Lecture 11. Environmental Mycobacteria; Mycobacteria ulcerans - Buruli Ulcer

Question 1

What are environmental or non-tuberculous mycobacteria (NTM)?

Answer 1

Ubiquitous in environment – saprophytic organisms
Cause disease infrequently – opportunistic pathogens
In immunocompetent and immunocompromised people
No person-person transmission
Pulmonary disease is most common especially with those who have pre-existing lung disease
Also localised infections in lymph nodes, skin, soft tissue and rarely bones
Often misdiagnosed as TB

Question 2

What can the risk factors of NTM disease be categorised as?

Answer 2

Host
Environmental
Organism

Question 3

Where has NTM increased in prevalence?

Answer 3

North America, Europe, Asia over last few decades

Question 4

What was the increase in NTM prevalence in the US from the early 1980s to 2013?

Answer 4

2.4 cases/100,000 to 15.2cases/100.000

Question 5

What was the increase in NTM prevalence in the UK from 1995 to 2006?

Answer 5

0.9 cases/100,000 to 2.9 cases/100,000

Question 6

What does disseminated mean?

Answer 6

Spreads throughout the body

Question 7

What does pulmonary mean?

Answer 7

Targets the lungs

Question 8

What classes of clinical diseases can environmental mycobacteria cause?

Answer 8

Pulmonary disease
Disseminated disease
Lymphadenitis (enlargement of lymph nodes, often infection)
Cutaneous disease
Nosocomial disease

Question 9

What are examples of diseases caused by environmental mycobacteria?

Answer 9

Cervical lymphadenitis (head and neck)
Swimming pool granuloma
Buruli ulcer (M. ulcerans)

Question 10

What is the treatment for NTMs?

Answer 10

Prolonged multi-drug therapy for pulmonary NTM (12-24 months)

Question 11

What are the complications that are caused by NTMs?

Answer 11

High rates of re-infection (rather than recurrence)
Frequent adverse drug reactions
20-40% of MAC-lung disease patients initially fail to respond
Drug resistance
Co-morbidity

Question 12

What is the causative agent of Buruli ulcer infection?

Answer 12

Mycobacterium ulcerans

Question 13

What is a Buruli ulcer?

Answer 13

An indolent necrotising disease of the skin, subcutaneous tissue and bone
Indolent = causing little or no pain
Necrotising = death of tissues

Question 14

Compared with TB and leprosy, how common and understood is Buruli ulcer?

Answer 14

Buruli is the third most common mycobacterial disease of humans, after tuberculosis and leprosy, and the least understood of the three

Question 15

What is Buruli ulcer (BU) characterised by?

Answer 15

Painless, necrotic skin lesions

Question 16

Which specie of mycobacterium is M. ulcerans closely related to?

Answer 16

M. tuberculosis

Question 17

What profoundly influences the replication and persistence of M. ulcerans in BU lesions?

Answer 17

Production of mycolactone (a cytotoxin that has immunosuppressive properties)

Question 18

Where is BU most prevalent?

Answer 18

West and Central Africa (Ivory Coast and Ghana most affected)
Previously reported cases in South America and China

Question 19

How are humans infected with BU?

Answer 19

Little evidence of human to human transmission
Humans are infected from the environment or vectors

Question 20

What is the environmental transmission route of BU into humans?

Answer 20

In endemic regions, the disease is highly focal and usually associated with wetlands or coastal regions
PCR testing of environmental samples, such as water, aquatic plants, soil, and detritus from swamps, has found evidence of M. ulcerans

Question 21

What is the vector transmission route of BU into humans?

Answer 21

Insects such as mosquitoes and water-residing biting arthropods have been associated with M. ulcerans epidemiologically and via PCR testing and therefore proposed as vectors for transmission
In Victoria, Australia, evidence suggests that native possums might be involved in transmission

Question 22

What were the key finding of detecting M. ulcerans DNA within the insect tissues from an endemic area?

Answer 22

Some types of insect more positive than others
Seasonal effect
Non-endemic area had no PCR positive insect pools

Question 23

What is one of the key differences between endemic and non-endemic areas of M. ulcerans?

Answer 23

Human activity was far greater in areas where M. ulcerans was endemic (deforestation, agricultural and fishing activities)
Interventions disrupting the environment have been identified, in several studies, as factors potentially favouring the establishment of M. ulcerans in remodelled environments

Question 24

What did a recent study testing mosquitos show?

Answer 24

66,000 mosquitos captured and tested, 5/1000 were positive for M. ulcerans (PCR)
Genome sequencing showed that M. ulcerans from people, possums and mosquitoes in same area were identical
Mouse model showed that mosquitoes can transmit M. ulcerans

Question 25

What are the proposed transmission pathways of M. ulcerans between the environment, mosquitos, possums and humans?

Answer 25

Possums ingest M. ulcerans from the environment and/or infected by an insect vector Possums amplify and shed M. ulcerans into the environment Insect vectors become contaminated with M. ulcerans from the environment and/or from contact with infected possums M. ulcerans transmitted to humans via insect vector and/or direct contact with contaminated environment Rare cases of direct transmission from possum to human after bite

Question 26

How does BU disease progress?

Answer 26

Resistance to M. ulcerans has been associated with the development of Th1 type responses Granuloma observed during progression to healing Progression of disease due primarily to mycolactone – an exotoxin

Question 27

Where do skin ulcers caused by BU form?

Answer 27

Lower legs Oedematous lesions still form well into antibiotic treatment and require debridement +/2 skin grafting

Question 28

What are the features that promote pathogenesis of M. ulcerans?

Answer 28

The low optimal temperature of growth (29–33°C) that makes the skin its preferential target M. ulcerans behaves as an intracellular pathogen triggering inflammatory cell responses and cell mediated immunity Unusual toxin production, mycolactone-associated high cytotoxicity that contrasts with the intracellular traits of its mycobacterial nature In advanced BU lesions, extracellular clusters of M. ulcerans reside in necrotic subcutaneous tissue and are protected from infiltrating leukocytes by the cytotoxic activity of secreted mycolactone Mycolactone also has immunosuppressive properties

Question 29

What is mycolactone?

Answer 29

A polyketide cytotoxin that has immunosuppressive properties

Question 30

What are the precise cellular targets of mycolactone?

Answer 30

Remain mysterious It induces apoptosis of infected host cells Inhibits production of the proinflammatory cytokine TNF-α by macrophages Suppresses dendritic cell priming of T cells

Question 31

What can mycolactone synthesised by M. ulcerans diffuse from and what does this do?

Answer 31

Mycolactone synthesised by M. ulcerans can diffuse from infected skin tissue to lymphoid organs within mononuclear cells, where it may exert some of these immune suppressive functions

Question 32

What contributes to the painlessness brought on by BU

Answer 32

Mycolactone causing nerve cell damage

Question 33

What genes are required for mycolactone synthesis and how were these genes acquired?

Answer 33

The genes required for mycolactone synthesis are carried on a virulence plasmid, pMUM001, that was apparently acquired by horizontal gene transfer from an unknown source

Question 34

Why were other mycobacterial virulence factors lost by reductive evolution within M. ulcerans?

Answer 34

Because mycolactone came to dominate the interaction of M. ulcerans with its human host, following acquisition of pMUM001 other mycobacterial virulence factors were apparently lost

Question 35

Summary of disease progression of BU

Answer 35

Infection Pre-ulcerative infection Ulcerative infection Spontaneous healing, treatment

Question 36

How is BU diagnosed?

Answer 36

Clinical diagnosis Microscopy - smear Culture IS2404 PCR-based tests DNA based

Question 37

How is BU treated?

Answer 37

Multi-drug therapy to reduce selection of drug-resistant strains Combination of rifampicin (10 mg/kg once daily) and clarithromycin (7.5 mg/kg twice daily) is now the WHO recommended treatment Surgery and hospital treatment of wounds for skin grafts In well-developed lesions with ongoing necrosis due to the effects of high mycolactone toxin load then the removal of this tissue is necessary Physiotherapy and rehabilitation

Question 38

How are wounds caused by BU managed?

Answer 38

Healing time can be months or years to heal even after antibiotic treatment Improved wound dressings emerging from other ulcer diseases eg diabetes

Question 39

How can BU infection be prevented and controlled?

Answer 39

Identification of high risk zones Vaccines Avoiding risky behaviours in endemic areas

Question 40

How are high risk zones of M. ulcerans identified?

Answer 40

Active surveillance, case management, treatment

Question 41

What is an example of how identification of a high risk zone prevented BU disease spread?

Answer 41

Republic of Congo Identification of this zone as a high-risk area for BU disease helped the MoH improve early detection, biological confirmation and treatment programs

Question 42

What vaccines are there for M. ulcerans?

Answer 42

Some protection seen with BCG but inconsistent Vaccines specific to M. ulcerans, targeting a mycolyl transferase (antigen 85A) of the bacteria being tested

Question 43

How can risky behaviours in endemic areas be avoided?

Answer 43

Swimming, fishing , agricultural works Mosquito nets may help if insect borne (eg Australia), good pre-existing wound care, protective clothing to prevent wounds

Question 44

What are the research priorities for NTMs recommended in the US and UK?

Answer 44

Rapid diagnostic tools fast identification of infecting species Methods in development for rapid point of care: ELISA (immune assay), LAMP (nucleic acid amplification), mycolactone specific tests Simple and cheap screening tool to identify patients Faster initiation of appropriate treatment and ultimately, superior care

Question 45

What is Telacebec?

Answer 45

A new anti-tuberculous drug developed by a Korean company Qurient Extreme potent activity against Mycobacterium ulcerans in animal studies, reducing treatment duration from 8 to 2 weeks

Question 46

What can shorten the duration of standard treatment time (8 weeks)?

Answer 46

Combining the standard treatment (rifampicin and clarithromycin) with amoxicillin/clavulanate shortens treatment time to 4 weeks