Elephant-human transmission of tuberculosis

You can probably tell from various posts that I'm a fan of the CDC's journal Emerging Infectious DiseasesIt's a great journal containing interesting infectious disease reports about people and animals (it's free too, which is a nice bonus). In addition to the ferret infection I wrote about, the last edition also has a very interesting report about tuberculosis (TB) in people associated with an elephant sanctuary.

Tuberculosis is a big problem in elephants. It is relatively common in captive elephants in North America (2-12% are thought to be infected), and it It can be a tricky infection to diagnose, so it's very hard to know who's infected an who isn't.

Beyond being a problem for the elephants (because infection can be fatal), elephants with TB pose a risk to people, and there have been reports of human infections associated with elephant contact. Close and prolonged contact is most often associated with transmission of TB from elephants to people, but this recent report shows that indirect transmission may also be a concern.

The report in question deals with an elephant sanctuary in Tennessee. Problems were identified in July 2009 when routine TB skin testing of some workers at the facility indicated exposure to TB. Further investigation revealed that a quarantined elephant had tested positive in December of 2008. The elephant was diagnosed by culture of a trunk wash - to do this, the elephant is trained to suck water into its trunk and then "exhale" it into a plastic bag. The sample is then cultured for Mycobacterium tuberculosis. Animals with positive trunk washes are considered infectious, just like people with positive sputum (spit) cultures.

The TB history at this facility goes back further. In 2004, they took in two elephants that were known to be infected with TB. They were handled as per USDA Guidelines for the Control of Tuberculosis in Elephants (yes, such a document exists). One died of TB and the other was later released from isolation after treatment, according to the guidelines. In 2006, they took in eight more elephants from the same facility. All were considered high-risk for TB and were tested annually. All samples between 2006 and 2009 were negative, except for that one December 2008 sample. There's concern that since the elephants tested negative, people might have become lax with infection control practices. It's a problem beyond elephant facilities, where compliance slips over time since people may not see the need to continue to do what's required. 

After being notified of the the 2009 human TB test results, Public Health personnel launched an investigation. They eventually determined that 9/46 people whom they were able to contact had positive TB skin tests during the 2006-2009 period, despite having negative tests before then (confirming that they were truly exposed during this period). The strange thing was that it wasn't only the people with prolonged, close contact who were infected. People who worked in the quarantined area in 2009 were significantly more likely to be positive, but of the 13 people that worked in the quarantine area, only one had close contact with any elephant. Furthermore, three of the people who were infected were administrators.

This is where is gets interesting. Normal cleaning practices included the use of high-pressure washing to clean the barn. We know that this increases the risk of spreading bacteria in the air over long distances, and it's generally frowned upon from an infection control standpoint. People in the quarantine area wore respirators to reduce the risk. However, the administrative area was attached to the quarantine area and investigators subsequently determined that there was airflow from the quarantine area into the administrative area. So, when people were cleaning the quarantine area with high pressure washing, they were creating infectious aerosols that spread into the administrative building. That presumably accounted for infection of the people who had no contact with elephants. Furthermore, one of the investigators was also exposed during time spent in the administrative area before the risk was identified.

This highlights a few issues:

  • Infectious diseases continue to surprise us. We may know what they tend to do, but new transmission patterns and new concerns can develop.
  • Power washing can contribute to the spread of infectious diseases. This is a concern in many types of facilities.
  • Infection control compliance needs to be maintained over time, even if it seems like the risks have dropped.
  • Captive elephants continue to pose a risk to people around them, and careful precautions are needed to reduce the risk of exposure to TB.

(click photo for source)

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