With all the talk about non avian reservoirs for H5N1, a little talk about flies might be in order, not because we think they are vectors for H5N1 (so far no evidence of that), but just because we like to talk about them. The subject came up recently in a New Zealand Medical Journal article (authors B. Harris and W Nelson), about which we’ve only seen news reports (we are away from our home base library, on vacation, which is one of the reasons we are slow on comments and light on posting; using a slow dial-up is pure torture).
According to the New Zealand docs, flies and too many cattle close to New Zealand cities is one of the explanations for their high rate of campylobacter food poisoning. Campylobacter is a bacterium responsible for a significant amount of food poisoning. New Zealand is reporting 14,000 cases a year and this is probably an underestimate (by a factor of ten to twenty, according to the authors). It is frequently found in chicken but it is easily destroyed by cooking. The paper analyzes temperature and chicken consumption in relation to campylobacter foodborne case reports. When temperature increases so do campylobacter reports. Campylobacter is commonly found in cattle feces. So how do they get to food?
Mr Harris said flies were the link between environmental sources of campylobacter and food. In New Zealand, the primary source of infection is thought to be the faeces of dairy and beef cattle.
It was the proximity to rural environments that allowed flies to transmit the bacteria to food, particularly during the warmer months when they are most active.
The bacteria is transferred through fly faecal deposits on common surfaces such as hand rails and door handles. Campylobacter deposited on fingertips can survive for at least an hour, and have been recovered from dry surfaces 24 hours after being deposited. (New Zealand Herald)
I haven’t read the original paper, so my criticism here is really unfounded. But I have a very hard time believing this, because t is pretty uncommon to trace disease outbreaks to flies (typhoid in the Spanish American War being one of the few historical exceptions). Now flies are admittedly pretty disgusting creatures. Insects have different kinds of mouthparts (biting, chewing, etc.) and domestic flies have sponging mouthparts. They essentially soak up their nutrients via a spongelike apparatus. In order to do this, the nutrients have to be liquified and flies do this by vomiting on the food to liquify it. After eating they also defecate on the food “(fly specks” come in light brown and darkbrown varieties, one being vomit, the other feces; thought you’d like to know). Flies also have very hairy legs and readily carry moist material from one place to another. Since they need to lay eggs in warm moist places, they frequently alight on masses of animal feces, picking up material when they do so.
I tell you these delightful things because I don’t much like flies. They are really hygienically nasty. But it’s very hard to find instances where they really have been shown to spread disease. The same with cockroaches. Cockroach guts are loaded with pathogens. I once took a class into the city hospital cafeteria and we trapped, ground up and plated out organisms from a cockroach (Periplaneta americana, for those interested in which one). We found the usual enteric organisms, including Salmonella spp. and if we’d assayed for enteric viruses we’d have found them, too. But it is very difficult to find instances in the medical literature that demonstrate cockroaches cause disease outbreaks. It is plausible they might. Most people think they do. But really showing it is another thing.
So when I see articles like this, which on their face present only a handwaving demonstration, I am skeptical, particularly when I see this:
The research, which appears in the New Zealand Medical Journal today, says eating chicken is also a “significant risk factor”.
The study warns that takeaway chicken meals are particularly risky if food is eaten without washing hands.
I can believe chicken is a risk factor, but not eating it without washing your hands as a risk factor is nonsense (to be fair, this is a newspaper rendition of the article; I don’t know what the original said or what the authors said to the reporter). Most foodborne illnesses require a significant inoculum of the organism to make someone sick, an inoculum usually only achieved by the organism growing to significant levels through a failure in food sanitation. This means the food is improperly cooked or has become contaminated by cross-contamination (e.g., contact with a cutting board used to cut raw meat) and afterward allowed to incubate for four hours or more. If just having it on your hands is enough to infect you, then eating cooked chicken is secondary. You could infect yourself in any way you put your hands in your mouth.
Campylobacter and many other pathogens increase as environmental temperature increases, so the relationship noted by the authors is plausible. But the implication flies are to blame is another matter. It could be true, but the medical literature rarely demonstrates it.