A survey of Gyrodactylus elegans on goldfish indicated that the temperature had an effect on the number of these parasites on the body but little or no effect on the number found on the gills. As the water became warmer the number of body parasites increased. The pectoral, pelvic and anal fins had more parasites per unit of area than the larger dorsal and caudal fins.

Fifteen identified species and 13 genera, of endoparasitic helminths (Digenea, Eucestoda, Nematoda, Acanthocephala) were collected from 207 white crappie and 189 channel catfish June, 1967, through September 1968, from a 3,300-acre, turbid reservoir in northcentral Oklahoma. Differences in the prevalence and intensity of helminths from six reservoir collection sites were not statistically significant. Statistically significant differences in intensity and prevalence of certain helminth were found among different age classes of the hosts. Ontogenetic changes in the food habits of channel catfish, from a diet of invertebrates to fish, were apparently the reason for changes in the occurrence of many enteric helminths. The occurrence of some helminths, however, was independent of age. Changes relating to age in the crappie were limited to the occurrence of Posthodiplostomum minimum where multiple generations of metacercariae accumulate in older fish. These metacercariae occurred in significantly higher numbers in males. Otherwise the occurrence of parasitism did not differ significantly between the sexes. Seasonal differences in parasitism, heretofore rarely studied, were pronounced, reflecting changes in feeding, metabolism, and the reproductive cycle of the host, and the annual life cycle characteristics of the parasite. Some parasites, such as Dacnitoides robusta, were absent in the channel catfish in the winter but abundant in the summer. Conversely, proteocephalid tapeworms from the channel catfish were abundant in the winter and infrequent in the summer. Observations on the pattern of seasonal variation in prevalence and degree of infection of Posthodiplostumum minimum in the white crappie suggests that it may contribute to summer mortality of its host.

White-tailed deer (Odocoileus virginianus) were sampled in a wilderness area of Southern Ontario in 1965, 1966, and 1967. Serological evidence of Leptospira pomona and L. grippotyphosa infection was found. Leptospirosis has not depressed the deer population. There was a positive correlation between age and reactor rate. Paper disc-absorbed whole blood has been tested comparatively with fluid serum and found to be a useful tool for field serological surveys.

Vibrio anguillarum was identified as the causal organism of an epizootic of vibriosis in juvenile chinook salmon (Oncorhynchus tshawytscha) reared in a salt-water impoundment on the Oregon coast.

The pathogenesis of epizootic hemorrhagic disease of deer (EHD) was studied in suckling white mice and white-tailed deer (Odocoileus virginianus) using these approaches: (1) measurement of hematological values, (2) assay of virus in various organs, and (3) detection of viral antigen within tissues using the direct fluorescent antibody technique. An increase in bleeding time was the only hematological change observed in mice. The virus content of the brains of infected mice increased rapidly during the early days of infection but little virus was found in other organs. Fluorescing viral antigen was detected only in the brains of acutely ill mice.

Infected deer had an increase in total erythrocyte counts as well as corresponding changes in erythrocyte-associated values. Although the percentage of neutrophils in the blood increased during infection, total leukocyte counts remained unchanged. Virus was isolated during the clinical illness from 13 of 16 organs tested. No viral antigen was demonstrated in any tissue by the direct fluorescent antibody technique.

Post-mortem examination of 181 elk (Cervus canadensis nelsoni) from the northern Yellowstone Park herd in 1967–68 revealed that 41% were infected with Thysanosoma actinioides. Infections occurred in all age classes of animals from seven areas in the Yellowstone, Gardner and Lamar drainages in the northern section of the Park.

Prevalence of the parasite was higher in calves and yearlings than in mature elk. Infections varied from 1 to 16 worms per animal, with an average intensity of 4.3 in 75 elk. Worms were confined to the first 6.2 feet of the small intestine, with no distinct habitat preference apparent within this area. Little evidence of T. actinioides or gross lesions associated with its presence was found in the liver or bile ducts of elk examined 20 to 40 minutes after death.

Herring gulls, Canada geese, and mallard ducks were orally vaccinated with attenuated duck plague virus and challenge inoculated with virulent virus. Herring gulls were unfnected by either virus; they did not die or produce detectable antibodies. Geese did not produce antibody to attenuated virus and 11 of 12 died after immunity challenge. Mallard ducks were more resistant as 7 of 9 survived challenge inoculation with virulent virus. No correlation between mortality and antibody produced to attenuated virus vaccine could be observed.