The State of Santa Catarina in southern Brazil is the most important Brazilian producer region. The Pacific oysters Crassostrea gigas and the brown mussels Perna perna are the main species currently produced. The occurrence and impact of parasites on bivalves from natural or culture populations from Brazil are still poorly studied. This work describes several diseases, parasites and commensals from four edible bivalve species, from cultured and exploited stocks, from five sites at Santa Catarina State. Unspecific histopathological symptoms of stress, parasites and commensals were found, including viral gametocytic hypertrophy and rickettsia-like organisms with very low prevalences; the protozoan Nematopsis sp. with high prevalence and, ovarian parasites with low prevalences; and, diverse metazoans, such as turbellarian, sporocysts and metacercariae of trematodes, metacestoda larvae and spionid polychaete. Ray's fluid thioglycollate medium assays for detection of Perkinsus spp. did not detect any infected bivalves. Polymerase chain reaction assays for Marteiliodies chungmuensis in oysters with ovarian lesions produced negative results.

Maintaining the puparial stage of successive generations of a population of tsetse 3 °C lower than normal reduced the numbers of rickettsia-like organisms (RLO) carried by emerging flies. The susceptibility of these flies to midgut infection with Trypanosoma congolense was also significantly reduced compared with control flies held at normal temperature. These results support the view that the relationship between RLO and susceptibility is quantitative – teneral flies with heavier RLO infections being more susceptible to trypanosome infection.

Rickettsia-like organisms (RLO) from tsetse midguts and mosquito cell cultures showed high levels of endochitinase activity. A line of Glossina morsitans morsitans highly susceptible to midgut trypanosome infection and with high incidence of RLO infection showed significantly greater chitinolytic activity than G. austeni which had low RLO incidence and were correspondingly refractory to midgut infection. Midgut infection rates of Trypanosoma brucei rhodesiense in G. m. morsitans showed a dose-related increase when flies were fed N-acetyl-D-glucosamine (GlcNAc) in the infective meal and for 4 subsequent days. A model is proposed for susceptibility to trypanosome infection based on the generation of GlcNAc by RLO endochitinase activity in tsetse pupae inhibiting midgut lectin in teneral flies.

A simple model of human and animal trypanosomiasis is proposed in which the Ross equation for disease transmission is supplemented by a differential equation describing the inheritance of susceptibility in the vector. The model predicts an equilibrium state of balanced polymorphism for the fraction, θ, of susceptible tsetse and the occurrence of periodic epidemics at roughly the observed intervals. A loss of infectivity to tsetse of mechanically transmitted strains of trypano some would seem to be a good evolutionary strategy for the trypanosome. The main implication for disease control is that measures initially reducing trypanosomiasis incidence could trigger off subsequent epidemics. Since θ leads incidence, monitoring θ could give several years advance warning of major epidemics.