What is OE?
Ophryocystis elektroscirrha (OE) is an obligate, protozoan parasite (shown by the green arrow above) that infects monarch and queen butterflies. In simpler language, OE isn’t an animal or a plant, but a protozoan. Protozoans are single celled organisms, living things that have many of the same characteristics as animals. In fact, protozoans are often called animal-like protists.
OE is considered an obligate parasite because it must live within a host to grow and multiply. Between infections OE survives as spores that are resistant to extreme environmental conditions. OE was first discovered infecting monarch and queen butterflies in Florida in the late 1960s. There are no known other hosts. It has since been found in all other monarch populations world-wide. Because of this world-wide range, all indications are that this parasite has coevolved with monarchs.
How common is OE in North American monarchs?
There are three major monarch populations in North America. Most research has focused on the population that breeds east of the Rocky Mountains and migrates every year to wintering sites in the transvolcanic mountains of central Mexico (shown in yellow on the map). After mating in the spring they migrate north to their summer breeding grounds in the United States and Canada. A second monarch population west of the Rocky Mountains has a shorter, less dramatic migration to their roosting areas on the coast of California (shown in orange on the map). There are also non-migratory population that breeds year-round in southern Florida (shown in green on the map), coastal Texas, Hawai'i, the Caribbean Islands, and Central and South America. Since milkweed plants grow here all year round, the butterflies do not need to leave the area. These resident monarchs reproduce throughout the entire year.
How common is OE in North America?
OE infects monarchs in all three North American populations. The eastern migratory monarchs have the lowest infection rate. Less than 8% of these butterflies are heavily infected with OE. More monarchs have OE west of the Rocky Mountains. About 30% of the western migratory population is heavily infected with OE. The highest rate of OE in North America occurs in the nonmigratory monarchs of South Florida. More than 70% of these monarchs have OE infections. The infection rates for monarch populations in North America have been constant for many decades.
OE spores are dormant cells found on the outside of infected monarchs. These tiny spores are sandwiched in between the scales that cover a butterfly’s body. The greatest concentration of spores is usually on the abdomen. Spores are much smaller than scales. In fact, a monarch scale is about 100 times larger than an OE spore. You must use a light microscope set at 40 to 100X to see a spore. Even at this magnification spores look like small, brown or black lemon-shaped objects. Here is a picture of OE spores at 400X:
The images below (taken by Chip Taylor) show scanning electron micrographs of parasite spores clustered on abdominal scales from a parasitized monarch.
Life Cycle and Transmission
The life cycle of OE is very closely related to the life cycle of the monarch butterfly. OE can only reproduce inside the insect’s body. Infected females pass on the parasite to their offspring when they lay eggs. Dormant spores on the outside of the female’s abdomen are scattered on the eggs and milkweed leaves. When a caterpillar hatches, its first meal is the egg shell. The caterpillar will eat OE spores along with the shell and milkweed.
How to tell if butterflies are infected with OE
An infected pupa may develop dark spots or blotches two or three days before the butterfly emerges. These abnormal dark areas are parasite spores. Spores form on the eyes, antennae, wing veins, but mostly on the abdomen. You can see the spores through the outside layer of the pupa a day or two before pigments that color the butterfly normally darken the pupa. Before a butterfly emerges from the chrysalis, pigments are laid down coloring the scales that cover the butterfly. This normal change in the color of the pupa is symmetrical. The color change of an infected monarch happens earlier and does not create a balanced pattern on the pupa.
Adults that are heavily infected with OE are weak and often have difficulty emerging from the chrysalis. Some monarchs die before emerging. Others emerge, but are too weak to cling to the pupal case. They fall to the ground before fully expanding their wings. These severely deformed monarchs do not survive long.
Mild OE infections also harm butterflies. Infected adults are often smaller than healthy monarchs. They weigh less and have shorter forewing lengths than normal. Parasites also damage the cuticle or outside layer of the monarch’s abdomen. This damage causes the butterfly to dry out and lose weight faster than normal. This is especially a problem if there is a shortage of nectar or water. Studies have shown that monarchs infected with OE cannot fly as far or as long as healthy butterflies. Since infected males are weak, they are less likely to mate and produce offspring than uninfected males. Infection does not appear to harm the ability of females to reproduce.
While these may all be symptoms of OE infection, many infected monarchs look healthy. They emerge normally and are not deformed. The only way to really know if your monarch is infected is to check for spores.
Sampling Monarchs for Parasites
Monarchs can be easily assessed for parasite loads by pressing a piece of ultraclear Scotch TM tape on their abdomens and counting the number of spores in a 1cm x 1cm area. This slide shows how spores appear relative to abdominal scales under the light microscope at 200x.
In our lab, we use this 'tape' method to categorize parasite loads on an approximate logarithmic scale of 0-5, with 5 being the most heavily infected class, and 0 being butterflies with no detectable spores. This method allows for rapid classification of disease status and the severity infection – and is highly correlated with the log of total infection loads estimated using a destructive wash-and count method.
We also create tape samples from swabs sent in by our MonarchHealth participants. For more information on how to test for OE parasites in monarch butterflies using the swab method click here. People in our lab have also developed more innovative methods for using digital image analyses to get more refined and continuous measures of spore densities (below).
More Resources :
Altizer, S.M. 2001. Migratory behaviour and host-parasite co-evolution in natural populations of monarch butterflies infected with a protozoan parasite. Evolutionary Ecology Research 3: 611-632. pdf
Altizer, S.M. and Oberhauser, K.S. 1999. Effects of the protozoan parasite, Ophryocystis elektroscirrha, on the fitness of monarch butterflies (Danaus plexippus). Journal of Invertebrate Pathology 74: 76-88. pdf
Altizer, S.M., Oberhauser, K.O., and Geurts, K.A. 2004. Transmission of the protozoan parasite, Ophryocystis elektroscirrha, in monarch butterfly populations. Pages in Oberhauser, K., and M. Solensky (eds.). The monarch butterfly: biology and conservation. Cornell University Press. Ithaca, NY.
Davis, A.K., S. M. Altizer and E. Friedle. 2004. A non-destructive, automated method of counting spores of Ophryocystis elektroscirrha in infected monarch butterflies (Lepidoptera: Nymphalidae). Florida Entomologist 87: 231-234.pdf
Leong, K. L. H., M. A. Yoshimura, H. K. Kaya, and H. Williams. 1997b. Instar susceptibility of
McLaughlin, R. E., and J. Myers. 1970. Ophryocistis elektoscirrha sp. n. a neogregarine pathogen of the monarch butterfly Danaus plexippus (L.) and the Florida queen butterfly Danaus gilippus berenice. Cramer. Journal of Protozoology 17: 300–305.
For more information on rearing monarchs without OE, click here.