March 11, 2020 - Volumes

Clear Call for Larval Source Management

Literature review establishes clear reasoning in support of larviciding as part
of an Integrated Vector Management strategy

Parasites & Vectors is an open access, peer-reviewed online journal dealing with the biology of parasites, parasitic diseases, intermediate hosts, vectors and vector-borne pathogens. On August 30, the journal published a peer-reviewed article by a team of authors from Kilimajaro Christian Medical College, Tumaini University in Tanzania titled “Bacterial larvicides for malaria vector control in sub-Saharan Africa: review of their effectiveness and operational feasibility.”

While one might say the  paper makes an unassailable case for a dramatic uptick in bacterial larviciding for malaria control in Sub-Saharan Africa (SSA), it also provides insights into the broader advancements in mosquito-borne disease control operations already underway in other parts of the world. The team from Tanzania points to  empirical data demonstrating the value of a change toward a new set of interventions that includes an intensified focus on larval source management rather than only focusing on adult mosquitoes.

The paper begins by establishing the need for alternative interventions to chemical insecticides which are under severe threat from the onset of insectide resistance, particularly in areas with heavy reliance on these interventions. In addition, it references how interventions such as Long Lasting Insecticide Treated Nets (LLINs) and Indoor Residual Sprays (IRS) are suseptible to behavioral adaptations by Anopheles mosquitoes that undermine the efficacy of these treatments. For example, in a space protected with both ITNs and IRS, Anopheles may adapt by changing their feeding habits to daytime hours (when hosts are no longer protected by nets) while also exhibiting spatial avoidance to nets and wall surfaces.

Data Utilization for LarvicidingFinally, the paper points out the limitations these same interventions face for outdoor applications. It heralds the research and development work that has expanded the capacity of larviciding and cites mounting evidence that clearly demonstrates the value of larviciding in a broader Integrated Vector Management strategy.  

“It has been argued that the current malaria control interventions constitute a necessary but insufficient set of measures to ensure sustainable control,” the report states, “and thus larviciding could play an important role when other vector control interventions have achieved their maximum practical impact.”

Innovations are the Key

ln addition to citing the many benefits of a larviciding component to a broader Larval Source Management (LSM) strategy, the review addresses perceived challenges to larviciding, mainly by pointing out technological innovations that have helped to make a larviciding strategy viable in many parts of the world. Central among these are proven product formulations with demonstrated efficacy and delivery systems that can introduce the product to the larval habitats where mosquitoes are highly vulnerable. The paper reported the strong results that could be achieved with low impact biological larvicides.

“Of the tested products, the highest larval reductions and the most prolonged effect was seen in studies that tested VectoMax® CG, with 98–100% reduction in late larval instars for 2 weeks.”

Ultimately, the report laid the foundation for the fundamental capacity of larvidicing to impact core objectives of the vector-borne disease program.

It found that at low rates, bacterial larvicides cause:

• a reduction in larval density
• a reduction in vector density
• a reduction in vector biting
• a reduction in disease transmission in most of the tested areas.

Challenges to Existing Chemical Malaria Interventions (LLINs and IRS) Related Benefits of Larviciding as
Part of an IVM Approach
Innovations for Overcoming Larviciding Challenges

Widespread Chemical Insecticide Resistance

Mosquito resistance to chemical insecticides has been identified as a global threat. According to the World Health Organization, “Urgent action is required to prevent the further development of resistance and to maintain the effectiveness of existing vector control interventions.”

Larvicides Extend the Useful Life of Chemical Adulticides

By reducing the size of the population being selected for resistance, biological mosquito larvicides promote the effects of chemical adulticide interventions when such applications are warranted.

Perceived Challenge: Effective Larviciding Requires a Well Adapted System

It is true that effective larviciding
requires a well adapted system. However, a well adapted system has other benefits that make the expected outcome an effective, informed, and well managed program. 

Behavioral Modification
of Target Species

Indoor mosquito species adapt to the use of Long Lasting Insecticide Treated Nets (LLIN)and Indoor Residual Sprays (IRS) through spatial avoidance and by altering the timing of their aggressiveness (changing feeding periods to earlier or later in the day).

Larvae Cannot Change
Their Behavior

Unlike adult mosquitoes, larvae cannot change their behavior to avoid interventions. Once habitats have been identified and targeted, interventions are highly successful because larvae are concentrated, immobile, and accessible.

Perceived Challenge:
Larviciding is Labor Intensive

A surplus of data shows that an LSM program that includes larviciding provides meaningful employment for youth and can make efficient use of personnel who also share responsibilities in other areas. Advancements in geo imaging technology and drones have increased capacity for field personnel to utilize larviciding and reduce overall program costs.

Limited Outdoor Application

The dynamics of mosquito control within a confined indoor space are fundamentally different than mosquito control outdoors. Insecticide-infused nets and surface sprays have little value for controlling adult vectors that prefer outdoor spaces where disease vectors pose a significant threat to human health (beyond just malaria).

Larviciding Works for Indoor
and Outdoor Species

Larviciding is an effective intervention against both outdoor and indoor vector species. Advancements in wide area larvicide application strategies have demonstrated that larvicides can be delivered to cryptic habitats in both urban and rural settings, providing excellent reduction data for adult mosquitoes.

Perceived Challenge: Product
Quality Variability among
Larvicides can be High

It is true that product quality variability among bacterial larvicides manufacturers can be high. WHO PQ is a program under which accredited products are certified with efficacy and impact data. Vector control programs need more products to be certified under the
WHO PQ program.

Negative Impact on
Non-Target Organism
s

Depending on the product and the application, chemical interventions may have (or may be perceived as having) a negative impact on non-target organisms such as birds, bees, fish, and people.

Bacterial Larvicides are Highly Specific in Their Activity

The activity of the larvicides Bacillus thuringiensis spp. israelensis (Bti) and Bacillus sphericus (Bs) are based on highly specific protein toxins that only break down in the gut of mosquitoes and other select dipteran larvae. Excellent safety data exists for these products and their low impact on  non-targets, such as birds, bees, fish, and people, all unaffected by these beneficial bacteria.

Perceived Challenge: Biological Larvicides do Not Persists Long Enough

Depending on the target species, habitats can be a moving target. For this reason, residuality may be less valuable because of the duration and nature of many larval habitats. The review notes that less persistent interventions may be more appropriate for transient larvae. Moreover, the target specificity and low impact nature of the larvicide is tied directly to its low persistence.

Progress on Malaria Reduction
has Slowed

According to the World Malaria Report 2018, only modest progress was made on global malaria reduction from 2015 – 2017.

Data Clearly Shows the
Positive Effects

There is increasing adoption and a growing amount of empirical data on the impact and value of bacterial larviciding as part of an Integrated Vector Management program in developing countries.

Challenge: There are Financial Constraints Associated
with Larviciding 

Evidence suggestst that a well run and properly resourced LSM program that includes larviciding reduces the public health burden downstream, and that the economic cost of LSM is in line with other interventions.