Ask The Entomologist

Tick diseases in Montana

Spring has sprung, and our first ticks are out and about.
When folks start talking about illnesses transmitted by ticks, the first to come up almost always seems to be Lyme Disease. While Lyme Disease is the most common tick-borne disease among Montanans, you don’t need to worry about picking it up around here – it tends to be something folks pick up on vacation.

The Black-Legged Tick, also known as the deer tick, is the primary culprit responsible for transmitting Lyme Disease. Black-legged ticks are not found in Montana – they are, however, found all across the eastern half of North America. A related tick on the west coast, the Western Black-Legged Tick, is a less-effective vector of Lyme Disease.

A black-legged tick feeding on you isn’t a guarantee of disease – to transmit the disease, the tick first needs to feed on a small mammal (usually a White-Footed Deer Mouse) which is infected with the bacterial agent responsible for the disease… Because these ticks eat blood meals only once per life stage (once each as a larva, nymph, and an adult), you’re most likely to get the disease from a black-legged tick nymph. The adults prefer deer to us humans, anyways.

The lifecycle of the Black-Legged Tick, sometimes known as the Deer Tick.
The common hosts for each life stage are pictured as well.
The CDC’s helpful graphic on the black-legged tick lifecycle.

Our northwestern corner of Montana is untroubled by tick-vectored disease. That said, if you venture into the southern portions of the state, we do have four tick-borne illnesses you could contract.

Rocky Mountain Spotted Fever
-About 6 cases reported in MT per year.
-This disease is potentially deadly, moreso than Lyme Disease. Symptoms include achiness and fatigue, as well as a distinctive mottled rash.
-This disease is vectored by larger ticks, the Rocky Mountain Wood Tick and the American Dog Tick, both of which should be easier to spot.

Tularemia
-About 4 cases reported in MT per year.
-Sudden high fever, swollen lymph nodes, and pervasive weakness.
-Tularemia can be vectored by the same two ticks that transmit Rocky Mountain Spotted Fever. However, more people catch it from contact with blood from infected rodents and especially rabbits. Something to keep in mind if you’re fond of skinning.

Colorado Tick Fever
-1 to 2 cases reported in MT per year.
-This feels much like flu – aching, fever, chills, fatigue – which makes sense, as flu and Colorado Tick Fever are both are caused by viruses. Not too dangerous, usually goes away after 1-3 days.
-Like the previous two diseases, this one is transmitted by both the Rocky Mountain Wood Tick and the American Dog Tick.

Tick-Borne Relapsing Fever
-Extremely rare, but it does occur in MT.
-Fever rapidly develops after the initial infection, but then subsides, and reocurs in cycles about four days long.
-Transmitted by soft-bodied ticks that specialize on chipmunks and pine squirrels – avoid contact with small woodland mammals and sleeping in dilapidated cabins and you should be okay.

I would strongly advocate wearing clothes treated with permethrin if you’ll be in good tick habitat for a while – especially if you’re someplace with more interesting tick-borne diseases, like Red Meat Allergy, in the American southeast.

The ticks you’re most likely to find here in Montana are these two… but if you come across something odd, there’s the CDC’s tick ID page, and beyond that I’d be more than happy to take a look at any bugs you might have for me!

A poster from the MT Department of Health & Human Services
Communicable Disease Epidemiology Program
Demography

Vaccination by the Pyramid

The term “population pyramid” goes back to a time when plotting populations by age really did produce a triangle, with a large base of young people and each older age cohort narrowing, until there were very few at the top.  As diseases became more controlled, and birth control entered the picture, the population pyramids changed shape,  The pyramid below is for the US in 2010.

(Data from http://www.proximityone.com/chartgraphics.htm )

Now, if we look at covid death rates by age cohort, Florida’s governor released survival rates back in September:

The numbers for the age cohorts are below – no need to extrapolate from the bars in the pyramid.  We’ll just take the complement of the survival rates, assume the vaccine is 100% effective, and calculate the potential lives saved in each cohort.

AgePopulationDeath RateLives Saved by Vaccination
70+22.8 million5.4%1,502,967
50-6985.8 million0.5%429,045
20-49127.5 million0.02%2,550
0-1983.3 million0.003%250
The Death Rate is 100% minus the survival rate for each age group. Lives Saved by Vaccination is the population of the age group multiplied by the death rate.

If everyone over the age of 70 were vaccinated (and the vaccine worked perfectly) 1,502,967 Lives would be saved. If everyone under the age of 20 were vaccinated, 250.

No editorializing here – just simple addition and multiplication with data from the census.  I know where I would put the first vaccinations if I had a limited supply.

Demography

America’s First Plagues

I wound up studying epidemics when I was given the task of teaching Indians of North America.  The data was limited, but pretty much irrefutable – European diseases, brought by ship to the islands and eastern coast of North America did far more than decimate the native population of the Americas.

By the end of my readings, I pretty much bought into Dobyns’ explanation: “Before Europeans initiated the Columbian Exchange of germs and viruses, the peoples of the Americas suffered no smallpox, no measles, no chickenpox, no influenza, no typhus, no typhoid or parathyroid fever, no diphtheria, no cholera, no bubonic plague, no whooping cough and no malaria.”

His research leads to the conclusion that European diseases race across the continent ahead of the European explorers, killing 80 to 95% of the population.

I settled for that explanation, until Covid statistics started to overwhelm me, then realized, I could take to the internet to find the R0 numbers for each of these diseases – and that those numbers would have been worse for virgin soil epidemics within a people who had no previous exposure.  R0 quantifies a germ’s ability to infect – the R0 for the 1918 Spanish Flu is estimated at 1.4 to 2.8, while the similar 2009 H1N1 was between 1.4 and 1.6.  Simply enough, the larger the R0 number, the more infectious the disease.

R0 for Covid was estimated at 5.7 by the CDC.

The lazy man’s way of capturing the R0 values is wikipedia. It shows:

DiseaseR0
Measles12-18
Chickenpox10-12
Mumps10-12
Polio5-7
Pertussis5.5
Smallpox3.5-6

As I looked at the data – often from Winter counts – I realized that Measles was likely at least as responsible for Native deaths as Smallpox, though the Native records really don’t distinguish. At any rate, the R0 provides a usable measure for understanding the unintended European  plagues had on the Native American populations.  Likewise, the chart lets me look back on my childhood diseases with a greater understanding of how virulent the diseases we encountered in the fifties actually were.

R0 provides a good way of ranking how infectious a disease can be. Wiki also provides a chart for fatality rates of many diseases.