By: Tracy Farone, DVM
This article originally appeared in the Autumn 2020 issue of BEEKeeping Your First Three Years
Emerging Disease in the Shadows, Tropilaelosis:
Zoonotic Disease and Public Health is a course I have had the joy of teaching to pre-health students for over 10 years. The course’s content covers all types of infectious diseases and how they affect and connect humans, animals, and the environment. Some of my students affectionately call it “Zoo” or even the “poo” class, because so many diseases can be transferred through fecal/oral contamination. Eww, but true! Many students are amazed to discover how many diseases surround us, how the health of our world is so interrelated, and under normal circumstances, how most of us are blessed with a wonderful immune system. Certainly, these same observations apply to honey bees and their health.
Danger emerging from the shadows…
Emerging infectious diseases is one topic we cover during the “zoo” course. Emerging infectious diseases are infections that have recently appeared in a population of humans or animals. Emerging diseases often arise when they are brought into new geographical ranges and/or species. Some causes of emerging disease may not have been previously known, while others may already be known, and pose a serious threat, if they are able to increase their geographic range. Ebola, Zika, Rocky Mountain Spotted Fever, Varroosis, and COVID-19 are all examples of emerging diseases. Many emerging diseases often originate from “foreign” or “exotic” diseases (or newly named “transboundary diseases”). Foreign, exotic or transboundary diseases are diseases that naturally exist in a certain country, continent, or areas of the world, but may cross borders, continents and/or oceans to infect new regions. If allowed to move into new geographical areas, foreign diseases can emerge in a population with little natural immunity against the disease agent. Therefore, these diseases can cause high morbidity and/or mortality when introduced to the new population of animals or humans. In our modern world, international trade and travel often accommodates hitch-hiking diseases and pests. To safeguard animal health in the US, a list of foreign animal diseases (FAD) is continuously monitored by the USDA and accredited veterinarians.
But what about bees? Do they have a current “FAD”? They do. It is a parasitic disease of honey bees that does not always make the headlines, but mirrors examples of other disease processes we see highlighted in our world. No, it is not the “murder” or Asian hornet, but a disease that is and should be on beekeepers’, entomologists’ and veterinarians’ radar: Tropilaelosis.
Tropilaelosis is a mite infestation of Apis mellifera (European honey bee) caused primarily by two major species: Tropilaelaps clareae or Tropilaelaps mercedesae. These mites’ natural honey bee hosts (Apis dorsata, Apis laboriosia, and Apis breviligula, “giant” honey bees) are better adapted host species of honey bees compared to Apis mellifera. Their natural range is found in Asia, Indonesia, and the Philippines. The mites have also been reported in parts of Africa, including Kenya and the Republic of the Congo. Tropilaelosis is currently a disease regulated world-wide and monitored by the OIE (The World Health Organization of Animals) as a notifiable disease and the USDA as a reportable disease. These mites are one reason why honey bee importation is limited in the US.
Tropilaelaps can be transmitted thorough swarms. Credit to Deidra Ressler.
The lifecycle of the mite is somewhat like Varroa with the reproductive cycle involving a gravid foundress mite invading a brood cell, egg laying, developing mites parasitizing and often killing the larvae/pupae, and re-emergence of new adult mites. Compared to Varroa, the reproductive cycle is relatively short, only about one week, and all mites emerge from the brood cell including the males. This feature allows the Tropilaelaps mites to populate a colony much faster than Varroa and therefore, take down a colony quickly. Tropilaelaps mites are unable to feed on adult bees, so their phoretic phase is much shorter than Varroa, usually only three days. This characteristic forces the mites back into the brood for yet another quick reproductive cycle, killing more brood and making more mites. Despite the short phoretic period, adult bees are still able to spread mites to other hives via swarms, package bees, exchange of frames of bees between hives, drifting, and robbing.
Mites are diagnosed and treated using similar methods to Varroa. Adult honey bee samples can be checked for mites with alcohol wash or sugar roll. While mite count levels have yet to be established for Tropilaelaps, any mites found would be significant. The mites are visible with the naked eye, but they are smaller and move faster than Varroa. They are easier to observe in capped drone brood. Sticky board or “bumping” frames to dislodge mites onto a white surface can also be used for detection. At the colony level, infestations will result in rapid colony collapse or absconding. Brood comb may be severely affected due to high mortality infected larvae and pupae. Treatment should involve an IPM approach. Treatments can include common acaricides used for Varroa, along with biological controls of inducing brood breaks, brood removal and caging the queen. Treatment timing protocols should consider the short phoretic period of the mites. Being unable to parasitize adult bees is one biological weakness of Tropilaelaps, that we can exploit. Natural broodless periods and overwintering are ways to limit or control these parasites. Luckily and so far the geographic range of Tropilaelaps has largely been limited due to this “tropical nature” of the mite. 
However, some honey bee colonies in South Korea, with a more temperate climate, have been found to support Tropilaelaps mites. There is more bad news. While rare, Varroa and Tropilaelaps can co-infect colonies, but Tropilaelaps usually out competes Varroa. Tropilaelaps has also been found to be a vector for viruses, like DWV. The good news: Tropilaelaps has not yet been reported in much of the world, including the US, Europe, Australia, and Canada. However, awareness and prevention of diseases are keys to keeping our honey bee population safe. How diverse animal species, humans, and diseases can be, yet how much is still shared and interconnected, amazes me. Studying and understanding these similar biological and epidemiological principles are paramount to understanding how we can all work together to best promote our collective health.
References & for further informational links:
De Guzman, Lilia I. , Williams Geoffrey R., et al., “Ecology, Life History, and Management of Tropilaelaps Mites”, Journal of Economic Entomology, Volume 110, Issue 2, April 201 7, Pages 319-332, https://doi.org/ 10.1093/jee/tow304 Published:08 March 2017.
Vidal-Naquet, Nicolas. Honeybee Veterinary Medicine: Apis mellifera L., Sm Publishing, 2015, pp. 138-142.
OIE policies on Tropilaelaps mites:
https://www.oie.int/fileadmin/Home/eng/Health_standards/tahc/current/chapitre_tropilaelaps_spp. pdf
Great picture of Tropilaelaps:
USDA bee mite ID, Tropilaelaps:
http://idtools.org/ id/mites/beemites/ factsheet.php?name=15241
Tropilaelaps info sheet:
https: //www.aphis.usda.gov/plant_health/plant_ pest_info/honey _bees/downloads/TropilaelapsInfoSheeta.pdf
USDA national honey bee survey information including surveillance for Tropilaelaps:
https: //www.aphis.usda.gov/plant_health/plant_pest_info/honey _bees/downloads/SurveyProjectPlan.pdf
USDA reportable bee diseases:
https://www.aphis.usda.gov/aphis/ourfocus/animalhealth/monitoring-and-surveillance/a_nahss/status-reportable-disease-us/tut/p /z l/ lZJNU4MwEIZ_Sw8cIRvaodQbIFNQqGMtirl0gqbADCVMEmT0 15vWkx9tMZd8zPPuZt9dRF COSEvf6pKqmre00fdn4mxXOFiCO8PJMrzG4EWL-DaaOwD3Nno6AsndLMD-A-g99MEL1M0DGMb8BSR_-kf10frN9lm5WI_9HBieTBOfwYg58Pf XEqgHbRFGqQllhlVlVm3O45ySbctraTUJ0VVL03BO i4ULRpmvtaSUcnMXurfkWPSU_6u7QvAoQE_gN8Oj6 ixbHjxNQSeW0xdXYxgOyaYsHqhnyulOnllgAHDMFgl 52XDrBe-N-AvScW1Qvl3EnX7LMvyjySCOjZJST54k8knxylGSgll/#bee
OIE interactive map of reportable animal disease distribution around the world:
https://www.oie.int/wahis_2/public/wahid.php/Diseaseinformation/Diseasedistributionmap?disease_ type _hidden=&disease_id_hidden= &selected_disease_name_hidden=&disease_type=0&disease_id_terrestrial=18l&species_t=0&disease_id_aquatic=-999&species_a=0&sta_method=semesterly&selected_ start_year=20l8&selected_report__period=2&selected_ start_month= l&date_submit=OK#
University of Florida Fact Tropilaelaps sheet:
http://entnemdept.ufl.edu/creatures/MISC/BEES/Tropilaelaps.htm
Great article with Tropilaelaps on comb video:
https://entomologytoday.org/2017/04/l 7/get-toknow-tropilaelaps-mites-another-serious-parasite-ofhoney-bees/
Another great article on Tropilaelaps:
https://www.sciencedirect.com/science/article/pii/S2214574517300810
Acknowledgements to my research students for research assistance: Katrina Bailey and Deidra Ressler.
