Toxoplasma gondii: A Cat Parasite in St. Lawrence Beluga Whales
In a scientific article published last fall in the journal Diseases of Aquatic Organisms, researchers report the detection of Toxoplasma gondii DNA in the tissues of 44% of the beluga carcasses tested.1 The samples used in this study were obtained from the St. Lawrence belugas mortality surveillance program managed by the CWHC-Quebec Regional Center. The researchers analyzed the heart and brain of 34 belugas found stranded during the years 2009 to 2012. Males had a higher infection rate than females and this parasite was more frequent in calves and juveniles compared to adults.
Toxoplasma gondii is a protozoan that needs two hosts to complete its cycle. Parasites are found in the intestine of the definitive host, which must be a feline (ex: domestic cat, lynx, cougar). The vast majority of mammals, including humans, and several bird species, act as intermediate hosts following the ingestion of cysts released into the environment through the feline’s feces. The protozoan will then become encysted in the organs of the intermediate host where it may remain dormant for a prolonged period. The feline definitive host will become infected by consuming organs of a carrier intermediate host. In the vast majority of cases, T. gondii infections are not associated with clinical signs in both the intermediate and definitive hosts. However, this parasite represents a risk for the fetus when the mother becomes infected during pregnancy. In addition, cases of fatal infections have been documented in several species of wild birds and mammals, including several species of marine mammals. Toxoplasma gondii infections are an important cause of death for two endangered species of marine mammals, the sea otter and the Hawaiian monk seal.
Of the 15 animals in which T. gondii was detected in the study cited here,1 infection by T. gondii was determined to be the cause of death in only one animal. The other cases were either asymptomatic infections (not causing disease), or significant infections but that could not be detected at post-mortem examination. This project demonstrates that the St. Lawrence beluga is highly exposed to a parasite that can potentially be pathogenic. Since the beginning of the St. Lawrence beluga mortality monitoring program in 1983, we have documented seven cases of beluga strandings caused by fatal T. gondii infections, which represents 4% of strandings for which the cause of death has been determined. Although this number seems small, it is undoubtedly an underestimation of the actual number of mortalities associated with this parasite in this population. Furthermore, it is well known that T. gondii can also cause sub-clinical (non-lethal) effects in intermediate hosts, such as behavioral changes, which may also contribute to diminishing the animals ability to survive and to reproduce. Therefore, although the impact of this parasite on St. Lawrence belugas is difficult to assess, its presence certainly does not help the recovery of this threatened population.
Toxoplasma gondii is unlikely to be a newcomer to the beluga ecosystem; before the arrival of the Europeans in America, it was probably already established in lynx. On the other hand, it is reasonable to assume that the arrival of domestic cats in North America has contributed to significantly increase the density of cysts of this parasite in the environment and therefore increasing the “infection pressure” on belugas. In addition, the exposure of belugas to high levels of contaminants known to have negative effects on the immune system (such as PCBs) may also reduce the natural resistance of beluga whales to this parasite and thus favor fatal infections. The reduction of PCBs levels in the environment is therefore good news for this cetacean population and should help improve their ability to cope with pathogens such as T. gondii. On the other hand, as these contaminants are extremely resistant, this improvement will spread over several decades. The best way to reduce the contamination of the environment by this parasite remains the responsible care of domestic cats. By having access to the outside environment without supervision, cats increase their risk of being infected by hunting and eating rodents and birds potentially carrying T. gondii cysts. By defecating outside, they will contribute to increase the density of this parasite in the environment and therefore the risks for potentially sensitive species such as beluga whales. Toxoplasma gondii cysts are extremely resistant and can survive in runoff to reach beluga habitat. A cat kept inside and allowed to go outside with supervision will have no risk of catching this parasite and will therefore not contribute to the problem. In addition, its well-being and life expectancy will be greatly improved. Similarly, because of their potential impact on wildlife or because of animal welfare issues (for cats), it is not recommended to support feral cat colonies.
- Iqbal A, Measures L, Lair S, Dixon B. Toxoplasma gondii infection in stranded St. Lawrence Estuary beluga Delphinapterus leucas in Quebec, Canada. Dis Aquat Organ. 2018;130(3):165-175.