Egusi seed allergy confirmed using the basophil activation test
To the Editor,
The vast majority of food allergies in children are caused by few foods,
namely cow’s milk, egg, wheat, soya, peanut, tree nuts, fish and
shellfish1. However, as a practising allergist, one
needs to have an open mind as allergy can be caused by virtually any
food. It is important to report rare allergies to increase awareness and
support the diagnosis of new cases should this be suspected.
Furthermore, with globalisation and dissemination of cuisines around the
world, knowledge about new foods and new allergens is very valuable.
We report the case of a 5-year old boy of Nigerian descent with a
history of immediate onset urticaria, periorbital angioedema and
vomiting after the ingestion of egusi melon seeds at the age of 6
months. He had a history of urticaria and vomiting on exposure to cow’s
milk at 3 months of age, and reported similar reactions to coconut,
peanut, almond and hazelnut in the first year of life. He had a
background of early onset atopic dermatitis, which was recently well
managed with mild topical corticosteroids, mild seasonal allergic
rhinitis and viral induced wheeze. There was a family history of atopy:
father had a history of asthma and allergic rhinitis.
Cow’s milk had since been successfully reintroduced into the diet, but
he continued to avoid egusi seed, all nuts and coconut, as well as
sesame seed due to evidence of sensitisation on blood testing. As egusi
seed formed an important part of the family’s diet, the family were keen
to reintroduce egusi seed if possible, and cashew, coconut or peanut
were in second line of priority.
Skin prick tests were performed in clinic and blood was collected for
specific IgE testing and the basophil activation test (BAT). The BAT was
performed to peanut to confirm the need for food challenge as part of an
ongoing research study (Research Ethics Committee reference 10/H0802/44)
and to egusi seeds to confirm the diagnosis, following parental consent
and child’s assent. The BAT was performed as previously
described2,3 using a peanut extract prepared by
ALK-Abello and an in-house prepared egusi seed extract alongside with
negative (Roswell Park Memorial Institute (RPMI) 1640 Medium alone) and
positive controls, namely anti-IgE and
N-formylmethionyl-leucyl-phenylalanine (fMLP) – for details on the
methodology see supplementary material. Briefly, dehulled egusi seeds
were ground and homogenised in phosphate buffered saline (PBS) for
overnight cold extraction. Crude extract was then centrifuged and
filtered using Whatman paper. Protein precipitation was performed using
saturated ammonium sulfate until 80% saturation was reached. The
resulting pellet was dissolved in ultrapure water then dialysed in
deionised water overnight at MWCO 3500. BCA assay was performed to
determine total protein content of aqueous extract and an SDS-PAGE
(Figure E1 ) was used to determine presence and approximate size
of proteins in the extract for use in the BAT.
The results of SPT and specific IgE are represented in Table 1 .
The BAT to peanut was positive, i.e. above the optimal diagnostic
cut-off (4.78% CD63+ Basophils) previously validated in our centre, and
the patient was therefore not referred for an oral food challenge, which
the family was reassured about as they were anxious about the idea of
exposing their child to peanut. The BAT to egusi seeds showed a
dose-dependent increase of the expression of
CD63 on the surface of basophils with
increasing concentration of egusi seed allergen extract with a peak of
73.71 % net CD63+ basophils at 10,000 ng/mL
(Figure 1 ). A similar dose-response was also observed for the
CD203c stimulation index. Basophil response to both anti-IgE and fMLP
controls were detectable, with 61.01 % and 12.71 % net
CD63+ basophils respectively. The combination of BAT,
SPT and the history confirm the diagnosis of egusi seed allergy.
Strict avoidance of peanut, all tree nuts, coconut, sesame seed and
egusi seed was advised and a written emergency management plan was given
along with emergency medication, consisting of 2 adrenaline
auto-injectors and cetirizine.
Egusi (Citrullus mucosospermus ), sometimes referred to as egusi
melon or egusi watermelon, is part of the Cucurbitacaeae family and is
closely related to watermelon4,5. It is found in the
tropics, from West Africa through to Sudan. Although the flesh of the
fruit is dry and bitter, the seeds can be dried and ground and are
commonly used to thicken soups in West African
cuisine6. To our knowledge, this is the first
documented case of egusi seed allergy. We were able to confirm the
diagnosis suggested by the clinical history with detection of functional
IgE to egusi seeds on SPT and the BAT.
In cases of suspected allergy to exotic foods or rare allergens,
modified skin prick testing using the implicated food may be the only
test available to confirm the allergy. It should also be noted, that in
our patient the skin prick test diameters were initially quite small –
despite a convincing history of reaction to egusi seeds - and then
became larger at age 6 years, probably reflecting a more established
immune allergic response and demonstrating the importance of a detailed
clinical history and the benefit of repeated skin prick testing in
children over time. We were fortunate to have access to the BAT, which
allowed to defer the OFC to peanut and to further confirm the diagnosis
of egusi seed allergy.
Our patient also had a clinical history and/or sensitisation pattern
suggestive of allergy to sesame seed, peanut and tree nuts including
hazel nut7. Cross-reactivity has been described in
seed allergy, with common allergenic structures identified between
sesame seed, poppy seeds, hazelnuts, rye grain and
kiwi8. Cross-reactivity has also been described
between other members of the Cucurbitacaeae family including pumpkin,
pumpkin seed, musk melon, watermelon, cucumber and
courgette9. Allergens identified in other seed
allergies include 2S albumin, oleosin, lipid transfer protein and
profilin, while profilin, malate dehydrogenase and triose phosphate
isomerase and profilin have been identified as major allergens in
watermelon allergy8,10. However, this requires further
research.
In summary, we report for the first time the case of allergy to egusi
seeds in an atopic child of Nigerian origin with allergy to other seeds
and nuts. This case highlights the need to know and explore less common
foods as potential allergens, the importance of modified skin prick
testing and the BAT to support the diagnosis of rare food allergies and
of awareness about world cuisine and exotic foods as potential
allergens.
Erika Harnik, MBBS1
Zainab Jama, BSc2,3,4
Matthew Kwok, BSc2,3,4
Adam T. Fox, MBBS MD1,2
Alexandra F. Santos, MD PhD1,2,3,4
1Children’s Allergy Service, Evelina London Children’s
Hospital, Guy’s and St Thomas’ Hospital, London, United Kingdom
2Department of Women and Children’s Health (Pediatric
Allergy), School of Life Course Sciences, Faculty of Life Sciences and
Medicine, King’s College London, London, United Kingdom
3Peter Gorer Department of Immunobiology, School of
Immunology and Microbial Sciences, King’s College London, London, United
Kingdom
4Asthma UK Centre in Allergic Mechanisms of Asthma,
London, United Kingdom