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Diagnosing Drowning | Estimating Time Since Death | Case Study

 

Limnology is the study of freshwater ecology. The algal community, particularly the diatoms, is especially useful in linking suspects and victims to crime scenes in or around fresh water (Miller Coyle et al., 2001). Diatom species are easily identified from the characteristic shape of the cells and unique refractive pattern caused by the silica in the cell wall (Miller Coyle et al., 2001). Seasonal variation in diatom abundance and diversity is used to generate floral profiles of aquatic habitats which can then be compared to specimens collected from body tissues or other materials recovered from the crime scene (Ludes et al., 1996). Diatom tests are used routinely in the diagnosis of drowning (Matsumoto and Fukui, 1993), and in estimating the post-mortem interval, or time since death (Casamatta and Verb, 2000).

 

DIAGNOSING DROWNING

 

A positive diagnosis of drowning can be very difficult (Ludes et al., 1996), particularly in decomposed bodies where physiological indications of drowning may be absent (Kobayashi et al., 1993). When a person drowns in fresh water, diatoms are taken not only into the lungs but are also dispersed among other internal organs (Miller Coyle et al., 2001). In the absence of other evidence, diatoms detected in the body tissues are the most reliable indicator of a freshwater drowning (Kobayashi et al., 1993). Even when there are only skeletal remains, diatoms can be detected in the bone marrow of drowned victims (Ludes et al., 1996). In cases of suspected drownings, 20 diatoms per 100 microlitres of pellet from a 10 gram lung sample, or five complete diatoms from other organs, are normally required for a positive diagnosis (Ludes et al., 1996).

Research in this area centres on efficacious extraction methods, and comparison of specimens recovered from body tissues to the floral profile of the site where the body was discovered. In the past, acid digestion was used to extract diatoms from tissues, but this method was dangerous and pollutive (Matsumoto and Fukui, 1993), and destroyed zooplankton (Kobayashi et al., 1993). Today, DNA extraction techniques have been adapted for use in isolating diatoms from lung tissue. Proteinase K and sodium dodecyl sulphate (SDS), traditionally used to digest proteins and cell plasma membranes in DNA extraction, has proven to be effective in isolating diatoms as well (Kobayashi et al., 1993). Ultrasonic digestion is also used (Matsumoto and Fukui, 1993). Both methods are simple, safe, and effective, and preserve diatoms and zooplankton better than acid digestion.

To make a positive diagnosis of drowning, the number and species diversity of diatoms sampled from the lungs and other circulatory organs should match that of the aquatic flora where the body was recovered (Ludes et al., 1996). In a study designed to establish a database for comparison between the flora of sites where drowning victims are frequently found and the diatoms recovered from the victims' tissues, Ludes et al. (1996) sampled lung, kidney, liver, and brain tissues of drowning victims and found good correlation between the taxa recovered from the samples and those present at a known drowning site.

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ESTIMATING TIME SINCE DEATH

(Casamatta and Verb, 2000)

One of the primary goals of forensic work is to determine the post-mortem interval (PMI), or time since death. The algal community is an excellent way to assess PMI for several reasons:

  • diatoms are ubiquitous in both lotic and lentic systems
  • algal populations, though variable, persist throughout the year
  • most diatoms can be identified to species with only a light microsope

Casamatta and Verb (2000) documented the algal colonization of immature rat carcasses in low-flow pool and high-flow riffle areas of a woodland stream. They found that the diversity of colonizing taxa increased in both habitats over the course of 31 days, suggesting that algal colonization patterns are a good tool for assessing the submersion interval for a body. Fewer than 20 colonizing taxa implies recent submersion, while more than 50 taxa indicates submersion for several weeks. Key indicator taxa can narrow the time frame down even further. For instance, in a riffle environment, Ankistrodesmus spp. do not appear until after 30 days' submersion, making this genus a key "late colonizer" taxon.

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CASE STUDY

(Siver et al., 1994)

Incident

In July of 1991, two young boys were attacked by a number of assailants while fishing at a local pond in Connecticut, U.S. The victims were bound with duct tape, beaten with a baseball bat, and left to drown in the pond. Fortunately, one of the boys was able to free himself and save his friend. Three suspects were apprehended.


Evidence

Reference samples from the pond were compared to mud collected from the sneakers of the victims and suspects. All samples contained numerous specimens of both diatoms and scaled chrysophytes, another type of siliceous planktonic algae. In all, 15 algal genera comprising 25 species were isolated from the samples, and 14 of the 15 genera were common to all samples. All species identified indicated a freshwater habitat.

The most abundant species in each sample was the scaled chrysophyte Mallomonas caudata, which is restricted in its distribution to ponds and lakes, i.e., it is not found in flowing streams or rivers. It is also rare to find M. caudata as the dominant algal species in northeastern U.S. lakes.

 

Quantitative analyses were conducted on the three species of Eunotia found in all the samples. Although a common freshwater diatom, it is rare for the population ratios of species from the same genus to be similar in communities that come from different freshwater habitats. A Chi square statistical test revealed that there was no difference in the population ratios of the three Eunotia species among the samples.

Interpretation

That all three classes of mud samples (victims' sneakers, suspects' sneakers, and pond reference) shared 14 of the 15 diatom genera identified, featured a rare dominant scaled chrysophyte in M. caudata, and showed no significant difference in Eunotia species composition, supported the conclusion that all samples originated from a common freshwater source, and provided strong evidence linking the suspects to the crime scene.

Outcome

All three suspects pleaded guilty to a variety of felony charges and were sentenced to time in prison.

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Eunotia micrograph, 6kb

Freshwater diatom, Eunotia sp. Cell is 45 microns long. Photo courtesy of the California Academy of Sciences Diatom Collection


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The Forensic Botany site was created in 2002 by Jennifer Van Dommelen as a project in the Web Literacy For the Natural Sciences class at Dalhousie University, Halifax, Canada. Terms in bold are defined within the body of the text. Highlighted terms and author citations are linked to a glossary and reference list, respectively, which open in new windows. All images have been used with permission. Header banners created by Jennifer Van Dommelen.

Last content update: April 2002
Last editorial/layout update: 17 June 2005

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