One of the profoundest mysteries of medical microbiology is why some people become deathly sick from rare infections while the vast majority remain unscathed. For instance, the brain-eating amoeba, Naegleria fowleri, is often acquired after swimming in a lake. But the fact that thousands or even millions of people swim safely in lakes every year makes this infection all the more puzzling. In America, merely 10 people come down with and (usually) die from this disease annually, according to the CDC. Why?
The answer, frustratingly, is that we simply do not know. Depending on the patient, it is probably some combination of bad luck (wrong place, wrong time), bad genetics, age, and health status. That the disease appears to strike randomly in previously healthy people makes it rather frightening.
Now, another victim has been killed by a bacterium that rarely infects humans. In its Morbidity and Mortality Weekly Report, the CDC details the extraordinary case of a 71-year-old woman who died from Streptococcus equi subspecies zooepidemicus, a bacterium that is commonly found in healthy horses but also occasionally causes illness in them. The woman had an upper respiratory tract infection for about a week. During that week, she was in close contact with a horse infected with S. zooepidemicus. One day, she developed diarrhea and vomiting and was dead in about 24 hours. The deceased's daughter also had an upper respiratory tract infection and was infected with S. zooepidemicus but recovered.
What explains the disparity in outcome? Possibly age. Older people are more susceptible to infectious disease, and the prior upper respiratory tract infection may have increased the victim's susceptibility. But the bottom line is that nobody really knows.
The report notes that there are 32 known cases of S. zooepidemicus infection in humans, seven of which were fatal. It is likely that far more human infections occur, but because they are probably mild or subclinical, they go unreported.
Is there a takeaway lesson from this case study? Not really. Dying of S. zooepidemicus is the microbiological equivalent of getting hit by a meteorite. Our furry friends can sometimes give us nasty diseases. If you are around animals often and develop an infection, keep in mind that there is a very slight chance that you could be infected with something potentially serious. Other than that, just hope that what happened to this poor lady doesn't happen to you.
Source: Kawakami V, Rietberg K, Lipton B, et al. "Notes from the Field. Fatal Infection Associated with Equine Exposure — King County, Washington, 2016." MMWR 65 (30): 788. Published: 5-Aug-2016. DOI: http://dx.doi.org/10.15585/mmwr.mm6530a5.