20 for 20 : Day 6 : Calculating local HDUs (Heartworm Development Units)

While trying to define heartworm development units (HDUs) for veterinary students in the year two Veterinary Parasitology course, I chanced upon a 2015 article in Veterinary Parasitology by Ledesma and Harrington, titled “Fine scale temperature fluctuation and modulation of Dirofilarial larval development in Aedes aegypti”. The HDU is a calculated measure that factors in temperature to determine the extrinsic incubation period of heartworm larvae in mosquitoes.

Mosquitoes are obligate biological vectors of  Dirofilaria immitis. They bite infected dogs, and take up circulating microfilaria. These microfilaria have to develop to the third stage larval form (called L3) in the mosquito before they can infect dogs and mature to adulthood. The maturation from microfilaria to L3 takes time and is dependent on temperature. The warmer it is, the faster the larvae mature, with no maturation taking place below 14 degrees Celsius. This is expressed as the HDU, with at least 130 HDUs  being necessary before the L3 migrate back to the mosquito head and labium, ready to infect the next animal that the mosquito feeds on.

The formula for calculating the HDU is ∑ Average daily temp - 14°C = Accumulated HDUs.

As I was preparing for my lecture tomorrow, I thought it would be fun to calculate and compare the HDUs afforded by the climate in different seasons in the town that I live in. I obtained daily temperatures (in degree celsius, because life loves the SI system) from January (winter), April (spring) and August (summer) and made my calculations.

130 accumulated HDUs can be ruled out in January, and there is no development of microfilaria to the L3 stage in mosquitoes that reside outside. However, near and inside buildings and parking lots close to areas with centralized heating, in the so-called “urban islands”, the story might be different.

In march, if a mosquito picked up microfilaria on the first day of the month, infective L3s will have theoretically developed by day 14, when 130 accumulated HDUs are reached. Similarly, in august, if microfilaria had been picked up on the first, theoretically L3s will have developed by day 8.

In terms of actual transmission risk, real life in this case probably follows theory. There is lower risk of a dog contracting canine dirofilariasis in the winter, than in the spring, which is only slightly lower than summer in this town. It is the duty of every owner then to ensure that their dogs receive the chemoprophylactic prescribed by their vet at the right time and the right dose to prevent infection.

This post is part of a goal to write for 20 mins for 20 days.