Host-parasite relationships in marine mammals: a fragile and sometimes fatal balance

Marine mammals, like most wild animals, are frequently infected with parasites. When investigating mortality in a marine mammal, the pathologist must carefully evaluate the potential impact of a parasitic infection on the animal’s health. In the vast majority of cases, these parasitic infections are not associated with mortality. While these parasites represent a physiological cost to the host, the host usually manages to accommodate the presence of these organisms. In most cases, there is a host-parasite balance, as the survival and transmission of the parasite usually depend on the host’s survival. However, under certain circumstances, these parasitic infections can be fatal. We present here three cases of fatal parasitic infections in marine mammals observed in 2022 in the St. Lawrence Estuary and Gulf in Quebec. These marine mammals were submitted to CWHC-Quebec by the Réseau québécois d’urgence pour les mammifères marins (RQUMM).

Verminous pneumonia caused by Halocercus sp. in a white-beaked dolphin

Last July, the carcass of a juvenile white-beaked dolphin (Lagenorhynchus albirostris) was found stranded in Blanc-Sablon, Quebec. Although this species is present in the Gulf of St. Lawrence, very few strandings of white-beaked dolphins have been reported by the RQUMM. In fact, this is the first individual of this species to be submitted for examination to the CWHC-Quebec. The animal was in very good body condition, suggesting that it died following a sudden event.

Lung from a white-beaked dolphin that died by a verminous pneumonia. Several yellow raised nodules, containing numerous nematode (Halocercus sp.), are observed at the surface of the lung. One of these nodules was ruptured, creating a pneumothorax.

The size of the animal and the presence of milk in the stomach indicate that it was a yearling that was still actively nursing from its mother. However, the presence of nematodes (roundworms) in the stomach suggests that this dolphin had also started to eat solid food (these gastric parasites are usually acquired through the ingestion of fish containing larvae). Considering all these elements, it is reasonable to assume that it was a calf approaching weaning. Post-mortem examination revealed the presence of a pneumothorax (an abnormal presence of air under pressure in the thoracic cavity) and a very large quantity of long whitish nematodes in the airways. These nematodes formed several irregular cavitated nodules measuring up to 4 cm in diameter in the lungs. One of these nodules was associated with a perforation of the pleura (the outer lining of the lung). Microscopic examination of the lung confirmed the presence of marked pyogranulomatous verminous bronchopneumonia. The morphology of the parasites is characteristic of nematodes of the genus Halocercus sp. Our examinations therefore established that the sudden death of this white-beaked dolphin was caused by a pneumothorax secondary to the rupture of a pulmonary nematode cyst of the genus Halocercus sp. The influx of air into the thorax caused compression of the lungs, preventing them from inflating and thus preventing the animal from breathing. Lung infections with nematodes of the genus Halocercus sp. are very common in cetaceans and usually have few significant clinical consequences. The severity of the infection in this case, leading to pneumothorax, was judged sufficient to be the cause of death. Although it is impossible to draw conclusions about risk factors associated with this infection based on a single case, one may question the effectiveness of this animal’s immune system. A change in the type of available prey (which are likely essential in the transmission of this parasite) could also influence the intensity of such an infection. The examination of more specimens of this species could help us better understand the importance of this parasite in the dynamics of the white-beaked dolphin population.

Verminous pneumonia caused by Otostrongylus circumlitus in a harbour seal

A second species of nematode, Otostrongylus circumlitus, caused the death of a harbour seal (Phoca vitulina). This female juvenile harbour seal was found dead in June 2022 in Sept-Îles, Quebec. The seal was severely emaciated, suggesting death from a chronic disease. During the examination of the lungs, an important quantity of large, white, filiform roundworms (2 mm x 15 cm long) obstructing the airways was observed in both bronchi. These nematodes had a characteristic location and morphology of the species Otostrongylus circumlitus.

Lungs from a harbour seal with a verminous bronchopneumonia. The airways are obstructed by numerous nematodes (round worms) characteristic of Otostrongylus circumlitus.

This parasite primarily affects young pinnipeds that become infected during the early stages of solid food feeding. The parasite mainly infects seals through the ingestion of fish that contain larval parasites encysted in their flesh. Young seals are particularly vulnerable to these parasitic infections because their immune systems are less developed than adults. Otostrongylus circumlitus is a nematode described in different seal species, including harbour seals and ringed seals. In severe infections, seals’ diving capacity could be diminished, limiting their success in capturing prey and potentially causing a loss of body condition. This handicap can threaten their survival, as was likely the case with this young seal. This type of case has rarely been documented in the harbour seal population of the St. Lawrence. However, very few specimens from the at-risk group (weaned juveniles) have been examined by our laboratory so far. Therefore, it is currently impossible to determine the importance of this parasitosis in the dynamics of this population.

Fatal Anisakis sp. verminous gastritis in a harbour porpoise

The third case of fatal parasitic infection was observed in an adult female harbour porpoise (Phocoena phocoena) found dead in September in Baie-des-Sables, Quebec.

Adult female harbour porpoise submitted for necropsy. The animal is in very good body condition, which is suggestive of a mortality following an acute event.

This porpoise had a good body condition, suggestive of a death due to a sudden event.  The most significant observation made on this animal was the presence of a large amount of coagulated blood in the first gastric compartment. This significant gastric bleeding was associated with the presence of numerous nematodes (roundworms) with characteristic morphology of Anisakidae.

The first gastric compartment of the harbour porpoise is filled with a large amount of blood and nematodes (round worms).

The gastric mucosa was sprinkled with a multitude of small ulcerative areas ranging from 1 to 4 mm in diameter covering a significant proportion of the mucosal surface (about 30%). These ulcers often contained nematodes well buried in the thickened wall.These observations lead us to believe that this dolphin died as a result of acute bleeding in the first gastric compartment. The presence of a large number of nematodes associated with gastric ulcers suggests that this parasitic infection was the cause of these fatal hemorrhages. Anisakis simplex is the most commonly described nematode species in the first compartment of harbour porpoise. Porpoises become infected by ingesting fish that act as paratenic (transport) hosts. These fish acquire these parasites after ingesting intermediate hosts mainly composed of invertebrates such as copepods and amphipods. These nematodes are actually very common in porpoises, and although they are often associated with chronic ulcerative gastritis, this condition is rarely identified as the cause of death. This case of mortality resulting from an infection by this species of gastric nematodes is therefore unusual. It can be thought that by “bad luck,” one or more parasites by burying themselves in the mucosa caused a rupture of a blood vessel leading to fatal bleeding by. As mortality cases are rare, this parasite probably has little impact on the dynamics of the common dolphin population.

CWHC-QUEBEC (Stéphane Lair, Charlotte Nury, Lysanne Pagé)