Tuesday, January 31, 2012

Sacculina carcini: Seafood Anyone?



Sacculina carcini is a parasite that carries on a symbiotic relationship with crabs [1]. This particular genus practices a form of parasitic castration whereby hindering the reproductive ability of its host [2]. According to our text, “Certain parasites appear to make an amazingly boneheaded move: they castrate their hosts (as do many trematodes in mollusks and crustaceans in other crustaceans) or they end up killing the host (most parasitoid insects [3].” Unlike the parasites referred to by Combes, the parasite S. carcini only partially castrates their hosts enough to control their sexual reproduction. It should also be noted, since crabs are found in a variety of marine habitats, that the parasite is vastly allocated in terms of global distribution. In regard to the ecological impact, the parasite prevents the crabs from reproducing eggs other than that of the parasite. This may in turn lead to the steady destruction of crab populations in certain parts of marine habitats [4]. 

Symbiont Description-

Until S. carcini finds a crab to infect, it looks like a small slug floating in the water [1]. The female parasite finds a break in the crab’s protective outer covering, usually in the crab’s joint, and injects itself into the crab [1]. The parasite will grow root-like structures inside the crab that hinder the crab from reproducing anymore, and will continue its life gathering nutrients for the eggs of the parasite [4].  

Host Description-

The definitive host or the host where the parasite reaches sexual maturity is the crab. The parasite, S. carcini, has no intermediate hosts and thus has no acting vector hosts [5]. The parasite only shares a relationship with one host, the crab, so it is considered a holoxenous parasite. The host specificity of S. carcini helps it keep selective pressures on simply one host, and there is then no need to adapt to different kinds of hosts [3].


Life Cycle-

The overall life cycle of S. carcini is rather simple. First, a free-swimming barnacle larva of the parasite finds a crab to inject its inner worm-like body [2]. Next, the parasite makes root-like tentacles to obtain nutrients from the host crab. Finally, after achieving the desired amount of nutrients, the parasite is able to make its eggs, and the host crab will care for the parasitic eggs as it were its very own fertilized eggs [2].


Initially, the consequences of this interaction in nature do not seem terribly severe. However, the prevalence of S. carcini is widespread since their hosts, crabs, are found in most all marine habitats. Most of the crab population is found in the marine habitats of China, Japan, and the United States. Another fair amount of crabs can also be found in the Atlantic Ocean around the United Kingdom [5]. Nearly half of certain crab populations are infected with this parasite [2]. This continued infection could result in the steady decrease of the crab population as a whole. 

An example of regular parasite distribution-

S. carcini is an example of regular parasite distribution because every individual host is infected the same amount as everyone else. Even male and female crabs have the same chance of being parasitized [2]. Likewise, a healthy crab and a sick crab have the same probability of being infected by the parasite because the parasite is only looking for any crab’s joint, and no other one particular attribute of a crab is more desirable to S. carcini than any other one specific attribute. 


[2] Piper, Ross. Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals. Westport, CT: Greenwood, 2007.

[3] Combes, Claude. Parasitism: The Ecology and Evolution of Intimate Interactions; Translated by Isaure De Buron and Vincent A. Connors; with a New Foreword by Daniel Simberloff. Chicago: University of Chicago, 2001.

[5] Zimmer, Carl. Parasite Rex: Inside the Bizarre World of Nature's Most Dangerous Creatures. London: Arrow, 2003.


Monday, January 30, 2012

Hooked on Necator americanus

Introduction to Necator americanus:


As a nematode, the hookworm, Necator americanus, is a parasite that mainly targets humans and is usually released into the soil through human feces. However, it can also be found in cats and dogs. Found in Africa and Europe it is also endemic to certain parts of China, particularly to the Sichuan Province [1].
The bane of human life in certain parts of the world, the infection of the worm can lead to iron deficiency anemia in humans resulting from blood loss. The severity of the infection is directly correlated to the number of worms present in the host. As the number increases the host experiences escalating symptoms such as fatigue, weight loss,  iron deficiency anemia, and possibly death. Children are at a higher mortality risk [2].

Description of N. americanus
Even as an adult N. americanus is a rather small organism, reaching a maximum length range of 9-11mm. However, it has a “bite.” It has two pairs of cutting plates located dorsal and ventral. In its adult form it is cylindrical and has a hook like body. It can live up to five years in the host organism [3].

Life Cycle

Once in the soil, the eggs of N. americanus hatches into its first form, the rhabditiform larva. Growing in the soil, it molts twice until it reaches the filariform stage where it is now ready to infect an unsuspecting human being. Upon coming into contact with a human, it burrows into the host skin. Once inside it travels to the lungs via the circulatory system, where it travels up the trachea and is swallowed. Its destination is the small intestine. It hooks itself onto an inner wall, causing increased blood loss and begins feeding on the host. It matures and reproduces in the small intestine. From there, the eggs pass through the feces and enter the soil where the cycle begins once again [3].  


Found mainly in tropical regions, N. americanus requires warm and moist conditions in order to hatch. Warm temperatures and high levels of rainfall seem to increase the level of transmission as was seen in a study conducted in along the coastal plain of Kwazulu-Natal province, South Africa [4]. Treatment of N. americanus involves administering the drug albendazole which generally wipes out about 90% of the infection. Iron supplements and a high protein diet speeds up the recovery process [5,6]. 

In Parasitism, Claude Combes discusses the advantages of specializing in one species of host. These advantages include limiting intraspecific competition, increasing probability of genetic exchange, and decreased pressure of transmission between hosts by limiting the cycle to one host species. By limiting itself to the human species, N. americanus decreased the stress involved in having multiple host species and the transmission from one particular species to another in the cycle. However, the disadvantage to this is that it limits geographic extension, resources, and increases chances of local extinction in the host. As it is with every strategy there are pros and cons.  

1. Changhua, L., Xiaorong, Z., Dongchuan, Q., Shuhua, X., Hotez, P.J., Defu, Z., Hulian, Z., Mingden, L., Hainan, R., Bing, Z., Haichou, X., Hawdon, J., and F. Zheng. Epidemiology of human hookworm infections among adult villagers in Hejiang and Santai Counties, Sichuan Province,China. Acta Tropica 1999; 73 (3):243-49.
2. Georgiev, V.S. Necatoriasis: Treatment and Developmental Therapeutics. Expert Opin Investing Drugs 2000; 9 (5): 1065-78.
4. Appleton, C., Maurihungirire, M., and E. Gows. The distribution of helminth infections along the coastal plain of Kwazulu-Natal province, South Africa. Annals of Tropical Medicine & Parasitology 1999; 93 (8): 859-68. 
5. http://www.britannica.com/EBchecked/topic/271350/hookworm-disease
6. Holzer, B. R. and F. J. Frey. Differential efficacy of mebendazole and albendazole against Necator americanus but not for Trichuris trichiura infestation. European Journal of Clinical Pharmacology 1987; 32 (6): 635-37.