Commentary

Response to: Bouwman, H. et al. hallogenated pollutants in terrestrialand aquatic bird eggs: Converging patterns of pollutant profiles,and impacts and risks from higher levelsEnviron. Res. (2013) http://dx.doi.org/10.1016/j.envres.2013.06.003

Donald R. Roberts a,n, Rajendra Maharaj b, Maureen Coetzee c, Richard H. Hunt c,John Govere d, Richard Tren e, Jasson Urbach f, Amir Attaran g, Lucille Blumberg h

a Uniformed Services University of the Health Sciences, Bethesda, MD, USAb Malaria Research Unit, South African Medical Research Council, Durban, South Africac Wits Research Institute for Malaria, Faculty of Health Sciences, University of the Witwatersrand, Johannesburg, South Africad Malaria Vector Control Specialist, Nelspruit, Mpumalanga, South Africae Africa Fighting Malaria, Washington, DC, USAf Africa Fighting Malaria, Durban, South Africag University of Ottawa, Faculty of Law and Medicine, Ottawa, Ontario, Canadah National Institute for Communicable Diseases, National Health Laboratory Service, Johannesburg, South Africa

a r t i c l e i n f o

Article history:Received 13 November 2013Received in revised form22 January 2014Accepted 24 January 2014

Keywords:DDTDDEBird egg shellsCattle EgretsMalariaSouth Africa

a b s t r a c t

Bouwman and coauthors present data and analyses of DDT and other halogenated pollutants inenvironmental samples and based on their data and analyses thereof, argue against the use of DDTfor malaria control. Regrettably, the analyses, presentations, and interpretations of data presented byBouwman and coauthors are biased and erroneous.

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1. Introduction

The authors report a study of chemical levels in bird eggs for anarea where DDT is sprayed inside houses for malaria control, andreport on DDT levels in bird eggs for sample sites in both sprayed andunsprayed areas. The weight of scientific merit of their study restsheavily on reporting: (1) fewer piscivore bird populations in theregion than they expected; (2) data for higher levels of DDT insprayed versus unsprayed areas; (3) greater shell thinning in eggs ofCattle Egrets in sprayed areas; and (4) regression analysis showingshells of Cattle Egret eggs were thinner when they contained higherconcentrations of DDE. We deal with each of these points below.

(1) The authors' suggestion that fewer piscivore bird populationsare present than they expected, is not supported by data toshow that this is actually the case or that DDT is causally linkedto the absence of birds in the region. Nor do they providereferences to past occurrences of birds in the study area.

(2) In the present study, Bouwman and co-authors use data on eggsof House Sparrows to show high DDT concentrations in birdpopulations of the DDT-sprayed area. Since the DDT-ladensparrows were unharmed and reproducing successfully, theauthors speculate that perhaps the sparrows had developedDDT tolerance. Their speculation is unwarranted because theyhave no evidence that sparrows have ever been harmed by DDTexposures. The weight of evidence suggests that the HouseSparrow, like the American Robin (see Harris et al., 2000. Arch.Environ. Contam. Toxicol. 39(2), 205–220; Gill et al., 2003.Ecotoxicology 12(1–4), 113–123), is not reproductively impairedby environmental accumulations of DDT residues.

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Environmental Research

http://dx.doi.org/10.1016/j.envres.2014.01.0100013-9351/& 2014 Elsevier Inc. All rights reserved.

DOI of original article: http://dx.doi.org/10.1016/j.envres.2013.06.003n Corresponding author.E-mail address: drdonaldroberts42@gmail.com (D.R. Roberts).

Please cite this article as: Roberts, D.R., et al., Response to: Bouwman, H. et al. hallogenated pollutants in terrestrial and aquatic birdeggs:.... Environ. Res. (2014), http://dx.doi.org/10.1016/j.envres.2014.01.010i

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(3) Bouwman et al.'s first table (Table 1) presents data on chemicallevels in eggs of different bird species at different samplingsites. For purposes of this study, a species of great importance isthe Cattle Egret. Cattle Egret (CE) eggs were collected at twosites in the unsprayed area (abbreviated in figures as CE Elmand CE Tsk) and one site where houses in the area were sprayedwith DDT (CE XDam). A summary of the statistics given inTable 1 for shell thickness of Cattle Egret eggs at the three sites.The first obvious problem with the above data is the smallsample size. It is extraordinary that the authors base a damningconclusion from an ecological study on a total of 15 eggs, fourfrom CE Elm, six from CE TSK, and only five from the DDT-sprayed area (CE XDam). Further, such a small sample cannot beconsidered normally distributed, and the authors make the veryelementary error of reporting parametric statistics such as themean or standard deviation, which is methodologically invalidwhen applied to non-normal distributions. Aside from this basicerror in attempting to find statistical significance, the authorsmake further errors.Bouwman et al. make use of data in Table 1 for Fig. 2G. The graphshows eggs from CE Elm as having thickest shells, followed byeggs from CE Tsk. In other words, they show graphically that thesite farther from DDT spraying had thicker shells than those fromthe area closer to the DDT sprayed area. They report that eggsfrom CE XDam, where DDT was being used, had the thinnestshells. However, data in Table 1 actually shows that eggshells inthe unsprayed area were thinner than eggshells collected in thesprayed area. Fig. 2G presents a completely reversed anderroneous picture of what the data in Table 1 portrays.

(4) In the next graph (Fig. 2H) the authors regress eggshellmeasurements on the log concentrations of DDE. Eggshellmeasurements are arranged along the x-axis with increasingshell thickness from left to right. DDE concentrations, logtransformed, are arrayed along the y-axis from lower to higherconcentrations. The authors interpret Fig. 2H as showingeggshell thinning is greatest with higher DDE concentrations.For example, the authors state;

“Considering the west–east transect of eggs collected at CattleEgret colonies away from (Elim), near (Tshakhuma) andwithin the DDT- sprayed area (Xikundu), it is clear that witha significant increase in p,p 0–DDE and p,p0–DDT there wasalso a significant thinning of the eggshells. The regression wasalso significant with a 33% difference between thinnest andthickest shells.” (cites for tables and graphs removed)

The authors used their interpretations to project harm to CattleEgrets, stating:

“It is obvious that there is good cause for concern about thereproductive performance of the Cattle Egrets in the study area,and probably also in other DDT-sprayed areas in Africa.”

This is an unjustified statement because they have presentedno data at all suggestive of any reproductive impairment of CattleEgrets in the study area.

Additionally, Table 1 data actually show a very different array ofmean DDE levels in eggs from the three sites. Thus, the negativeslope of their regression line in Fig. 2H is wrong. Although datapoints along the y-axis meet their requirements for showing anassociation of thinning eggshells with increasing concentrations ofDDE, the data array along the x-axis has been transposed. Todemonstrate this, the outlier data point corresponding to 0.16 mmeggshell thickness is actually for CE Elm (see Table 1), and rangelimits of DDE concentrations for CE Elm are 6 and 29 ng/g. Logtransformations of these values are 0.78 and 1.46, not a logconcentration approaching 3 as depicted in Fig. 2H. Additionally,although the authors do not explain which data points in Fig. 2Hbelong to which collecting site, we can identify all five eggs fromthe DDT-sprayed area (CE XDam) because they fall within therange limits of eggshell thickness from 0.22 mm to 0.24 mm. Datapoints in Fig. 2H erroneously show that eggs from CE XDam hadlowest concentrations of DDE while Table 1 data shows that eggsfrom CE XDam had range limits of DDE concentrations of 50 and720 ng/g. These values for CE XDam represent, on average, thehighest concentrations, not the lowest.

As a final point, we take issue with the way Bouwman et al.report a negative impact of DDT in the form of urogenital birthdefects in baby boys in Limpopo Province (see: Bornman et al. BJUInt. 2010. 106, 405–411). Bouwman et al.'s statement is a clearinference that DDT causes birth defects. This inference is counterto Professor Richard Grady's editorial commentary accompanyingthe cited paper in which he states, “issues of causality andassociation cannot be distinguished in this study.” We remindBouwman and co-authors that Bornman et al. reported data on sixurogenital birth defects. Only one was weakly associated (P of0.04) with DDT exposure. Two of the six birth defects wereproportionately less common in DDT sprayed populations thanin unsprayed populations. For grouped data, the association ofbirth defects with DDT exposed populations was not statisticallysignificant. Overall Bornman et al. describe a weak association thatis more likely due to bias and confounding factors than a cause-effect relationship. Clearly, such a weak association is not adequatefor inferring DDT is the cause of birth defects.

2. Conclusions

It can be argued that points (1) and (2) are a matter of opinionand that authors have a right to express their opinions. Still thereare the large errors in data, analyses and interpretations that mustbe addressed.

It appears that in order to show increasing eggshell thinningalong a transect of three sample sites from unsprayed to sprayedareas, the authors transposed the data on eggshell thickness tocreate Fig. 2G. Then, to maintain coherence of their message, theytransposed DDE concentrations to show increasing DDE concen-trations with thinning of eggshells in Fig. 2H. In their interpreta-tions the authors ignore data in Table 1 showing that increasingconcentrations of DDE are associated with increasing eggshellthickness and that the thickest eggshells were in eggs from theDDT sprayed area with the highest DDE concentrations. The majormisrepresentations of data in this paper make the author's inter-pretations and discussions null and void.

Considering that DDT is a highly emotive topic and that SouthAfrica is using DDT to protect health and save lives, data on thesubject and its interpretation should be dealt with in an objective,scientifically unbiased manner.

Table 1Summary statistics on eggshell thickness of Egret eggs in unsprayed and DDTsprayed study sites.

Summarystatistics

Unsprayed area's DDT-sprayed area

Eggshell thicknessin mm at CE Elm(n¼4)

Eggshell thicknessin mm at CE Tsk(n¼6)

Eggshell thickness inmm at CE XDam(n¼5)

Mean 0.19 0.205 0.228Median 0.2 0.21 0.23Maximum 0.2 0.22 0.24Minimum 0.16 0.19 0.22

D.R. Roberts et al. / Environmental Research ∎ (∎∎∎∎) ∎∎∎–∎∎∎2

Please cite this article as: Roberts, D.R., et al., Response to: Bouwman, H. et al. hallogenated pollutants in terrestrial and aquatic birdeggs:.... Environ. Res. (2014), http://dx.doi.org/10.1016/j.envres.2014.01.010i