The Animals That Can Smell Cancer

February 18, 2016

In a small, hot room in a compound located in Tanzania’s lush southern highlands are three white-clad technicians, a glass-and-metal chamber, and a large brown rat named Charles.

After being gently dropped into the chamber, Charles aims his long snout towards the first of a series of 10 sliding metal plates in the chamber’s base. A technician swiftly opens it, revealing a small hole. Charles sniffs at it … and moves on.

The hole is re-closed, and there’s a clink of metal as the next plate is yanked back. This time, Charles is gripped. He sniffs hard, scratching at the metal, the five claws on each paw splayed with the pressure. The technician calls out, “Two!”

Over by the window, her colleague is holding a chart, which he keeps raised so the others cannot see it. He inserts a tick. I glance over. The chart is a grid of small boxes, 10 across by 10 down, each marked with an alphanumeric code. Two of the boxes in each line are shaded gray. The tick has been placed in one that is white. It’s highly possible that Charles has just saved someone’s life.

Charles is an African giant pouched rat, a species endemic to sub-Saharan Africa. He’s also a pioneer, one of 30 of his species that live and work here in Morogoro, a few hundred kilometers west of Tanzania’s largest city, Dar es Salaam, on a program to sniff out tuberculosis (TB).

TB is a disease that can destroy the lungs. About 9 million new cases are diagnosed worldwide every year, one-quarter of them in Africa. Africa also has the highest TB death rate per head of population. Antibiotics can cure TB, but it’s fatal if untreated, and many patients are never diagnosed. This is partly because the 125-year-old microscope-based test used across Tanzania (and in many other cash-strapped countries) picks up only about 60 percent of cases, a figure that drops as low as 20 percent for people also infected with HIV.

This is where Charles the rat comes in. Charles and his rat colleagues sniff cough-and-spit samples provided by suspected TB patients. The rats aren’t infallible, but they do detect about 70 percent of cases, and it doesn’t matter to them if a patient has HIV—which matters a great deal in Tanzania, where about four in every 10 people with TB are HIV positive.

This particular morning Charles has sniffed 100 samples, missing one that has been identified as positive by the public clinic—shaded gray on the chart—but identifying 12 new suspected cases, which will now go for secondary checking.

The next rat brought into the testing room, a sleeker, bigger-eared, 3-year-old named Vladić (after a Bosnian Croat footballer; many of the rats are named after footballers), is even speedier than Charles. There’s a rapid clatter of metal plates being pulled back and replaced. The two technicians manning the chamber call out numbers: “Three! … Nine!” Ticks rapidly accumulate on a fresh copy of the same chart. About 15 minutes later, Vladić has correctly identified eight out of 10 clinic-positives, and also 15 new suspects.

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