every instance, it was shown that sufficient 

 testing had not been done to insui'e that all 

 carrier cases had been removed prior to the 

 vector season. Clinical cases and death losses 

 were extensive in the nonsegregated and non- 

 tested herds in the area. This indicated that 

 conditions for natural spread of the disease 

 were present. 



Several herds in the area are being studied 

 for the effects of insecticide and antibiotic con- 

 trol. Preliminary findings indicate that I'egular 

 use of insecticides and antibiotic feeding sup- 

 port the segregation program. 



Trials to continue 



Field trial studies in the Delta Region will be 

 continued until more detailed recommendations 

 on control can be made. Research and field 

 studies will also be continued in the sections 

 of the country where ticks are the primary 

 vector. 



Where anaplasmosis is not too prevalent and 

 the natural vectors of primary importance are 

 biting insects, the disease can be controlled by 

 application of the complement-fixation test, re- 

 moval of reacting carrier animals by segre- 

 gation or slaughter, and proper herd manage- 

 ment practices. 



CATTLE GRUBS 



Cattle grubs, also called "warbles" or 

 "wolves," are the larval stage of the heel fly 

 or warble fly. This pest of economic impor- 

 tance has plagued domestic animal producers 

 throughout recorded history. 



The two species present in this country — 

 Hypoderma lineatum, the common cattle grub, 

 and H. hovis, the northern cattle grub — were 

 probably imported with the fii'st cattle brought 

 in by the early American settlers. Their sub- 

 sequent spread paralleled movement of cattle 

 accompanying settlers during their westward 

 migration. 



The common cattle grub is found throughout 

 the United States. The northern grub, for- 

 merly limited to the Northern States, is gradu- 

 ally spreading south. 



The grub story 



The two species have a similar life history. 



Eggs are attached near the base of the hair on 

 the host. The adult of the northern species de- 

 posits her eggs singly. Her buzzing approach 

 panics cattle, and the fly in pursuit strikes the 

 thighs and the rump. The adult of the com- 

 mon species — more stealthy in its approach — 

 manages to place several eggs on the heels or 

 the underside of the host. 



Eggs hatch in 3 to 6 days and the newly 

 hatched larvae penetrate the skin. They mi- 

 grate through the host to the gullet, dia- 

 phragm, and abdominal viscera. Nine months 

 after penetration of the host the grubs appear 

 under the skin of the animal's back. They come 

 to rest near the median line between the 

 shoulders and hips, cut a breathing hole 

 through the skin, and encyst. The injury at 

 this site is considered most damaging by the 

 leather industry. 



Soon after the larva reaches the back, it 

 molts into the second instar. During this stage, 

 which lasts about 3 weeks, the larva grows and 

 causes considerable tissue inflammation, us- 

 ually accompanied by pus, in and around the 

 cyst. A second molt follows, and the larva 

 enters the third and final stage of its develop- 

 mental period. After some 20 days, the mature 

 larva emerges from the cyst and falls to the 

 ground. It crawls under a protective object 

 and pupates. 



The pupal period lasts fi'om 16 to 75 days, 

 depending on temperature and humidity. At 

 the end of this period, the adult fly emerges 

 from its pupal case and immediately seeks a 

 mate. Adults do not feed, and egg laying 

 promptly follows mating. 



Losses 



Livestock Conservation Incorporated reports 

 annual losses from cattle grubs of over $55 

 million to growers and feeders; $32 million to 

 the tanning and leather industry; and $13 mil- 

 lion to the meat packing industry — a total of 

 more than $100 million annually. 



Control 



The effectiveness of rotenone in controlling 

 cattle grubs has long been recognized. Some 

 workers rate it at a little less than 80 percent 

 as compared with the 85- to 90-percent effec- 

 tiveness claimed for more recently developed 



31 



