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Mycobacteriosis due to members of the Mycobacterium avium complex in swine: Significance, diagnosis and identification of possible sources of infection

Julio Álvarez
January 27th, 2009

Mycobacterium avium complex comprises two bacterial species of great importance, M. intracellulare and M. avium; the last one is currently subdivided in four subspecies (M. avium subsp. avium, M. avium subsp. paratuberculosis, M. avium subsp. hominissuis and M. avium susbsp. silvaticum). All of them can infect a wide range of host species. In addition, while some of these species/subspecies are strict pathogens and are rarely found outside the host (M. avium subsp. avium, M. avium subsp. paratuberculosis and M. avium susbsp. silvaticum) the others are saprophytic bacteria that can survive in the environment and are widely distributed (M. intracellulare and M. avium subsp. hominissuis). Usually these “environmental” bacteria do not cause significant pathological problems, although in certain situations they can induce severe clinical diseases and macroscopical lesions (more often in immunocompromised or stressed hosts). In this sense, M. avium complex members are important pathogens in Public Health whose relevance has been significantly increased in the last decades due to a combination of several factors: on one hand, the emergence of the HIV epidemic, the progressive aging of the population in developed countries and the generalized use of immunosuppressive therapies for certain pathologies have conducted to an increase of the immunocompromised population, susceptible to these bacteria (Falkinham, III, 2003b); on the other hand, the extended use of water disinfection treatments worldwide can lead to a selection of bacteria which are resistant to chlorination, such as the M. avium complex members (Falkinham, III, 2003a; Primm etal., 2004).

Regarding infections in swine, there are two members of the Mycobacterium avium complex that are more frequently isolated in this host species:

M. avium susbp. avium: Is the causative agent of avian tuberculosis, and therefore birds are considered its preferred host, although it can infect other species; in this sense, it has been isolated from a wide variety of animal species including deer, sheep, cows, rodents, swine and humans. However, the number of isolations of this subspecies from humans and swine is usually lower compared to those of M. avium subsp. hominissuis. This bacterium is considered a strict pathogen, and therefore the chance of infection from environmental sources is less likely.

Granulomatous lesions due to M. avium in a pig’s mesenteric lymph node
Figure 1. Granulomatous lesions due to M. avium in a pig’s mesenteric lymph node.
© Given by Alejandro Skoknic

M. avium subsp. hominissuis: It is the most heterogeneous subspecies of M. avium, as all molecular characterization techniques performed on isolates from this subspecies reveal a greater variability compared to the strict pathogens M. avium subsp. avium and M. avium susbp. paratuberculosis. Although it also has a wide host range, M. avium subsp. hominissuis is more frequently isolated from human and swine; in swine can induce the development of granulomatous lesions, usually located in head and mesenteric lymph nodes. Even though pigs do not typically show clinical symptoms, finding these lesions in slaughterhouses can lead to an important decrease in the economical value of the infected animal. The location of lesions suggests an oral route of infection, although they have been also described in respiratory lymph nodes. M. avium subsp. hominissuis infection in swine has been reported in all continents.

When an outbreak caused by one of these pathogens is suspected the isolation of the causative agent is of paramount importance, as the macroscopical aspect of lesions caused by Mycobacterium avium complex members can be sometimes misidentified with those produced by Mycobacterium tuberculosis complex members, causative agents of human and animal tuberculosis. Therefore, the in vitro culture of the causative agent of the outbreak is the only way for the unequivocal confirmation of the implication of M. avium subspecies, thus excluding an infection by a M. tuberculosis complex member.

Ubiquity of environmental members of the M. avium complex often complicates the establishment of the origin of infection, as usually several potential routes of infection can be identified. A complete epidemiological study, aimed at the identification of all potential risk factors involved, is generally the only way to trace back the origin of the outbreak; afterwards, culture of the aetiological agent from the potential sources of infection would be attempted, and then environmental and clinical isolates would be compared by means of the application of molecular characterization tools. Culture of these bacteria must be performed in specific culture media, designed to support the growth of M. avium subspecies.

M. avium subsp. hominissuis growth in Coletsos medium
Figure 2. M. avium subsp. hominissuis growth in Coletsos medium.
© VISAVET

Identification and comparison of environmental and clinical isolates can be achieved using several methods, usually based in the analysis of bacterial DNA. Sometimes specific insertion sequences, which can discriminate to a species/subspecies level, are used as the target of molecular techniques (IS901 and IS1245 are often employed for M. avium complex members) (Kunze etal., 1991; Guerrero etal., 1995); other potential targets are polymorphic sequences among subspecies and/or strains [as the gene that encodes the heat-shock protein hsp65 (Turenne etal., 2006) and several polymorphisms involving large sequences (Semret etal., 2006). Another technique is based on the analysis of the variable number of tandem repeats (VNTRs), consisting on the detection and quantification of tandem repeats (short DNA sequences from 14 to 100 nucleotides that are normally repeated in the bacterial chromosome from 4 to 40 times in a specific locus) (Bull etal., 2003; Thibault etal., 2007). The number of repeated units varies among strains, and thus determining the exact number of repetitions can constitute a powerful tool for the characterization of isolates. 

The techniques showing the highest discrimination power for isolate’s comparison in order to determine if there is an epidemiological link between them are those based on the analysis of a larger proportion of the bacterial chromosome. Among them, the pulsed field gel electrophoresis (PFGE) analysis has been widely used: this method is based on the enzymatic digestion of the whole bacterial DNA and the latter analysis of the resulting fragments; first the DNA is embedded in agarose plugs and subjected to a chemical lysis in order to free the DNA from the bacterial cells. DNA is distributed homogeneously on the plugs, and is then digested by the selected restriction enzyme (normally low-frequency cleavage enzymes). Finally, the resultant fragments are separated by means of a multi-directional electrophoresis performed on the digested DNA during a determined period of time. While pulsed field gel electrophoresis can separate very large fragments (up to 10 Mb) yielding characteristic fragments, conventional electrophoresis is only able to separate bands of up to 50 kb. The distinct band patterns obtained are produced by genetic changes such as mutations that create or eliminate cut points of the enzyme, or deletions or insertions that increase or decrease the size of a band.

The application of these techniques has allowed the identification of the source of infection on several mycobacterial outbreaks in swine (Matlova etal., 2004; Matlova etal., 2005). Among the risk factors that can contribute to the beginning of an outbreak due to M. avium complex members, colonization of sawdust used for the bedding of the piglets, of water distribution systems and of cooling systems are some of the most frequently involved, and should be therefore taken into account in management practices on swine farms.


Julio Álvarez

VISAVET Health Surveillance Centre
Complutense University





References

Bull, T. J., Sidi-Boumedine, K., McMinn, E. J., Stevenson, K., Pickup, R., y Hermon-Taylor, J. (2003). Mycobacterial interspersed repetitive units (MIRU) differentiate Mycobacterium avium subspecies paratuberculosis from other species of the Mycobacterium avium complex. Mol.Cell.Probes 17, 157-164.

Falkinham, J. O., III (2003a). Factors influencing the chlorine susceptibility of Mycobacterium avium, Mycobacterium intracellulare, and Mycobacterium scrofulaceum. Appl.Environ.Microbiol. 69, 5685-5689.

Falkinham, J. O., III (2003b). The changing pattern of nontuberculous mycobacterial disease. Can.J.Infect.Dis. 14, 281-286.

Guerrero, C., Bernasconi, C., Burki, D., Bodmer, T., y Telenti, A. (1995). A novel insertion element from Mycobacterium avium, IS1245, is a specific target for analysis of strain relatedness. J.Clin.Microbiol. 33, 304-307.

Kunze, Z. M., Wall, S., Appelberg, R., Silva, M. T., Portaels, F., y McFadden, J. J. (1991). IS901, a new member of a widespread class of atypical insertion sequences, is associated with pathogenicity in Mycobacterium avium. Mol.Microbiol. 5, 2265-2272.

Matlova, L., Dvorska, L., Ayele, W. Y., Bartos, M., Amemori, T., y Pavlik, I. (2005). Distribution of Mycobacterium avium complex isolates in tissue samples of pigs fed peat naturally contaminated with mycobacteria as a supplement. J.Clin.Microbiol. 43, 1261-1268.

Matlova, L., Dvorska, L., Palecek, K., Maurenc, L., Bartos, M., y Pavlik, I. (2004). Impact of sawdust and wood shavings in bedding on pig tuberculous lesions in lymph nodes, and IS1245 RFLP analysis of Mycobacterium avium subsp. hominissuis of serotypes 6 and 8 isolated from pigs and environment. Vet.Microbiol. 102, 227-236.

Primm, T. P., Lucero, C. A., y Falkinham, J. O., III (2004). Health Impacts of Environmental Mycobacteria. Clin.Microbiol.Rev. 17, 98-106.

Semret, M., Turenne, C. Y., de Haas, P., Collins, D. M., y Behr, M. A. (2006). Differentiating host-associated variants of Mycobacterium avium by PCR for detection of large sequence polymorphisms. J.Clin.Microbiol. 44, 881-887.

Thibault, V. C., Grayon, M., Boschiroli, M. L., Hubbans, C., Overduin, P., Stevenson, K., Gutierrez, M. C., Supply, P., y Biet, F. (2007). New variable-number tandem-repeat markers for typing Mycobacterium avium subsp. paratuberculosis and M. avium strains: comparison with IS900 and IS1245 restriction fragment length polymorphism typing. J.Clin.Microbiol. 45, 2404-2410.

Turenne, C. Y., Semret, M., Cousins, D. V., Collins, D. M., y Behr, M. A. (2006). Sequencing of hsp65 distinguishes among subsets of the Mycobacterium avium complex. J.Clin.Microbiol. 44, 433-440.

To know more:

Thorel MF, Huchzermeyer H, Weiss R, Fontaine JJ. Mycobacterium avium infections in animals. Literature review. Vet Res. 1997 Sep-Oct;28(5):439-47.

Thorel MF, Huchzermeyer HF, Michel AL. Mycobacterium avium and Mycobacterium intracellulare infection in mammals. Rev Sci Tech. 2001 Apr;20(1):204-18.




Article data

Title:
Mycobacteriosis due to members of the Mycobacterium avium complex in swine: Significance, diagnosis and identification of possible sources of infection
Author:
Julio Álvarez
Online publication date:
January 27th, 2009
Keythems:
Tuberculosis, Paratuberculosis, Swine mycobacteriosis, Mycobacterium avium

Author data
Julio Álvarez
VISAVET. Health Surveillance Centre
Complutense University
Madrid (Spain)
Julio Álvarez BiographyDatos de contactoI+D+i data
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