Phocids, also known as earless seals or true seals, are common coastal marine mammals. Since 1972, seals have been protected under the Marine Mammal Protection Act and are cared for frequently in rehabilitation settings. Beyond human interference, seals encounter a variety of diseases and infections while in the wild. Parasitic infections are regularly found in wild seals and, in some cases, can have detrimental effects on the seal’s health. When trying to maintain healthy population sizes, it is important for animal care professionals to understand the typical parasites that threaten seal populations. Heartworms are common parasites infecting phocid hosts, but all aspects of their infection are not fully understood.
A parasitic organism is one that obtains nutrition through host infection, negatively impacting the host in the process. There are three main groups of parasites: protozoa, helminths, and ectoparasites. Protozoa are single-celled internal parasites, transferred via contaminated food or water. Helminths are internal parasitic worms such as roundworms and flatworms, often transferred to the host through ectoparasitic vectors. Lastly, ectoparasites are blood-sucking arthropods that live on the exterior of an organism, such as lice and mosquitos (CDC 2020). An internal parasite’s life cycle involves infecting intermediate hosts as the parasite progresses through its larval stages. The life cycle ends at a single definitive host where the parasite matures and reproduces (Anderson 2000). Having an intermediate host in close proximity to the definitive host is critical for parasite development.
The heartworm lifecycle usually involves only one intermediate host and one definitive host. The intermediate host is infected with heartworm microfilariae, the earliest stage of the nematode lifecycle. Larvae mature to L1, L2, and L3 stage larvae in the intermediate host within two weeks. At L3, the larvae are infectious and transferred to the definitive host through an open wound (Anderson 2000). Once in the definitive host, the larvae mature to L4 larvae and travel through the blood until they reach the heart. The heart, specifically the pulmonary artery, is where the heartworms develop into sexually mature adults. As the heartworms reproduce, the microfilariae enter the bloodstream and are transferred back to the intermediate host, continuing the cycle (Anderson 2000). Unlike intestinal worms, heartworms cannot be found in feces samples. They can only be detected using blood tests looking for the microfilariae (Keroack et al. 2018).
Acanthocheilonema spirocauda, aka “seal heartworm” is the species of heartworm that infects phocids. While seals are the definitive host of A. spirocauda, the intermediate host is not fully understood. Common symptoms of heartworm in seals are fatigue, anorexia, coughing, and erratic breathing patterns. The presence of worms in the heart can lead to swelling, obstruction of blood flow throughout the body, and an increased chance of developing macroscopic lesions. Low blood flow and lesions have often been found to be fatal, especially in young, weak seals (Eley 1981). In wild seals, where heartworm usually goes untreated, surviving the infection can lead to further complications. The most severe long-term complication of heartworm infection is reduced lung capacity, which can harshly restrict a seal’s ability to dive and forage for food (Geraci & Aubin 1986). In the rehabilitation setting, Ivermectin is frequently used to treat heartworm and other internal and external parasites.
Two key questions about phocid heartworm have yet to be completely answered: What is the intermediate host for seal heartworm? And why has heartworm never been definitively detected in grey seals, Halichoerus grypus? These two questions go hand in hand, as one of the hypotheses for why heartworm does not occur in grey seals is that they do not interact with the heartworm intermediate host. However, no evidence has been found to support this hypothesis. In a 2007 study by Leidenberger et al., the authors correlated the co-occurrence of seal heartworm with seal lice, Echinophthirius horridus. Seal lice is an ectoparasite that survives on the blood of its host. Seal lice are known to transfer pathogens and bacterial diseases between seals, so they may also possess the ability to transfer parasites. Leidenberger (2007) found that seals infected with heartworm had a significantly higher chance of also being infected with seal lice. Their frequent co-occurrence raised speculation about seal lice as heartworm vectors. The only issue faced by researchers is the inability to actually find heartworm microfilariae in seal lice. Geraci et al. (1981) were able to isolate A. spirocauda L1, L2, and L3 stage larvae in the lice, but without the presence of microfilariae, seal lice cannot definitively be classified as a phocid heartworm vector.
If seal lice act as a transmission pathway for heartworm in phocids, the lack of heartworm in grey seals remains a mystery. Grey seals have been found to have a similar parasitic burden of lice as harbor seals, hooded seals, harp seals, and ringed seals, all of which are commonly infected with heartworm (Leidenberger 2007). Despite this, there has been no verified case of heartworm in grey seals. Some other hypotheses for why heartworm has never been detected in grey seals are insufficient sampling, extreme mortality from infection, and resistance to infection. To further study this phenomenon, Keroack et al. (2018) developed a highly sensitive PCR assay for detecting heartworm in phocids. They used this assay to find what is considered to be the first case of heartworm in grey seals. Keroack et al. (2018) suggested that grey seals may have some sort of resistance to heartworm, causing the microfilariae concentrations to be abnormally low and not detected by standard blood tests. While the results have yet to be replicated, the findings do present novel technology that can be used to more accurately track heartworm infection in phocid populations.
Despite extensive research, heartworm in phocids is still not fully understood. While it is clear that heartworm larvae mature in seals as a definitive host, the intermediate host is only speculated to be seal lice. The absence of heartworm in grey seals has also raised questions about biological differences among seal species and the efficiency of heartworm blood tests. Having a grasp on parasitic infections and their impacts on phocid populations can improve rehabilitation techniques and promote healthy, growing seal communities.
Research review paper written by fall intern, Meaghan K. Meaghan is a student at Colgate University studying Marine and Freshwater Biology.
Anderson, R. C. (2000). Nematode parasites of vertebrates: their development and transmission. Journal of Wildlife Diseases, 17(1):65–67
“CDC – Parasites – About Parasites.” Centers for Disease Control and Prevention. Centers for Disease Control and Prevention, September 18, 2020. https://www.cdc.gov/parasites/about.html.
Eley, T. J. (1981). Dipetalonema spirocauda in Alaskan marine mammals. Journal of Wildlife Diseases, 17(1), 65-67.
Keroack, C. D., Williams, K. M., Fessler, M. K., DeAngelis, K. E., Tsekitsidou, E., Tozloski, J. M., & Williams, S. A. (2018). A novel quantitative real-time PCR diagnostic assay for seal heartworm (Acanthocheilonema spirocauda) provides evidence for possible infection in the grey seal (Halichoerus grypus). International Journal for Parasitology: Parasites and Wildlife, 7(2), 147-154.
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Geraci, J. R., Fortin, J. F., Aubin, D. S., & Hicks, B. D. (1981). The seal louse, Echinophthirius horridus: an intermediate host of the seal heartworm, Dipetalonema spirocauda (Nematoda). Canadian Journal of Zoology, 59(7), 1457-1459.