Veterinary Forensic Entomology

by PETER BATES, Ph.D. MSB. C.Biol. FRES – Veterinary Entomologist

Forensic entomology is used by law enforcement agencies throughout the world to estimate the point of death (post-mortem interval or PMI) of a human corpse through the analysis of its invertebrate fauna. Forensic entomology can also be used to aid in the enforcment animal health regulations.

Under the Animal Welfare Act (2006) (updated 2007) it is an offence to cause unnecessary suffering to any animal. Reasonable steps must be made to ensure that the animal’s needs are continually met, including protection against pain, injury, suffering and disease (including ectoparasite infestations). The welfare of farmed animals is additionally protected by the Welfare of Farm Animals (England) Regulations 2007, which allows for Codes of Recommendations for the welfare of animals to be produced. Although these Codes are not statutory requirements, livestock farmers are required by law to ensure that all those attending to their livestock have access to the relevant Codes for the species farmed. Although their main aim is to encourage farmers to adopt high standards of husbandry, Codes may also be used to back-up legislative requirements. Where a person is charged with a welfare offence, failure to comply with the provisions of a welfare code may be relied on by the prosecution to establish guilt. Anyone found guilty under the Act may be banned from owning animals, fined up to £20,000 or sent to prison.

Domestic livestock can be attacked by a number of ectoparasites (parasitic insects, mites or ticks living on the skin of the host), all capable of causing considerable distress and possible death of the host. Consequently actions required to prevent or treat ectoparasites are included in the relevant Code of Recommendations. Failure to follow the Codes can result in considerable animal suffering, which can be considered unnecessary as effective chemical treatments are available to prevent or cure infestations. However, chemical treatment is not foolproof. It is important that the ectoparasite is identified correctly and the correct treatment applied. Skill is required in applying a treatment effectively, some products do not claim 100% efficacy and in some cases the ectoparasite has developed resistance to the treatment. Consequently, it is of paramount importance to differentiate cases of treatment failure from those of definite neglect.

Ectoparasites affecting sheep include scab and blowfly strike, both widespread throughout the UK and making up the bulk of veterinary forensic entomology investigations. Sheep Scab, caused by the mite Psoroptes ovis, is a form of debilitating allergic dermatitis resulting in wool loss, intense irritation, epileptiform seizures, scab formation and death. In addition to the Animal Welfare Act 2006 the disease also falls under the Sheep Scab Order (1997), giving Local Authorities (LAs) the means to improve the control of scab and prosecute when owners of infested sheep do not take appropriate control measures voluntarily. Ageing the duration of scab infestation (and therefore the period of neglect) is crucial to a successful prosecution. Unfortunately this is not easy. Early lesions are virtually undetectable, this ‘sub-clinical’ phase (characterised by low mite numbers and small lesions) can last for a matter of days, weeks, months or even years before the lesion progresses into the active (visible) clinical phase, eventually covering the whole of the sheep. The duration of the sub-clinical phase can be influenced by breed of sheep, parasite virulence and previous exposure to scab. Thus the animal with the largest lesion is not necessarily the animal with the oldest lesion.

Blowfly strike (invasion of living sheep tissue by larvae (maggots) of the greenbottle, Lucilia sericata), if not treated can cause considerable suffering and mortality within a flock. A method must therefore be available for the authorities to estimate the duration of strike on an individual animal in order for a prosecution to be successful. One such method compares the species, life-stage (instar) and size of infesting larvae against standard larval growth curves for varying temperature ranges for the major fly species associated with strike in the UK. Although targeted to strike in live sheep, the method has also been used to age larvae taken from strike cases affecting cats, dogs, poultry, pigs and wildlife.

Only a small proportion of the L. sericata population will strike sheep, the vast majority are involved in the environmentally useful tasks of disposing of dead bodies and carrion. Consequently, where dead sheep are concerned it is important to know if the animal died from the effects of blowfly strike or was struck after death.

The EU Animal By-Products Regulations brought into effect in May 2003 prohibits the on-farm burial or burning of fallen stock (animals that have died through natural causes) due to the risk of disease spread through groundwater or air pollution. Animals must be taken to/collected by an approved agent for incineration. Prosecutions can occur when carcases are left in situ or illegally dumped. In these cases it is possible to determine the PMI of these carcasses and therefore how long the carcase has been abandoned.