How does screwworm affect
my animal?
The screwworm eggs laid in the
wound hatch into larval screwworm,
which feed on the living tissue of the
animal. Screwworm infestations are
diffi cult to detect at fi rst. Slight movement
might be seen inside a wound,
but the larvae feed and the wound
gradually enlarges and deepens.
A blood tinged fl uid with a distinct
odor may seep from the wound. As
many as 200 vertically oriented larvae
may be packed deep inside the
wound.
Infested animals usually separate
from the herd and lie down in shady
areas. Discomfort, decreased appetite
and lowered milk production are
common. Untreated animals may die
in 7 to 14 days from toxicity or secondary
infections.
Life Cycle (Back to Top)
Eggs: Female D. hominis adults deposit their mature eggs on a blood-feeding arthropod, usually a mosquito or a tick, that is captured by the bot fly in flight. This behavior is known as phoresy (Safdar et al. 2003). As the vector takes a blood meal, the bot fly eggs react to the change in temperature and hatch.
Larvae: The larvae enter the skin through the bite wound or hair follicles, where it then burrows into the skin. The larvae breathe through two posterior spiracles which lie flush with the skin of the host. A study by Pereira et al. (2001) showed that in rats, it took two days for the larvae to reach the subcutaneous tissue. After the seventh day of infestation, the larvae molt to the second instars, and then to third instar after eighteen days. After approximately thirty days, the third instar larvae, which can grow to be relatively large (Fig. 7), crawl out of the host to pupate in the soil. In general, the life of the larvae inside the host is five to 12 weeks (Acha and Szyfres 1994). The larva feed on tissue exudates (Haruki et al 2005 and Kahn 1999).