JOHNE'S DISEASE: RISKS OF INTERSPECIES TRANSMISSION

FRA PRAKSISNYTT, VOLUM 2, Nr 1, April 1997

Sakset fra the IVth International Congress for Sheep Veterinarians:

JMSharp

Moredun Research Institute, Edinburgh, EH 1 7 7JH, Scotland

Introduction

It has been accepted for many years that the causal agent of paratuberculosls '(Johne's disease) is Mycobacterium avium subspecies paratuberculosis. For most of this time the disease has been perceived, almost exclusively, as a problem in ruminants and little cognisance has been given to the presence of the organism in other hosts. However, there is a growing appreciation of the wider host range of M.a. paratuberculosis and the potential implications for veterinary- and medical science. T'his paper will review the host range of M.a.paratuberculosis, evidence for reservoirs in non-domestic species, transmission of the organism between species and the implications for control of paratuberculosis and human health.

Host range

Paratuberculosis has been described in all of the domesticated ruminant species and this pathological diagnosis confirmed by isolation of M.a.paratuberculosis (Table l).

Table 1 Domestic ruminants affected by paratuberculosis

Animal

Not surprisingly, the organism also has been isolated from several other ruminant species, either in the wild or in zoos on all of the major continents (Table 2). Although some of these reports note the potential of these species as reservoirs for M.a.paratuberculosis no real emphasis has been accorded to these observations and their signficance for control strategies has been overlooked.

It is clear from Tables 1 and 2 that M.a.paratuberculosis can produce a natural infection in a wide range of ruminant species. This host range has been extended to include monogastric species by means of experimental inoculation, although generally clinlncal disease does not develop and lesions do not resemble paratuberculosis (Table 3). In addition, naturally-occurring infections and disease have been reported in primates, including man, and wild rabbits (see below). T'his recently, perceived wide host range of M.a.paratuberculosis raises far-reaching issues about the importance of reservoirs, transmission of infection between species and the attendant implications for control strategies based on destocking and culling of infected animals.

Table 2 Non-domestic ruminants affected by paratuberculosis

Animal

Table 3 Monogastric animals susceptible to infection by M.a.paratuberculosis

Animal

Reservoirs in non-domestic species.

Wild ruminants have been implicated by several authors as potential reservoirs of mycobacterial infections for domestic livestock, a view which has been highlighted by the marked susceptibllity of deer, especially farmed deer, to mycobacterial infections. However, infection of wild ruminants by M.a. paratuberculosis is of no significance unless this attains a high prevalence. There is little information on this point- but a recent extensive survey of culled wild deer in Scotland found that approximately 2% were infected with M.a.paratuberculosis. A more recent finding that may be of much greater significance was the microbiological and histological evidence for paratuberculosis in 67% of a small sample of wild rabbits (Oryctolagus cuniculus). The relatively high prevalence infection with M.a.paratuberculosis in wild deer and particularly rabbits suggests that they could have a role in the epildemiology of these infections. However, it is not yet clear that these specics are reservoirs of mycobacterial infections and whether they disseminate these infections to other domestic livestock.

lnterspecies transmission

A crucial issue in any debate concerning the transmission of infection between species is whether strains of M.a.paratuberculosis can move naturally between different host species or wether they are species-specific. Currently, it is not known whether strains M.a.paratuberculosis isolated from different hosts and different geographical locations are the same. It is very clear that experimentally the organism can be forced to infect species other than its natural host (Table 3). However, this issue is not so clear outside the laboratory and experience of naturally-occurring paratuberculosis suggests, so far, that in the field interspecies transmission may not be a frequent event. In other words. it may be more accurate to consider that the various strains of M.a.paratuberculosis express a host preference that limits transmission between hosts, but that this preference can be overcome experimentally by, inoculation with large numbers of organisms. An essential prerequisite to resolving these questions will be the development of techniques that can reveal the maximum genetic diversity in M.a.paratuberculosis and exploit these to differentiate strains or demonstrate that identical strains are circulating between species At present, progress towards addressing these issues is impeded by our inability to adequately differentiate strains of M.a.paratuberculosis. Restriction fragment length polymorphism, based on M.a.paratuberculosis specific insertion element IS900, has indicated that there are some polymorphisms but because this insertion element has specific integration targets the number of polymorphisms is rather limited. Thre is a great need to develop non-targeted techniques, which may reveal greater diversity.

lmplications of interspecies transmission

The chronic granulomatous enteritis of man, Crohn's disease, shares a number of pathological features with paratuberculosis, which has prompted a search for M.a.paratuberculosis, in Crohn's disease. M.a.paratuberculosis has been isolated from patients with Crohn's disease on only a few occasions, but the organism has been detected by some research groups in a high proportion of resected lesions using highly sensitive molecular techniques. However, considerable controversy surrounds the debate on the aetiological significance of these findings. The debate has been confused further by the demonstration of M.a.paratuberculosis DNA in pasteurised milk, which has led to one view that Chron's disease arises as a result of consumption of such contaminated milk. If this hypothesis is correct, then it can be predicted that the strains of M.a.paratuberculosis isolated from man will have the same polymorphisms as those isolated from cattle. This debate and the potential implications for food hygiene and human health serve only to underline the need to develop techniques that can differentiate strains and support good epidemiological studies.

Control of paratuberculosis is undertaken currently by variations on the themes of detection and culling of infected animals or vaccination. The choice of control strategy has been influenced traditionaliy by factors such as the commercial value of the species involved and whether paratuberculosis is endemic in the herd or area. The existence of wild life reservoirs and their importance in the epidemiologi of paratuberculosis has been largely ignored. However, if wildlife is involved in the epidemiologi of paratuberculosis and can act as a source of infection for domestic stock, current control strategies employing a detection and cull policy will prove inefficient in the long term and vaccination may be a more acceptable approach.