New Study: An Early Warning System for Lice Infections

Summary

Argulus (freshwater lice) is one of the most significant ectoparasites affecting Indian Major Carps in freshwater aquaculture. We have recently begun investigating if a simple, low-cost solution—a pole installed at a fish farm which the farmer can easily take out of the water, and replace afterwards, to observe for the presence/absence of lice eggs—can serve as an effective sentinel system for early detection of lice infection. We designed a three-phased approach to investigate if this concept is worthy to take forward to a pilot project. Here we report on the findings from the first two phases, and our plans going forward.

The overarching objective of the first phase—an exploratory, fact-finding phase built around interviewing key stakeholders—was to establish whether the sentinel pole concept warranted further field testing. We conducted this phase in June. Information collected from the various sources provided us with confidence to move forward to the next phase (see our internal report for full details).

The overarching objective of the second phase—the initial proof-of-concept phase—was to show that a pole could serve as a sufficiently attractive substratum onto which Argulus adults can lay their eggs. In July, we tested three different types of poles installed at each of seventeen fish farms. We successfully detected lice eggs at a high rate on sentinel poles made from all three materials tested. The speed at which we detected eggs, coupled with the finding that the presence of lice was suspected at only one of the fourteen farms at which eggs were detected on sentinel poles, supports the hypothesis that this sentinel system has potential to serve as an early-warning system (see our internal report for full details). This phase of the study provided the evidence we needed to move to the next phase of the study.

The second phase provided evidence that a sentinel pole can serve as an early warning system for lice infections. Understanding if farmers make use of the sentinel poles to inform their treatment decisions, and to assess if early treatment translates into a quicker elimination of the infection and meaningful welfare improvements for the fishes will be the focus of the next—and final—phase of the study. We will initiate this study later this year. This final stage is intended to provide the critical information as to whether this concept is worthy for FWI to take forward to a pilot program.

Background

Argulus (freshwater lice) is one of the most significant ectoparasites affecting Indian Major Carps in freshwater aquaculture. In addition to the direct negative impacts to fishes resulting from lice attaching and feeding, feeding sites provide access to secondary infections from other parasites, bacteria, fungi, and viruses, thereby representing indirect welfare threats to fishes.

Farmers that are part of FWI’s Alliance for Responsible Aquaculture (ARA) are typically alerted to lice outbreaks—and therefore, the need to treat—by the behaviour of their fishes, most notably tail splashing and fish jumping. Additionally, infections may be revealed when farmers conduct monthly sampling, where a small number of fishes are captured by use of cast nets to assess their weight gain and general health (although, the number of fishes that farmers typically observe during routine cast netting is low, and it’s possible that low-prevalence infections are missed due to insufficient sample sizes). By the time farmers typically detect lice in their ponds, the infections are usually well established amongst the fishes. This makes treatment more difficult, compared to treating at an early stage of infection, and repeated applications of harmful chemicals are typically required to control the infection. Farmers typically apply chemical treatments to ponds to combat Argulus outbreaks without proper veterinary advice, using harmful pesticides not intended for aquaculture. This likely has detrimental effects on the environment, other animals in the pond, and—depending on the chemicals used—the fishes themselves. It can also have potential human health implications, given that the fishes will ultimately be consumed.

Two adult-stage Argulus (arrows) attached to the underside of a rohu fish.

FWI recently conducted a data campaign using continuous monitors—probes installed in a pond to capture water quality 24 hours a day—to collect water quality data for our remote sensing project. While this data campaign focused purely on water quality parameters and was not designed in any way around Argulus, routine inspections of the water quality probes revealed that lice eggs were quickly being laid on water quality probes submerged in the water. The photograph below shows numerous Argulus eggs clearly visible on a dissolved oxygen probe that had been submerged for three days (the device had been left submerged for three days by the time this photograph was taken, but it’s possible eggs may have been observed as early as one or two days after the probe was submerged in the water). This observation prompted the following idea: can a simple pole installed in an aquaculture pond serve as a “sentinel system” for early detection of lice infection?

Clusters of Argulus eggs deposited on the surface of a dissolved oxygen probe. This probe had been submerged in an aquaculture pond for three days.

Developing and Testing an Early Warning System for Lice

Prompted by our observations of lice eggs deposited on probes from our continuous monitors, we have recently embarked on a series of studies to determine if a simple low-cost pole installed in an aquaculture pond can be a sufficiently attractive substratum onto which Argulus adults can lay their eggs, and if this can serve as an effective sentinel system for early detection of lice infection.  The idea is for this to be a low-tech, simple-to-use device installed at a fish farm which the farmer can easily take out of the water—and replace afterwards—to observe for the presence/absence of lice eggs.

Our hypothesis is that if we can create a simple and inexpensive monitoring device that allows farmers to identify the presence of lice in their ponds earlier than their current methodologies allow, and that prompts them to take action quicker, the infection could be controlled quicker and with less quantities of harmful chemicals than would otherwise be applied. This would lessen the infection (good for fishes), lessen the economic impacts to farmers by reducing the amount of chemicals needed (good for farmers), and lessen the risk to fishes, other aquatic life, the environment, and the end consumer from exposure to harmful chemicals (good for fishes and other wildlife, the environment, and the public). 

Using sentinel poles for early detection of lice would not prevent infection, but the rationale is that farmers could take action quicker, thereby minimising the effects of lice infection on their fishes. This represents the primary mechanism of improving fish welfare. Welfare of fishes could also be improved due to the fishes being exposed to lower quantities of harmful chemicals (the hypothesis being that eliminating a more established infection would require more chemicals than an early-stage infection). The incentive for the farmer to use this monitoring device is that this would reduce treatment costs (due to lower quantities of chemicals being required) and reduce their losses at the time of harvest (lower mortalities or healthier fishes).

Ultimately, we want to assess if this concept has potential as a scalable program to improve fish welfare in aquaculture farms in India. If we can gather the evidence to show that such a sentinel system for early detection of lice has meaningful benefits to the fishes and the farmers, we could develop a program around providing these devices to farmers and showing them how to effectively use them.

A Phased Approach To Development

We designed our usual phased approach to test this concept, with the decision to move to each subsequent phase determined by the outputs from the preceding phase.

Findings from Phase 1: Initial Exploration

The overarching objective of Phase 1—an exploratory, fact-finding phase—was to establish whether the sentinel pole concept warranted further field testing. In June, we gathered information from three different sources: a structured interview with 27 farmers across four regions of Andhra Pradesh, interviews with five experts and five medicine suppliers, and a literature review. Information collected from the various sources supported the notion that sentinel poles could have utility (see our internal report for full details). Guided by the findings from Phase 1, we proceeded to Phase 2.

R&D Senior Manager Vivek, with support from ExPros Lead Kowshik, interviewing a farmer as part of Phase 1 of the sentinel pole study.

Findings from Phase 2: Initial Proof-of-Concept

The overarching objective of Phase 2—the initial proof-of-concept phase—was to show that a pole could serve as a sufficiently attractive substratum onto which Argulus adults can lay their eggs. During a two-week period in July, we tested three different types of poles installed at each of seventeen fish farms. Poles were monitored at approximately weekly intervals for the presence of lice eggs by FWI associates. Farmers were not asked to monitor the poles themselves at this stage of the study.

Three types of poles were tested. The two blue poles were made from polyvinyl chloride (PVC). The light blue “rough” PVC variant (left) was made by using sandpaper to roughen the surface of the commercially available “smooth” PVC pipe (middle). The third pole was made from bamboo (right). We installed poles at each of seventeen ponds in a side-by-side configuration, such that all three pole types were installed at the same location. Members of our ARA team, along with the farmer, are visible on the dyke.

We successfully detected eggs at a high rate—at fourteen out of the seventeen fish farms—on sentinel poles made from all three materials tested, albeit in different numbers/densities. The speed at which we detected lice—within one week of installation at 71% of farms, and within two weeks at 12%; no eggs were detected within two weeks of installation at 17% of the farms—coupled with the finding that the presence of lice was suspected at only one of the fourteen farms at which eggs were detected on sentinel poles, supports the hypothesis that this sentinel system has potential to serve as an early-warning system for detecting lice (see our internal report for full details). This phase of the study provided the evidence we needed to move to the next phase of the study.

Eggs detected on three different sentinel poles at the same pond. These three images show the smooth PCV (left), rough PCV (middle) and bamboo (right) sentinel poles from one of the study ponds at the first monitoring visit. Using a 4-point scoring system, the PVC smooth variant was assigned a score of 3, the PCV rough variant a score of 2, and the bamboo pole a score of 1. Similar analysis from the poles inserted at all seventeen study ponds showed that the smooth PVC variant performed the best in terms of detecting lice eggs.

Plans for Phase 3: Informing Decision to Conduct a Pilot Program

Phase 2 provided the evidence that a sentinel pole can serve as an early warning system for lice infections. But, an early-warning system is meaningless if farmers don’t use that information and take appropriate action. The hypothesis behind the whole concept is that early detection of lice will allow farmers to clear infections quicker and with less quantities of harmful chemicals. Understanding if farmers make use of the sentinel poles to inform their treatment decisions, and to assess if early treatment translates to a quicker elimination of the infection, will be the focus of the next—and final—phase of the study. 

We plan to conduct a study using two groups of fish farms: one group (“treatment”) supplied with sentinel poles and necessary training, and the other group (“control”) relying on existing methodology for detecting lice. Over the course of a full farming cycle, we will track various metrics and compare the results from the two groups at the end. The key questions we will aim to address are: 

• Do farmers use the sentinel poles as intended, such that this system represents a concept that could be taken forward to a program?

• When utilised by farmers, do sentinel poles allow for detecting the presence of lice earlier than the existing methods used by farmers?

• When utilised by farmers, does the use of the sentinel pole system offer benefits to the fishes and farmers compared to existing methodologies for detecting lice? 

We are currently finalising the protocol, and we expect to initiate this study in December (because of the effects of temperature/seasons on Argulus reproduction, it’s best to wait until then rather than starting now). At each study pond, the study will run for a full cycle—from the time the farm is stocked with fishes until the fishes are harvested—which is typically in the region of seven months (although durations can vary based on various factors). Given the uncertainty with the duration and when all study farms will be ready to start (we’ll likely follow a phased approach, with farms starting at different times), we can’t accurately state when the study will end. However, we expect to be completed by September 2027; earlier may be possible, based on when the recruited farmers start their cycles.

This final stage is a field-based effectiveness study, whereby farmers will be tasked with using the sentinel poles themselves, with FWI’s role focused on data collection. As an effectiveness study, this phase is intended to provide the critical information as to whether this concept is worthy for FWI to take forward to a pilot program.