PATHOGENESIS AND EPIDEMIOLOGY
The pathogenesis of MPPD is outlined in Fig. 32.1.
2.1 Microbiology
A range of aetiological agents have been cultured from MPPD lesions, most commonly anaerobes including Fn (Burton 1981; Samuel 1983; Oliphant et al.
1984) and Bac- teroides spp. (Taylor et al. 1978; Oliphant et al. 1984). Such pathogenic bacteria produce cytotoxic proteins and various other substances that contribute to disease progression (Nagaraja et al. 2005). Actinomyces spp., the genus associated with lumpy jaw in cattle, and Nocardia spp. have been reported in association with MPPD (Taylor et al. 1978; Miller et al. 1980; Samuel 1983; Kane et al. 2017). Of three red kangaroos (Osphranter rufus) with advanced disease, Pseudomonas spp. were consistently cultured from all cases, while only one case yielded Fn (Brookins et al. 2008). Bertelsen et al. (2012) cultured Pasteurella mul- tocida from gingivitis and osteomyelitis lesions in multiple red-necked wallabies (Notamacropus rufogriseus).Burton (1981) and Samuel (1983) cultured Fn from the majority of affected macropod mouths sampled and Samuel (1982) failed to isolate it from healthy mouths. It was questioned whether high loads of Fn initiated, or were the result of, oral disease and whether Fn was part of the normal flora. Burton (1981) experimentally induced disease in tammar wallabies (N. eugenii) by inoculating Fn- spiked sterile feed mash into surgically formed periodontal pockets, with a lag period of 6-105 d; Fn was recovered from lesions in Fn-inoculated wallabies, and also from the single control wallaby inoculated with non-spiked mash. Fn has been proposed as the aetiological agent of MPPD, ostensibly acting as a primary pathogen and transmitting as a contagious disease. However, Fusobacterium spp., including Fn, are considered part of the normal oral and gastrointestinal flora of humans and other animals and hence are generally considered opportunistic pathogens (Nagaraja et al.
2005). Asperger (2002) did isolate Fn from healthy mouths of red-necked wallabies, eastern grey kangaroos (Macropus giganteus) and red kangaroos, and from only 27% of affected mouths.Culture studies, although valuable, have several limitations (Antiabong et al. 2013a; Bird et al. 2015). In humans and domestic animals, molecular methods have revealed far more diverse oral bacterial communities than had previously been identified with culture methods (Bird et al. 2015).
The leucotoxin gene sequence lktA, encoding the major virulence factor of Fn, was detected by PCR in 100% (n = 10) of affected yellow-footed rock-wallaby (YFRW) (Petrogale xanthopus) and tammar wallaby (TW) mouths, supporting previous studies that Fn is an important pathogen in MPPD. Faint bands, confirmed to be lktA by sequencing, were also found in 21% of healthy mouths (n = 48) (Antiabong et al. 2013b), suggesting the presence of Fn is not sufficient to cause disease.
Fig. 32.1. Pathogenesis of macropod progressive periodontal disease.
Four Fn biotypes are recognised, not all of which carry leucotoxin genes. Six wallabies (YFRW and TW) with gingivitis all had Fn and IktA detected by PCR. Of six wallabies with more advanced disease, four had Fn and lktA, one had Fn without IktA and one had no detectable Fn (Antiabong et al. 2013c). This suggests that Fn is not necessary to cause disease.
Fn and Porphyromonas gingivalis are considered synergistic in human periodontal disease; P. gingivalis may trigger changes that favour disease-associated bacteria (Antiabong et al. 2014). The majority of affected YFRW and TW were PCR-positive for Fn (92%) and P. gulae (58%), in contrast to unaffected wallabies, suggesting the potential for similar pathogen synergism (Antiabong et al. 2013c). The closely related P. loveana was recently described from the oral cavity of marsupials (Bird et al. 2016); it is probable that the P.
gulae detected by Antiabong et al. (2013c) was in fact P. loveana, as the genotypic tests used were not sufficiently discriminatory (D Trott pers. comm.).The microbial communities in healthy and diseased macropod mouths have been compared using molecular fingerprinting and sequence analysis (Antiabong et al. 2013d) and high-throughput sequencing (Yip et al. 2021). Healthy mouths exhibited a diverse microbiome dominated by facultative anaerobes and aerobes, including Pasteurellaceae and Moraxellaceae. In diseased mouths, the bacterial community structure was very different, being dominated by anaerobes, notably Fusobacteriaceae, Bacteroidaceae and Porphyromonadaceae. Gingivitis cases were associated with an increase in detectable bacterial diversity and a functional organisation consistent with a balanced community. The bacterial diversity was decreased in more advanced cases, with a more specialised, yet fragile, bacterial community structure (Antia- bong et al. 2013d).
Yip et al. (2021) characterised 27 and 66 bacterial genera with significantly different abundance between healthy mouths and those with gingivitis and periodontitis ± osteomyelitis, respectively; 18 of these differences in genera were common to both gingivitis and periodontitis-osteomyelitis.
Bacterial-fungal synergism in the healthy macropod oral microbiome was suggested by Antiabong et al. (2013e). The fungal population was significantly reduced, and less diverse, in affected YFRW and TW mouths, because of significant fungicidal activity of the anaerobic bacterial population from diseased mouths (Antiabong et al. 2013e). Further work is required to understand the role of fungi, and of bacterial-fungal interaction, in the healthy oral microbiome and in the pathogenesis of MPPD.
It has been proposed that MPPD in macropods follows the ecological plaque hypothesis, whereby disruptions to microbial ecology trigger inflammatory and local pH changes favouring pathogenic species (Antiabong et al.
2013d; Marsh et al. 2015). It has also been hypothesised that ‘keystone pathogens’ play an important role by destabilising host defences, allowing pathogenic microbes to prosper (Antiabong et al. 2014). Certainly the evidence supports a polymicrobial pathogenesis of MPPD, with the bacterial diversity and associated microbial interactions more complex than has been proposed historically (Yip et al. 2021).1.1 Proximate risk factors
Many factors have been reported to contribute to the development of MPPD.
1.1.1 Plaque
Plaque-mediated gingivitis, with gingival recession and impaction of feed material in resulting pockets, is the likely pathogenesis in the majority of cases; the site of infection can generally be recognised at the gingival margin (Burton 1981). Certain feeds, such as pellets high in readily digestible carbohydrate, promote plaque formation (Burton 1981).
1.1.2 Trauma
Food sources that are coarse or spikey may cause oral mucosal trauma, precipitating infection of the gingiva and deeper tissues. However, some macropod species are observed to naturally consume potentially injurious plants without apparent adverse effects.
Traumatic injuries to teeth and/or the peridontium, notably of the lower incisors, predisposes to periodontal infection, abscessation and osteomyelitis.
1.1.3 Other dental abnormalities
MPPD in a wild eastern grey kangaroo was proposed to have been secondary to the contralateral molars apparently failing to develop (Barber et al. 2008).
1.1.4 Molar progression
Molar progression is a normal physiological process in macropods. In grazing and some intermediate browsergrazer macropods, premolar and molar teeth progressively become non-functional at the rostral end of the quadrant and are shed (Vogelnest and Portas 2008; Lentle
Fig. 32.2. Oblique lateral skull radiograph, red kangaroo (Osphranter rufus) aged 15 yr in the advanced stages of molar progression with only the 4th molar remaining in all four quadrants.
There is lucency around atrophic roots of the maxillary 4th molar bilaterally (arrows). (Inset) Intraoral dental radiograph of the right maxillary 4th molar. Alveolar lucency is most pronounced around the atrophic caudal tooth root (arrowhead). Purulent exudate at the caudal gingival margin was only appreciated with use of a dental mirror; directly visible aspects of the tooth were grossly normal.and Hume 2010; Fiani 2015) (Fig. 32.2). This normal tooth loss should be differentiated from pathological processes. However, accumulation of plaque on post-functional rostral molars and disruption of periodontal tissues during tooth loss are potential risk factors for MPPD (Miller and Beighton 1979). Burton (1981) did not demonstrate a pathogenic role for molar progression, though the premolar and rostral molars were the most common sites of infection. In browsing macropods, the large third premolar is retained and likely blocks molar progression, though lingual or buccal molar drift of rostral molars (Fig. 32.3a) can allow them to be squeezed out (Vogelnest and Portas 2008; Lentle and Hume 2010). This should be considered when evaluating mobility/loss of the first molar in these species and is also a risk factor for MPPD (Fig. 32.3).
1.1.5 Age
Increasing incidence of MPPD with age has been reported in managed red-necked wallabies (Kido et al. 2013) and free-ranging eastern grey kangaroos (Borland et al. 2012). Rendle et al. (2020) found macropods ≥10 yr were over seven times more likely to develop MPPD than animals <1 yr. Conversely, Burton (1981) documented a higher prevalence in younger animals.
1.1.6 Sex
Male macropods were two times more likely to develop MPPD than females in European, but not Australian, zoos (Rendle et al. 2020).
1.1.7 Previous episodes of MPPD
Macropods that recover from MPPD subsequently have a higher incidence than previously unaffected macropods (Lewis et al. 1989; Kido et al. 2013). Rendle et al. (2020) found up to 34% of cases went on to experience at least one additional occurrence of MPPD, and up to 54% of recurrent cases eventually succumbing to MPPD.
This may be related to altered mastication and plaque accumulation because of the missing teeth, a compromised oral microbiome less resilient to detrimental shifts and/or recrudescence of infection in the event of incomplete resolution.1.1.8 Stress
Social stressors (e.g. inappropriate groupings, excessive stocking density, intensive management practices, excessive contact with zoo visitors, pest species), concurrent disease and adverse environmental conditions likely predispose to MPPD. Griner (1983) reported MPPD in 21% of managed macropods over a 14-yr period, yet wallabies maintained in a large, naturalistic enclosure were largely unaffected. Cold weather has been associated with increased incidence of periodontal lesions (Burton 1981; Oliphant et al. 1984; Kido et al. 2013). Conversely, a review of cases at an Australian zoo found no seasonal pattern (Vogelnest and Portas 2008). Borland et al. (2012) reported 54% prevalence in skulls of free-ranging eastern grey kangaroos collected during drought conditions with limited pasture availability and heavy faecal contamination.
1.1.9 Animal moves
The risk of developing MPPD in macropods housed in Australian zoos was found to increase with the number of inter-zoo transfers; animals moved once were 1.7 times more at risk, through to a 44 times greater risk for seven moves (Rendle et al. 2020). Similarly, the number of moves between enclosures within a zoo increased the risk of developing MPPD, from 1.6 times for one move to 16 times for ≥11 moves (Rendle et al. 2020).
Fig. 32.3. Yellow-footed rock-wallaby (Petrogalexanthopus) aged 9 yr. (a) Buccal drift of the right mandibular 1st molar (arrowhead). The tooth may be squeezed out between progressing caudal molars and the fixed premolar and is a risk factor for macropod progressive periodontal disease (MPPD). The 1st molars in all four quadrants were similarly affected to some degree, with associated gingival recession and enlarged periodontal pockets. No tooth mobility was appreciated. Affected teeth were cleaned but no further treatment was instituted. (b) Rostral aspect, (c) right side and (d) left side of a 3D reconstruction of a post mortem CT scan of the skull of the same wallaby euthanased 2 mo later with severe MPPD, including extensive mandibular osteomyelitis with pathological fractures and multiple periodontal lesions in all quadrants. The missing right 1st-3rd mandibular molars and supporting section of mandible fell away during oral examination (arrows). Extraction of affected molar teeth on initial presentation may have averted progression of disease.
1.1.10 Enclosure hygiene
Faecal contamination of enclosures, especially feed areas, may increase oral exposure to opportunistic pathogens such as Fn (Burton 1981); however, non-spore forming bacteria such as Fn can only persist for extended periods in the environment under favourable conditions.
1.1.11 Inappropriate species-specific zoo-based management
Some species, notably red kangaroos and red-necked wallabies, have been purported to be particularly susceptible to MPPD (Calaby and Poole 1971; Schurer 1980). However, macropods are a diverse taxon; environmental and management conditions will likely be more, or less, appropriate for different species at a given institution. An association between periods of cold wet weather and cases of MPPD has been observed in red kangaroos, while western grey kangaroos (M. fuliginosus) under equivalent conditions have remained relatively unaffected (I Hough pers. comm.). Several Australian zoos have experienced a high incidence of MPPD in red kangaroos, yet others, including one in central Australia (within the species’ natural range), have seen little if any disease in this species (B Pascoe pers. comm.; T Portas pers. comm.). Such examples cast doubt over inherent species susceptibility. Where a particular species is more prone to MPPD at a particular institution, attention should be directed at the suitability of prevailing environmental and management factors for that species.
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