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Chapter 31 - Heartworm of Swans and Geese
Authors: Rebecca A. Cole.
Filarial heartworm, Sarconema, Sarconema eurycerca
Heartworm in swans and geese is caused by a filarial nematode or a roundworm of the superfamily Filarioidea which is transmitted to the bird by a biting louse. The nematode and the louse both are parasites. Sarconema eurycerca is the only one of several species of microfilaria or the first stage juvenile of the parasite found in the circulating blood of waterfowl that is known to be pathogenic or cause clinical disease.
Sarconema eurycerca has an indirect life cycle (Fig. 31.1) that requires the parasite larvae to develop in an intermediate host before they can become infective for and be transmitted to a definitive host, where they mature and reproduce. Female adult heartworms release microfilariae into the bloodstream of the definitive host bird. The microfilariae infect a biting louse, Trinoton anserinum, that subsequently feeds upon the bird. The larvae go through three stages of development within the louse, and the third stage is infectious to birds. A new host bird becomes infected when the louse bites it to feed on its blood and the third-stage larvae move into the birds bloodstream. The larvae migrate through the bloodstream to the myocardium, which is the middle and thickest layer of the heart wall composed of cardiac muscle. They are nourished by and develop to sexual maturity within the myocardium. The cycle continues as this next generation of mature heartworms release microfilariae into the bloodstream.
Infection with the parasite is not synonymous with disease;that is, the parasite may infect and develop within the bird but not debilitate it. Return to top of page Species Affected
Sarconema eurycerca was first identified from a tundraswan (whistling swan) in the late 1930s. It has since been reported from trumpeter, Bewicks, and mute swans and, from Canada, snow, white-fronted, and bean geese. Varying percentages of swans (420 percent) have been found to be infected on the basis of blood smears that were taken from apparently healthy birds during field surveys. Canadian investigators have reported a prevalence of approximately 10 percent of snow geese that were examined at necropsy and which had died from other causes. This parasite has not received sufficient study for its full host range, its relative frequency of occurrence in different species, or its significance as a mortality factor for wild birds to be determined. Return to top of page Distribution
Heartworm is found throughout the range of its swan andgoose hosts. Seasonality
It is suspected that while swans and geese are on the breedinggrounds, louse infestation and colonization on birds is prevalent. Therefore, the possibility of infection by heartworm is highest while birds are on the breeding grounds. Field Signs
Field signs are not always present in infected birds, andinfection cannot be determined by the presence of clinical signs alone. Chronic types of debilitating diseases, such as lead poisoning, may exacerbate louse infestation because birds become lethargic and do not preen. No specific field sign is diagnostic for infection. Return to top of page Gross Lesions
The severity of infection dictates the lesions that are seenat necropsy. Birds may be emaciated or in comparably good flesh. The heart may be enlarged and have pale foci or spots within the myocardium. The thin, long thread-like worms may be visible under the surface layer or epicardium of the heart or the worms may be embedded within the deeper muscle tissue of the myocardium (Fig. 31.2).
A diagnosis of heartworm as the cause of death must besupported by pathologic lesions seen during examination of the heart tissues with a microscope and consideration of other causes. Therefore, whole carcasses should be submitted for diagnostic assessments. If the transit time is short enough to avoid significant decomposition of the carcass and if the carcass can be kept chilled during transit, then chilled whole carcasses should be submitted to qualified disease diagnostic laboratories. If those conditions cannot be met, then carcasses should be submitted frozen. Control
Control of heartworm is not practical for free-rangingbirds. Decreasing the opportunity for heavy infestation of the louse intermediate host will result in reduced opportunity for heartworm infection. Human Health Considerations
Sarconema eurycerca has not been reported to infect humans.Return to top of page Supplementary Reading
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