The phylogenetic relationship between Mycoplasma mycoides subsp. mycoides Small Colony (MmmSC), causative agent of contagious bovine pleuropneumonia (CBPP), and the other members of the M. mycoides cluster, especially Mycoplasma mycoides subsp. mycoides Large Colony (MmmLC) and M. mycoides subsp. capri (Mmc), has always generated interest among the veterinary scientific community. This interest was further stimulated to the point of concern by the occurrence of MmmLC in goats and calves in New Zealand in 2001, reported through a declaration to the OIE on 5 November 2001. This presentation by the International Cooperation Centre on Agrarian Research for Development (CIRAD) is prompted by the need to give an update on these issues, which may not be familiar to colleagues working in countries where CBPP is considered an exotic disease and/or in countries where dairy goat production may not be important.
Researchers G. Cottew and F. Yeats (1978) provided a distinction between the two M. mycoides types, based on growth characteristics (much faster for MmmLC); survival at 45 degrees centigrade (much higher for MmmLC) or casein digestion (much higher for MmmLC).
A further classification of the M. mycoides cluster was given in 1987 (Cottew et al., 1987). Two subgroups were clearly identified: capricolum (consisting of M. capricolum type F38 and Leach's serogroup 7) and mycoides (MmmSC, MmmLC and Mmc). Since 1987, newer laboratory techniques such as polymerase chain reaction (PCR), by facilitating the specific identification of MmmSC, have enabled a clearer distinction between MmmSC and MmmLC (Dedieu et al., 1994; Bashiruddin et al., 1994). To examine the M. mycoides cluster further, a phylogenetic study of the cluster was elaborated through the systematic sequencing of an intergenic DNA sequence (Thiaucourt et al., 2000). The results showed a good agreement with the conventional classification used for M. mycoides cluster in two subgroups. Furthermore, they showed that MmmLC and Mmc should be considered as a single entity.
From a more practical standpoint, this means that the isolation of MmmLC strains in a CBPP-free country, as was the case with New Zealand, should not be considered as a major threat to cattle. Yet, in a more distant perspective, analysing the phylogenetic relations of a number of MmmLC strains collected worldwide could be quite interesting, since many authors have shown that the mycoides group is heterogeneous. For example, the origin of the MmmSC specific Insertion Element (IS 1634) certainly merits attention in order to understand how MmmSC strains acquired it. It could originate from a particular group of yet unidentified MmmLC strains or through a lateral DNA exchange with a phylogenetically more distant bacteria.
Goats seem to be the natural hosts of mycoplasmas of the M. mycoides cluster. Various authors (Cottew and Yeats, 1978; DaMassa and Brooks, 1991) have shown that these bacteria could be isolated from the ear canal of goats without symptoms of an ear infection, or from parasites colonizing the ear canal. These mycoplasma strains are indistinguishable from those that are isolated from pathogenic events in goat herds, suggesting that there may be some kind of an ecological niche where they survive. It also suggests that additional factors could explain why they become pathogenic at some point.
Mycoplasma strains of the M. mycoides cluster cause disease symptoms that resemble "contagious agalactia", with mastitis, arthritis and keratitis constituting the prominent features in adults with pneumonia and septicaemia common in kids. (Thiaucourt et al. 1996). This resemblance to the contagious agalactia syndrome caused by M. agalactiae is certainly due to an evolutionary convergence and not to a phylogenetic proximity, as M. agalactiae differs widely from the members of the M. mycoides cluster.
Practically, the serological or genetic tools to identify M. agalactiae are very specific and should not lead to cross-reactions with members of the M. mycoides cluster. This should constitute an interesting point of reflection that could lead to the recognition/identification of M. agalactiae as the sole etiological agent of "contagious agalactia". In conclusion, it is quite likely that a systematic search of mycoplasmas in goats will lead to numerous isolations anywhere in the world even if no disease occurs. The normal hosting of mycoplasmas in the ear canal without any detectable seroconversion may lead to difficulties in defining ways to implement sound control strategies.
An outbreak of polyarthritis due to Mycoplasma mycoides subsp. mycoides (Large Colony) (MmmLC) occurred in November 2001 in goats and very young calves. In unusual circumstances, the calves were fed milk from infected goats. As a precaution, all calves exposed to the infected milk were slaughtered or held under containment for research purposes, in particular to determine whether horizontal transmission between cattle could occur. Investigations during the outbreak revealed that MmmLC had been present in New Zealand dairy goats for some years without causing significant disease outbreaks. M. agalactiae has never been isolated in New Zealand. (For further information, see OIE Disease Information, 17 May 2002, Vol. 15, No. 20, available at www.oie.int, online publications.)
Bashiruddin, J.B., Taylor, T.K. & Gould, A.R. 1994. A PCR-based test for the specific identification of Mycoplasma mycoides subspecies mycoides SC. J. Vet. Diag. Inv., 6(4): 428-434.
Cottew, G.S. & Yeats, F.R. 1978. Sub-division of Mycoplasma mycoides subsp. mycoides from cattle and goats in two types. Aust. Vet. J., 54: 293-296.
Cottew, G.S., Breard, A., Da Massa, A.J., Erno, H., Leach R.H., Lefevre, P.C., Rodwell, A.W. & Smith, G.R. 1987. Taxonomy of the Mycoides cluster. Isr. J. Med. Sci., 23: 623-625.
DaMassa, A.J. & Brooks, D.L. 1991. The external ear canal of goats and other animals as a mycoplasma habitat. Small Ruminant Research, 4, 85-93.
Dedieu, L., Mady, V. & Lefevre, P.C. 1994. Development of a selective polymerase chain reaction assay for the detection of Mycoplasma mycoides subsp. mycoides S.C. (Contagious bovine pleuropneumonia agent). Vet. Micro., 42: 327-339.
Thiaucourt, F., Bolske, G., Leneguersh, B., Smith, D. & Wesonga, H. 1996. Diagnosis and control of contagious caprine pleuropneumonia. Rev. Sci. Tech. (O.I.E.), 15(4): 1415-1429.
Thiaucourt, F., Lorenzon, S., David, A. & Breard, A. 2000. Phylogeny of the Mycoplasma mycoides cluster as shown by sequencing of a putative membrane protein gene. Vet. Microbiol., 72 (3-4): 251-268.
Contribution from the FAO collaborative centre, International Cooperation Centre on Agrarian Research for Development, Animal Production and Veterinary Medicine (CIRAD-EMVT).
