|What have I got to do with this?
Hantavirus Pulmonary Syndrome is a ZebraSin Nombre hantavirus infection is thought to be extremely rare in humans. Only about 700 cases have been documented. That makes hantavirus a zebra disease.
Zebras are also rare compared to horses. Unless you happen to be on the Serengeti plains.
When doctors are diagnosing a difficult illness, they are trained to have a rule-of-thumb in the back of their minds. "When you hear hoof beats, don't look for zebras." The odds that your particular patient will turn out to have an extremely rare, "zebra" of an illness, are very low. So don't even look for them, keep your eyes peeled for horses or you will waste valuable time and delay the patient's diagnosis.
But there can be a problem with that. If you don't look for something, you have a reduced chance of finding it. When a disease is a zebra disease, it tends to be misdiagnosed, or missed entirely.
, : "During medical training, students and residents are being taught a quintessential maxim of clinical medicine—“When you hear hoofbeats, think of horses, not zebras”. Common diseases are what physicians should expect to encounter. Unfortunately, in a world of horses, the hoofbeats of zebras too often go unrecognized or misdiagnosed."
Could hantavirus pulmonary syndrome (HPS) be being misdiagnosed, because it is a rare, zebra of a disease that few practitioners have ever encountered? What measures are in place to capture HPS cases?
Urgent Care PresentationFrom the Centers for Disease Control's page on hantavirus pulmonary syndrome, the primary source of guidance on HPS for Emergency and Urgent Care rooms around the country:
"The earlier the patient is brought in to intensive care, the better. If a patient is experiencing full distress, it is less likely the treatment will be effective.
Therefore, if you have been around rodents and have symptoms of fever, deep muscle aches, and severe shortness of breath, see your doctor immediately. Be sure to tell your doctor that you have been around rodents—this will alert your physician to look closely for any rodent-carried disease, such as HPS."
Hantavirus infection should also be suspected if a particular combination of blood test results is obtained. The combination is called a tetrad of symptoms because there are four changes that, together, suggest hantavirus infection. If that tetrad is spotted in the blood count test tests by someone knowledgeable enough to look for it, hantavirus should be suspected. The patient would then likely be tested for antibodies.
Even in the modern, excellent medical facility where my wife recovered, none of our doctors, nurses, or techs had ever encountered hantavirus before as is likely the case with most Hanta patients. It was almost literally the last thing anyone thought of or discussed during a week of brain-wracking over what was going wrong. They did not know what was wrong, so, they treated for everything they could, and provided supportive care. That was the right treatment, (fortunately!) but it does not hinge on, nor support, the diagnosis of hantavirus.
Diagnosis of hantavirus requires testing for antibodies to it, and that is not a trivial thing.
Hantavirus Antibody Testing
Signal to Noise RatioEven if you aren't expecting a zebra, if one walks up and presents itself, it would be hard to miss. Right?
What if that zebra was engulfed in teaming throngs of horses, gazelles, and zebu? What if HPS resembled other diseases that were far more common? When something could be lost in a cloud of similar things, you have to think about something called signal to noise ratio, (S/N).
|Just one of the gang.
HPS patients present with symptoms that resemble a variety of other diseases, syndromes and categories of illness. Influenza, or flu-like illness, is a common early diagnosis. Community acquired pneumonia (CAP), a category, is the catch-all term for respiratory problems that the hospital didn't transmit. CAP can be caused by numerous pathogens, viral and bacterial. Identification of the particular pathogen in a patient is unusual.
HPS is one of many "insults" to the system that can cause what is called Acute Respiratory Distress Syndrome, (ARDS), a relatively new and still controversial classification of sudden respiratory collapse that may result from any of a variety of "insults" to the patient. These collapses follow a general pattern, and have a mortality rate of about 40%, about the same as HPS. Somewhere between 50,000 and 100,000 people per year are struck by ARDS in the U.S.
The huge range in these numbers reflects widely varying estimates of the number of severe illnesses and deaths that fall into this syndrome. A study of post mortem victims of respiratory collapse found that about half of all patients that met the criteria for ARDS were not diagnosed as having had ARDS while they were alive. That's a matter of multiple, somewhat different definitions and criteria.
|Zebra herd on the Serengeti plain.
From National Geographic
Differentiating ARDS caused by hantavirus from ARDS caused by requires some means of differentiation. Unfortunately, there are only general reasons for guidance. Hantavirus can be ruled out for cases of ARDS that have a clear source, such as a recent surgery or other illness. But hantavirus is only suggested as a faint possibility in cases with known exposure to rodents, and patients often have no way of knowing that they had that exposure, some weeks previously.
Misdiagnosis of hantavirus was recently reported in a case in Brazil, where the HPS syndrome is a newly identified condition. Hantavirus misdiagnosed as Dengue In places where the medical community is not trained to look for hanta, it tends to get missed. A kind of "latching" effect may appear in the statistics for this reason, with states reporting hantavirus enough to get press coverage tending to find more of it afterward.
ConclusionI can't be certain that people have been contracting hantavirus through infestations of their cabin air systems, but neither can anyone be assured that they are not.
There are reasons to believe that significant numbers of cases of hantavirus go undiagnosed in both Mexico and the United States.
Effort should be undertaken to better determine the scope of undiagnosed hantavirus from any source, and the particular question of whether vehicle cabin air systems are a mode of infection that has slipped "under the radar" of hantavirus detection.
Some comments on related webpages and excerpts from related references
The CDC webpage on hantavirus transmission makes no mention of vehicles, let alone of vehicle cabin air system infestations.
The King County Public Health site now has a pretty good update on hantavirus posted here:
This advisory is mainly directed at health care providers. It now specifically calls out vehicle cabin air systems as suspected transmission mechanisms. These changes probably would have never happened if it were not for this website calling attention to these issues.
Unfortunately, the general hantavirus information page and the Health Insider article make no mention of checking cabin air filters for deer mouse infestation. It does not mention that this common site of Deer mouse infestation even exists.
This needs to be changed, and the information for health care providers needs to be disseminated on a national basis. It would also be reasonable to develop some form of instruction that for patients presenting with symptoms resembling hantavirus, someone with proper ventilator mask and gloves should check the cabin air filters of vehicles they have used for signs of rodent infestation.
|Hantavirus cases by state of reporting from CDC. Note that many states
have never reported hantavirus, even though they have plenty of mice.
Same with three northernmost states of Mexico, as described above.
Before making any firm conclusions however, it should be noted that although the results of our preliminary study are suggestive of under recognition of ARDS/ALI, they are subject to a number of important limitations."
The incidence of ARDS in Maryland is in the range of 10-14 cases per 100,000 people. The ARDS mortality rate is 36% to 52%, similar to that calculated in previous studies.
* Note These posts will use Hanta to specifically indicate the Sin Nombre strain of hantavirus.