In a recent paper, Nagpal et al. [1] voiced concerns about the limited or biased use
of scientific evidence to support public health interventions to control neglected
tropical diseases (NTDs). Visceral leishmaniasis (VL), also known as kala-azar, is
one of the major NTDs and does not escape this problem. Transmission is vector-borne
and the Indian subcontinent is the region reporting most of the VL cases worldwide.
In this region, the main causative species is Leishmania donovani and Phlebotomus
argentipes is the vector. Transmission is considered anthroponotic and peridomestic—occurring
at night when female sand flies bite people sleeping inside their house. The World
Health Organization and the governments of India, Nepal, and Bangladesh set out in
2005 to eliminate VL from the region by 2015 through a combination of early treatment
of cases and vector control. However, while recent advances in diagnostic tools and
drugs have significantly improved case management strategies, the available vector
control tools against P. argentipes remain limited. The elimination initiative promotes
the use of indoor residual spraying (IRS) of households and cattle sheds to reduce
vector density, but the evidence underpinning the effectiveness of IRS in this region
is scanty. Historical observations show that L. donovani transmission declined concomitantly
with dichlorodiphenyltrichloroethane (DDT) spraying during the 1950s–60s to eradicate
malaria. In the aftermath of this malaria eradication campaign, very few VL cases
were observed in endemic regions until the mid-seventies, when there was resurgence
of a VL epidemic in India [2]. To date, there are no randomized trials showing the
effect of IRS on the incidence of clinical VL [3,4], though some studies showed a
reduction in vector density. When the VL elimination initiative was launched in 2005,
there were no clear alternatives for IRS as a vector control strategy. Insecticide
treated nets (ITNs) were proposed as an alternative or complement to IRS on the basis
of analogy arguments regarding their given efficacy against malaria [5] or on data
from observational studies suggesting ITNs reduce the risk of VL [2]; but as for IRS,
there were no randomized trials evaluating the effect of ITNs on L. donovani transmission.
In this context, a number of field studies were conducted in the Indian subcontinent
in the past decade to evaluate the effectiveness and impact of ITNs and other vector
control tools on VL. Most of these studies have been reviewed in detail in two recent
papers [3,4]. The only two studies evaluating the impact of vector control interventions
on clinical outcomes found conflicting results. First, the KALANET project, a cluster
randomised controlled trial (CRT) in India and Nepal, showed that mass-distribution
of ITNs did not reduce the risk of L. donovani infection or clinical VL [6]. Then,
an intervention trial in Bangladesh suggested that widespread bed net impregnation
with slow-release insecticide may reduce the frequency of VL [7]. Technical (e.g.,
type of nets and insecticides, lack of replicas and randomisation in Bangladesh) and
biological factors (e.g., insecticide susceptibility and sand fly behaviour) may explain
the different results observed. This apparent contradiction raises the question about
the role that ITN may play in controlling VL in the Indian subcontinent but has also
triggered a lot of discussion on methodology and evidence levels required when evaluating
vector control tools for VL. In this paper, we would like to summarise the lessons
learned from the KALANET CRT in terms of methodology to inform the generation of future
evidence and discuss interpretation of findings against this background.
The KALANET trial was designed to evaluate the distribution of ITNs as a public health
intervention to prevent VL in the Indian subcontinent. The objective was to answer
the question: “in the current context in India and Nepal, would free mass distribution
of ITN significantly reduce the incidence rate of VL in endemic regions?” This was
a question asked not at the individual level, whether sleeping under an ITNs protects
an individual against VL compared to an individual not sleeping under an ITN, but
a question asked at program level: does such prevention measure reduce VL incidence
in communities. Furthermore, the question was about effectiveness in real life conditions
and not a question about efficacy of ITNs in “laboratory conditions,” a difference
that is clear for public health experts but not necessarily for all readers. The answer
to this question may, for instance, be entirely different in a country such as Bangladesh,
where no vector control program was operating for many years, no spraying was implemented
at the time, and fewer households were using untreated nets, compared to India and
Nepal. The first lesson we learned was about the importance of clarifying the research
question itself.
To answer the above question, we adopted a study design used previously in a successful
intervention trial on zoonotic VL transmission [8]. We designed a CRT to demonstrate
a 50% reduction on the risk of L. donovani infection associated to the village-wide
distribution of ITN [6]. Long-lasting insecticidal nets (LNs) were chosen as intervention
as they remain effective for three to four years in the field [2]. Incident L. donovani
infection, measured as seroconversion in the Direct Agglutination Test (DAT), was
used as the main outcome. Measuring the impact of LN on the risk of clinical VL would
have been the preferred primary outcome, but its low incidence and the long incubation
period precluded this. Incidence of VL cases was nonetheless measured as secondary
outcome. The trial was conducted in 26 high-incidence clusters (16 in India and 10
in Nepal) with over 20,000 inhabitants followed over 24 months. After randomisation,
LNs were distributed in all households in the 13 intervention clusters, with the number
of LNs proportional to household size, to make sure that all household members could
sleep under the nets. Participants in the control clusters were allowed to continue
using their untreated nets. The effect of LNs on the incidence rate of seroconversion
and VL was compared after 24 months between intervention and control clusters. No
LNs were used in the control clusters. The results of the trial, analysed as suggested
by Hayes and Moulton [9], showed that the large scale distribution of LN did not reduce
the risk of L. donovani infection [6]. These results were consistent across several
endpoints measured, as no difference was observed in (1) incidence rate of clinical
VL [6], (2) seroconversion in rK39 ELISA (a second serological marker) [10], and (3)
mean P. argentipes exposure measured at cluster level by a sand fly saliva antibody
detection ELISA [11]. Similarly, the reduction of P. argentipes density indoor in
the study clusters was limited (24.9%) [12]. The main conclusion of the trial was
that “there is no evidence that using LNs as a public health intervention provides
additional protection against VL at community level compared with existing control
practices in India and Nepal (e.g., irregular use of untreated nets and IRS). This
does not mean that the use of LNs in those VL endemic regions should be dismissed,
as they may provide some degree of personal protection against sand flies [13] and
have been shown to reduce the risk of malaria [6]. However, the VL elimination initiative
in India and Nepal cannot rely on the stand-alone use of LNs to effectively control
transmission.”
The above message is complex and was disappointing for many, in the first place for
the researchers themselves, as the hopes for a user-friendly, household-controlled
tool to control VL in the Indian subcontinent were given a serious blow. Moreover,
results from a negative trial are hard to communicate. Criticism of peers focused
on four main areas: 1. the biological rationale for the intervention, 2. the choice
and number of units of analysis, 3. the choice of endpoint and 4. the adherence to
the intervention. Stockdale and Newton also identified these methodological issues
as key factors to evaluate studies testing preventative methods against human leishmaniasis
infection [3].
Rationale for the Intervention
In theory, LNs could be an effective tool to prevent L. donovani transmission, as
P. argentipes are supposed to bite people indoors while they sleep [14]. However,
recent entomological findings in India indicate that L. donovani vectors are more
exophilic and exophagic than previously reported [15,16]. If P. argentipes bite people
outdoors (e.g., in the early evening when and where bed nets are not deployed), LNs
will have a limited impact on L. donovani transmission. Moreover, as P. argentipes
is also zoophagic [17], LNs will have a limited impact on vector survival and thus
on transmission [18]. We hypothesised that these were the main factors explaining
the KALANET trial results as participants used the mosquito nets correctly (see below);
vectors were susceptible to the insecticide used in the nets (e.g., deltamethrin [19]),
and LNs provide an effective barrier effect against P. argentipes [20]. Unfortunately,
the KALANET trial was not designed to study P. argentipes behaviour. For example,
the effect of LNs on vector density was only measured indoors [21]. Entomological
studies are urgently required to document the transmission dynamics of L. donovani
in the Indian subcontinent.
The Unit of Analysis
In KALANET, the unit of analysis was a cluster with 350 to 1,500 people corresponding
to a hamlet (“tola” in India, “ward” in Nepal). These clusters were selected based
on their previous history of VL: at least one VL case in the last three years and
minimum VL incidence of 0.8% during that period. Clusters were pair-matched based
on their prior VL incidence. The number of clusters was calculated assuming a 2% yearly
L. donovani infection incidence rate and a coefficient of variation between clusters
(κ) of 0.25 [6]. It is known that VL cases are clustered in space and time. VL cases
tend to occur in microepidemics, affecting one village, lasting three to five years,
fading out only to reappear in another area. This phenomenon is supposed to be related,
among other factors, to herd immunity at village level. The criteria used to select
the clusters in the KALANET study may have resulted in a variety of villages at different
stages in those “microepidemic cycles,” with some villages still on the increasing
slope of the incidence curve and others on the decrease. This did not invalidate the
study design or trial outcomes, as clusters were randomly allocated to both study
arms. However, the inclusion of some clusters in the late phases of the local epidemic
may have decreased the power of the study as more incident infections and VL cases
are expected in a more “naïve” population. Future trial designs should take this into
account and try to include clusters as early in the cycle as possible. Sample size
calculations at design stage were based on an expected L. donovani infection incidence
of 2% which proved correct—but we underestimated the clustering of incident infections
as the observed coefficient of variation for L. donovani infection was 0.56 instead
of 0.25. This observed k-value may be of use for the planning of future intervention
trials. In the KALANET trial, we increased the number of clusters by 30% (13 clusters
per arm instead of 10 initially planned) to increase the power of the study.
The Endpoint
Using VL cases as the primary outcome for a community intervention trial, instead
of L. donovani infection, is the better option. However, this would have necessitated
a much larger number of clusters, and the low VL incidence may result in clusters
having low counts (e.g., less than five VL cases). We therefore considered several
L. donovani infection markers as alternatives. Seroconversion in the DAT test is strongly
associated with clinical VL [22,23]. We chose it as the main outcome of the KALANET
trial based on results from previous VL trials [8]. rK39 ELISA, used as a secondary
serological marker, showed poor agreement with DAT [24] and presented different kinetics
in past VL cases compared to DAT [25]. High rK39 titres are equally associated with
progression to clinical disease [22]. The Leishmanin Skin Test (LST), initially postulated
as an alternative or complement to serological tests, was discarded because of problems
from a source good manufacturing practices (GMP)-manufactured antigen and some erratic
results observed in the study area [26]. As stated above, analysis of all endpoints
in KALANET gave consistent results.
Adherence to the Intervention
From the start of the study project, we included a large research component on “acceptability”
of the intervention, using mixed methods, including Knowledge-Attitude-Practices surveys,
observation, and focus group discussions. The acceptability of bed nets was a priori
not considered problematic in this region as bed net coverage and use is high in rural
villages in the Indian subcontinent [7,27,28]. People like to protect themselves from
insect nuisance at night by sleeping under nets, as it enhances quality of sleep.
This pattern is season dependent though, with less use in the hotter months. The availability
of commercial bed nets in the communities living in the KALANET clusters was widespread
before we started the trial: 70% to 80% of households in Nepal and India had at least
one net at baseline [6]. Those nets were all untreated, many were damaged, and most
of the families did not have enough nets to protect all household members. Nevertheless,
untreated mosquito nets were commonplace in the study villages and most households
used untreated nets [27,28].
However, even if communities in the study area were familiar with the use of bed nets,
we conducted a series of activities to ensure the correct and regular use of LNs distributed
in intervention clusters. First, we selected from the available LN brands the product
that best met the people’s preference based on a formal comparative evaluation of
several brands [29], and we took into account cultural preferences regarding colour
and size of the nets. Enough LNs were provided per household to ensure all family
members could sleep under a treated net while at the same time respecting existing
sleeping patterns. We did not take away the existing commercial nets in the control
clusters, but did so in the intervention clusters, in exchange for the new LNs. It
is important to remember that untreated nets were already in use before the trial
with no apparent effect on L. donovani transmission in those VL endemic communities.
Forbidding the use of untreated nets in control clusters would have been unethical.
To enhance the correct use of LNs, field workers organised meetings in the villages
and distributed Information-Education-Communication (IEC) materials (e.g., pictorial
diagrams in local language) to promote the correct use (e.g., net deployment, washing
frequency) of the LNs. The content of these IEC messages was largely inspired from
prior findings in focus group discussions on perception of the disease and attitudes
with regard to preventive measures. Finally, quarterly house-to-house surveys were
conducted during the trial to monitor and promote the regular use of LN in the intervention
clusters. So by the end of the trial, the use of LNs in intervention clusters was
very high: 91% of the individuals in those clusters slept more than 80% of the nights
under a treated net [6]. This figure contrasts with the 30% of people in control clusters,
where no LNs were distributed, who reported regular use of their untreated nets during
the trial [6]. Some peer reviewers argued that the use of untreated nets in control
clusters may have masked the possible effect of LNs in VL incidence, and this is correct
to a certain extent, but cannot have led to a huge impact given their low, irregular,
and inconsistent use.
The KALANET study was a huge collaborative endeavour of seven research teams in six
countries, conducted at a marginal cost of 2 million €, a budget provided by the 6th
Framework Programme of the European Union (INCO/RTD). One can ask whether this huge
research effort is (a) required to underpin public health policy and (b) cost-effective?
We think that this level of evidence from properly conducted randomized controlled
trials testing effects on human morbidity and mortality is indeed required before
adopting novel vector control tools as public health policy, as effects on vector
density only are not sufficient to demonstrate health impact. A randomized controlled
design over a sufficient number of study units is required in this highly variable
and hyper-clustered disease. CRTs remain the preferred design to evaluate public health
interventions at community level.
Whether these large research projects are also “cost-effective” or value for money
is not easy to answer, as the opportunity cost of ineffective health policy should
be put in the balance. Moreover, the benefits of a CRT are often not limited to a
single outcome measure. The KALANET study allowed us to better understand the epidemiology
of VL in India and Nepal, and the data generated were used to develop a mathematical
model evaluating the L. donovani transmission parameters and control measures against
VL in the Indian subcontinent [30,31]. This transmission model suggests that integrated
vector management (e.g., combining IRS and LNs) is the best approach to overcome the
limitations of the current vector control strategy [31]. Mathematical modelling can
help designing new vector control methods that then need to be evaluated in the field.
The KALANET project not only highlighted the huge need for innovation in vector control
in VL, but pointed also to the fundamental knowledge gaps in this domain. Better understanding
of the vector bionomics (e.g., biting rhythm, population dynamics, endophagy and exophagy,
and endophily and exophily) and human behaviour (e.g., sleeping habits) is essential
to develop new control measures.
Nonetheless, if we want to foster innovation in vector control, we should scrutinize
and simplify the evaluation methodology of new vector control tools for VL, as the
huge resources and time required for the comprehensive evaluation approach adopted
in the KALANET project cannot be mainstreamed for every single study. As for drugs
and diagnostics, a “pipeline approach” to the development of new vector control interventions,
with proof of principle leading to evaluation in several staged design phases (I,
II, III, and IV), should be promoted [32]. Alternative methods for a quick evaluation
of the entomological efficacy of new P. argentipes control tools under field conditions
are needed [13]. An individual marker of P. argentipes exposure as a sand fly saliva
antibody test—if validated—could allow the direct evaluation of vector control measures.
For the last stage of the pipeline, the impact evaluation, alternative CRT designs
(e.g., crossover or stepped wedge designs) and sample size calculation methods should
be explored to take into account the spatiotemporal clustering of rare events. We
also suggest selecting clusters in the early stages of the local epidemic cycle and
using a higher k-value (e.g., 0.50) for sample size calculation as well as analysis.
The use of new end points to measure the impact of those interventions at population
level in a more efficient way would make a difference [33]. New and better markers
of L. donovani infection (e.g., cellular immunity markers) should be developed and
evaluated.
In conclusion, the KALANET trial is thus far the only CRT evaluating the impact of
ITN on VL. The CRT design should be taken into account when the KALANET results are
evaluated and compared to other studies using a less robust methodology (e.g., a nonrandomized
trial comparing two clusters, one area with bed net impregnation and one without in
Bangladesh [7]). In the context of VL in the Indian subcontinent, entomological and
epidemiological studies should be conducted to better understand L. donovani transmission
in endemic villages. IRS, which remains the main vector control strategy in the region,
needs to be reassessed, and integrated vector methods (e.g., IRS combined with LNs)
should be evaluated using a CRT design. Finally, a randomized controlled design is
essential to produce evidence for health policy in this field, and methodological
innovation is urgently needed to make the Research & Development (R&D) pipeline process
more efficient.