in feedlot calves with upper respiratory disease where it is believed that viral and viral damage to the larynx and medial angles of the arytenoids cartilage provides F. necrophorum with a portal of entry. The necrotic exotoxin produced by F. necrophorum causes a local necrotic lesion in the pharynx and larynx that can spread outwards and descend into the lungs causing  bronchopneumonia.

This results in fever, open-mouthed breathing, dyspnoea and inspiratory stridor. Animals usually have a foul breath and hyper-salivate. Untreated animals can become toxaemic and develop obstruction of the airways leading to death. In chronic cases aspiration pneumonia and distortion of the larynx may occur.

The disease is usually diagnosed by the clinical appearance.

To confirm the diagnosis histopathology can be done on biopsies where the typical gram-negative filaments are observed often together with other bacteria. It is rare that samples are submitted for culture.

Antibiotics such as penicillin and symptomatic therapy are used to treat the disease. Control is usually aimed at reducing the effect of predisposing factors.

Bacterial and fungal rumenitis in intensively reared cattle and sheep and its consequences

Cattle, such as feedlot and dairy cattle, fed diets rich in carbohydrates changes the rumen microflora to lactic acid producers. This decreases the pH of the rumen resulting in damage to the rumen mucosa. The lack of dietary fiber contributes to this. Bacteria, such as Fusobacterium necrophorum, that thrive under acidic conditions proliferate and invade the damaged mucosa. The necrotoxins and endotoxins of these bacteria lead to inflammation and necrosis of the rumen compounding the damage already present. The bacteria move via the portal veins to the liver resulting in multifocal areas of necrosis of the liver.  Animals seldom show overt clinical signs, however, they do eat less and have a drop in milk production.

Less commonly, bacteria escaping from the liver enter the caudal vena cava where they can cause areas of thrombosis of the caudal vena cava. Bacterial emboli detach from the vena cava where they are trapped by the capillary beds of the lung. Lung abscessation and pulmonary haemorrhage is a consequence. Lung haemorrhage is observed clinically as nose bleeds (epistaxis) and anaemia.

Direct spread of the bacterium to the peritoneum results in the development of peritonitis.

The acidic conditions of the rumen also favour the growth of plant-associated fungi such as Mucor and Absidia. They tend to affect the blood vessels of the rumen causing ischaemic necrosis. Occasionally the fungi enter the bloodstream and localise in the placenta of pregnant cows. Placental damage leads to starvation of the foetus and abortion. Occasionally the fungus will infect the skin of the foetus leading to fungal plaques. Fungal infections of ruminants are more common in temperate climates where cattle are housed indoors in the winter and fed hay and silage.

Similar disease has been recorded in sheep.

Alveolar osteomyelitis in wallabies and kangaroos. Also called “oral necrobacillosis”, “Lumpy Jaw “or “Kangaroo Disease”

Alveolar osteomyelitis is a relatively common, sporadic disease of both captive and free-living macropods. Infection of the jaw via contaminated plant material that entered through the dental alveoli has been recorded in macropods. Several predisposing factors have been identified including coarse food, mature animals and faecal contamination of grazing areas.  In captive animals, it is believed that the continuous feeding of soft foods changes the chewing action from a horizontal action used with grasses to a vertical one. The vertical movement of the jaw is believed compact the food into the dental alveoli.

There is usually a mixed infection with synergy between obligate anaerobic bacteria and aerobes or facultative aerobes.  These bacteria are usually part of the normal oral or gastrointestinal microflora. Anaerobic bacteria that have been most isolated from these lesions include Fusobacterium necrophorum, Porphyromonas species, Bacteroides species and Prevotella species – all gram-negative, anaerobic bacteria. Aerobic or facultatively anaerobic bacteria isolated include Trueperella pyogenes, Pseudomonas species and Streptococcus species.

Treatment involves both the local application of antiseptics and systemic antibiotics. Another treatment is to impregnate polymethylmethacrylate beads with antibiotics such as clindamycin or lincomycin and implant them directly into the lesion. In advanced cases, it may be necessary to surgically remove the affected tooth’s root and damaged bone, curette and disinfect the area further using laser treatment and seal the area with bone cement.

For more information on this disease, refer to the article below. It is accessible by the JCU library’s One Search

Conor P. Kilgallon, M.V.B., M.Sc., Beth Bicknese, D.V.M.,M.P.V.M., and David A. Fagan, D.D.S. (2010) Successful Treatment of Chronic Periapical Osteomyelitis in a Parma Wallaby (Macropus parma) Using Comprehensive Endodontic Therapy with Apicoectomy. Journal of Zoo and Wildlife Medicine, 41(4):703-709.

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