Friday, March 3, 2017

What is Hantavirus?

Hantavirus micrograph. About 150 nm diameter.

I. Why Should I Care?

Hantavirus is a family of viruses that lives in rodents throughout the world.  Anywhere from five to twenty percent of the host species have antibodies for hantavirus, meaning that they either have it, or have had it. Hantavirus doesn’t seem to do rodents much harm. When hantavirus makes the jump from rodent to human, however, it often causes severe illness or death.

Despite the fact that these viruses have been around for thousands of years, they have only recently been discovered to be killing people in the United States. These new hantavirus infections resembled much more common diseases like the flu. Death came when those flu-like symptoms, called the “prodrome,” gave way to a sudden and swift respiratory collapse.

There are many species of hantavirus around the world, and variations in the genes of those species as well. It appears that hantavirus species have evolved separately in many locations, each together with its favored rodent host species. Most hantavirus species attack the kidneys in humans, causing a condition called Hantavirus Fever with Renal Syndrome (HFRS). HFRS has a mortality rate that ranges from 1 – 5%, and it causes damage to the internal organs of many other infected people. Hundreds of thousands of people worldwide suffer from HFRS every year.

The Sin Nombre hantavirus, (SNV), formerly known as Four Corners virus, was only recognized in the U.S. Southwest in 1993 after a cluster of cases happened to tip off observant doctors.  In contrast to most hantaviruses around the world, this “new” form of virus attacked the lungs, causing what is now called Hantavirus Pulmonary Syndrome, (HPS). The mortality rate of HPS remains about 40% despite aggressive, supportive treatment.

Total numbers of HPS cases since 1993 by state. (CDC)
Although it was at first associated with the desert Southwest and is still most commonly reported there, SNV hantavirus has been found just about everywhere in the continental United States and in Canada. Tests from stored blood samples of respiratory collapse victims have shown cases going back to the 70’s and Navajo lore in the area refers to an illness that was likely HPS, with outbreaks going back to 1918 and before. 
Deer mouse, the most common SNV host.
The deer mouse, Peromyscus maniculatus, (Figure 1.) is the primary host species for SNV in most of the U.S. Where deer mouse roam, they are often so successful that they outnumber all other rodent species combined. A great article on deer mice can be found below, going into ecology, behavior, identification, etc. Don’t let the “biokids” deter you, this is a good site.

In areas where the deer mouse isn’t the “king rat” some other, similar species rules that niche of the rodent roost, and finds itself host to its own species of hantavirus. In the southeast it is the cotton-footed rat and the rice rat. The white-footed mouse, in the northeastern U.S.  may be the most like the deer mouse, but carries the New York Virus. Pictures of these other rodents that help identify them along with maps showing their turfs can be found here:
White-footed mouse, from the northeast U.S.

Deer mice, or their ecological equivalents, are extremely common, are commonly are infected with SNV, and commonly come into close proximity with people. We have a disease with an extremely high mortality rate and no specific treatment that is present in vast quantities all around us. If you live in the country, it’s hosts are probably scampering around in your backyard at this moment. They may be building a nest inside your automobile, or setting up a food bank in your child’s backyard playhouse.

And yet, only about 700 HPS cases have been documented in the nearly 30 years in the U.S., less than 25 cases per year for the entire country. Why are we so “lucky?” Why aren’t hantavirus cases far more common?

How is it possible that there are only be 25 cases/year in the entire country? Is it extremely unlikely that anyone could ever catch it, and so surprising that there are any cases at all? Or is there a single bottleneck somewhere in the infection process? Perhaps a single unlikely step in the infection process prevents an epidemic from erupting.

Or, is it possible that SNV is actually not that rare at all? A study performed in Mexico found that no cases were reported, despite SNV antibodies being common among rodents. They theorized that many HPS cases were being missed Mexico.

I have become convinced that a substantial number of SNV victims may be misdiagnosed in the U.S. as well. Along with presenting general information about hantavirus, this site will explore that possibility, and consider what may be done to prevent it.