Mathematics of STI Spread

(DISCLAIMER: This post is a little more technical, and hopefully will be enjoyed by my dear friends in math, engineering, or computer science).

Let:

R° = the average number of secondary cases infected by each person who has an STI (units: people)

β = the percentage probability that a person with an STI will spread the infection to a current susceptible sexual partner.

c = the average number of new sexual partners made (units: people/month)

D = the average duration of infectiousness for a particular STI (units: months)

M = the average number of people with an STI cured from infectiousness each month by medical intervention (units: people/month)

The change in prevalence of an STI in the population is determined by R°:
- If R° < 1, the STI is declining in prevalence
- If R° = 1, the STI is in equilibrium
- If R° > 1, the STI is becoming more prevalent 

R° can be expressed in terms of the other variables: 
     R° = βcD

Given a particular M, and noticing that β and D are constant for each STI, R° varies with c. Studies of sexual practices show that c follows a Poisson distribution that is heavily skewed towards the left. Thus:
- In most populations (the left-skew of the distribution), c is low enough that R° < 1
- There exist “core” populations (the right-sided tail of the distribution) where c is high enough that R° > 1. 
- Demonstrated core populations are: young people, sex-trade workers, and drug users.
- By estimating β and D, we can determine the threshold c for which R° = 1 (c = 1 / βD), and above which there is increasing prevalence of the STI. 

Some interesting research findings regarding core groups and epidemics:
- Like-with-like sexual activity (= within a core group) is a risk factor for a fast-developing, but limited STI epidemic.
- When core groups are small enough, there is self-limiting of the epidemic since sexual contacts become very likely to have previously contracted the infection
- Like-with-unlike sexual activity (= between a core group member and non-core group member) is a risk factor for a slow-developing, but ultimately more prevalent epidemic. 

STI prevention focuses on:
- Increasing M, which causes a decrease in D
- Advocating safe-sex and protective behaviours that reduce β
- Identifying core groups for focused medical attention and STI screening

Whoever said that mathematicians have no hope of ever becoming familiar with sex ? Course this isn’t quite the type of familiar I think they were hoping for…

Surprise Visitors

This morning’s topic was clotting disorders, such as Hemophilia. Nearing the end of the lecture, ushered into the room were two visitors who nobody was expecting. They were dogs from the Queen’s University Hemophilia-A colony. This colony has been raised at Queen’s for over 20 years, and so we were meeting at least the 16th generation of Hemophiliac dogs. They provide a model for research into better therapies for Hemophilia, including the much-anticipated gene therapy.  Because they are prone to spontaneous bleeding, exceedingly good care is devoted to these dogs. They were so cute!

Who Ever Said Med. Students Can’t Cook?

As is true for everything that I cook, all credit goes to the source of the recipes, the one who taught me how, and the best cook I know: my mom.

Made with a few classmates (yes! all med. students – can you believe it?)…

Hemolyze This!

We have an awesome faculty !

On Wednesday the topic of the morning was hemolytic anemias. Hemolysis is the disease process whereby red blood cells are destroyed either mechanically, or by the immune system. The result is anemia, or not enough oxygen reaching all of your tissues.

Anyway, our professor had challenged his kids over the weekend to build this incredible model of a paper mache red blood cell. Best of all – it was a pinata ! In large bold letters on the front: “Meds 2012: Hemolyze This!”. So right there, at the end of lecture, we played pinata and hemolyzed that giant red blood cell spilling lovely hemoglobin candies (zoo animals!).

Seriously. Awesome faculty.

UPDATE: Watch the video posted by a classmate on Facebook!

Bloodletting

Bloodletting was a common medical procedure of antiquity that involved draining sometimes large quantities of blood from ill patients. It was believed to be helpful for a plethora of diseases, including everyday ailments like fever and headaches. There were detailed medical protocols for how much blood to take, how often, and from which anatomical locations. In infants,  the entry-point of choice for bloodletting was from the anterior fontenelle, or the “soft spot” on the top of the baby’s still-forming skull.

Modern medicine has demonstrated that bloodletting has little therapeutic benefit for most of the diseases that it was standardly used for. Additionally, bloodletting introduced all sorts of horrific complications like infections, hypovolemic shock, and even brain damage when the needle went too far into the infant’s head.

That being said, I was surprised to discover that there is at least one disease for which bloodletting is effective, and still used today! The disease has to do with iron in the blood. Most people think about blood-iron in the context of iron-deficient anaemia, or not having enough iron to make adequate red blood cells. But, it is also possible to have too much iron in the blood.

Iron overloading in North America is most often due to a genetic disease called hereditary hemachromatosis. It involves a mutation for a protein that normally regulates iron absorption in the gut. As a consequence, the gut absorbs too much iron and the blood becomes “overloaded”. Iron overload can have serious consequences causing damage to the liver, pancreas, pituitary gland, and skin.

Since the body has limited capacity for excreting iron, the best treatment for hereditary hemachromatosis is regular bloodletting to remove the excess iron. Of course, nowadays we use the fancy term “plebotomy”, but the principle is still the same! Neat, eh?

A Day in the Life: Phase IIA

06:30 am – Alarm, Breakfast

07:00 am – Gym

07:30 am – Shower, pack for class

08:30 am – Class begins

12:30 pm – Lunch break / bagged lunch with speaker

01:30 pm - Class resumes

02:30 pm – 04:30  pm - Class ends (variable), dinner break.

6:00 pm - In the library.

11:00 pm - Library closes.

12:00 am - Asleep

How to Date a Medical Student

Shared by a classmate, this amusing (but SO accurate!!!!) article: How to Date a Medical Student

Crazy Sexy Cancer

Watched an insightful movie this morning (yes – a movie in class!) about a young woman living with a rare, slow-growing metastatic cancer. This movie comes highly recommended for a real, in-your-face perspective on how having cancer can change your life.

Watch the trailer:

The Cancer Menu

As we storm through our whirlwind week on cancer, it’s interesting to consider which cancers you are most likely to get. Please don’t become paranoid, however – this is an exercise in epidemiology, and not doom-and-gloom:

Women are most likely to get:

1. Breast cancer
2. Lung cancer
3. Colorectal cancer

Men are most likely to get:

1. Prostate cancer
2. Lung cancer
3. Colorectal cancer

Note that for both men and women, lung cancer is far more deadly than either breast cancer or prostate cancer. So for both sexes, the cancer you are most likely to die from is lung cancer. That being said, if you are a non-smoker and are able to minimize your environmental exposures (second-hand smoke, asbestos, radon, etc.), your lifetime risk for lung cancer is far lower than these lists might suggest.

Prevent Cancer with Good Health

Not long ago I posted the Five Top Steps to Good Health.

As an interesting follow-up, here’s what the Canadian Cancer Society had to recommend for their  seven steps to prevent cancer (in red are those that were included in the Five Top Steps):

1. Be a non-smoker and avoid second-hand smoke
2. Eat 5-10 servings of fruit and vegetables each day. Choose high fibre, lower fat foods. If you drink alcohol, limit your intake to 1-2 drinks a day.
3. Be physically active on a regular basis.
4. Protect yourself and your family from the sun.
5. Get yourself screened for common cancers.
6. Visit your doctor or dentist if you notice any change in your normal state of health.
7. Follow instructions when using, storing, and disposing of hazardous materials.

Here’s where you can find the CCS’s Recommendations.