Here in the UK, there are few haematophagous arthropods which trouble me. While mosquitoes provide a minor irritation, the Scottish midge Culicoides impunctatus is massively annoying, principally because of the sheer number that typically assail one. For me the bite is more irritating as it occurs rather than an itching after-effect. Generally I come into contact with the Scottish midge while on cycling holiday, and swarms of the blighters can make even the most lovely campsite intolerable. However the beast that irritates me more than any other is Neotrombicula autumnalis, commonly known as the harvest mite or berry bug. The common names reflect both the season in which N. autumnalis larvae become active and the activities which bring people into contact with them. This blog article was inspired by a pair of bites I picked up this week.
Most people who suffer from N. autumnalis first become aware of quite severed itching and raised reddened lumps that are typically found around clothing constrictions, typically underwear elastic. But by this time the little devils are usually long gone, leaving several days of irritation behind them. So what are harvest mites?
The biology of trombiculid mites
Harvest mites are small arthropods, with a life cycle as shown in the figure (from Wikipedia). It’s a typical mite life cycle: the offending stage is the 6 legged larval stage, of which more later: one phase of the nymph stage and the adults are active predators on other small arthropods.
This life cycle diagram is a slight simplification. The nymph stage is actually composed of three stages, the protonymph (an inactive stage entered after the larva has finished feeding and left its host), the deutonymph (the active nymph stage during which the animal feeds on other, presumably smaller, arthropods) and the tritonymph (a second inactive stage after which the adult emerges).
For this purpose, it’s the larval stage that is of interest: this is the only stage in which the animal feeds on a vertebrate host (see image to left). These beasts lurk on the tips of grass and other leaves awaiting a passing animal. When the animal brushes the leaf, the larval mite climbs aboard and crawls about, seeking a suitable attachment point. For some animals this will be areas devoid of hair or thin skin, such as around the eyes or in the ears. In the case of humans, this is frequently at points of clothing constriction – usually the last place you want to develop an itch!
Larval trombiculid mites don’t actually bite, or drink blood (so my depiction of them as haematophagous is rather stretching the point): rather they pierce the skin, injecting digestive juices and sucking up the resulting fluids. This goes on for a few days after which the mite drops off the host and falls to the substrate. This process causes a tube of hardened skin called the stylostome to form, and it’s through this that the mite continues to feed.
Apparently it’s only after the mite has departed that the bite becomes noticeable, by which time the culprit is long gone. Fortunately in temperate areas such as the UK, trombiculid mites only have one annual generation, with larvae being active in summer and early autumn (hence the association with berry picking, crop harvests and, in my case, mushroom collecting). In tropical areas, trombiculids breed rather more frequently, with the life cycle being completed in around 40 days.
Trombiculid mites and disease
Here in the UK, harvest mites don’t transmit disease, but this isn’t the case elsewhere. In south east Asia, trombiculid mites are knwon to transmit scrub typhus (also known as Tsutsugamushi), caused by the bacterium Rickettsia tsutsugamushi (aka R. orientalis) – rather than being a typhus, it’s actually a Rickettsiosis.
Treatment of harvest mite bites
Severe attacks may lead to dermatitis as the rashes and pustules merge. I’m not personally aware of any particularly effective treatment for the “berry bug itch”. The Pied Piper pest control website has a section on harvest mites that recommends the following – both seem to be very sensible advice. If you’ve visited a site known for harvest mite infestation, wash your clothes in hot soapy water, and have a good hot shower. The larval mites will be wandering around your body and clothes, the hot wash should kill them. Unfortunately the realisation that one may have been in infested terrain may only surface once the mites have gone and you have the itch… Scratching can cause secondary infections: temporary relief may be provided by ointments of benzocaine, hydrocortisone, calamine lotion, or other things that may be recommended. For what it’s worth, I’ve tried calamine lotion, it works for a very short time!
The pet health website PetWellBeing.com recommends (for dogs) the topical application of steroids to alleviate the severe itching. Ultimately, my opinion is that the best treatment is probably to ensure you don’t scratch the lesion!
Wikipedia entry Trombiculidae
Lane & Crosskey “Medical Insects and Arachnids” Pub 1993 Chapman & Hall.
Wikipedia entry Tsutsugamushi
Note: this article should not be considered medical advice