Making delusions of infestation understandable: A worthy goal?

Prevailing views in psychiatry have described delusions as ‘un-understandable’ (Sims, 1995). This perspective preferences the phenomenology, and therefore the form of the delusions, but excludes considerations of both content and aetiology. Our focus is on delusions of infestation and we explore why insects and parasites form the content of this specific delusion. In doing so, we examine two aetiological propositions and argue the therapeutic and treatment benefits attention to the content of delusions of infestation can afford.

Delusions of infestation are the fixed belief that one is infested by insects or parasites, despite medical evidence to the contrary. They are a somatic subtype of delusional disorder within Diagnostic and Statistical Manual of Mental Disorders (DSM-5). Individuals with delusions of infestation are known to frequently attend general practice, emergency departments, dermatologists, infectious disease specialists and veterinarians, while those few who do present to psychiatric services are considered difficult to treat. To date, the aetiology of this affliction has received almost no attention outside of biological factors. Moreover, current psychological aetiological models for other delusions (e.g. persecutory type) do not allow us to answer the question: why do insects form the content of infestation delusions? First, we detail below an evolutionary and a plausibility/learning theory argument to work towards answering this question.

An evolutionary perspective argues that insects represent evolutionary disadvantage. Insects can be bearers of dangerous and infectious diseases, associated with the deaths of millions of people over millennia. There exist two variants of the evolutionary model. A no-trial learning model suggests that fear/disgust is totally innate and the response presents fully formed without prior learning (Lockwood, 2013). The second preparedness model proposes one-trial learning, whereby the fear/disgust is latent, and a single aversive learning experience evokes the negative response that subsequently proves resistant to extinction (Seligman, 1971).

There are, however, limitations to an evolutionary perspective. First, the majority of the cases reviewed by the authors to date were of post-menopausal women, and thus, reproduction/species survival would not be impacted by infestation. Second, while mosquitoes cause the greatest morbidity and mortality, we have not identified a single case in which patients expressed conviction of mosquito infestation. Third, Lockwood (2013) astutely notes that while our fear of insects may be a function of their ability to carry diseases, epidemics of insect-transmitted disease began when humans started living in high-density communities approximately 10,000 years ago. This appears to be too short a period of time to support an evolutionary argument.

The plausibility hypothesis affords an alternate explanation for why insects form the focus of infestation delusions. This hypothesis takes into account learning theories and posits that through direct and vicarious experiences of feeling insects on the skin and/or being bitten, individuals learn to fear, avoid and actively rid one’s self of insects and possible infestations. The feeling of something in or on your skin is ego-alien, and these uncomfortable sensations are aversive and evoke anxiety, distress or disgust. Second, through cultural information, the sharing of experiences and beliefs, and media for example, one vicariously learns the ‘fear evoking’ properties of insects outlined by Lockwood (2013). Specifically, insects are autonomous organisms with a ‘mind’ of their own. They are largely uncontrollable and unpredictable to the person and breed rapidly and prolifically (Lockwood, 2013). The person can be a living food platform and this can add greatly to the fear and consternation. Therefore, it is important to rid oneself of any sensation that may be attributed to an insect as this sensation carries information that the person is at risk of infestation. The case studies reviewed by the authors to date appear to fit with this framework, as individuals presenting with this delusion appear more concerned with the proximal fear of being infested rather than the ultimate fear of contracting and transmitting infection.

We thus argue that in the presence of real tactile or hallucinatory tactile sensations and itches, individuals’ past direct and vicarious experiences with bugs and insects lend plausibility to the assumption that such sensations are the function of an insect infestation. An attentional bias for benign somatic experiences likely exists and results in heightened psychological and physiological arousal and distress, and subsequent attempts to rid one’s self of the assumed infestation through any means available to the individual.

The above hypotheses present general, distal factors detailing why insects may form the content of infestation delusions. Examination of the specific content of delusions as opposed to the form, however, also allows for identification of the idiosyncratic factors impacting someone’s psychiatric presentation and thus the development of an individualised case formulation. A shared and collaborative case formulation for those with psychotic illnesses has consistently been identified as a predictor of treatment engagement and outcome and takes the content of the delusion and its impact from ‘un-understandable’, to understandable, meaningful and modifiable. Examination of the available case study literature lends evidence for this position.

Those with abnormal personality styles and a history of social isolation appeared particularly predisposed to the later development of an infestation delusion. In such individuals, four themes of precipitating factors have been identified. First, the delusion was precipitated by an actual or perceived insect bite. Second, the delusion was precipitated by interactions with animals or ‘dirty’ household items (e.g. old rugs, bed sheets) that were perceived to be the source of the infestation. Third, the delusion was precipitated by unexpected life events (e.g. divorce). Finally, the delusion was precipitated by highly traumatic events causing experiences of violation, invasion, dirtiness, unworthiness and disgust that, within a psychodynamic framework, thematically inform the content of an infestation delusion. Infestation delusions then appear perpetuated by isolation and withdrawal from others which limits opportunities for reality testing, and secondary morbidities (e.g. infection following skin picking) that an individual interprets as further evidence of their infestation.

A comprehensive case formulation also provides information regarding a patient’s risks to self and others. For example, those with an infestation delusion will often gouge at their skin (leading to infection requiring medical treatment), wash with harsh chemicals such as bleach or take drastic actions such as burning furniture (see Freudenmann and Lepping, 2009). Hostility and anger towards medical professionals have also been noted when patients are provided with negative results (Freudenmann and Lepping, 2009). A thorough understanding of the type of infestation the person perceives, the source of their perceived infestation, and their past or planned attempts to treat the infestation are vital aspects of risk management and good clinical care and are directly informed by delusion content as opposed to form.

Although the first-line treatment for delusions of infestation is undoubtedly antipsychotic medication, Freudenmann and Lepping (2009) reported high rates of partial response, no response, and relapse in those with delusions of infestation treated with antipsychotic medications. They also noted frequent poor compliance and the high proportion of individuals who are rejecting of medication. As noted in our previous article by O’Connell and Jackson (2018), psychological interventions that focus on development of a strong therapeutic alliance, provide opportunities for exploration of the individual’s explanatory model and understanding of the delusion within the person’s life story, and attend to the consequences of the delusion (e.g. distress, risk and impact to social and vocational functioning) are worthy adjunctive interventions for those with infestation delusions in addition to medications.

Conclusion

Conceptualisation and understanding of infestation delusions to date has derived largely from case series and individual case reports. While this disorder is rare in the psychiatric community, those affected frequently present to other medical specialists where antipsychotic medications are considered a first-line treatment. As such, there has been negligible interest in the content of this disorder (as opposed to form), despite the great impact on social and occupation functioning and significant distress experienced by sufferers. Similarly, little work has focused on establishing testable theories that account for the aetiology and maintenance of this phenomenon. In the absence of this lack of argument or theoretical speculation, we have offered two possibilities – an evolutionary theory and a plausibility theory.

We argue that as an initial distal predisposing factor, it is hard to ignore the ancestral, evolutionary notion of preparedness. This suggests that humans may have some innate fear or disgust towards insects due to both their specific alien perceptual features and the evolutionary disadvantage they may pose. Unlike an evolutionary perspective, we also propose a plausibility model. This model suggests that individual experiences with insects and sociocultural learning, prime distress and disgust of insects and ensures action to protect one’s self from their impact. We also consider the more proximal and idiosyncratic aetiological factors that inform the content of infestation delusions including isolation, stressful life events, trauma, experiences with insects or animals, and somatic attentional biases. Most importantly, we note that while phenomenology is important for diagnosis and consideration of differentials, close attention to the content of delusions provides opportunities for psychological engagement and intervention in the absence of any current strong evidence that antipsychotic medications are the only suitable treatment for individuals with infestation delusions.