What is a larval index, and how do vector control programmes use it?

If you've sat through a vector-borne disease surveillance meeting, you've heard these numbers thrown around like they're self-explanatory: "Breteau's at 42 in that ward" or "House Index came back under 5." They're not self-explanatory if you're new to the programme, and even people who've run surveys for years sometimes mix up what each one is actually counting.

A larval index is just a ratio: positive containers or houses divided by something you inspected, turned into a rate you can compare across time or between sites. The three you'll run into most often in Aedes surveillance are the house index, the container index and the Breteau index. They all come from the same house-to-house larval survey, but they answer slightly different questions.

The three indices, and what each one counts

House index (HI) is the percentage of houses or premises inspected that have at least one container positive for larvae or pupae. It tells you how widely spread infestation is across a neighborhood. A ward with HI of 30 has breeding in roughly a third of the houses a team walked, whether that house has one wet bucket or ten.

Container index (CI) is the percentage of water-holding containers inspected that are positive. This one tells you about container management, not spread. A low HI with a high CI usually means breeding is concentrated in a few houses with a lot of bad containers. A high HI with a low CI means the problem is spread thin across many premises, each with maybe one offending tire or drum.

Breteau index (BI) is positive containers per 100 houses inspected, not per container inspected. It's the one most WHO guidance leans on for risk categorization, because it folds container density and house infestation into a single number. A BI above 5 in some guidance, above 20 or 50 in others depending on the vector and the region, gets treated as a trigger for stepped-up control. Programmes argue about the exact cutoffs more than you'd think, because thresholds validated in one climate and housing type don't always transfer cleanly to another.

None of the three tells you species composition or which containers are driving the count unless your field sheet breaks that out separately, which most do: tires, drums, flower pots, discarded containers, tree holes, each logged on its own line.

What the numbers are good for, and where they lag

Larval indices work because they're cheap, standardized, and comparable across years when the same crews walk the same type of container count the same way each survey. A ward that's run BI 15 for three Augusts running and shows 60 this year is worth digging into: heavier rainfall, a new habit of storing water in open drums, or a gap in solid waste pickup could each explain the jump.

Where they lag is timing. A larval survey tells you what was breeding in the houses your team walked, that week. By the time the Breteau Index comes back elevated, the containers that produced it have usually already cycled a generation or two of adults. The index is a lagging indicator of conditions that built up over the prior weeks, not a forecast of where the next hotspot will be once the rains shift or an irrigation cycle changes. Teams end up spraying the ward that scored high last survey, which isn't always the ward where conditions are turning worst right now.

That's the gap a pre-season habitat layer is built to close. Breeding Habitat Map reads multispectral satellite imagery for standing water, irrigation ponding and vegetation signatures before the transmission season starts, so a programme can route larviciding and spray teams toward where conditions favor breeding this season, instead of waiting for the next round of house-to-house indices to confirm it after the fact. It doesn't replace the larval survey. It gives the survey a better starting map to walk.

Larval indices stay the ground truth once teams are out there counting containers. The question worth asking before the season turns is which neighborhoods to send them to first.

If your pre-season routing is still based on last year's hotspot list, it's worth seeing what a seasonal habitat-suitability layer adds to that picture.