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Vector control in cutaneous leishmaniasis of the old world: A review of literature

  • Author(s): Kassi, Masoom
  • Kasi, Pashtoon Murtaza
  • Marri, Shah Muhammad
  • Tareen, Iqbal
  • Khawar, Talha
  • et al.
Main Content

Vector control in cutaneous leishmaniasis of the old world: A review of literature
Masoom Kassi1, Pashtoon Murtaza Kasi2, Shah Muhammad Marri1, Iqbal Tareen3,Talha Khawar1
Dermatology Online Journal 14 (6): 1

1. Department of Pathology, Bolan Medical College, Quetta, Pakistan
2. Department of Medicine, Sandeman Provincial Hospital, Quetta, Pakistan. pashtoon.kasi@gmail.com
3. Department of Dermatology, Bolan Medical College, Quetta, Pakistan


Abstract

Cutaneous Leishmaniasis (CL), a disfiguring disease, is prevalent in many parts of Pakistan and neighboring Afghanistan. Leishmaniasis is second only to malaria in terms of the number of people affected; it is a major public health issue with significant social stigma. Although the different methods to diagnose and treat the disease are well discussed in the literature, the role of vector control in the prevention of CL has been underemphasized. Both Pubmed and Ovid search engines were used to obtain articles on prevention and control of cutaneous leishmaniasis. These materials were then screened for articles pertaining to vector control only. The World Health Organization's website along with the Cochrane database were also searched for relevant text. From this qualitative review, it can be seen that many effective interventions exist. Considering the multitude of factors involved in transmission of CL and the various effective control measures tried and tested by investigators, an interdisciplinary approach involving more than one of the above interventions would make sense. The interventions selected would then depend on the incidence of CL in that particular area, the population being targeted, the reservoir, the particular vector, the environment, the acceptability/popularity of the intervention, and the availability of funds.



Introduction

Cutaneous Leishmaniasis (CL), a disfiguring disease, is prevalent in many parts of Pakistan and neighboring Afghanistan (Fig. 1) [1, 2, 3]. Both regions fall in the leishmaniasis endemic zones according to the World Health Organization [4]. Second only to malaria in terms of the number of people affected, it is a major public health issue with significant social stigma [5]. Children and women are particularly affected [6].

Although the different methods for diagnosis and treatment are well represented in the literature [7], the role of vector control in the prevention of CL has been underemphasized [8, 9, 10]. In the absence of an effective vaccine, the avoidance of sandfly bites is of primary importance [11]. Unfortunately, even though our understanding of the disease has significantly increased over the past decade, effective control measures are not given their due importance and are underutilized [11]. The objective of our article is to review the available evidence on the different methods of vector control of CL and suggest recommendations accordingly for our region.


Figure 1Figure 2
Figure 2. The sand fly vector along with lesions of leishmaniasis which are mainly on the exposed parts of the body, particularly the face.

The vector

Cutaneous leishmaniasis is transmitted by the bite of 2-3mm long sandflies and many factors are involved (Fig. 2). There are 3 features that make sandflies easy to recognize: "when at rest, they characteristically hold their wings at an angle above the abdomen; they are hairy; and, when coming to engorge, they typically hop around on the host before settling down to bite [12]." A range of different sandflies can act as carriers for the protozoan parasite, Leishmania [13, 14, 15]. The Leishmania species responsible for causing CL in Pakistan and neighboring Afghanistan (Old World CL) include predominantly L major, L tropica, and L (L) aethiopica [14, 16]. Various measures to control sandflies have been described and implemented in various parts of the world. These include spraying houses and adjacent areas with insecticides, placing screens on windows, and using uniforms or top sheets impregnated with insecticides. Major alterations in the environment have also been suggested. A detailed discussion on the biology of the vector is available [13].


Methods

Both Pubmed and Ovid search engines were used to obtain articles on prevention and control of cutaneous leishmaniasis. These were then screened for articles pertaining to vector control only. Firstly, only randomized controlled trials (RCTs) were searched. A total of 96 articles were retrieved in Pubmed for the term "Cutaneous Leishmaniasis" with "Randomized Controlled Trials" and "English." Of these, 17 were related to various aspects of prevention, of which 3 were related to vector control [17, 18, 19]. In Ovid, the term "Cutaneous Leishmaniasis" retrieved a total of 2366 articles. Of these, 191 were related to prevention and control, of which 21 were related to vector control. The Cochrane Database yielded 3 articles; one pertained to interventions for CL. The bibliographies of the selected articles were also searched for relevant articles. An attempt was made to acquire all the relevant articles. The authors were also contacted when copies of an article were unavailable. The World Health Organization's website (www.who.int) was also searched for relevant articles.


Results


Randomized Controlled Trials

Of the randomized controlled trials, 2 were conducted in Venezuela while 1 was conducted in Kabul, Afghanistan. The use of house sprays, insecticide impregnated curtains, and insecticide treated bednets, chaddars, or top sheets were evaluated by these investigators [18, 19, 20].


House spraying

In the first study in Venezuela, the treated group (20 houses) was sprayed with 25 mg/m2 of the pyrethroid insecticide, lambdacyhalothrin, based on the susceptibility of Lutzomyia ovallesi. The sandfly densities were then compared with the control group (19 houses using CDC light traps) 7-79 days later [18]. Although there was an initial significant reduction in the total number of fed females, females, and males, the levels of the latter two rose to those of the controls 7 and 11 weeks later. Based on this, the authors have suggested indoor sprayings twice, separated by a period of 2 months, based on the transmission season.

House spraying was also evaluated by the study done in the city of Kabul, Afghanistan [20]. Here the interior rooms were sprayed with 30 mg/m2 of lambdacyhalothrin and the incidence of CL was evaluated 15 months later. As compared to the controls (7.2%), the incidence of CL in people living in the sprayed houses was 4.4 percent. People living in the sprayed houses were 0.6 times more likely to have CL than the controls (95% CI 0.3-0.95), thus showing a protective effect.


Insecticide impregnated curtains

Kroeger et al looked at the effect of insecticide impregnated curtains in Venezuela [19]. The target concentration of lambdacyhalothrin selected was 12.5 mg/m2. All new cases occurring during a period of 12 months were recorded; light traps were used to record sandfly densities. The houses with impregnated curtains (241) were then compared with 2 control groups: houses with non-impregnated curtains (222) and houses with no curtains at all (106). At the end of the period, significantly fewer sandflies were observed in areas of impregnated curtains (mean difference of 13 sandflies per trap 95% CI 9-17, p<0.001) compared with areas having non-impregnated curtains. At the same time, the cumulative incidence in the group that received impregnated curtains was 0 "(mean difference in 12 month incidence 8 cases per sector; 4 to 12; p=0.001)". The authors concluded that these were effective in preventing bites and thus reducing the transmission of the disease.


Insecticide treated bednets

In the RCT in Kabul, Reyburn et al studied the effect of insecticide treated bednets with other methods to prevent CL. The insecticide selected was permethrin at a dosage of 0.5 mg/m2. The incidence of CL at the end of 15 months was 2.4 percent (OR 0.31, 95% CI 0.2-0.5) as compared to the controls (7.2%).

The use of insecticide treated bednets also resulted in statistically significant reductions in the incidence of CL in the study by Alten and colleagues [20]. The insecticide used to impregnate bednets in this study was K-OTAB (deltamethrin-tablet formulation). The reported reductions in the incidence of CL were from 1.87 percent to 0.035 percent in one area and from 2.3 percent to 1.32 percent in the other [6].


Reducing the reservoir

Measures have also been taken to decrease the reservoir populations for the sandfly vector. In one study in Iran, the use of 2.5 percent zinc phosphide to control rodent populations resulted in statistically significant differences in the incidence of CL between the intervention and the control group [12]. The method and timing suggested by the authors is to destroy all rodent holes within a radius of 500m of the houses once every two years before April i.e. before the active season of the vector [12]. For regions that are comparable in their incidence of CL, these interventions should be done once a month for four months followed by the aforementioned pattern of control measures before the active season of sandflies. A field trial by the same authors earlier resulted in about a 12-fold reduction in the incidence of CL. One-fifth of the baseline levels were maintained two years later, showing that such measures are very effective in selected settings [21].


Other methods

Various other items have been impregnated with insecticides to confer protection from sandfly bites. In the same RCT described above in Kabul, the authors also impregnated the chaddars or top sheets; the users were 0.33 times more likely to have CL (protective effect) with an incidence of 2.5 percent (OR 0.33, 95% CI 0.2-0.6) [20].

In another study, Asilian and colleagues tried to evaluate the effect of impregnating uniforms of military soldiers. The items impregnated included shirts, undershirts, pants, socks, and hats. Permethrin was used to impregnate clothing at a concentration reaching 850 mg/m2. The study, however, could not demonstrate a protective effect of the uniforms; there was no statistically significant difference in the incidence of CL in the intervention and the control groups. However, a protective effect has been demonstrated in a similar study done on Columbian soldiers [22]. This study, however, had a shorter period of follow-up as compared to the previous one.

The side effects reported by the use of insecticide impregnated nets and other clothing items have been minimal.

Along with the requirement of female sandflies for blood, both male and female sandflies require sugar meals of plant materials or aphid and coccid-secreted honeydews [23]. This property of the vector has been utilized by Schlein and colleagues to identify plants that when eaten by the sandfly, would kill the parasite within the vector. The lifespan of the sandflies also decreased by feeding on these noxious plants [24].


Discussion

From this qualitative review, it can be seen that several effective interventions exist to control vector-spread cutaneous leishmaniasis (Fig. 1). Some have even been implemented on a large scale; the distribution of insecticide treated bednets in Kabul by the WHO is one example [24]. Bednets are particularly effective where sandflies are endophagic (mainly feed indoors) [25]. Long lasting insecticide treated nets are under development; cost is, however, an issue [26]. Utilization of locally made nets might be an alternative (Fig. 3).


Figure 3Figure 4
Figure 3. The sand fly vector along with lesions of leishmaniasis which are mainly on the exposed parts of the body, particularly the face.
Figure 4. Bednets for protection against leishmaniasis and malaria available in all shapes and sizes. Some have been impregnated with insecticides as well.

In order to assure compliance, it is necessary that health education measures be undertaken at the same time, or preferably before, interventions are implemented. These programs could potentially increase the impact of the aforesaid interventions to a great extent and also ensure its sustainability.

Interventions placed should be in concordance with the biology and properties of the vector. For example, the flight-range of sandflies has been suggested to be 300m; thus destruction of reservoir populations should cover an area with a radius of 500m. Next, the height to which sandflies can rise is 5m; people should thus avoid sleeping on the floor [11, 22]. The size of wings of the vector is less than 3mm. Therefore, untreated bed nets would be effective only if the mesh is narrow, 156 holes per square inch [14]. The use of ornamental plants that can prove harmful for both the vector and the parasite can also be considered [24]. However, such studies must be replicated and the results need to be evaluated with respect to our environment. A search for additional plants in our region that could serve the same purpose would be advised.

For areas in Pakistan and Afghanistan that have not been studied in much detail, it is imperative to identify the vectors as well as the reservoirs. High-risk areas should be identified first [3]. In such sites, the identification and reduction of reservoir populations, as shown in studies in similar settings, should be highly effective in reducing the incidence of CL [12, 22]. It is, however, important to understand that when reservoirs are targeted, the areas, such as the villages, should be 'separated' and the entire vicinity should be covered under the operation [22].

When implementing the suggested interventions, it is imperative to assess the behavior and knowledge of people residing in the area. Education programs should inform the population and mobilize the community to participation in the intervention. An intervention that is not acceptable to the people living in the area endemic to CL will result in low compliance and failure. On the other hand, an intervention that is accepted will be much more efficacious. For example, in the RCT done in Afghanistan, insecticide treated bed nets were the most popular, followed by house spray, and then impregnated chaddars [20]. Identification of such preference patterns is important to consider in making sure that the interventions placed are sustainable as well.

For areas like Pakistan and Afghanistan, where little money is invested into healthcare, the interventions selected need to be cost-effective. The prices of insecticide treated bednets, for example, must be brought down and the countries in which these nets would be distributed should not tax them [27].

Finally, once such interventions have been implemented, it is vital to accurately monitor and assess the actual impact of each of the interventions. Ways to precisely calculate the impact of the interventions should be in place before the implementation of control programs.

Considering the many factors involved in transmission of CL and the various effective control measures tested by investigators, an interdisciplinary approach involving more than one of the above stated interventions should be employed. The interventions selected would then depend on the incidence of CL in that particular area, the population targeted, the reservoir, the particular vector, the environment, the acceptability/popularity of the intervention, and the availability of funding to begin and sustain the endeavor.


The Road Ahead

Methods to control the vector can lower the burden of disease to a great extent and be cost-effective at the same time [28]. Public health interventions can play a key role in the prevention of leishmaniasis and other major vector-borne diseases such as malaria, dengue fever, and Chagas disease. In addition, programs aimed at other vector borne diseases such as malaria should be combined and implemented side by side.

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