India’s marine mammal diversity is recognized as one of the highest in the Indian Ocean Sanctuary, as declared by the International Whaling Commission (Kumaran 2012). Previous reports have documented the presence of 25 species of marine mammals in Indian waters (Kumaran 2002) out of a total of 40 cetacean species recorded in the Indian Ocean region. Despite this rich diversity, marine mammal research remains limited and that related to their parasites is almost non-existent. Wildlife populations are known to harbour a variety of parasitic organisms, and the frequency of parasitic illnesses, as well as the emergence of new diseases, can serve as valuable bioindicators of marine animal health and ecology. Therefore, it is imperative to gather data on the parasite diversity of marine mammals, which will establish a vital baseline for evaluating their impact on both host and ecosystem ecology (Lehnert et al. 2019).
Worldwide, there have been an increasing number of reports of cetacean stranding (Alvarado-Rybak et al. 2020). Numerous human and environmental variables, including parasite infections, have been suggested as contributing to cetacean stranding behaviours. The causes is still a topic of current debate, as according to some authors, parasites should be included among the potential causes of the cetacean debilitation and death. Multiple variables, such as the parasite species, its number, and the host's health state, influence the harm and death that parasitic diseases bring to individuals and populations (Oliveira et al. 2011).
In addition to affecting host behaviour and fitness, parasites can control host population levels, which can sometimes have significant implications on trophic interactions, food webs, competition, biodiversity, and keystone species (Preston and Johnson 2010). Determining the influence of parasites on the ecology and health of marine mammals, and ultimately the size of the cetacean population, is therefore regarded a critical step towards the implementation of appropriate management and conservation measures (Poulin et al. 2016; Lehnert et al. 2012). As any fluctuation in parasite diversity may be an indication of "ecosystem distress syndrome," parasites can be utilised as markers of environmental changes. (Rapport 2007). Marine mammal parasitological researches in this part of the world are relatively scanty. Inevitably, reports and descriptions of parasites appear after the host’s biology. It is, therefore, no surprise that there are very few published reports on parasites of marine mammals.
By influencing variables like survival, reproductive efficiency, or behaviour, parasites appear to have a significant impact on the population dynamics of their hosts. (Morand and Deter 2009). This may result in a drop in host populations or have an impact on hosts in many subtle ways, by consuming resources and changing metabolic rate, territorial behaviour, phenology, intra- and interspecific relationships, mating and foraging success, among other subtle effects, (Møller 1997).
Marine mammals are prone to infection by a wide range of parasites, both endo- and ectoparasites. The respiratory, cardiovascular, and even the auditory systems of mammals are all often parasitized by Metastrongyloidea (Nematoda: Strongylida), known as lungworms, or metastrongyles (Measures 2001). Parasitism of these host compartments by metastrongyles could be associated with negative health consequences for the host or even stranding and death (Measures 2001; Bergeron et al. 1997; Seibel et al. 2010). Nevertheless, metastrongyle nematodes are one of the most diagnosed parasites in marine mammals, and in many circumstances are considered incidental findings of little to no pathologic importance (Geraci and Aubin 1987). The impact of metastrongyle infections on the health of marine mammals is not yet fully understood. Most notably, there is conflicting evidence regarding the connection between metastrongyle infections and cetacean strandings (Bergeron et al. 1997). The presence of the nematodes in the respiratory system could facilitate secondary bacterial infections with undesirable outcomes for the hosts.
Three families of metastrongyles are prominently found in marine mammals, spanning seven genera, with a considerable diversity of species within each genus. Pseudaliidae was the most reported metastrongyle family in marine mammals (Jared R Fischbach and Mauricio A. Seguel). Delphinids and porpoises in both hemispheres frequently have pseudaliid nematodes in their pterygoid sinuses and respiratory tracts (Lehnert et al. 2005). Data regarding the specificity of these nematodes are very rare, partly because host sampling is very difficult and experimental work is almost impossible (Pool et al. 2021); as a result, nothing is known about their life cycle or route of transmission (Faulkner et al. 1998).
Opportunistic finding and molecular identification of a lung worm Halocercus lagenorhynchi, in a stranded marine striped dolphin, Stenella coeruleoalba is reported here. To the best of our knowledge, this is the first report of the infection of Halocercus lagenorhynchi, and the molecular analysis of the same.
Strandings offer important information on the existence and relative abundance of cetacean species as well as on their physiology, behaviour, and health state. (Soares-Castro et al. 2019, García de los Ríos et al. 2021). However, only a very few studies have dealt on parasitic diversity and prevalence in marine mammals stranded in Indian waters.