El injerto y la absorción de nutrimentos

V.1. Serological surveys in pinnipeds

Bacterial isolates in free-ranging pinnipeds include a wide variety of opportunistic and potentially zoonotic micro-organisms (see Section IV.3). Given the increasing risk of transmission of such pathogens to humans (Harvell et al., 1999) and reciprocally from terrestrial sources to marine animals (Grimes, 1991; Harvell et al., 1999; Stoddard et al., 2005), surveys have been conducted in wild and rehabilitated pinnipeds to monitor the prevalence of different bacteria and viruses present in the marine ecosystem and this is commonly undertaken using serological techniques.

Some care has to be taken in interpreting results when using serological techniques adapted from domestic animals to pinnipeds as only a few studies have characterized pinniped Igs (Cavagnolo and Vedros, 1978; Carter et al., 1990). They have shown that these were generally of a similar size and structure to other mammalian immunoglobulins. Hence it is reasonable to predict that most techniques used in other mammals could be adapted to use in pinnipeds.

When studying bacterial strains in wild pinniped populations, the Western Blot (WB) method presents several advantages compared to other serodiagnostic techniques. Antigens of infectious organisms are immobilized on a solid phase and the test serum probes these directly. The presence of pinniped antibodies aggregated on antigenic particles is revealed after incubation with a combination of enzyme/conjugate (staphylococcal protein A and/or streptococcal protein G) reagent and the corresponding substrate/chromogen reagent. Other techniques such as ELISAs require more effort to determine and validate cut-off values and may also depend on the degree of antigen purity (Aldridge et al., 2001; King et al., 2001).

Probably the main advantage in using WB is that it does not require a cut-off to be determined and results of the test are easier to interpret (Priest et al., 2001). However, WB remains more difficult to apply to a large number of samples and less sensitive than ELISA. There are also likely to be issues with cross-reactivity between serotypes (see Section IV.4.2). An alternative to serology is to search directly for the presence of organisms, its DNA, or its antigens.

V. 2. Passive transfer of immunity in pinnipeds

V.2.1. Placental versus colostral transfer

Passive immunity in mammals is transferred before birth through the placenta and post- partum through colostrum for a short period of time only. Pinnipeds have an endotheliochorial placenta like many carnivores including dogs (Dierauf et al., 1986; Carter and Enders, 2004). In such animals, transfer of Igs via the colostrum is expected to predominate over prenatal transfer, as endotheliochorial placentas are impermeable to Igs (Tizard, 1987). In a first study in southern elephant seals, Marquez et al. (1995) hypothesized that transfer of all IgGs occurred mainly before birth as none were detected in the milk throughout the whole lactation period. However, in a later study, after they had refined their technique, the authors then found out that all classes of Ig (G, M and A) were present in the milk but IgG was the predominant type from birth to the end of the lactation (Marquez et al., 2003). This suggests that transfer of Igs through colostrum is possible. Numerous studies support the evidence that colostrum is the major source of IgG in pinniped pups for the species so far studied (harbour seals: Ross et al., 1993, 1994; grey seals: Carter et al., 1990; northern fur seals: Cavagnolo and Vedros, 1979; northern elephant seals: King et al., 1998; southern elephant seals: Marquez et al., 2003). Nevertheless, the levels of Igs in serum of suckling pinniped pups (Carter et al., 1990; Ross et al., 1994; Beckmen et al., 2003) were generally lower compared to terrestrial species such as canids (10-20 mg/ml, Foale et al., 2003).

V.2.2. Absorption of maternal IgGs by the newborn

Absorption of Igs from the colostrum in pinnipeds presumably only occurs for a limited time as for other mammalian neonates (Casal et al., 1996). Intestinal closure is complete by 25 hours post-partum in bovine calves and feline kittens (Bush and Staley, 1979; Stott et al., 1979; Casal et al., 1996). After the phase of absorption, levels of maternal Igs may decline more or less rapidly, depending on the species.

V.2.3. Factors influencing the passive transfer of IgGs

There are various factors that can potentially influence passive transfer of Igs to pinniped neonates. Inexperience of the dam has been suggested to negatively affect transfer of immunity in pinnipeds. Indeed, northern fur seal pups born to young females were reported to have lower Ig levels compared to pups of older females (Beckmen et al., 2003). On the other hand, in a study with bovine species, it was reported that the levels of IgGs in calves’ serum was lower in lactating females on diet restriction, even though there was no difference in IgG levels in the colostrum (Hough et al., 1990). This suggests that restricting the diet of pinniped lactating dams (see Section III.1) may influence the absorption of maternal Igs in the young. However, there are no studies investigating the influence of such a factor on passive immune transfer and on absorption of colostral Igs by pinniped neonates.

V.3. Immunocompetence of pinniped pups

Pups, like other mammalian neonates, benefit from maternal immunoglobulins for a limited period of time (Banks, 1982). Meanwhile they have to develop their own specific immune response to environmental pathogens. Infectious diseases endemic to a population tend to affect neonates when they are no longer protected by maternal Igs and while their immune system is still functionally immature (Morein et al., 2002). In addition, innate deficiencies in the immune system of mammalian neonates and other causes of impairment

of cellular immunity have been reported to diminish their immune response (Winter et al., 1983; Holan et al., 1991; DeSwart et al., 1994). As a consequence, maternally acquired Igs are crucial for pinniped pups to cope with their environment soon after birth; yet this protection is only temporary.

There are no techniques available to discriminate maternally-derived from innate Igs but it is possible to monitor the fluctuations of serum antibodies in neonates after experimental challenge of their immune system by vaccination against bacterial and viral pathogens (Spencer and Burroughs, 1992; Ross et al., 1994; Beckmen et al., 2003). Using this method, it was demonstrated that pinniped neonates are capable of developing a rapid, strong and highly specific humoral immune response (Ross et al., 1994; Beckmen et al., 2003). Harbour seal newborns even showed a stronger cellular immune response to rabies antigens than dog and cat neonates (Ross et al., 2004). Apart from studies using vaccination, there is no information on the ability of young pinnipeds to mount an immune response towards infectious antigens.