Is infection from dogs probable?
In his article (1), Landvik cites Sherlock Holmes «When you have eliminated the impossible, whatever remains, however improbable, must be the truth». Landvik believes that the evaluation committee, having excluded animals as a likely source of infection, therefore landed upon sewage leakage as the only possible cause, solely because they had eliminated everything else. This also implies that the sewage leakage theory is unlikely. However, many outbreaks of waterborne parasitic infections, where the source of infection has actually been identified, demonstrate contamination from sewage (3). In other words, an association between sewage leakage and waterborne disease outbreaks is not uncommon, and there appears to be no clear explanation of why Landvik believes it to be unlikely in this particular case.
Based on publications about previous waterborne disease outbreaks, faeces from dogs have never before been identified as a source of infection. Landvik himself emphasises how unusual this is by calling it «the first drinking water epidemic due to faeces from dogs».
Although we agree that direct investigations of sewage from the relevant houses ought to have been conducted after the outbreak was detected, something that was also recommended, we were nevertheless obliged to respect the regulations intended to protect the privacy of individuals. In the wake of the large waterborne cryptosporidiosis outbreak in Östersund, Sweden in 2010, sewage leakage from a building containing several apartments was considered the most likely source of infection, but closer investigations to identify the specific apartments were not conducted (4). Additionally, because contamination of the water occurred several months before the outbreak was discovered, the results of such an investigation in Bergen would probably have been misleading.
However, one thing is clear – the sewer outlet from the buildings in the neighbourhood was located only a short distance (200–300 m) from the raw water intake, while run-off of dog faeces from the area along the footpath around the lake would depend on where they are deposited and the terrain down to the lake. After the outbreak was identified, between 5–10 kg of dog faeces were collected from this footpath. Ten sub-samples were analysed at the Veterinary School, NMBU for both Cryptosporidium and Giardia; all were negative (2). These results do not exclude Giardia infection in dogs in the area, but neither do they provide any evidence for greater suspicion.
Rather than presenting these findings, Landvik has concentrated on a survey of young dogs (5). Although the cumulative prevalences found in this study were, as quoted, relatively high (44 % for Cryptosporidium and 21 % for Giardia), the individual prevalences were only half as high in puppies (23 % for Cryptosporidium and 12 % for Giardia) and far lower in adult bitches (< 3 % for Giardia and < 4 % for Cryptosporidium). It is known that these parasites are most common in younger animals, and therefore the age distribution amongst the dogs that were being walked around Svartediket should be taken into consideration. Furthermore, the intensity of the infection should be taken into account – of the puppies that were positive in the study by Hamnes and colleagues (5), approximately 50 % had high numbers of Giardia cysts in their faeces, while only 17 % had a high number of Cryptosporidium oocysts. The infection pressure is not only determined by the quantity of dog faeces on the footpath that contain Giardia cysts or Cryptosporidium oocysts, but also the number of cysts and oocysts per gram. Although the estimated amount of dog faeces (130 – 521 kg) in the area around Svartediket can be perceived as alarming, interpretation of its significance regarding the amount of Giardia or Cryptosporidium is far more complex. Furthermore, studies at Norwegian sewage works have shown that Giardia and Cryptosporidium occur frequently in high concentrations (from 4,000 to over 20,000 cysts/oocysts per litre of sewage) and the results from Bergen (in samples taken before the outbreak) were particularly high (6).