Zoonoses are not a one-way street…April 3, 2011
When discussing zoonoses it is very easy to think of only those diseases that pass from animals to humans and completely forget those that go the other way. Indeed, some people refer to diseases that pass from humans to animals as ‘reverse zoonooses’ (although the WHO defines zoonotic diseases as ‘a group of infectious diseases that are naturally transmitted between vertebrate animals and humans’ – emphasis my own).
As an aside: I don’t actually like the term ‘reverse zoonosis’. To me the thing that is important about these diseases is the ability to cross the human/non-human animal boundary, regardless of which direction this is in. I’d love to hear what any of you think, though.
Aside over, this paper particularly interested me as a possible example of these diseases that transfer from humans to animals. (It also allows me to put an extremely cute picture up courtesy of dnatheist )
Did transmission of Helicobacter pylori from humans cause a disease outbreak in a colony of Stripe-faced Dunnarts (Sminthopsis macroura)?
The discovery of H. pylori in human stomachs in 1982 and its characterisation as a human pathogen won two researchers, Dr Barry Marshall and Dr Robin Warren, a Nobel Prize in 2005. It is known that the presence of these bacteria in the stomach is key in the development of gastric cancer and peptic ulcer disease. Many other Helicobacter spp. have been found in other mammals with some also being linked to gastrointestinal pathology.
This paper seems to have initially been sparked by an increase in the number of deaths in a breeding colony of Stripe-faced Dunnarts (awwww!) at the University of Melbourne during the breading season of 2003. Post mortems were performed to try to identify the cause of death but the only pathology observed was that nearly half of the dead and culled animals had severe gastroenteritis.
A few months later the researchers tested 2 apparently well females for Helicobacter spp. using stomach biopsies for the CLO test and they came up positive. When they looked at the biopsies under the microscope they noticed some mild inflammation, although they could see no bacteria.
During the breeding season of 2004 a second spate of deaths occurred with 30% of the colony dying. 25% of the dead Dunnarts showed evidence of bleeding into their stomachs.
From 2005 to 2007 the mortality rate in the colony returned to normal and the researchers saw no more cases of gastric bleeding. During this time period the researchers used a variety of methods to test for Helicobacter spp. and whilst they were unable to culture any bacteria, using PCR they found the presence of Helicobacter spp. in both faeces and stomach contents. They also amplified up the 16S ribosomal RNA gene and partially sequenced it. The sequenced corresponded to H. pylori strains originally isolated from human gut. Finally, researchers showed that some of the H. pylori sequences they isolated had two important (virulence factors) which allow the bug to cause disease in humans.
In 2008 the researchers again collected samples from the colony and all 18 females tested negative. The researchers state that there was no obvious explanation for this loss of infection as there were no husbandry changes and no antibiotics were used.
So what do all of these results mean? Well the authors suggest that the clearing of H. pylori from the colony between 2007 and 2008 shows that the bug is not part of the normal flora of the Dunnarts. They speculate that it may have entered the colony via an infected human handler.
Because H. pylori was present in the colony in the absence of disease the authors suggest that it alone did not cause the outbreaks, but, because many of the mortalities showed bleeding into the stomach, it might have contributed to the outbreak with another pathogen. Stress also seems to have contributed to the outbreaks, as both occurred during the breeding season (a period where the Dunnarts typically lose 12-16% of their body weight, even if they are entirely healthy).
The researchers also speculate about the reason for the persistence of the bug for years followed by its apparent disappearance: they suggest that the loss was unlikely to be due to the Dunnarts’ native flora suddenly out-competing it after 4 years, and instead suggest that perhaps the colony was being constantly reinfected by a human handler and that the loss of bug was due to a change in handler.
We are not told what protocols the handlers had to follow when working with the colony, but this paper should certainly serve to raise awareness that the hand-washing and protective clothing used to protect ourselves from bugs from animals can also help protect them from us: something known and well-understood in the biomedical research world, but perhaps considered less fully in places like petting zoos and children’s farms.
Anyone working with, or simply cuddling an animal should not only be aware of the zoonotic bugs they can catch from the animal, but also the zoonotic bugs they could give to it.
Every, A., Selwood, L., Castano-Rodriguez, N., Lu, W., Windsor, H., Wee, J., Swierczak, A., Marshall, B., Kaakoush, N., Mitchell, H., & Sutton, P. (2011). Did transmission of Helicobacter pylori from humans cause a disease outbreak in a colony of Stripe-faced Dunnarts (Sminthopsis macroura)? Veterinary Research, 42 (1) DOI: 10.1186/1297-9716-42-26
Warren JB & Marshall B. (1983). Unidentified curved bacilli on gastric epithelium in active chronic gastritis. Lancet 321, 1273-1275
Helicobacter in other spp.:
Harbour S & Sutton P. (2008). Immunogenicity and pathogenicity of Helicobacter infections of veterinary animals. Vet Immunol Immunopathol. 122, 191-203
Hassebrouck et al. (2009). Gastric Helicobacters in domestic animals and nonhuman primates and their significance for human health. Clin Microbiol Rev. 22, 202-223