I’ve held off writing about this, but needed to get to it sooner or later. A recent article in the journal Emerging Infectious Diseases (Nelson et al 2016) has spurred a serious of over-exaggerated and sometimes downright comical headlines. “Kittens can cause death; US study” is an Australian example of over-the-top reporting, probably by people who didn’t really read the manuscript.
The paper is a study of cat scratch disease (CSD), a bacterial infection caused by Bartonella henselae. Cats are the main reservoir for this bacterium, which is spread between cats by fleas. The bacterium can live in the cat’s bloodstream for prolonged periods of time without causing any problems for the cat. As the name of the disease implies, cat scratches are a main route of exposure for people, but fleas play a role as well. The fleas ingest the bacterium in the blood when feeding on the cat, and then pass it in their feces. People can be infected if a scratch gets contaminated with Bartonella-ladden flea feces, inoculating the bacterium into the body. Cat bites might also play a role, as another way to inoculate a person with infected flea poop, or potentially even infected blood directly from the cat. Most often, inoculation of the bacterium probably doesn’t do anything – the body simply fights it off and produces antibodies against the bug. However, disease can obviously occur too depending on the circumstances.
This CDC study evaluated CSD in the US by looking at a health insurance claims database. The findings are different from some previous studies and perceptions, but not evidence of impending doom. Here are some highlights:
- The average incidence of CSD was about 4.7 per 100,000 people.
- There was a steady decrease in cases of CSD that did not result in admission to hospital (outpatient visits) over time, from a high of 5.7/100,000 in 2005 to 4.0/100,000 in 2013.
- Not surprisingly, areas that tend to have lower flea populations had fewer CSD cases.
- The highest incidence was in kids 5-9 years of age, at 9.4 /100,000. Overall, kids less than 14 years of age accounted for about a third of cases. (One thing to note is that the database did not have information on people over 65 years of age. They are another potentially increased risk group and one that would likely be at higher risk of being hospitalized from CSD).
- Overall, most cases occurred in the southern US in the late summer and fall.
Much of what was reported in this study is consistent with what we already know. The distribution of disease across the US and the highest risk times of year are what would be expected for a flea-associated infection. The estimate of the incidence of disease is higher than some studies and lower than others. The incidence of hospitalization was a lot lower than other studies.
What does this study tell us, in the big picture? It’s a reminder that CSD is uncommon but still something that warrants attention, particularly as cat ownership increases. Bartonella henselae is a strange bug that is increasingly associated with problems other than CSD, so measures to reduce exposure (and ensure accurate diagnosis) are important.
Some take away messages:
- Like many diseases, children are over-represented. This may be, in part, biological (being at greater risk of disease after exposure). However, a lot of it is probably due to an increased risk of kids being scratched or bitten by cats. Good supervision and education of kids about how to act around cats can reduce the risk.
- This is one more reminder of the importance of flea control.
Controlling fleas and avoiding bites and scratches can make this a largely preventable disease.